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1
Lögdberg, Ola. "Turbulent Boundary Layer Separation and Control." Doctoral thesis, KTH, Linné Flow Center, FLOW, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-9821.
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Boundary layer separation is an unwanted phenomenon in most technical applications, as for instance on airplane wings, ground vehicles and in internal flow systems. If separation occurs, it causes loss of lift, higher drag and energy losses. It is thus essential to develop methods to eliminate or delay separation.In the present experimental work streamwise vortices are introduced in turbulent boundary layers to transport higher momentum fluid towards the wall. This enables the boundary layer to stay attached at larger pressure gradients. First the adverse pressure gradient (APG) separation bubbles that are to be eliminated are studied. It is shown that, independent of pressure gradient, the mean velocity defect profiles are self-similar when the scaling proposed by Zagarola and Smits is applied to the data. Then vortex pairs and arrays of vortices of different initial strength are studied in zero pressure gradient (ZPG). Vane-type vortex generators (VGs) are used to generate counter-rotating vortex pairs, and it is shown that the vortex core trajectories scale with the VG height h and the spanwise spacing of the blades. Also the streamwise evolution of the turbulent quantities scale with h. As the vortices are convected downstream they seem to move towards a equidistant state, where the distance from the vortex centres to the wall is half the spanwise distance between two vortices. Yawing the VGs up to 20° do not change the generated circulation of a VG pair. After the ZPG measurements, the VGs where applied in the APG mentioned above. It is shown that that the circulation needed to eliminate separation is nearly independent of the pressure gradient and that the streamwise position of the VG array relative to the separated region is not critical to the control effect. In a similar APG jet vortex generators (VGJs) are shown to as effective as the passive VGs. The ratio VR of jet velocity and test section inlet velocity is varied and a control effectiveness optimum is found for VR=5. At 40° yaw the VGJs have only lost approximately 20% of the control effect. For pulsed VGJs the pulsing frequency, the duty cycle and VR were varied. It was shown that to achieve maximum control effect the injected mass flow rate should be as large as possible, within an optimal range of jet VRs. For a given injected mass flow rate, the important parameter was shown to be the injection time t1. A non-dimensional injection time is defined as t1+ = t1Ujet/d, where d is the jet orifice diameter. Here, the optimal t1+ was 100-200.QC 20100825
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Lögdberg, Ola. "Turbulent boundary layer separation and control /." Stockholm : Mekanik, Kungliga Tekniska högskolan, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-9821.
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Lögdberg, Ola. "Vortex generators and turbulent boundary layer separation control." Licentiate thesis, KTH, Mechanics, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4152.
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Boundary layer separation is usually an unwanted phenomenon in most technical applications as for instance on airplane wings, on ground vehicles and in internal flows such as diffusers. If separation occurs it leads to loss of lift, higher drag and results in energy losses. It is therefore important to be able to find methods to control and if possible avoid separation altogether without introducing a too heavy penalty such as increased drag, energy consuming suction etc.
In the present work we study one such control method, namely the use of vortex generators (VGs), which are known to be able to hinder turbulent boundary layer separation. We first study the downstream development of streamwise vortices behind pairs and arrays of vortex generators and how the strength of the vortices is coupled to the relative size of the vortex generators in comparison to the boundary layer size. Both the amplitude and the trajectory of the vortices are tracked in the downstream direction. Also the influences of yaw and free stream turbulence on the vortices are investigated. This part of the study is made with hot-wire anemometry where all three velocity components of the vortex structure are measured. The generation of circulation by the VGs scales excellently with the VG blade height and the velocity at the blade edge. The magnitude of circulation was found to be independent of yaw angle.
The second part of the study deals with the control effect of vortex generators on three different cases where the strength of the adverse pressure gradient (APG) in a turbulent boundary layer has been varied. In this case the measurements have been made with particle image velocimetry. It was found that the streamwise position where the VGs are placed is not critical for the control effect. For the three different APG cases approximately the same level of circulation was needed to inhibit separation. In contrast to some previous studies we find no evidence of a universal detachment shape factor H12, that is independent of pressure gradient.
4
Williams, Simon. "Three-dimensional separation of a hypersonic boundary layer." Thesis, Imperial College London, 2005. http://hdl.handle.net/10044/1/11450.
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Lögdberg, Ola. "Vortex generators and turbulent boundary layer separation control /." Stockholm : Department of Mechanics, Royal Institute of Technology, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4152.
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Stringer, Marc Alexander. "Separation of air flow over hills." Thesis, University of Reading, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.269964.
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Angele, Kristian. "Experimental studies of turbulent boundary layer separation and control." Doctoral thesis, KTH, Mechanics, 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3565.
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The object ofthe present work is to experimentally study thecase ofa turbulent boundary layer subjected to an AdversePressure Gradient (APG) with separation and reattachment. Thisconstitutes a good test case for advanced turbulence modeling.The work consists ofde sign of a wind-tunnel setup, developmentofP article Image Velocimetry (PIV) measurements and evaluationtechniques for boundary layer flows, investigations ofs calingofb oundary layers with APG and separation and studies oftheturbulence structure ofthe separating boundary layer withcontrol by means ofs treamwise vortices. The accuracy ofP IV isinvestigated in the near-wall region ofa zero pressure-gradientturbulent boundary layer at high Reynolds number. It is shownthat, by careful design oft he experiment and correctly appliedvalidation criteria, PIV is a serious alternative toconventional techniques for well-resolved accurate turbulencemeasurements. The results from peak-locking simulationsconstitute useful guide-lines for the effect on the turbulencestatistics. Its symptoms are identified and criteria for whenthis needs to be considered are presented. Different velocityscalings are tested against the new data base on a separatingAPG boundary layer. It is shown that a velocity scale relatedto the local pressure gradient gives similarity not only forthe mean velocity but also to some extent for the Reynoldsshear-stress. Another velocity scale, which is claimed to berelated to the maximum Reynolds shear-stress, gives the samedegree of similarity which connects the two scalings. However,profile similarity achieved within an experiment is notuniversal and this flow is obviously governed by parameterswhich are still not accounted for. Turbulent boundary layerseparation control by means ofs treamwise vortices isinvestigated. The instantaneous interaction between thevortices and the boundary layer and the change in the boundarylayer and turbulence structure is presented. The vortices aregrowing with the boundary layer and the maximum vorticity isdecreased as the circulation is conserved. The vortices arenon-stationary and subjected to vortex stretching. Themovements contribute to large levels ofthe Reynolds stresses.Initially non-equidistant vortices become and remainequidistant and are con- fined to the boundary layer. Theamount ofi nitial streamwise circulationwas found to be acrucial parameter for successful separation control whereas thevortex generator position and size is ofseco ndary importance.At symmetry planes the turbulence is relaxed to a nearisotropic state and the turbulence kinetic energy is decreasedcompared to the case without vortices.
Keywords:Turbulence, Boundary layer, Separation,Adverse Pressure Gradient (APG), PIV, control, streamwisevortices, velocity scaling.
8
Araki, Daisuke. "Boundary-layer separation on a moving surface in supersonic flow." Thesis, University of Manchester, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.488392.
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Atcliffe, Phillip Arthur. "Effects of boundary layer separation and transition at hypersonic speeds." Thesis, Cranfield University, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.336458.
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Cohen, Giel S. "Control of shock-induced boundary layer separation at supersonic speeds." Thesis, Queen Mary, University of London, 2007. http://qmro.qmul.ac.uk/xmlui/handle/123456789/1724.
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The results of a systematic experimental investigation into the effects that Sub- Boundary Layer Vortex Generators (SBVGs) have on reducing normal shock-induced turbulent boundary layer separation are presented. The freestream Mach number and Reynolds number were M. = 1.45 and Re= 15-9xlO6/m, respectively. All measurement instruments and modifications to the wind tunnel were designed and manufactured as part of the project, specifically for these experiments. Boundary layer, wall pressure measurements and flow visualisation were used in the results analysis. The effects of SBVG height, lateral spacing and location upstream of the shock were investigated. A novel, curved shape SBVG was also evaluated and comparisons against the flat vane SBVG were made. The results show that in all but two cases, separation was completely eliminated. As expected, the largest SBVGs with height, h= 55%5, provided the greatest pressure recovery and maximum mixing. However, the shock pressure rise was highest for this case. Reducing the distance to shock to 108 upstream showed an improvement in the flow quality in the interaction region only. The distortion created by the vortices was also found to be closer to the wall in this case. Increasing the spacing of the SBVG pair to n-- 3 provided the greatest improvement in downstream boundary layer flow quality although this resulted in a small separated region at the foot of the shock. In order to achieve an overall improvement in flow quality, it was suggested that a compromise is required between an increase in wave drag and the extent of reduction of boundary layer separation. The effect of curving the SBVGs provided an improved near wall mixing with an improved static and surface total pressure recovery downstream of the separation region. However, an increased viscous drag resulted from these devices.
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Zare, Shahneh Abolghasern. "Investigation of a sub boundary layer vortex generator for the control of separation in boundary layer-shock wave interaction." Thesis, Queen Mary, University of London, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.485561.
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It is well known that in transonic flows, shQtk waves are fonned over the wings of aircraft. Depending on the strength of the shock wave and the state of the boundary layer, a region of boundary layer separation can occur downstream of the shock. This separation can lead to drastic changes in the flow over the wing which in tum can h~ad to an increase in drag, reduction of lift and buffeting. The aim of this investigation is to assess the potential of sub-boundary layer devices (Sub Boundary Vortex Generators, SBVG) to control a nonnal shock wave/ boundary layer interaction. The experiments have been perfonned at a nominal Mach number of 1.4 and a fre~strcam Reynolds mm1ber of 16x 106 pcr Uii.it length. Detr.ih;d measurements of a fully developed flat plate turbulent boundary layer were used to assess the perfonnance of 8 different SBVG configurations. The SBVG perfonnance was assessed by comparing flow before separation and after the reattachment. Mean flow data such as static and surface total pressure distributions, boundary layer total pressure and velocity profile, surface oil flow visualisation and schlieren method were used in evaluating the perfonnance of SBVGs. The experimental results indicate that the optimum SBVG for the current flow condition is a pair of Tetrahedral Vortex Generator with 30mm length with base and· height dimensions of 3mm by 3mm, which the height is 40% of the boundary layer thickness. The leading and trailing edge of this configuration were 18mm and 3 mm correspondingly.
12
Balzer, Wolfgang. "Numerical Investigation of the Role of Free-Stream Turbulence on Boundary-Layer Separation and Separation Control." Diss., The University of Arizona, 2011. http://hdl.handle.net/10150/204289.
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The aerodynamic performance of lifting surfaces operating at low Reynolds number conditions is impaired by laminar separation. Understanding of the physical mechanisms and hydrodynamic instabilities that are associated with laminar separation and the formation of laminar separation bubbles (LSBs) is key for the design and development of effective and efficient active flow control (AFC) devices. For the present work, laminar separation and its control were investigated numerically by employing highly-accurate direct numerical simulations (DNS).For a LSB on a curved plate, the primary and secondary instability of the uncontrolled flow were investigated. An inviscid Kelvin-Helmholtz (KH) instability was found to be responsible for the shedding of predominantly two-dimensional (2D) vortices. The onset of transition was caused by temporally-growing three-dimensional (3D) disturbances inside the separated region, which were supported by elliptical and hyperbolic secondary instabilities. The hyperbolic instability was demonstrated to be of absolute/global nature. High-amplitude forcing using pulsed vortex generator jets and 2D time-periodic blowing was found to exploit the KH instability and lead to a significant reduction in bubble size. In addition, the 2D forcing was found to suppress the secondary instabilities such that transition to turbulence was delayed.The role of free-stream turbulence (FST) in the transition process was investigated for a LSB on a flat plate. FST was shown to cause the formation of streamwise-elongated streaks inside the boundary layer. For the uncontrolled LSB, increasing the FST levels led to accelerated transition and a reduction in bubble size. The stage of linear disturbance growth due to the inviscid KH instability was not ``bypassed''. Flow control by means of 2D periodic excitation was found to remain effective, since it could exploit the KH instability and suppress secondary absolute instabilities. Transition was initiated by an interaction of the 2D wave introduced by the forcing and the streamwise boundary-layer streaks. The interaction led to a spanwise modulation of the 2D wave, which was amplified due to a convective elliptical instability.
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Yumashev, Dmitry. "Viscous-inviscid interaction in a transonic flow caused by a discontinuity in wall curvature." Thesis, University of Manchester, 2010. http://www.manchester.ac.uk/escholar/uk-ac-man-scw:87020.
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The work addresses an important question of whether a discontinuity in wall curvature can cause boundary layer separation at transonic speeds. Firstly an inviscid transonic flow in the vicinity of a curvature break is analysed. Depending on the ratio of the curvatures, several physically different regimes can exist, including a special type of supersonic flows which decelerate to subsonic speeds without a shock wave, transonic Prandtl-Meyer flow and supersonic flows with a weak shock. It is shown that if the flow can be extended beyond the limiting characteristic, it subsequently develops a shock wave. As a consequence, a fundamental link between the local and the global flow patterns is observed in our problem. From an asymptotic analysis of the Karman-Guderley equation it follows that the curvature discontinuity leads to singular pressure gradients upstream and downstream of the break point. To find these gradients, we perform computations and employ both the hodograph method and the phase portrait technique. The focus is then turned to analysing how the given pressure distribution affects the boundary layer. It is demonstrated that the singular pressure gradient, which appears to be proportional to the inverse cubic root of the distance form the curvature break, corresponds to a special resonant case for the boundary layer upstream of the singularity. Consequently, the boundary layer approaches the interaction region in a pre-separated form. This changes the background on which the viscous-inviscid interaction develops, allowing to construct an asymptotic theory of the incipient viscous-inviscid interaction for our particular problem. The analysis of the interaction which takes place near a weak curvature discontinuity leads to a typical three-tier structure. It appears to be possible to obtain analytical solutions in all the tiers of the triple deck when the curvature break is small. As a result, the interaction equation may be derived in a closed form. The analytical solution of the interaction equation reveals a local minimum in the skin friction distribution, suggesting that a local recirculation zone can develop near the curvature break. In fact, the recirculation zone is formed when the ratio of the curvatures is represented as a series based on negative powers of the logarithm of the Reynolds number. This proves that a discontinuity in wall curvature does evoke boundary layer separation at transonic speeds. The result is fundamentally different from the effect of a curvature break at subsonic and supersonic speeds, as no separation takes place in these two regimes (Messiter & Hu 1975).
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Servini, Pietro. "Roughing up wings : boundary layer separation over static and dynamic roughness elements." Thesis, University College London (University of London), 2018. http://discovery.ucl.ac.uk/10043177/.
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The separation of a boundary layer from an aeroplane wing can have severe effects on aeroplane safety and efficiency, as its occurrence directly results in decreases in lift and increases in drag. Similar considerations apply to other technologies that rely on airfoils, such as drones, helicopters, propellers and wind turbines. Hence recent experimental and numerical work on dynamic roughness elements---small bumps that are made to oscillate up and down at a given frequency---is exciting, as it suggests that these elements are able to delay separation or increase the angle of attack at which it occurs, provided that the Reynolds number is such that the flow remains laminar (Grager et al. (2012), Huebsch (2006), Huebsch et al. (2012), Rothmayer & Huebsch (2011)). Our aims are to gain further insight into whether this is indeed the case; to determine the possible impact of the roughness parameters on the separation of a boundary layer from a surface; and to attempt to understand the physical mechanisms that may be involved, with our focus very much on the pressure gradient. To this end, we will make use of a mathematical approach and exploit asymptotic methods throughout. Three scenarios will be considered, and we will study both dynamic and static roughnesses. The first consists of small roughness elements, which are able to modify the mean flow pressure gradient, on a flat plate. The second will revolve around flow over a hump within a condensed boundary layer, first described by Smith & Daniels (1981), but with the addition of roughness elements on its lee side, in the region in which local separation occurs and the advent of full breakaway separation is seen. The final scenario is set near the leading edge of an airfoil, inclined to the oncoming flow at or near the critical angle of attack, where marginally separated flow exists and a small separation bubble is possible (Ruban (1982), Stewartson, Smith & Kaups (1982)).
15
Skote, Martin. "Studies of turbulent boundary layer flow throughdirect numerical simulation." Doctoral thesis, KTH, Mechanics, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3089.
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Harper, David Keneda. "Boundary Layer Control and Wall-Pressure Fluctuations in a Serpentine Inlet." Thesis, Virginia Tech, 2000. http://hdl.handle.net/10919/32841.
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In this thesis, the benefits of boundary layer control (BLC) in improving aerodynamic performance and engine stability were examined in a compact, serpentine inlet exhibiting flow separation. A 1/14-scale turbofan engine simulator provided the flow through the inlet. The inletâ s mass flow was measured to be 759 scfm (0.939 lbm/s) with an average throat Mach number of 0.23 when the simulator speed was 40 krpm. Boundary layer suction, blowing, and their combination were used to minimize the inletâ s flow separation. The effectiveness of the suction alone and the blowing alone was shown to be approximately equivalent, and the effectiveness of the combined use of both was seen to be better than either one by itself. With blowing and suction flowrates around 1% of the simulatorâ s core flow, the inletâ s distortion was lowered by 40.5% (from 1.55% to 0.922%) while the pressure recovery was raised by 9.7% (from 87.2% to 95.6%). With its reduction in distortion, BLC was shown to allow the simulator to steadily operate in a range that would have otherwise been unstable. Minimizing the flow separation within the inlet was shown to directly relate to measurements from flush-mounted microphones along the inlet wall: as the exit distortion decreased the microphone spectrum also decreased in magnitude. The strong relationship between the aerodynamic profiles and the microphone signal suggests that microphones may be used in an active flow control scheme, where the BLC effort can be tailored for different engine operating conditions. Unfortunately, the sensing scheme used in this experiment showed the microphone signal to continue to decrease even when the separation is overly compensated; therefore refinements must be made before it would be practical in a real application.Master of Science
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Jabbal, Mark. "Understanding the behaviour of synthetic jets in a boundary layer for flow separation control." Thesis, University of Manchester, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.504739.
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Forecasts in air traffic growth, coupled with increases in output of atmospheric carbon pollution, have placed greater demands on the aircraft manufacturing industry to pursue design optimisations that will minimise operating costs and improve aircraft efficiency. Internationally agreed goals, such as the European Vision for 2020 have been set to help alleviate the environmental impact of future aircraft. The development of active flow control technologies, consisting of dynamic actuators with the ability to respond to changes in now conditions at full-scale night, as opposed to the static nature of passive flow control, is believed to be one of the key measures towards the realisation of these goals. Synthetic jet actuators (SJAs) are a promising form of active flow control technology, with a proven capability of demonstrating flow separation control at laboratory scale (Amitay et ai, 1998; Seifert and Pack 1999; Glezer and Amitay, 2002). These devices produce zero-net-mass-flux jets that negate the need for bleed air supply and complex piping. Therefore, SJAs open up possibilities for the efficient, low-energy input control of separated nows on aircraft with the potential for significant improvements in performance, leading to reduced fuel consumption and reductions in the release of environmentally deleterious emissions. Despite the potential of SJAs, little is still known about the detailed flow physical processes of interaction between a SJA and a boundary layer. For practical flow control purposes, the typical vortical structures formed by the interaction of synthetic jets with a boundary layer, their impact near the wall and their relative effectiveness for flow separation control must be considered. The aims of the present research were directed towards furthering understanding in these areas. PlY measurements of circular synthetic jets in quiescent air were undertaken. Firstly, the innuence of the jet now parameters was investigated. The Reynolds number and stroke length influenced the strength and spacing of the ensuing vortex rings respectively. The results were also found to support a performance prediction model for SJAs. Secondly, the influence of SJA geometry for vortex circulation enhancement was investigated. The now behaviour in the orifice duct due to varying orifice depth and the onset of Helmholtz resonance in the cavity due to varymg cavity height were mechanisms responsible for circulation enhancement.
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Clapperton, Surfleet Ben Lewis. "Drag reduction of bluff bodies by passive control of boundary layer transition and separation." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/48456.
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In many sporting activities the athletes and their associated equipment often operate in a range of Reynolds number, Re, that is close to the critical regime where the boundary layer flow undergoes transition from a laminar to a turbulent state. It is well known from numerous studies on circular cylinder and sphere flows that boundary layer transition can be forced to occur at lower Re, and hence drag reduced by delaying flow separation, through the application of, for example, roughness to the surface. This thesis is aimed at increasing understanding of how passive flow control methods might be employed to influence boundary layer flow in order to reduce the drag of bluff bodies. A wind tunnel based research programme was undertaken to study these aspects, including a review of a selection of commonly studied boundary layer tripping methods. The main body of the thesis is devoted to the investigation of two novel passive flow control concepts, developed for this research, which were found to significantly reduce the drag coefficient in the sub-critical Re flow regime of a plain cylinder. Of these two concepts, the main research focus was on identifying the drag reducing mechanisms of a system of passive, continuously blowing jets. It was found that the interaction of the jets and cross-flow induced a very high frequency instability which leads to the downstream formation of tornado-like vortices which are shed into, and identified in the near-wake. It is postulated that the introduction of stream-wise vorticity into the separating shear layer develops favourable drag reducing mechanisms. Discrete cylindrical surface protrusions were additionally found to develop similar effects on flow topology and drag reduction, and were more effective than the passive jets at low Reynolds numbers.
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Ahmed, Ishtiaq. "The interaction of synthetic jets with attached and separating turbulent boundary layer." Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/the-interaction-of-synthetic-jets-with-attached-and-separating-turbulent-boundary-layer(1c09d1b2-ca0f-4e60-afc9-8279d879747b).html.
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Like virtually every other human activity, air transport has an impact on the environment and similar to all other industries environmental impacts and economic issues are exerting more pressure on aircraft sector to meet the demands and implicated conditions. Secondly in today’s competitive industrial performance index new modern techniques are being introduced to improve the aerodynamics so that the efficiency of the newly designed aircrafts could be enhanced. The active flow control techniques have been proved vital towards achieving more effective air flow on the aircraft wing and that eventually helps to increase the lift coefficient at full scale flight. Synthetic jet actuators have been experimentally proved a promising technique towards achieving flow separation delay on the surface they have been deployed on. For the operation the synthetic jet actuators offer a unique characteristic in that they make use of the ambient work fluid and that deny the need of any extra fluid from outside the system and that helps in two fold. Firstly the need to make additional arrangements for air supply through the complex piping system has been ruled out completely. Secondly in the system the addition of any new weight that usually associates with the introduction of any new technique has been avoided. In this work firstly the dye visualization technique is used to study the interaction of the synthetic jet with both types of boundary layers that is laminar and turbulent. Secondly PIV measurements are performed to quantitatively analyze the evolution of vortical structures in the boundary layer. The aim is to understand the fluid dynamics involved in the interaction of the vortical structures with the neal wall fluid that ultimately re-attach the flow with the surface. Lastly an artificial flow separation is generated on the deflected flat plate surface and the synthetic jet is deployed to observe the separation delay on the surface. Various vortical structures have been generated by operating the actuator at varying parameters and issued into the boundary layer upstream of the separation line. The effectiveness of each type of vortices has been evaluated quantitatively to work out the optimum parameters at which the actuator must be operated to achieve the best control effect at the given free-stream condition.
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Xiang, Xue. "Corner effects for oblique shock wave/turbulent boundary layer interactions in rectangular channels." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/287477.
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In a rectangular cross-section wind tunnel a separated oblique shock reflection is set to interact with the turbulent boundary layer (oblique SBLI) both on the bottom wall and in the corner formed by the intersection of the floor with the side-walls. In such a scenario, shock-induced separation is often seen in each of the streamwise corners, resulting in a highly three-dimensional flow field in the near-wall region. To examine how the corner separations can affect the `quasi-two-dimensional' main interaction and by what mechanism this is achieved, an experimental investigation has been conducted. This examines how modifications to the corner separation influence an oblique shock reflection. The nature of the flow field is studied using flow visualisation, Pressure Sensitive Paint and Laser Doppler Anemometry. A nominal freestream Mach number of 2.5 is used for all experiments with a unit Reynolds number of $40\times10^6$m$^{-1}$, and the shock-generator angle is set to $8^\circ$. The flow conditions are chosen to result in substantial separations both in the corners and along the centreline for the baseline case, which is thought to be a good starting point for this study. The results show that the size and shape of central separation vary considerably when the onset and magnitude of corner separation change. The primary mechanism coupling these separated regions appears to be the generation of compression waves and expansion fans as a result of the displacement effect of the corner separation. The presence and strength of the expansion waves have been overlooked in previous studies. This is shown to modify the three-dimensional shock-structure and alter the adverse pressure gradient experienced by the tunnel floor boundary layer. It is suggested that a typical oblique SBLI in rectangular channels features several zones depending on the relative position of the corner waves and the main interaction domain. In particular, it has been shown that the position of the corner `shock' crossing point, found by approximating the corner compression waves by a straight line, is a critical factor determining the main separation size and shape. Thus, corner effects can substantially modify the central separation. This can cause significant growth or contraction of the separation length measured along the symmetry line from the nominally two-dimensional baseline value, giving a fivefold increase from the smallest to the largest observed value. Moreover, the shape and flow topology of the centreline separation bubble is also considerably changed by varying corner effects.
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Das, Shyama Prasad. "An Experimental Study Of Instabilities In Unsteady Separation Bubbles." Thesis, Indian Institute of Science, 2006. https://etd.iisc.ac.in/handle/2005/290.
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The present thesis is an experimental study of some aspects of unsteady two dimensional boundary layers subject to adverse pressure gradient. An adverse pressure gradient usually leads to boundary layer separation or an instability which may result in transition to turbulence. Unsteady boundary layer separation is not yet fully understood and there is no specific criterion proposed in literature for its occurrence. The details of separation depend on the Reynolds number, the geometry of the body (streamlined or bluff) and the type of imposed unsteady motion (impulsive, oscillatory etc.). Similarly there are many unknowns with respect to instability and transition in unsteady boundary layers, especially those having a streamwise variation.
For unsteady flows it is useful to break up the pressure gradient term in the unsteady boundary layer equation into two components:(Formula) is the velocity at the edge of the boundary layer. The first term of the right hand side of this equation may be called the temporal component (Πt) which signifies acceleration or deceleration in time of the free stream and the second term is the spatial component (Πx) which represents the spatial or convective acceleration of the free stream. Many of the studies on instability in unsteady flows found in literature are carried out in straight tubes or channels, where the Πx term is absent. However, in many cases, especially in biological systems both terms are present. An example is the unsteady flow over the moving body of a fish.
To study the effects of Πt and Πx on unsteady separation and instability we have built an unsteady water tunnel where the two components can be systematically varied. The flow is created by a controlled motion of a piston. By a suitable combination of the geometry of the model and the piston motion, different types of separation bubbles may be generated. In our studies the piston motion follows a trapezoidal variation: constant acceleration from rest, followed by constant velocity and then deceleration to zero velocity.
We have chosen two geometries. One is a bluff body and thus has a high value of Πx and other is a small angle diffuser with a divergence angle 6.2° and thus having a small value of Πx. Upstream and downstream of the diffuser are long lengths of constant cross section.
We have performed experiments with the above mentioned geometries placed in the tunnel test section. Flow is visualized using the laser induced fluorescence technique by injecting a thin layer of fluorescein dye on the test wall. Numerical simulations have been done using the software FLUENT. Boundary layer parameters like boundary layer, displacement and momentum thicknesses are calculated from the simulations and used to analyze the experimental results. For the flow in the diffuser, quasi-steady stability analysis of the instantaneous velocity profiles gives a general idea of stability behavior of the flow.
Two types of experiments have been done with the bluff body. One is the unsteady boundary layer separation and the formation of the initial vortex for a flow that is uniformly accelerated from rest. We have found some scalings for the formation time (tv) of the separation vortex. The second type of experiment was to study the vortex shedding from the separating shear layer after the boundary layer has fully separated. At high enough Reynolds number shear layer vortices are seen to shed from the separation bubble. The Strouhal number based on the momentum thickness and the velocity at the edge of the boundary layer just upstream of the separation point is found to vary between 0.004 and 0.008. This value is close to the Strouhal number value of 0.0068 found in laminar separation bubbles on a flat plate.
The second part of the study concerns with the evolution of the flow in the small angle diffuser with a mild variation of the spatial component of the pressure gradient. From the experimental visualizations we have found that the ratio of Πx and Πt at the start of the deceleration phase of the piston motion is an important parameter that determines the type of instability. This value of Πx/Πt is controlled by controlling the piston deceleration: a large deceleration gives a low Πx/Πt value and a low deceleration gives a large Πx/Πt value. Three types of instabilities have been observed in our experiments. In Type I, the first vortex forms at the maximum pressure gradient point (MPGP) and which grows disproportionately with time. However, instability vortices are seen later at other locations around the MPGP. In type II an array of vortices over a certain length are observed; the vortices grow with time. In Type III, which we observe for low decelerations, we observe initial vortices only in the diffuser section in the deceleration phase of the piston motion. Type III instability is similar to the one observed in dynamic stall experiments. In all cases the instability is very localized - it occurs only over some length of the boundary layer. Transition to turbulence, which is also localized, is observed at higher Reynolds numbers. The non-dimensional time for vortex formation is not very different from that found in straight channel experiments. Quasi-steady linear stability analyses for the boundary layer at the MPGP both for the top and the bottom walls show that the flow is absolutely unstable for some cases.
In summary, the thesis looks at in a unified way the separation and instability of unsteady boundary layers with reverse flow. It is hoped that the results will be useful in predicting and understanding onset of separation and instability in practically occurring unsteady flows.
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Das, Shyama Prasad. "An Experimental Study Of Instabilities In Unsteady Separation Bubbles." Thesis, Indian Institute of Science, 2006. http://hdl.handle.net/2005/290.
Full textAbstract:
The present thesis is an experimental study of some aspects of unsteady two dimensional boundary layers subject to adverse pressure gradient. An adverse pressure gradient usually leads to boundary layer separation or an instability which may result in transition to turbulence. Unsteady boundary layer separation is not yet fully understood and there is no specific criterion proposed in literature for its occurrence. The details of separation depend on the Reynolds number, the geometry of the body (streamlined or bluff) and the type of imposed unsteady motion (impulsive, oscillatory etc.). Similarly there are many unknowns with respect to instability and transition in unsteady boundary layers, especially those having a streamwise variation.
For unsteady flows it is useful to break up the pressure gradient term in the unsteady boundary layer equation into two components:(Formula) is the velocity at the edge of the boundary layer. The first term of the right hand side of this equation may be called the temporal component (Πt) which signifies acceleration or deceleration in time of the free stream and the second term is the spatial component (Πx) which represents the spatial or convective acceleration of the free stream. Many of the studies on instability in unsteady flows found in literature are carried out in straight tubes or channels, where the Πx term is absent. However, in many cases, especially in biological systems both terms are present. An example is the unsteady flow over the moving body of a fish.
To study the effects of Πt and Πx on unsteady separation and instability we have built an unsteady water tunnel where the two components can be systematically varied. The flow is created by a controlled motion of a piston. By a suitable combination of the geometry of the model and the piston motion, different types of separation bubbles may be generated. In our studies the piston motion follows a trapezoidal variation: constant acceleration from rest, followed by constant velocity and then deceleration to zero velocity.
We have chosen two geometries. One is a bluff body and thus has a high value of Πx and other is a small angle diffuser with a divergence angle 6.2° and thus having a small value of Πx. Upstream and downstream of the diffuser are long lengths of constant cross section.
We have performed experiments with the above mentioned geometries placed in the tunnel test section. Flow is visualized using the laser induced fluorescence technique by injecting a thin layer of fluorescein dye on the test wall. Numerical simulations have been done using the software FLUENT. Boundary layer parameters like boundary layer, displacement and momentum thicknesses are calculated from the simulations and used to analyze the experimental results. For the flow in the diffuser, quasi-steady stability analysis of the instantaneous velocity profiles gives a general idea of stability behavior of the flow.
Two types of experiments have been done with the bluff body. One is the unsteady boundary layer separation and the formation of the initial vortex for a flow that is uniformly accelerated from rest. We have found some scalings for the formation time (tv) of the separation vortex. The second type of experiment was to study the vortex shedding from the separating shear layer after the boundary layer has fully separated. At high enough Reynolds number shear layer vortices are seen to shed from the separation bubble. The Strouhal number based on the momentum thickness and the velocity at the edge of the boundary layer just upstream of the separation point is found to vary between 0.004 and 0.008. This value is close to the Strouhal number value of 0.0068 found in laminar separation bubbles on a flat plate.
The second part of the study concerns with the evolution of the flow in the small angle diffuser with a mild variation of the spatial component of the pressure gradient. From the experimental visualizations we have found that the ratio of Πx and Πt at the start of the deceleration phase of the piston motion is an important parameter that determines the type of instability. This value of Πx/Πt is controlled by controlling the piston deceleration: a large deceleration gives a low Πx/Πt value and a low deceleration gives a large Πx/Πt value. Three types of instabilities have been observed in our experiments. In Type I, the first vortex forms at the maximum pressure gradient point (MPGP) and which grows disproportionately with time. However, instability vortices are seen later at other locations around the MPGP. In type II an array of vortices over a certain length are observed; the vortices grow with time. In Type III, which we observe for low decelerations, we observe initial vortices only in the diffuser section in the deceleration phase of the piston motion. Type III instability is similar to the one observed in dynamic stall experiments. In all cases the instability is very localized - it occurs only over some length of the boundary layer. Transition to turbulence, which is also localized, is observed at higher Reynolds numbers. The non-dimensional time for vortex formation is not very different from that found in straight channel experiments. Quasi-steady linear stability analyses for the boundary layer at the MPGP both for the top and the bottom walls show that the flow is absolutely unstable for some cases.
In summary, the thesis looks at in a unified way the separation and instability of unsteady boundary layers with reverse flow. It is hoped that the results will be useful in predicting and understanding onset of separation and instability in practically occurring unsteady flows.
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Chan, Lucilla. "Turbulent boundary-layer flow separation as portrayed by a two-dimensional, second-order closure model." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp05/NQ66343.pdf.
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Dogan, Eda. "Experimental Investigation Of Boundary Layer Separation Control Using Steady Vortex Generator Jets On Low Pressure Turbines." Master's thesis, METU, 2012. http://etd.lib.metu.edu.tr/upload/12614485/index.pdf.
Full textAbstract:
This thesis presents the results of an experimental study that investigates the effects of steady vortex generator jets (VGJs) integrated to a low pressure turbine blade to control the laminar separation bubble occurring on the suction surface of the blade at low Reynolds numbers. The injection technique involves jets issued from the holes located near the suction peak of the test blade which is in the middle of a five-blade low speed linear cascade facility. Three injection cases are tested with different blowing ratio values ranging from low to high. Surface pressure and particle image velocimetry (PIV) measurements are performed. The results show that steady VGJ is effective in eliminating the laminar separation bubble. Also it is observed that to have fully developed attached boundary layer, blowing ratio should be chosen accordingly since a very thin separation zone still exists at low blowing ratios.
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Ashcraft, Timothy Allen. "Control of Boundary Layer Separation and the Wake of an Airfoil using ns-DBD Plasma Actuators." Thesis, The University of Arizona, 2016. http://hdl.handle.net/10150/605119.
Full textAbstract:
The efficacy of nanosecond pulse driven dielectric barrier discharge (ns-DBD) plasma actuators for boundary layer separation and wake control is investigated experimentally. A single ns-DBD plasma actuator is placed at the leading edge of a NACA 0012 airfoil model. Both baseline and controlled flow fields are studied using static pressure measurements, Particle Image Velocimetry (PIV) and Constant Temperature Anemometry (CTA). Experiments are primarily performed at Re = 0.74 x 10⁶ and α = 18°. CP, PIV and CTA data show that a forcing frequency of F⁺ = 1.14 is optimal for separation control. CTA surveys of the wake at x/c = 7 indicate three approximate regimes of behavior. Forcing in the range 0.92 < F⁺ < 1.52 results in the best conditions for separation control over the airfoil, but has no dominant signature in the wake at x/c = 7. Excitation in the range of 0.23 < F⁺ < 0.92 produces a single dominant frequency in the wake while F⁺ < 0.23 shows behavior representing a possible impulse response or nonlinear effects. PIV data confirm these observations in all three regimes. Cross-correlations of CTA data are also employed to evaluate the two-dimensionality of the excited wake. The initial results presented here are part of an ongoing effort to use active flow control (AFC), in the form of ns-DBDs, as an enabling technology for the study of unsteady aerodynamics and vortex-body interactions.
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Nilsson, Erik Olof. "Fluxes and Mixing Processes in the Marine Atmospheric Boundary Layer." Doctoral thesis, Uppsala universitet, Luft-, vatten och landskapslära, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-195875.
Full textAbstract:
Atmospheric models are strongly dependent on the turbulent exchange of momentum, sensible heat and moisture (latent heat) at the surface. Oceans cover about 70% of the Earth’s surface and understanding the processes that control air-sea exchange is of great importance in order to predict weather and climate. In the atmosphere, for instance, hurricane development, cyclone intensity and track depend on these processes. Ocean waves constitute an obvious example of air-sea interaction and can cause the air-flow over sea to depend on surface conditions in uniquely different ways compared to boundary layers over land. When waves are generated by wind they are called wind sea or growing sea, and when they leave their generation area or propagate faster than the generating wind they are called swell. The air-sea exchange is mediated by turbulent eddies occurring on many different scales. Field measurements and high-resolution turbulence resolving numerical simulations have here been used to study these processes. The standard method to measure turbulent fluxes is the eddy covariance method. A spatial separation is often used between instruments when measuring scalar flux; this causes an error which was investigated for the first time over sea. The error is typically smaller over ocean than over land, possibly indicating changes in turbulence structure over sea. Established and extended analysis methods to determine the dominant scales of momentum transfer was used to interpret how reduced drag and sometimes net upward momentum flux can persist in the boundary layer indirectly affected by swell. A changed turbulence structure with increased turbulence length scales and more effective mixing was found for swell. A study, using a coupled wave-atmosphere regional climate model, gave a first indication on what impact wave mixing have on atmosphere and wave parameters. Near surface wind speed and wind gradients was affected especially for shallow boundary layers, which typically increased in height from the introduced wave-mixing. A large impact may be expected in regions of the world with predominant swell. The impact of swell waves on air-sea exchange and mixing should be taken into account to develop more reliable coupled Earth system models.
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von, Stillfried Florian. "Computational fluid-dynamics investigations of vortex generators for flow-separation control." Doctoral thesis, KTH, Turbulens, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-94879.
Full textAbstract:
Many flow cases in fluid dynamics face undesirable flow separation due to ad-verse pressure gradients on wall boundaries. This occurs, for example, due togeometrical reasons as in a highly curved turbine-inlet duct or on flow-controlsurfaces such as wing trailing-edge flaps within a certain angle-of-attack range.Here, flow-control devices are often used in order to enhance the flow and delayor even totally eliminate flow separation. Flow control can e.g. be achieved byusing passive or active vortex generators (VGs) for momentum mixing in theboundary layer of such flows. This thesis focusses on such passive and activeVGs and their modelling for computational fluid dynamics investigations. First, a statistical VG model approach for passive vane vortex genera-tors (VVGs), developed at the Royal Institute of Technology Stockholm andthe Swedish Defence Research Agency, was evaluated and further improvedby means of experimental data and three-dimensional fully-resolved computa-tions. This statistical VVG model approach models those statistical vortexstresses that are generated at the VG by the detaching streamwise vortices.This is established by means of the Lamb-Oseen vortex model and the Prandtllifting-line theory for the determination of the vortex strength. Moreover, thisansatz adds the additional vortex stresses to the turbulence of a Reynolds-stresstransport model. Therefore, it removes the need to build fully-resolved three-dimensional geometries of VVGs in a computational fluid dynamics mesh. Usu-ally, the generation of these fully-resolved geometries is rather costly in termsof preprocessing and computations. By applying VVG models, the costs arereduced to that of computations without VVGs. The original and an improvedcalibrated passive VVG model show sensitivity for parameter variations suchas the modelled VVG geometry and the VVG model location on a flat plate inzero- and adverse-pressure-gradient flows, in a diffuser, and on an airfoil withits high-lift system extracted. It could be shown that the passive VG modelqualitatively and partly quantitatively describes correct trends and tendenciesfor these different applications. In a second step, active vortex-generator jets (VGJs) are considered. They were experimentally investigated in a zero-pressure-gradient flat-plate flow atTechnische Universitä̈t Braunschweig, Germany, and have been re-evaluated for our purposes and a parameterization of the generated vortices was conducted. Dependencies of the generated vortices and their characteristics on the VGJsetup parameters could be identified and quantified. These dependencies wereused as a basis for the development of a new statistical VGJ model. This modeluses the ansatz of the passive VVG model in terms of the vortex model, theadditional vortex-stress tensor, and its summation to the Reynolds stress ten-sor. Yet, it does not use the Prandtl lifting-line theory for the determinationof the circulation but an ansatz for the balance of the momentum impact thatthe VGJ has on the mean flow. This model is currently under developmentand first results have been evaluated against experimental and fully-resolvedcomputational results of a flat plate without pressure gradient.QC 20120511
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Rao, Martin Kiran. "An experimental investigation of the use of air jet vortex generators to control shock induced boundary layer separation." Thesis, City University London, 1988. http://openaccess.city.ac.uk/7551/.
Full textAbstract:
Air jet vortex generators were originally investigated by R.A. Wallis. Results showed that their effectiveness in delaying shock induced boundary layer separation was not as good as conventional vane type generators. Recent low speed wind tunnel tests carried out at City University indicated that the strength of the vortex could be increased considerably by using rectangular jet exits rather than round ones as used by Wallis. On this basis an investigation into air jet vortex generators was undertaken to find out whether similar improvements in vortex strength may be gained at transonic speeds and hence achieve a more effective method of controlling shock induced boundary layer separation. It was felt that in order to design air jet vortex generators it would be necessary to understand the mechanism by which an air jet forms a vortex, and to evaluate the effects of various jet parameters on vortex size, strength and position. The parameters investigated in this thesis were: (i) exit shape (ii) exit size (iii) jet direction (iv) jet inclination and (v) blowing pressure. The tests were conducted using a combination of high speed wind tunnel tests and flow visualisation in a water tunnel. The wind tunnel tests used the half aerofoil or 'bump technique' as used by Wallis. Bumps with thickness to chord ratios of 8%, 10% and 14% were tested. Increasing the thickness of the bumps resulted in higher local Mach numbers ahead of the shock and hence an increase in the severity of the shock induced separation. Vane vortex generators designed using the criteria laid down by H.H. Pearcey were used to establish a datum of control effectiveness. As a result of this investigation a method by which an air jet forms a vortex has been proposed together with a hypothesis on the influence of the various jet parameters. The results have shown that air jet vortex generators can be designed to be more effective than conventional vane type generators. Based on the work reported in this thesis a set of design guidelines has been proposed together with suggestions for further work.
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Weier, Tom. "Elektromagnetische Strömungskontrolle mit wandparallelen Lorentzkräften in schwach leitfähigen Fluiden." Forschungszentrum Dresden, 2010. http://nbn-resolving.de/urn:nbn:de:bsz:d120-qucosa-28435.
Full textAbstract:
Die vorliegende Arbeit widmet sich der, vorwiegend experimentellen, Untersuchung der Wirkung wandparalleler Lorentzkräfte in Strömungsrichtung auf Grenzschichtprofile und Körperumströmungen. Die Themen - Beeinflussung der Grenzschicht an ebenen Platten mit stationären Lorentzkräften - Kontrolle von Strömungsablösungen an Zylindern und symmetrischen Profilen mit stationären Lorentzkräften - Beeinflussung von Zylindernachläufen und abgelösten Tragflügelumströmungen mit zeitlich periodischen Lorentzkräften werden behandelt.
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Weier, Tom. "Elektromagnetische Strömungskontrolle mit wandparallelen Lorentzkräften in schwach leitfähigen Fluiden." Forschungszentrum Rossendorf, 2006. https://hzdr.qucosa.de/id/qucosa%3A21670.
Full textAbstract:
Die vorliegende Arbeit widmet sich der, vorwiegend experimentellen, Untersuchung der Wirkung wandparalleler Lorentzkräfte in Strömungsrichtung auf Grenzschichtprofile und Körperumströmungen. Die Themen - Beeinflussung der Grenzschicht an ebenen Platten mit stationären Lorentzkräften - Kontrolle von Strömungsablösungen an Zylindern und symmetrischen Profilen mit stationären Lorentzkräften - Beeinflussung von Zylindernachläufen und abgelösten Tragflügelumströmungen mit zeitlich periodischen Lorentzkräften werden behandelt.
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Östlund, Jan. "Supersonic flow separation with application to rocket engine nozzles." Doctoral thesis, KTH, Mechanics, 2004. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3793.
Full textAbstract:
The increasing demand for higher performance in rocketlaunchers promotes the development of nozzles with higherperformance, which basically is achieved by increasing theexpansion ratio. However, this may lead to flow separation andensuing instationary, asymmetric forces, so-called side-loads,which may present life-limiting constraints on both the nozzleitself and other engine components. Substantial gains can bemade in the engine performance if this problem can be overcome,and hence different methods of separation control have beensuggested. However, none has so far been implemented in fullscale, due to the uncertainties involved in modeling andpredicting the flow phenomena involved.
In the present work the causes of unsteady and unsymmetricalflow separation and resulting side-loads in rocket enginenozzles are investigated. This involves the use of acombination of analytical, numerical and experimental methods,which all are presented in the thesis. A main part of the workis based on sub-scale testing of model nozzles operated withair. Hence, aspects on how to design sub-scale models that areable to capture the relevant physics of full-scale rocketengine nozzles are highlighted. Scaling laws like thosepresented in here are indispensable for extracting side-loadcorrelations from sub-scale tests and applying them tofull-scale nozzles.
Three main types of side-load mechanisms have been observedin the test campaigns, due to: (i) intermittent and randompressure fluctuations, (ii) transition in separation patternand (iii) aeroelastic coupling. All these three types aredescribed and exemplified by test results together withanalysis. A comprehensive, up-to-date review of supersonic flowseparation and side-loads in internal nozzle flows is givenwith an in-depth discussion of different approaches forpredicting the phenomena. This includes methods for predictingshock-induced separation, models for predicting side-loadlevels and aeroelastic coupling effects. Examples are presentedto illustrate the status of various methods, and theiradvantages and shortcomings are discussed.
A major part of the thesis focus on the fundamentalshock-wave turbulent boundary layer interaction (SWTBLI) and aphysical description of the phenomenon is given. Thisdescription is based on theoretical concepts, computationalresults and experimental observation, where, however, emphasisis placed on the rocket-engineering perspective. This workconnects the industrial development of rocket engine nozzles tothe fundamental research of the SWTBLI phenomenon and shows howthese research results can be utilized in real applications.The thesis is concluded with remarks on active and passive flowcontrol in rocket nozzles and directions of futureresearch.
The present work was performed at VAC's Space PropulsionDivision within the framework of European spacecooperation.
Keywords:turbulent, boundary layer, shock wave,interaction, overexpanded,rocket nozzle, flow separation,control, side-load, experiments, models, review.
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Hansen, Laura C. "Phase Locked Flow Measurements of Steady and Unsteady Vortex Generator Jets in a Separating Boundary Layer." Diss., CLICK HERE for online access, 2005. http://contentdm.lib.byu.edu/ETD/image/etd763.pdf.
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Zhou, Jue. "Numerical investigation of the behaviour of circular synthetic jets for effective flow separation control." Thesis, University of Manchester, 2010. https://www.research.manchester.ac.uk/portal/en/theses/numerical-investigation-of-the-behaviour-of-circular-synthetic-jets-for-effective-flow-separation-control(6faae20e-82d6-4efb-8044-c02b089d667c).html.
Full textAbstract:
The stringing regulation on greenhouse gases emissions coupled with the rising fuel price and the growth in aviation transportation have imposed increasing demands on the aircraft industry to develop revolutionary technologies to meet such challenges. Methods of delaying flow separation on aircraft high lift systems have been sought which can lead to an increase in the aircraft performance and ultimately a reduction in aircraft operational costs and its impact on the environment. Synthetic jet actuators are a promising method of delivering flow control for aircraft applications due to their ability to inject momentum to an external flow without net mass flux and their potential in being integrated in MEMS through micro-fabrication with relative ease. It has been demonstrated in many laboratory experiments that synthetic jets are capable of delaying flow separation on aerodynamic bodies of various shapes. However, currently the operating conditions of synthetic jets are mostly chosen by trial-and-error, and thus the flow control effectiveness varies from one experiment to another. In order to deliver an effective flow separation control which achieves a desired control effect at minimum energy expenditure, a better understanding of the fluid mechanics of the behaviour of synthetic jets and the interaction between synthetic jets and a boundary layer are required. The aims of the present research were to achieve such a goal through a series of purposely designed numerical simulations. Firstly, synthetic jets issued from a circular orifice into quiescent air were studied to understand the effect of dimensionless parameters on the formation and the extent of roll-up of vortex rings. The computational results confirmed that the Stokes number determines the strength of vortex roll-up of a synthetic jet. Based on the computational results, a parameter map was produced in which three different operational regimes of synthetic jets were indentified and a criterion for vortex roll-up was also established. A circular synthetic jet issued into a zero-pressure-gradient laminar boundary layer was then investigated. The capability of FLUENT in modelling the key characteristics of synthetic jets was validated using experimental data. The formation and evolution of coherent structures produced by the interaction between synthetic jets and a boundary layer, as well as their near-wall effect in terms of the wall shear stress, were examined. A parameter map illustrating how the appearance of the vortical structures and their corresponding shear stress patterns vary as the synthetic jet operating condition changes was established. In addition, the increase in the wall shear stress relative to the jet-off case was calculated to evaluate their potential separation control effect.Finally, the study moved one step forward to investigate the flow separation control effect of an array of three circular synthetic jets issued into a laminar boundary layer which separates downstream on an inclined plate. The impact of synthetic jets on the boundary layer prior to separation and the extent of flow separation delay on the flap, at a range of synthetic jet operating conditions, were examined and the correlation between them was investigated. Furthermore, the optimal operating conditions for this synthetic jet array in the current study were identified by considering both the flow control effect and the actuator power consumption. The characteristics of the corresponding vortical structures were also examined.The findings from this work have produced some further insights of the behaviour and the interaction between synthetic jets and a boundary layer, which will be useful for ensuring an effective application of synthetic jets in practical settings.
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Perry, Michael. "The Effect of Freestream Turbulence on Separation at Low Reynolds Numbers in a Compressor Cascade." Thesis, Virginia Tech, 2007. http://hdl.handle.net/10919/35834.
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A parametric study was performed to observe and quantify the effect of varying turbulence intensities on separation and performance in a compressor cascade at low Reynolds numbers. Tests were performed at 25o and 37.5o stagger angle, negative and positive angles of incidence up until the point of full stall, Reynolds numbers from 6 x 104 to 12.5 x 104, and turbulence intensities from approximately 0.7% â 8%. Additionally, oil flow techniques were combined with static tap data to visualize the boundary layer characteristics at various test conditions. The overall performance of the cascade was presented and evaluated through mass-averaged total pressure loss coefficients.
The results of the study showed that the best efficiency (lowest pressure loss coefficient) was determined by separation characteristics for any angle of attack. While adding turbulence generally delayed separation, in some cases, adding turbulence to a separated airfoil resulted in decreased performance. Very similar separation characteristics were observed for the full range of Reynolds numbers and stagger, with the higher stagger setting giving slightly better performance. It was shown that a large percentage of total pressure losses can be recovered by applying the appropriate turbulence intensity at any angle of attack, which is relevant to possibilities for active control of such flows.Master of Science
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DELLACASAGRANDE, MATTEO. "Experimental study of the boundary layer separation and transition processes under turbine-like conditions by means of advanced post-processing techniques." Doctoral thesis, Università degli studi di Genova, 2019. http://hdl.handle.net/11567/944948.
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In this work the transition process of the boundary layer (BL) evolving under turbine-like
conditions has been experimentally investigated in details. The effects of the Reynolds
number (Re), the free-stream turbulence intensity (Tu) and the adverse pressure gradient
(APG) imposed to the flow have been studied for a large variation of these parameters, since
they are known to strongly influence the separation and transition processes of the boundary
layer. Emphasis has been put on both the statistical and the dynamic behaviour of the flows
at hand, that have been experimentally characterized by means of advanced and ad-hoc
developed post processing techniques.
The study of the effects of the Reynolds number and the Tu level on the development of
laminar separation bubbles (LSB) under fixed APG is presented in the first part of this work.
The mechanisms by which the variations of Re and Tu act on the bubble size were found to
be substantially different and the coexistence of different amplification mechanisms has been
observed in the LSBs for high Tu levels.
In case of by-pass transition, the effects of the APG has been investigated with respect to
the zero pressure gradient condition. The transition process has been found to be more rapid
due to the APG imposed to the flow with respect to the zero pressure gradient case. The
profiles of the mean streamwise velocity and velocity fluctuation rms obtained by means of
Hot-Wire instrumentation showed a self-similar behavior in the laminar part of the boundary
layer. For what concern the effects of the Tu level on the velocity and rms of velocity
fluctuation profiles, the high free-stream turbulence has been found to reduce the effects of
the pressure gradient on the curvature of the mean velocity profiles and shifting the maximum
of the turbulence peak towards the wall.
In order to shed light on the effects of the APG variation on the statistical and dynamic
behaviour of LSB, as well as to provide a complete experimental database containing
information about the effects of Re, Tu and APG in case of both attached and separated
flows, a new test section has been designed in the second part of this work allowing the
continuous variation of the pressure gradient imposed to the flow. In case of separated flows,
the separation position was found to move downstream when the APG is reduced and the
bubble becomes longer. However, the bubble thickness is reduced with respect to the higher
APGs conditions.
Proper Orthogonal Decomposition (POD) has been adopted to reduce the large amount
of experimental data collected, obtaining a statistical treatment of the main dynamics at hand
in terms of their energy content. Moreover, with the aim of characterizing the coexistence
of structures with different energy within the flow (e.g., boundary layer streaks, Kelvin-
Helmholtz and free-stream vortices) a variant of the classical POD procedure has been
proposed. The application of this technique in case of both attached and separated flows
highlighted the presence of free-stream structures near the edge of the boundary layer where
the transition process has been found to occur, suggesting that free-stream structures can
actually play a crucial role in the evolution and breakdown of structures growing into the
boundary layer, thus leading transition.
Finally, the analysis of the statistical quantities of the flows at hand (i.e. BL integral
parameters) has been carried out for all the acquired conditions with the aim of developing
new empirical correlations for the prediction of the transition onset and length in case of
separated flows. Data collected during both the measuring campaigns allowed the tuning of
the proper coefficients in order to take into account for the variation of all the parameters
considered in this work. The proposed correlations have been found to fit both the collected
data as well as other experimental data available in literature.
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Thompson, Andrew C. "Investigation and Simulation of Ion Flow Control over a Flat Plate and Compressor Cascade." Thesis, Virginia Tech, 2009. http://hdl.handle.net/10919/33078.
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An investigation of ion flow control was performed to determine the effect of a positive, DC corona discharge on the boundary layer profile along a flat plate and to examine its ability to prevent separated flow in a low-speed compressor cascade. Flat plate tests were performed for two electrode configurations at free-stream velocity magnitudes of 2.5, 5, 7.5, and 10 m/s. Boundary layer velocity profile data was taken to measure the performance of the electrode pairs. Ion flow control was also tested in the compressor cascade for a stagger angle of 25° at angles of attack equal to 6° and 12°. The cascade tests were performed at free-stream velocities of 5 and 10 m/s. Static tap data was used to characterize separated flow behavior and the effect of ion flow control on flow reattachment. A computational model was developed using the commercial CFD software Fluent. This model simulates ion flow control as a body force applied to the flow through user-defined functions.
The study showed that the corona discharge has the ability to increase near-wall velocities and reduce the thickness of the boundary layer for flow over a flat plate. Ion flow control successfully prevented trailing edge separation in a compressor cascade for angles of attack of 6° and 12°; however, the flow control scheme was not able to prevent leading edge separation for angle of attack equal to 12°. The ion flow control CFD model accurately predicted flow behavior for both the flat plate and cascade experiments. The numerical model was able to simulate the boundary layer velocity profiles for flat plate tests with good accuracy, and was also able to predict the flow behavior over a compressor blade. The model was able to show the trends of separated and reattached flow over the blade surface.Master of Science
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Fakhari, Ahmad. "Wall-Layer Modelling of massive separation in Large Eddy Simulation of coastal flows." Doctoral thesis, Università degli studi di Trieste, 2015. http://hdl.handle.net/10077/11104.
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2012/2013The subject of modelling flow near wall is still open in turbulent wall bounded flows, since there is no wall layer model which works perfectly. Most of the present models work well in attached flows, specially for very simple geometries like plane channel flows. Weakness of the models appears in complex geometries, and many of them do not capture flow separation accurately in detached flows, specially when the slope of wall changes gradually. In many engineering applications, we deal with complex geometries. A possible way to simulate flows influenced by complex geometry using a structured grid, is to consider the geometry as immersed boundary for the simulation. Current wall layer models for the immersed boundaries are more complex and less accurate than the body-fitted cases (cases without immersed boundaries). In this project the accuracy of wall layer model in high Reynolds number flows is targeted, using LES for attached flows as well as detached flows (flows with separation). In addition to the body fitted cases, wall layer model in the presence of immersed boundaries which is treated totally different also regarded. A single solver LES-COAST (IE-Fluids, University of Trieste) is used for the flow simulations, and the aim is to improve wall layer model in the cases with uniform coarse grid. This is in fact novelty of the thesis to introduce a wall layer model applied on the first off-wall computational node of a uniform coarse grid, and merely use LES on the whole domain. This work for the immersed boundaries is in continuation of the methodology proposed by Roman et al. (2009) in which velocities at the cells next to immersed boundaries are reconstructed analytically from law of the wall. In body-fitted cases, since smaller Smagorinsky constant is required close to the walls than the other points, wall layer model in dynamic Smagorinsky sub-grid scale model using dynamic k (instead of Von Karman constant) is applied to optimize wall function in separated flows. In the presence of immersed boundaries, the present wall layer model is calibrated, and then improved in attached and also detached flows with two different approaches. The results are also compared to experiment and resolved LES. Consequently the optimized wall layer models show an acceptable accuracy, and are more reliable. In the last part of this thesis, LES is applied to model the wave and wind driven sea water circulation in Kaneohe bay, which is a bay with a massive coral reef. This is the first time that LES-COAST is applied on a reef-lagoon system which is very challenging since the bathymetry changes very steeply. For example the water depth differs from less than 1 meter over the reef to more than 10 meters in vicinity of the reef, in lagoon. Since a static grid is implemented, the effect of wave is imposed as the velocity of current over the reef, which is used on the boundary of our computational domain. Two eddies Smagorinsky SGS model is used for this simulation.
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Östlund, Jan. "Flow Processes in Rocket Engine Nozzles with Focus on Flow Separation and Side-Loads." Licentiate thesis, KTH, Mechanics, 2002. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-1452.
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Ozturk, Burak. "Combined effects of Reynolds number, turbulence intensity and periodic unsteady wake flow conditions on boundary layer development and heat transfer of a low pressure turbine blade." [College Station, Tex. : Texas A&M University, 2006. http://hdl.handle.net/1969.1/ETD-TAMU-1150.
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Adler, Michael C. "On the Advancement of Phenomenological and Mechanistic Descriptions of Unsteadiness in Shock-Wave/Turbulent-Boundary-Layer Interactions." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1553543774661509.
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Poulain, Arthur. "RANS & WMLES Simulations of Compressor Corner Separation." Thesis, KTH, Mekanik, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-279819.
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In axial compressor, corner separation phenomenon can occur between the blade surface and the hub. This leads to high total pressure losses, blockage and may worsen to surge. Various studies on NACA65-009 blade were previously performed experimentally and numerically to predict the corner separation. The LMFA showed that RANS simulations tend to overestimate it while the Wall-Resolved LES (WRLES) was able to well capture it. The conclusions drawn on RANS are validated here with another solver software. An extensive parametric study is performed on RANS which highlights the good performance of two non-linear turbulence models k − ω Wilcox QCR and EARSM k − kl for for predicting the topology and the intensity of corner separation. They are however very dependent on the mesh and the numerics. A Wall-Modeled LES (WMLES) is then computed. It reproduces well the topology of the separation given by the experiments and predicts similar anisotropy to the WRLES. Nevertheless it shows high sensitivity to the level of turbulence close to the endwall and the boundary layer profile of the upstream flow. Finally, this confirms that the WMLES is a promising alternative to the WRLES in order to study the corner separation on more costly geometries (several blades for instance).I axiell kompressor kan hörnseparationsfenomen uppstå mellan bladytan och navet. Konsekvenserna är stora totala tryckförluster och kompressor blockering. Olika studier på NACA65-009 bladet utfördes tidigare experimentellt och numeriskt för att förutsäga hörnseparationen. LMFA visade att RANS simuleringar tenderar att överskatta den hörnseparationen medan Vägg-Löst LES (WRLES på engelska) kunde fånga bra den. Slutsatserna som dras om RANS valideras här med en annan lösningsprogramvara. En omfattande parametrisk studie utförs på RANS som belyserde goda prestandan för två icke-linjära turbulensmodeller k − ω Wilcox QCRoch EARSM k − kl för att förutsäga topologin och intensiteten för hörnseparation. Dock är de mycket beroende av nät och numerik. En Vägg-Modell LES (WMLES på engelska) beräknas sedan. Det reproducerar väl topologin för separationen som ges av experimenten och förutsäger liknande anisotropi som WRLES. Dock visar det hög känslighet för turbulensnivån nära ändväggen och gränsskiktsprofilen för uppströmsflödet. Slutligen bekräftar detta att WMLES är ett lovande alternativ till WRLES för att studera hörnseparationen på dyrare geometrier (till exempelflera blad).
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Rohr, Allen R. "A Computational Fluid Dynamics (CFD) Analysis of the Aerodynamic Effects of the Seams on a Two-Dimensional Representation of a Soccer Ball." DigitalCommons@CalPoly, 2018. https://digitalcommons.calpoly.edu/theses/2013.
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Most major sports today use a dedicated ball or projectile with specific shape, size, and surface geometry, except for soccer. Over the history of the sport, the surface geometry and design stayed relatively unchanged, sewn together using 32 pentagonal and hexagonal panels. However, recent innovations in panel designs differ substantially from the traditional 32 panel ball. The effects these new designs have on the aerodynamic characteristics of the ball have remained largely unknown, even with the influx of experimental research completed in the past decade. Experimental studies have been broad in scope, analyzing an entire ball in wind tunnels or full flow paths in trajectory analyses. Computational efforts have been too assumptive in flow conditions, such as a fully turbulent flow field, which has not yielded accurate representations of the flow phenomenon. This study investigates the aerodynamic effects of the seam on a two-dimensional representation of a non-rotating soccer ball using Computational Fluid Dynamics (CFD). By applying a transitional solver to the narrowed scope of a two-dimensional flow domain, with a single seam in cross-flow, the effects of the seam on the boundary layer and overall transient flow structure can be more accurately modeled. Data analysis suggests the seam produces a local effect on skin friction, however, that effect does not materialize into a premature boundary layer transition or delayed separation point, as predicted by literature. A detailed flow visualization is consistent with this result, displaying expected symmetric vortex shedding similar to a smooth cylinder, but not fully capturing the effects of the seam, reinforcing the need for expanding computational research efforts in this field.
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Törnblom, Olle. "Experimental and computational studies of turbulent separating internal flows." Doctoral thesis, KTH, Mekanik, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4071.
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The separating turbulent flow in a plane asymmetric diffuser with 8.5 degrees opening angle is investigated experimentally and computationally. The considered flow case is suitable for fundamental studies of separation, separation control and turbulence modelling. The flow case has been studied in a specially designed wind-tunnel under well controlled conditions. The average velocity and fluctuation fields have been mapped out with stereoscopic particle image velocimetry (PIV). Knowledge of all velocity components allows the study of several quantities of interest in turbulence modelling such as the turbulence kinetic energy, the turbulence anisotropy tensor and the turbulence production rate tensor. Pressures are measured through the diffuser. The measured data will form a reference database which can be used for evaluation of turbulence models and other computational investigations. Time-resolved stereoscopic PIV is used in an investigation of turbulence structures in the flow and their temporal evolution. A comparative study is made where the measured turbulence data are used to evaluate an explicit algebraic Reynolds stress turbulence model (EARSM). A discussion regarding the underlying reasons for the discrepancies found between the experimental and the model results is made. A model for investigations of separation suppression by means of vortex generating devices is presented together with results from the model in the plane asymmetric diffuser geometry. A short article on the importance of negative production-rates of turbulent kinetic energy for the reverse flow region in separated flows is presented. A detailed description of the experimental setup and PIV measurement procedures is given in a technical report.QC 20100923
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Reimann, Daniel D. "Effects of Spanwise and Discrete Disturbances on Separating Boundary Layers on Low Pressure Turbine Blades." Diss., CLICK HERE for online access, 2007. http://contentdm.lib.byu.edu/ETD/image/etd1761.pdf.
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DeMoss, Joshua Andrew. "Skin Friction and Cross-flow Separation on an Ellipsoidal Body During Constant Yaw Turns and a Pitch-up Maneuver with Roll Oscillation." Diss., Virginia Tech, 2010. http://hdl.handle.net/10919/29063.
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The skin friction and cross-flow separation location on a non-body-of-revolution (non-BOR) ellipsoidal model performing constant-yaw turns and a pitch-up maneuver, each with roll oscillation were studied for the first time. The detailed, low uncertainty, flow topology data provide an extensive experimental database on the flow over non-BOR hull shapes that does not exist anywhere else in the world and serves as a crucial tool for computational validation. The ellipsoidal model was mounted on a roll oscillation machine in the Virginia Tech Stability Wind Tunnel slotted wall test section. Hot-film sensors with constant temperature anemometers provided skin friction magnitudes on the body's surface for thirty-three steady flow model orientations and three unsteady maneuvers at a constant Reynolds number of 2.5 million. Cross-flow separation locations on the model were determined from span-wise minima in the skin friction magnitude for both the steady orientations and unsteady maneuvers. Steady hot-film data were obtained over roll angles between ±25° in 5° increments with the model mounted at 10° and 15° yaw and at 7° pitch with respect to the flow. The roll oscillation machine was used to create a near sinusoidal unsteady roll motion between ±26° at a rate of 3 Hz, which corresponded to a non-dimensional roll period of 5.4. Unsteady data were obtained with the ellipsoidal model mounted at 10° and 15° yaw and at 7° pitch during the rolling maneuver. Cross-flow separation was found to dominate the leeside flow of the model for all orientations. For the yaw cases, the separation location moved progressively more windward and inboard as the flow traveled downstream. Increasing the model roll or yaw angle increased the adverse pressure gradient on the leeward side, creating stronger cross-flow separation that began further upstream and migrated further windward on the model surface. For the pitch flow case, the cross-flow separation remained straight as the flow moved axially downstream. The strongest pitch cross-flow separation was observed at the most negative roll angle and dissipated, moving further downstream and inboard as the modelâ s roll angle was increased. The unsteady flow maneuvers exhibited the same flow topology observed in the quasi-steady conditions. However, the unsteady skin friction and separation locations lagged their quasi-steady counterparts at equivalent roll angles during the oscillation cycle. A first order time lag model and sinusoidal fit to the separation location data quantified the time lags that were observed.Ph. D.
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Fadla, Fawzi. "Caractérisation expérimentale de la dynamique du décollement de couche limite induit par un gradient de pression adverse et un effet de courbure." Thesis, Valenciennes, 2014. http://www.theses.fr/2014VALE0022/document.
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Ces travaux de recherche portent sur la caractérisation des phénomènes instationnaires associés aux écoulements décollés induits à la fois par un gradient de pression adverse et un effet de courbure. Ce type de décollement est très couramment rencontré, en particulier dans le secteur des transports. Cette étude repose sur une approche purement expérimentale réalisée en canal hydrodynamique à l’aide de techniques de mesure non intrusives permettant de ne pas dénaturer la dynamique très sensible du phénomène de décollement de couche limite. Le décollement est, dans notre cas de figure, provoqué par un obstacle 2dne présentant pas de rupture de pente. Le régime d’écoulement étudié est principalement turbulent et la gamme des nombres de Kármán analysée s’étale de 60 à 730. L’objectif principal de cette étude est d’évaluer les effets Reynolds sur l’étendue et l’existence même du phénomène de décollement de couche limite, mais également sur la dynamique des instabilités, identifiées à plus bas régime dans la littérature. Les mesures effectuées dans le cadre de ces travaux ont tout d’abord permis de constituer une base de donnéesexpérimentale étoffée, et d’établir que le décollement de couche limite ainsi que les instabilités induites par celui-ci, identifiées en régime laminaire, persistent à plus haut nombre de Kármán. Les fréquences associées aux instabilités ont également été identifiées ainsi que les paramètres caractéristiques pilotant leur dynamique. La dynamique spatio-temporelle de ces instabilités et en particulier celle du phénomène debattement du bulbe décollé a été détaillée notamment par le biais d’une analyse stochastique. Finalement, la répartition relativement étendue des grandes échelles tourbillonnaires associées aux mécanismes instables (soulignée notamment par leur émergence spectrale large bande) a également été mise en évidence, ainsi que certains phénomènes dynamiques secondaires. L’ensemble de ces résultats et en particulier l’identification des paramètres clés pilotant la dynamique du décollement de la couche limite s’avèreront très utiles en vue de concevoir par la suite des modèles simplifiés reproduisant le plus fidèlement possible la dynamique des décollements afin de mieux pouvoir les contrôlerThese investigations concern the characterization of unsteady phenomena associated to the boundary layer separation induced by both an adverse pressure gradient and a curvature effects. This kind of separation is very usual, particularly in the transport field. This study, essentially based on an experimental approach, is carried out in an hydrodynamic channel using non intrusive measurement techniques. They respect the very sensitive dynamics of the boundary layer separation phenomenon. The separation is, in our case, induced by a 2d obstacle without sharp corner. The studied flow regime is mainly turbulentand the analyzed Kármán number ranges from 60 to 730. The main aim of this study is to estimate the Reynolds number effects on the boundary layer separation length and even on the existence of such phenomenon, but also on the instabilities dynamics, identified in the literature especially for laminar flow regime. The measurements made within the framework of these works allowed, first to built a large experimental database, and secondly to establish that the boundary layer separation and also the associate instabilities, identified for laminar flow, persist even for higher Kármán number. The frequencies associated to the instabilities phenomena have been also identified as well as the characteristic parameters driving their dynamics. The instabilities space-time dynamic, in particular those of the flapping phenomenon were detailed using stochastic analysis. Finally, the large scales distribution associated with the unstable mechanisms (underlined by their spectral broadband frequency range) were also highlighted, as well asothers secondary dynamic phenomena. All these results, especially the identification of the key parameters driving the boundary layer separation, will turn out very useful to design afterward simplified models reproducing as faithfully as possible the separation dynamics and to be able to control them better
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Dinsenmeyer, Alice. "Probabilistic approach for the separation of the acoustic and aerodynamic wall pressure fluctuations." Thesis, Lyon, 2020. http://theses.insa-lyon.fr/publication/2020LYSEI087/these.pdf.
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Avec l'apparition des MEMS et la diminution globale du coût des capteurs, les acquisitions multivoies se généralisent, notamment dans le domaine de l'identification de sources acoustiques. La qualité de la localisation et de la quantification des sources peut être dégradée par la présence de bruit de mesure ambiant ou induit par le système d'acquisition. En particulier, dans le cas de mesures en présence d'un écoulement, la couche limite turbulente qui se développe sur le système de mesure peut induire des fluctuations de pression de niveau bien supérieur à celles des sources acoustiques. Il devient alors nécessaire de traiter les acquisitions pour extraire chaque composante du champ mesuré. Pour cela, on propose de décomposer la matrice spectrale mesurée en la somme d'une matrice associée à la contribution acoustique et d'une matrice pour le bruit aérodynamique. Cette décomposition exploite les propriétés statistiques de chaque champ de pression. En supposant que la contribution acoustique est fortement corrélée sur les capteurs, le rang de la matrice interspectrale associée se limite au nombre de sources décorrélées équivalentes. Concernant la matrice du bruit aérodynamique, deux modèles statistiques sont proposés. Un premier modèle fait l'hypothèse d'un champ totalement décorrélé sur les capteurs, et un second repose sur un modèle physique préexistant. Ce problème de séparation est résolu par une approche d'optimisation bayésienne, qui permet de prendre en compte les incertitudes sur chaque composante du modèle.Les performances de cette méthode sont d'abord évaluées sur des mesures en soufflerie puis sur des données industrielles particulièrement bruitées, provenant de mesures microphoniques effectuées sur le fuselage d'un avion de ligne en volWith the emergence of MEMS and the overall decrease in the cost of sensors, the acquisitions multichannel are becoming more widespread, particularly in the field of acoustic source identification. The quality of source localization and quantification can be degraded by the presence of ambient or electronic noise. In particular, in the case of in flow measurements, the turbulent boundary layer that develops over the measuring system can induce pressure fluctuations that are much greater than those of acoustic sources. It then becomes necessary to process the acquisitions to extract each component of the measured field. For this purpose, it is proposed in this thesis to decompose the measured spectral matrix into the sum of a matrix associated with the acoustic contribution and a matrix for aerodynamic noise. This decomposition exploits the statistical properties of each pressure field. Assuming that the acoustic contribution is highly correlated on the sensors, the rank of the corresponding cross-spectral matrix is limited to the number of equivalent uncorrelated sources. Concerning the aerodynamic noise matrix, two statistical models are proposed. A first model assumes a totally uncorrelated field on the sensors, and a second is based on a pre-existing physical model. This separation problem is solved by a Bayesian optimization approach, which takes into account the uncertainties on each component of the model. The performance of this method is first evaluated on wind tunnel measurements and then on particularly noisy industrial measurement, coming from microphones flushmounted on the fuselage of an inflight large aircraft
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Passaggia, Pierre-yves. "Instabilités d'écoulements décollés et leur contrôle." Thesis, Aix-Marseille, 2012. http://www.theses.fr/2012AIXM4790/document.
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La dynamique d'instabilité d'un écoulement laminaire décollé est étudiée expérimentalement et son contrôle par le biais de la simulation numérique. La configuration étudiée est une couche limite laminaire décollée au dessus d'une géométrie de type bosse.Pour une certaine gamme de paramètres, l'écoulement de recirculation en aval de la bosse est caractérisé par un battement basse fréquence. L'étude expérimentale de cette dynamique a permis de retrouver les différents régimes d'instabilité mis a jour par voie numérique. Ces résultats prouvent notamment que les instabilités basse fréquence, dont l'existence a été surtout mise en évidence dans des configurations d'écoulements compressibles, sont un phénomène générique pour des bulles de recirculations allongées. Le contrôle du battement basse fréquence est ensuite étudié par voie numérique suivant deux approches complémentaires. Un asservissement en boucle fermée de la dynamique de perturbation linéaire est tout d'abord proposé. Les modes d'instabilité linéaires sont utilisés afin de construire des modèles réduits de la dynamique de perturbation. Cette réduction de modèle donne lieu à des estimateurs de faible dimension capables d'estimer la dynamique et de la contrôler. Ainsi la dynamique d'instabilité linéaire peut être supprimée en couplant le système de Navier-Stokes linéarisé avec le contrôleur.Le contrôle de la dynamique non linéaire est ensuite étudié en utilisant une méthode d'optimisation Lagrangienne. Cette méthode permet de calculer les lois de contrôle à partir de la dynamique non linéaire des équations de Navier-StokesThe dynamics and control of a separated boundary-layer flow have been investigated. Separation is triggered by a bump mounted on a flat plate and the transition dynamics has been investigated experimentally. For a certain parameter range, the recirculation region is subject to self-sustained low-frequency oscillations, and results from the numerical simulation for the same geometry are recovered. These results show that low frequency oscillations, observed mainly in compressible flow regimes, are inherent to elongated recirculation bubbles.The control of this low-frequency instability has been investigated using modern control theory based on two complementary approaches. Feedback control of the linear perturbation dynamics is first considered. Global instability modes are used to build reduced-order estimators. This model reduction gives rise to low-dimensional compensators capable of controlling the unstable dynamics. Once coupled to the unstable linearised Navier-Stokes system, the compensator is seen to succesfully control the unstable dynamics. The control of the nonlinear dynamics is then investigated using adjoint-based optimisation procedures. This method is used to compute control laws based on a complete knowledge of the nonlinear dynamics. Although the low-frequency instability is clearly attenuated, it seems difficult to control the flow towards its steady state, using only a few blowing/suction actuators localized on the wall
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Mello, Hilton Carlos de Miranda. "Estudo dos efeitos de um jato sintético simulado numericamente no atraso da separação de uma camada limite sobre um aerofólio hipotético." Universidade de São Paulo, 2005. http://www.teses.usp.br/teses/disponiveis/18/18135/tde-26022006-111816/.
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A realização deste trabalho tem como objetivo fundamental estudar os efeitos dos atuadores de jato sintético no escoamento de uma camada limite desenvolvida sobre uma placa plana e um aerofólio hipotético. A interação dos jatos sintéticos com um escoamento transversal pode conduzir a uma aparente modificação da forma aerodinâmica de corpos rombudos e, dessa forma, fornecer uma maneira de controle da separação na camada limite. Estudos recentes demonstram que tipos diferentes de escoamentos podem ser produzidos pelo atuador dependendo da oscilação da membrana. Um método numérico para solução das equações de Navier-Stokes incompressíveis bidimensionais na formulação vorticidade-velocidade é utilizado neste trabalho. As equações governantes são discretizadas utilizando-se métodos de diferenças finitas compactas de sexta ordem para as derivadas espaciais. A equação de Poisson para a componente da velocidade normal é resolvida por um método iterativo de sobre-relaxação em linhas sucessivas usando um esquema com malha composta para acelerar a convergência. Os resultados de simulações com diferentes valores de freqüência, amplitude e comprimento de fenda foram verificados através de uma análise de Fourier temporal. Através desta análise é verificado qual a melhor situação para se atrasar a separação da camada limiteThis work has as a fundamental objective the study of the effects of synthetic jet actuators on the boundary layer flow on a flat plate and on a hypothetical airfoil. The interaction of synthetic jets with transverse flow can lead to an apparent modification in the aerodynamic shape of blunt bodies and, in that way, supply a means of control of transition within the boundary layer. Recent studies demonstrate that different types of flow may be produced by the actuator, depending on the amplitude of oscillation of the membrane. A numerical method for the solution of two-dimensional incompressible Navier-Stokes equations written in vorticity-velocity formulation is used in this work. The spatial derivatives are discretized with a sixth order compact finite differences scheme. The Poisson equation for the normal velocity component is solved by an iterative line successive over relaxation method and uses a multigrid full approximation scheme to accelerate the convergence. The results of simulations with different values of frequency, amplitude and slot length were verified through a temporal Fourier analysis. By way of this analysis it is verified which are the better parameters for the controlled delay of boundary layer separation
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Bonne, Nicolas. "Stabilité de l'intéraction onde de choc/ couche limite laminaire." Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLX025/document.
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Le phénomène d’interaction onde de choc/couche limite (CL) est omniprésent en aérodynamique. De manière générale, il génère des oscillations basses fréquences qui peuvent être néfastes pour les machines. L’exemple typique est le tremblement de l’onde de choc sur profil d’aile en régime transsonique, dangereux car il peut exciter les modes de structure de l’aile et potentiellement la rompre. Ce type de phénoménologies a été largement étudié en condition de CL amont turbulente, ce qui a donné lieu à des scénarios physiques crédibles et des méthodologies d’étude efficaces, notamment les analyses de stabilité sur champs turbulents moyennés (RANS). Toutefois la technologie laminaire, c’est-à-dire l’utilisation de CL laminaires en vue de réduire la consommation des aéronefs représente un nouveau challenge scientifique sur cette problématique. La physique est en effet fortement impactée par la nature laminaire de la CL, notamment du fait de la faible résilience de celle-ci aux gradients de pression adverses et à la transition turbulente. Cette thèse a ainsi porté les méthodes d’analyse de stabilité sur champ RANSpour les situations de CL laminaire. L’originalité et l'apport de l’étude résident dans la prise en compte des modèles de transition dans l’approchelinéarisée sur champ RANS. Les modèles utilisés (RANS et transition) ont donc été linéarisés afin de réaliser des études de stabilité en perturbant toutes les variables aérodynamiques. La validation de la méthode a été réalisée par comparaison avec des résultats expérimentaux et de simulation (LES) sur deux conifurations d'application. La première configuration est le cas de la réflexion d’un choc oblique sur une plaque plane. La deuxième est celle du choc droit à l’extrados d’un profil en condition transsonique. Ces deux cas sont en condition de CL laminaire à l'amont du choc.Des analyses de stabilité et de résolvent ont été réalisées.Ces approches ont permis de caractériser le comportement d’oscillateur/amplificateur des écoulements en question et d'analyser la physique des instationnarités observées dans les expériences.Le cas de la réflexion de choc est caractérisé par trois fréquences. L'analyse de stabilité montre que celles-ci ne correspondent pas à des modes globaux instables mais à une dynamique d'amplificateur de l'écoulement. L'analyse de résolvent identifie bien ces trois fréquences. L’analyse des réponses optimales, couplée à une analyse de stabilité locale, a ensuite permis de proposer des scénarios physiques de ces dynamiques.Dans le cas du choc droit sur profil en régime transsonique, l'écoulement apparaît globalement instable. Deux modes d'instabilité sont identifiés. Le premier à basse fréquence correspond au phénomène de tremblement observé en conditions turbulentes. Le deuxième apparaît à plus haute fréquence, et correspond à un mode d'oscillation de la bulle de séparation présente sous le pied de choc.Plus largement, la thèse permet de suggérer que certaines dynamiques dans ce type d’interaction procèdent de mécanismes similaires liés à la respiration de la bulle de séparation laminaireThe shock wave boundary layer (BL) interaction phenomenon is ubiquitous in aerodynamic. In general this interaction generates some low frequency oscillations which can be disastrous for the machines. The typical example is the buffet phenomenon on an airfoil in transonic conditions. Buffet is dangerous since its low frequency can excite the structural modes of the airfoil and break it. The phenomenology has been wildly studied when the incoming BL is turbulent. These studies have derived several credible scenarii and efficient methodologies to capture its dynamic, especially the stability analysis tools on an averaged turbulent flow (RANS). However laminar technologies, the use of laminar BL to reduce the fuel consumption of planes, represent a new scientific challenge on this problematic. In fact, the physic of the interaction is importantly impacted by the laminar nature of the BL especially because of its weak resilience to an adversed pressure gradient and of the transition to turbulence.The thesis deals with the methodologies for the stability analysis on a RANS base flow in the case of a laminar BL. The originality and the contribution of this work have been to take into account a transition criteria in the linearised dynamic on a RANS base flow. The model used (RANS and transition) have then been linearized in order to make a stability analysis which take into account all the aerodynamic varaibles. The validation of this methodology has been made by comparison to expermient and simulation (LES) on two configurations of application. The first one is a weak reflected shock wave on a flat plate. The second one is the strong shock around an airfoil in a transonic regime. In both cases the incoming BL is laminar.Stability and resolvent analysis have been made. These approches have been able to caratirized the ocillator/noise amplifier behavior of the flow and to enabled a physical analysis of the unsteadinesses observed in the experiments.The case of the reflected shock wave is caracterized by three frequencies. The stability analysis shows that they don't correspond to globally unstable modes but to a noise amplifier behavior of the flow. The resolvent analysis identifies the three frequencies. The analysis of the optimal response, coupled with a local stability analysis, enables to proposed physical scenarii of these dynamics.In the case of the strong shock on an airfoil in transonic regime, the flow is globally unstable. Two unstable modes have been identified. The first one, at low frequency, correspond to the buffet phenomenon also observed in the turbulent case. The second one appears at higher frequency and correspond to the oscillation of the separation bubble formed at the feet of the shock.More generally, this thesis suggests that some dynamics of these two interactions result from the same mecanism linked to the breathing motion of the laminar separation bubble