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1
Yuile, Adam. "Swept boundary layer transition." Thesis, University of Liverpool, 2013. http://livrepository.liverpool.ac.uk/14613/.
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Boundary layer transition has been investigated for incompressible three-dimensional mean flows on a flat plate with a 60° swept leading edge for a nominally zero, a positive, and a negative pressure gradient for three freestream turbulence intensities using a low speed blower tunnel with a 1.22 x 0.61 m working section at the University of Liverpool. The freestream turbulence intensities were generated using grids upstream of the leading edge, producing turbulence levels of approximately 0.2 %, 1.25 % and 3.25 %. For each of these nine (3 x 3) test cases detailed boundary layer traverses were obtained at ten streamwise measurement stations, at a fixed spanwise location, using single-wire constant temperature hot-wire anemometry techniques and digital signal processing. The location for the onset and end of transition was obtained for each case, in terms of distance from the leading edge and local momentum thickness Reynolds number. These results are compared with the 2-D unswept empirical transition correlations of Abu-Ghannam and Shaw (1980) and the differences in the results between the two flows are highlighted. It was found that transition starts and ends earlier than for similar unswept flows, complementing the transition observations of Gray (1952) for swept wings. Further to this the receptivity of the swept boundary layers to freestream turbulence (in the bypass transition regime) was determined by comparing near wall and local freestream spectra, for the pre-transitional boundary layers. These experimental results were compared with numerical predictions from a fourth order accurate computational fluid dynamics method which considered a multitude of perturbation waveforms. This numerical approach was also able to identify the waveform frequency and orientation combinations which drive receptivity in swept boundary layer transition and indicate the manner in which receptivity scales with momentum thickness Reynolds number. It was found that the most receptive waveforms correspond to the streamwise streaks which are frequently observed in flow visualisations and direct numerical simulation studies of pre-transitional boundary layers. Additionally it was also found that the numerical receptivities to freestream turbulence were highest for the positive pressure gradient and, in contrast, lowest for the negative pressure gradient – a similar finding to that in 2-D boundary layers. Transition was seen to commence prior to the advent of the intended non-zero pressure gradients in the experiments and thus direct comparisons are not strictly available. The results obtained, and synthesis undertaken for this thesis, contribute towards an improved understanding of the transition process, particularly with respect to receptivity, in regard to flat plates with swept leading edges in various pressure gradients and highlight the differences between swept and unswept flows. Furthermore, additional avenues have been identified for future work on more complicated topologies where potential problems have also been highlighted.
2
Riley, S. "Three-dimensional boundary layer transition." Thesis, University of Liverpool, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.356291.
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Gardiner, I. D. "Transition in boundary layer flows." Thesis, University of Abertay Dundee, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.376973.
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An experimental investigation of transition in boundary layer flows under the influence of various freestream conditions is described. Velocity profiles are obtained automatically by means of a stepper-motor driven traverse mechanism which carries a hot wire probe connected to a constant temperature anemometer and associated instrumentation. This was achieved by use of a data acquisition and control facility centred around a microcomputer with a Eurocard rack mounted extension. The automatic boundary layer traverse is software controlled and the data obtained is stored in a disc file for subsequent analysis and graphical display. As an integral part of this facility a successful method of obtaining reliable intermittency values from a hot wire signal was developed. The influence of freestream turbulence and pressure gradient upon transition within a boundary layer developing on a flat plate is elucidated by a series of controlled experiments. From the data accumulated, the concept of statistical similarity in transition regions is extended to include moderate non-zero pressure gradients, with the streamwise mean intermittency distribution described by the normal distribution function. An original correlation which accounts for the influence of freestream turbulence in zero pressure gradient flows, and the combined influence of freestream turbulence and pressure gradient in adverse pressure gradient flows, on the transition length Reynolds number R, is presented. (The limited amount of favourable pressure gradient data precluded the extension of the correlation to include favourable pressure gradient flows). A further original contribution was the derivation of an intermittency weighted function which describes the development of the boundary layer energy thickness through the transition region. A general boundary layer integral prediction scheme based on existing established integral techniques for the laminar and turbulent boundary layers with an intermittency modelled transition region, has been developed and applied successfully to a range of test data.
4
Grimaldi, Margaret Elizabeth. "Roughness-induced boundary layer transition." Thesis, Massachusetts Institute of Technology, 1994. http://hdl.handle.net/1721.1/47353.
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Högberg, Markus. "Optimal Control of Boundary Layer Transition." Doctoral thesis, KTH, Mechanics, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3245.
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Högberg, Markus. "Optimal control of boundary layer transition /." Stockholm, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3245.
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Berlin, Stellan. "Oblique waves in boundary layer transition." Doctoral thesis, Stockholm, 1998. http://www.lib.kth.se/abs98/berl0529.pdf.
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Hachem, Farouk H. "Boundary layer transition on concave surfaces." Thesis, University of Liverpool, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.279702.
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Leoutsakos, George. "Boundary layer transition on concave surfaces." Thesis, Imperial College London, 1987. http://hdl.handle.net/10044/1/47060.
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Ozkan, Musa. "Boundary layer transition over rotating disks." Thesis, University of Warwick, 2016. http://wrap.warwick.ac.uk/87170/.
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This thesis summarizes results of a combined numerical and experimental study investigating the effects of surface roughness, and of the geometry of the ow domain (confinement) on the boundary{layer transition over rotating disks. Numerically, a three{dimensional enclosed cavity ow in a rotor{stator flow configuration is simulated. The effects induced by surface roughness of the rotor disk and the effects induced by the stator geometry enclosing the ow domain are investigated. The steady{state velocity pro les of the boundary{layer ow on the rotating disk are obtained, subjected to a linear stability analysis and compared to relevant data from the literature. Experimentally, the ow over rotating disks is studied for smooth disk surfaces and for disks with concentric grooves representing distributed roughness. The disks are mounted submerged inside a water {filled tank. Due to the surrounding perimeter wall of the tank and the liquid surface this arrangement resembles the classic rotor{stator flow configuration. Comprehensive measurements of the boundary{layer ow and its laminar{turbulent transition were performed by means of an hot{ lm anemometer. The computational results suggest that, for the rotor{stator ow investigated, the roughness{induced effects are very similar to the geometry{induced effects, both in nature and magnitude. This suggests that it may be di cult to distinguish between both effects in experiments where the ow domain is restricted. Nevertheless, in comparison to previous hot{ lm measurements employing the same experimental facility, the data of the current study have been significantly improved by means of introducing a new calibration technique. The new experimental data discussed here confirm recent theoretical results of our research group in that they corroborate that an increase in the roughness level can reduce the number of stationary vortices and also stabilize the Type{I (cross{ ow) instability mode. However, the detailed analysis of the experimental data, in comparison to the theoretically predicted magnitude of the roughness{induced and the geometry{induced effects, reveal that future studies would greatly bene t from the availability of a new air{based rotating{disk apparatus.
11
Su, Yi-Chung. "Longitudinal vortices in boundary layer transition /." The Ohio State University, 1999. http://rave.ohiolink.edu/etdc/view?acc_num=osu1488190109870444.
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Kral, Linda Dee. "Numerical investigation of transition control of a flat plate boundary layer." Diss., The University of Arizona, 1988. http://hdl.handle.net/10150/184621.
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A numerical model has been developed for investigating boundary layer transition control for a three-dimensional flat plate boundary layer. Control of a periodically forced boundary layer in an incompressible fluid is studied using surface heating techniques. The spatially evolving boundary layer is simulated. The Navier-Stokes and energy equations are integrated using a fully implicit finite difference/spectral method. The Navier-Stokes equations are in vorticity-velocity form and are coupled with the energy equation through the viscosity dependence on temperature. Both passive and active methods of control by surface heating are investigated. In passive methods of control, wall heating is employed to alter the stability characteristics of the mean flow. Both uniform and nonuniform surface temperature distributions are studied. In the active control investigations, temperature perturbations are introduced locally along finite heater strips to directly attenuate the instability waves in the flow. A feedback control loop is employed in which a downstream sensor is used to monitor wall shear stress fluctuations. Passive control of small amplitude two-dimensional Tollmien-Schlichting waves and three-dimensional oblique waves are numerically simulated with both uniform and nonuniform passive heating applied. Strong reductions in both amplitude levels and amplification rates are achieved. Active control of small amplitude two-dimensional and three-dimensional disturbances is also numerically simulated. With proper phase control, in phase reinforcement and out of phase attenuation is demonstrated. A receptivity study is performed to study how localized temperature perturbations are generated into Tollmien-Schlichting waves. It is shown that narrow heater strips are more receptive in that they maximize the amplitude level of the disturbances in the flow. It is also found that the local temperature fluctuations cause mainly a strong normal gradient in spanwise vorticity. Control of the early stages of the nonlinear breakdown process is also investigated. Uniform passive control is applied to both the fundamental and sub-harmonic routes to turbulence. A strong reduction in amplitude levels and growth rates results. In particular, the three-dimensional growth rates are significantly reduced below the uncontrolled levels. Active control of the fundamental breakdown process is also numerically simulated. Control is achieved using either a two-dimensional or three-dimensional control input.
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Rioual, Jean-Luc. "The automatic control of boundary layer transition." Thesis, University of Southampton, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.259625.
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Alzwayi, Ali Saad. "Transition of free convection boundary layer flow." Thesis, University of Glasgow, 2013. http://theses.gla.ac.uk/4803/.
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Transition of natural convection flow on a heated flat plate and inside a heated channel is studied numerically. Three different RANS based turbulent k-ε models namely standard, RNG and Realizable with an enhanced wall function are employed in the simulations. Additionally, a Large Eddy Simulation (LES) technique is also applied to particularly investigate the flow field and transition in a heated plate facing upward. Numerous published papers presented the typical characteristic behaviour of natural convection flow inside a channel, however, none of these provided information about the transition behaviour of flow inside a channel, and importantly, how the transition is affected by the merging of two growing boundary layers which occur inside a channel. Therefore, taking the above important things into consideration, the aim of the study is to carry out in-depth investigations of the transition of the free convection flow inside a channel with an effect of its width, angular orientation and several important thermal and boundary conditions. Moreover, the transition phenomena of the free convection flow developing in a heated channel facing both downward and upward are thoroughly investigated in the thesis. Numerically predicted results are compared with available experimental data in the published literatures. Fluid properties are assumed to be constant except for the density which changes with temperature and gives rise to the buoyancy forces and is treated by using the Boussinesq approach. Air with a Prandtl number of 0.7 is used as a test fluid in all the simulations. In the RANS based models, the results show that the Realizable model with an enhanced wall function predicts numerical results well compared to the experiment than those obtained by the other two models (standard and RNG), therefore this model was selected to perform all the other RANS based numerical simulations in this work. The results particularly indicate that the inclination of the channel has major effects on the transition stage. As the inclination angle is increased, the transition stage moves further downstream of the channel. However, the predicted local heat flux, reached its minimum further upstream of the channel, does not agree with that of the velocity and turbulent kinetic energy in a small channel width. Natural convection from an isothermal plate is conducted by using a Large Eddy Simulation method. The results show that with an increase in the angle the peak of the thermal and velocity boundary layers move from the near outlet of the plate for the vertical case to the middle of the plate when θ = -70°. So the thermal and boundary layers become fatter which causes an early transition.
15
Bottini, Henny. "Experimental investigation of induced supersonic boundary layer transition." Doctoral thesis, Universitat Politècnica de Catalunya, 2014. http://hdl.handle.net/10803/145254.
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Turbulence onset within an initially laminar flow is one of the most common phenomenon in Fluid Mechanics, yet is an open field of research. This is due to the many and diverse causes that can trigger turbulence, which often add to each other, change their effect upon the flow velocity, and are difficult to single out in real-world situations. This is why laminar-toturbulent transition experiments have been and still are a fundamental tool for the researcher.
This thesis work has addressed the study of turbulence onset in supersonic flows from an experimental point of view. Two test campaigns have been carried out each in a dedicated supersonic wind tunnel. The first aimed at tracking turbulence onset triggered by a single tetrahedral roughness element. It has been carried out in a multiple-Mach-number supersonic wind tunnel whose qualification campaign has been completed within this thesis work. Two roughness heights were tested, both for two Mach numbers, 1.6 and 2.3. The second test campaign has investigated the effects of an oblique shock wave impinging onto a Mach-2 transitional boundary layer. The transitional state of a boundary layer is that state during which the boundary layer passes from fully laminar to fully turbulent. This passage can be either induced or natural, and it was represented in this thesis work by the trigger wake and by the boundary layer flow downstream of the release point of an electric spark, respectively.
Surface sensors, as thin-films and piezoelectric pressure transducers, were used to measure steady and unsteady highfrequency flow evolutions. Different wall temperatures were set for the thin-films insert as to allow the experimental estimation of the recovery temperature. Convective heat-flux trends have been extracted from the steady measurements, which, together with the recovery temperature trends, allowed the calculation of the Stanton number trends. All these quantities let to conclude on the general state of the boundary layer investigated in the first test campaign.
The post-processing of the unsteady measurements yielded temperature and pressure fluctuations spectra and RMS streamwise evolutions, along with spectral time evolution at a given position. For the first test campaign, they allowed the characterization of the unsteadiness produced by the roughness within the supersonic boundary layer at different downstream locations. In so doing, they helped conclude on the state of the boundary layer, thus on the effectiveness of the roughness in triggering transition to turbulence. For the second test campaign, they allowed to single out the unsteady effects of the shock impinging downstream of the single roughness and downstream of the electric spark release point. In this way, differences in the shock effect between the roughness configuration and the clean-plate configuration have been highlighted, and the effects of different spark release frequencies compared.El desarrollo de la turbulencia en el interior de un flujo inicialmente laminar, a pesar de ser uno de los fenómenos más comunes en la mecánica de fluidos, continúa siendo un campo abierto de investigación. Esto es debido a las muchas y diversas causas que condicionan la transición de flujo laminar a régimen turbulento, a menudo actuando de modo combinado, cuyo efecto cambia con la velocidad del flujo y las cuales son difíciles de aislar en situaciones reales. Este es el motivo por el cual los experimentos que estudian la transición de régimen laminar a turbulento han sido y continúan siendo una herramienta fundamental para el investigador. Esta tesis doctoral ha abordado el estudio del comienzo de la turbulencia en flujos supersónicos desde un punto de vista experimental. Dos series de experimentos fueron realizados, cada uno en un túnel de viento supersónico específico. La primera serie tuvo como objetivo el seguimiento del inicio de la turbulencia causado por un único elemento de rugosidad de forma hexaédrica. Este tipo de experimentos fue realizado en un túnel de viento supersónico capaz de operar en un cierto rango de números de Mach, cuya caracterización fue completada en paralelo a esta investigación. Los experimentos fueron realizados a dos números de Mach, 1.6 y 2.3, y dos niveles de rugosidad diferentes variando la temperatura de pared. El objetivo de la segunda serie de medidas consistió en investigar los efectos del impacto de una onda de choque oblicua en una capa límite transitoria, en un flujo a Mach 2. El estado transitorio de una capa límite es aquel durante el cual la capa límite pasa de enteramente laminar a enteramente turbulenta. Esta transformación puede ser tanto natural como inducida. Ambos escenarios han sido reproducidos en esta tesis. La transformación natural ha sido representada mediante la presencia de un elemento de rugosidad, mientras que la transformación inducida se corresponde con el desarrollo de la capa límite aguas abajo del punto de liberación de chispas eléctricas. Para medir la evolución de flujos estacionarios y no estacionarios de alta frecuencia se emplearon sensores superficiales tales como thin-films y transductores piezoeléctricos de presión. Para determinar experimentalmente la temperatura de recuperación (recovery temperature) se aplicaron distintas temperaturas de pared a la pieza contenedora de los thin-films. De las medidas estacionarias se extrajeron las diferentes distribuciones del flujo de calor por convección, las cuales, junto con la distribución de la temperatura de recuperación, permitieron el cálculo de la distribución espacial del número de Stanton. Estos resultados permitieron llegar a una conclusión sobre el estado general de la capa límite investigada en la primera serie de experimentos. Las medidas no estacionarias proporcionaron espectros de fluctuación de temperatura y presión, y evoluciones longitudinales de residuos cuadráticos medios (MSR), así como espectros de evolución temporal en una posición dada. Estos datos permitieron caracterizar, en la primera serie de experimentos, la inestabilidad producida por la rugosidad en la capa límite supersónica en distintas posiciones aguas abajo. Ayudaron, por tanto, a determinar el estado de la capa límite y, co n ello, concluir en la efectividad de la rugosidad para provocar la transición a régimen turbulento. En la segunda serie de experimentos se pudieron señalar los efectos no estacionarios de la onda de choque incidiendo aguas debajo del elemento de rugosidad, y los efectos no estacionarios de la onda incidente aguas abajo del punto de liberación de chispas eléctricas. De este modo, se han identificado las diferencias entre el efecto de una onda de choque en presencia de un elemento de rugosidad y en el caso de la configuración limpia, y se han comparado los efectos del uso de diferentes frecuencias de descargas eléctricas
16
Metcalfe, S. J. "A blow-up mechanism in boundary layer transition." Thesis, Keele University, 2013. http://eprints.keele.ac.uk/193/.
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Robinson, D. P. "Experimental investigation of attachment-line boundary layer transition." Thesis, Queen's University Belfast, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.398120.
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Graham, Derek. "Boundary layer transition in simulated turbine blade flow." Thesis, University of Abertay Dundee, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.280568.
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Roberts, L. S. "Boundary-layer transition on wings in ground effect." Thesis, Cranfield University, 2017. http://dspace.lib.cranfield.ac.uk/handle/1826/12789.
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The competitiveness of a high-performance racing car is extremely reliant on aerodynamics. Due to the current economic climate, track testing is often forsaken and the majority of aerodynamic development carried out using sub-scale wind tunnel testing and computational simulations. It is important, therefore, that experimental and computational approaches represent real-world conditions as closely as possible. Although racing cars travel at much higher speeds than typical passenger cars, in comparison to aircrafts they still operate at relatively low Reynolds numbers and, consequently, laminar and transitional phenomena are evident. Despite this, the bulk of relevant literature available for racing-car aerodynamics is undertaken with little regard to the influence of Reynolds number, and in the case of computational studies, the omission of laminar and transitional phenomena all together. The present work has demonstrated, using a super-scale two- dimensional wind-tunnel model, that laminar and transition flow phenomenon are important at Reynolds numbers equivalent to a full-scale racing car. Moreover, the influence of these aspects increased as the wing’s ground clearance reduced; meaning that in ground effect they are even more important. Further experiments with three-dimensional models of varying complexity, from a simple single-element wing to a highly complex F1-specification wing, showed that laminar phenomena are important for F1 applications as well as for lower-downforce capability racing cars. A transition-sensitive eddy-viscosity turbulence model, k-kL-w, was used to simulate inverted wings operating in ground effect. It was shown that that laminar and transitional flow states could be simulated easily inside a commercial solver, and that the model offered a substantial improvement over the classical fully-turbulent k-w SST in terms of both force coefficient prediction and surface-flow structures. This experiments and computational simulations described in this thesis show the Reynolds number sensitivity of, and importance of laminar phenomenon on, wings operating in ground effect. It has been shown that laminar boundary layers are an important aspect of the flow characteristics of wings in ground effect, at both full-scale and model-scale Reynolds numbers. As such, it is recommended that future studies incorporate laminar and transitional phenomena.
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Lillywhite, Spencer Everett. "MICROPHONE-BASED PRESSURE DIAGNOSTICS FOR BOUNDARY LAYER TRANSITION." DigitalCommons@CalPoly, 2013. https://digitalcommons.calpoly.edu/theses/1064.
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An experimental investigation of the use low-cost microphones for unsteady total pressure measurement to detect transition from laminar to turbulent boundary layer flow has been conducted. Two small electret condenser microphones, the Knowles FG-23629 and the FG-23742, were used to measure the pressure fluctuations and considered for possible integration with an autonomous boundary layer measurement system. Procedures to determine the microphones’ maximum sound pressure levels and frequency response using an acoustic source provided by a speaker and a reference microphone. These studies showed that both microphones possess a very flat frequency response and that the max SPL of the FG-23629 is 10 Pa and the max SPL of the FG-23742 is greater than 23 Pa. Several sensor-probe configurations were developed, and the three best were evaluated in wind tunnel tests. Measurements of the total pressure spectrum, time signal, and the root-mean-square were taken in the boundary layer on a sharp-nose flat plate in the Cal Poly 2 foot by 2 foot wind tunnel at dynamic pressures ranging between 135 Pa and 1350 Pa, corresponding to freestream velocities of 15 m/s to 47 m/s. The pressure spectra were collected to assess the impact of the probe on the microphone frequency response. The two configurations with long probes showed peaks in the pressure spectra corresponding to the resonant frequencies of the probe. The root-mean-square of the pressure fluctuations did not vary much between the different probes. The root-mean-square of the pressure fluctuations collected in turbulent boundary layers were found to be 10% of the local freestream dynamic pressure and decreased to 3.5% as the freestream dynamic pressure was increased. The RMS of the pressure fluctuations taken in both laminar boundary layers and in the freestream varied between 0.5% and 2.5% of the local freestream dynamic pressure. The large difference between the RMS of the pressure fluctuations in laminar and turbulent boundary layers taken at low dynamic pressures suggests that this system is indeed capable of distinguishing between laminar and turbulent flow. The drop in the RMS of the pressure fluctuations as dynamic pressure increased is indicative of insufficient maximum sound pressure level of the microphone resulting in clipping of the pressure fluctuation; this is confirmed through inspection of the pressure time signal and spectrum. Thus, a microphone with higher maximum sound pressure level is needed for turbulence detection at higher dynamic pressures. Alternatively, it may be possible to attenuate the total pressure fluctuation signal.
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Vasudevan, K. P. "Transition Zone In Constant Pressure Boundary Layer With Converging Streamlines." Thesis, Indian Institute of Science, 2000. https://etd.iisc.ac.in/handle/2005/272.
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The laminar-turbulent transition in viscous fluid flows is one of the most intriguing problems in fluid dynamics today. In view of the enormous applications it has in a variety of fields such as aircraft design, turbomachinery, etc., scientists have now realized the importance of tackling this problem effectively. Three-dimensional flows are usually associated with pressure gradient, streamline curvature, streamline convergence / divergence etc., all acting simultaneously. Towards a better understanding of the transition process and modeling the transition zone, it is important to study the effect of each of these parameters on the transitional flow. The present work aims at studying experimentally the effect of lateral streamline convergence alone on the laminar-turbulent transition zone under constant stream-wise pressure.
The experimental setup consists of a low turbulence wind tunnel with its test section modified to cause lateral streamline convergence under constant pressure. This is achieved by converging the side-walls and appropriately diverging the roof, thus maintaining a constant stream-wise pressure. The half angle of convergence is chosen as 100 , which is approximately the same as the half of the turbulent spot envelope in constant pressure two-dimensional flows.
Experiments are carried out to analyze the development of the laminar and transitional boundary layers, intermittency distribution in the transition zone and the overall characteristics of an artificially induced turbulent spot.
The laminar velocity profiles are found to be of the Blasius type for two-dimensional constant pressure flows. However, the converging streamlines are found to contribute to an increased thickness of the boundary layer as compared to the corresponding two-dimensional flow.
The intermittency distribution in the transition zone is found to follow the universal intermittency distribution for two-dimensional constant pressure flow. A simple linear-combination model for two-dimensional flows is found to perform very well in predicting the measured velocity profiles in the transition zone.
An artificially introduced turbulent spot is found to propagate along a conical envelope with an apex cone angle of 220 which is very nearly the value for a corresponding constant pressure two-dimensional flow. The spot shapes and celerities are also comparable to those in two-dimensional flow.
In summary, the present study brings out many similarities between a constant pressure laterally converging flow and a constant pressure two-dimensional flow.
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Vasudevan, K. P. "Transition Zone In Constant Pressure Boundary Layer With Converging Streamlines." Thesis, Indian Institute of Science, 2000. http://hdl.handle.net/2005/272.
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The laminar-turbulent transition in viscous fluid flows is one of the most intriguing problems in fluid dynamics today. In view of the enormous applications it has in a variety of fields such as aircraft design, turbomachinery, etc., scientists have now realized the importance of tackling this problem effectively. Three-dimensional flows are usually associated with pressure gradient, streamline curvature, streamline convergence / divergence etc., all acting simultaneously. Towards a better understanding of the transition process and modeling the transition zone, it is important to study the effect of each of these parameters on the transitional flow. The present work aims at studying experimentally the effect of lateral streamline convergence alone on the laminar-turbulent transition zone under constant stream-wise pressure.
The experimental setup consists of a low turbulence wind tunnel with its test section modified to cause lateral streamline convergence under constant pressure. This is achieved by converging the side-walls and appropriately diverging the roof, thus maintaining a constant stream-wise pressure. The half angle of convergence is chosen as 100 , which is approximately the same as the half of the turbulent spot envelope in constant pressure two-dimensional flows.
Experiments are carried out to analyze the development of the laminar and transitional boundary layers, intermittency distribution in the transition zone and the overall characteristics of an artificially induced turbulent spot.
The laminar velocity profiles are found to be of the Blasius type for two-dimensional constant pressure flows. However, the converging streamlines are found to contribute to an increased thickness of the boundary layer as compared to the corresponding two-dimensional flow.
The intermittency distribution in the transition zone is found to follow the universal intermittency distribution for two-dimensional constant pressure flow. A simple linear-combination model for two-dimensional flows is found to perform very well in predicting the measured velocity profiles in the transition zone.
An artificially introduced turbulent spot is found to propagate along a conical envelope with an apex cone angle of 220 which is very nearly the value for a corresponding constant pressure two-dimensional flow. The spot shapes and celerities are also comparable to those in two-dimensional flow.
In summary, the present study brings out many similarities between a constant pressure laterally converging flow and a constant pressure two-dimensional flow.
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Savin, Deborah Jane. "Linear and nonlinear aspects of interactive boundary layer transition." Thesis, University College London (University of London), 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.243306.
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Fabbiane, Nicolò. "Transition delay in boundary-layer flows via reactive control." Doctoral thesis, KTH, Stabilitet, Transition, Kontroll, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-187173.
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Transition delay in boundary-layer flows is achieved via reactive control of flow instabilities, i.e. Tollmien-Schlichting (TS) waves. Adaptive and model-based control techniques are investigated by means of direct numerical simulations (DNS) and experiments. The action of actuators localised in the wall region is prescribed based on localised measurement of the disturbance field; in particular, plasma actuators and surface hot-wire sensors are considered. Performances and limitations of this control approach are evaluated both for two-dimensional (2D) and three-dimensional (3D) disturbance scenarios. The focus is on the robustness properties of the investigated control techniques; it is highlighted that static model-based control, such as the linear-quadratic- Gaussian (LQG) regulator, is very sensitive to model-inaccuracies. The reason for this behaviour is found in the feed-forward nature of the adopted sensor/actuator scheme; hence, a second, downstream sensor is introduced and actively used to recover robustness via an adaptive filtered-x least-mean-squares (fxLMS) algorithm. Furthermore, the model of the flow required by the control algorithm is reduced to a time delay. This technique, called delayed-x least-mean-squares (dxLMS) algorithm, allows taking a step towards a self-tuning controller; by introducing a third sensor it is possible to compute on-line the suitable time-delay model with no previous knowledge of the controlled system. This self-tuning approach is successfully tested by in-flight experiments on a motor-glider. Lastly, the transition delay capabilities of the investigated control con- figuration are confirmed in a complex disturbance environment. The flow is perturbed with random localised disturbances inside the boundary layer and the laminar-to-turbulence transition is delayed via a multi-input-multi-output (MIMO) version of the fxLMS algorithm. A positive theoretical net-energy- saving is observed for disturbance amplitudes up to 2% of the free-stream velocity at the actuation location, reaching values around 1000 times the input power for the lower disturbance amplitudes that have been investigated.I den här avhandlingen har reglertekniska metoder tillämpats för att försena omslaget från ett laminärt till ett turbulent gränsskikt genom att dämpa tillväxten av små instabiliteter, så kallade Tollmien-Schlichting vågor. Adaptiva och modellbaserade metoder för reglering av strömning har undersökts med hjälp av numeriska beräkningar av Navier-Stokes ekvationer, vindtunnelexperiment och även genom direkt tillämpning på flygplan. Plasmaaktuatorer och varmtrådsgivare vidhäftade på ytan av plattan eller vingen har använts i experimenten och modellerats i beräkningarna. Prestanda och begränsningar av den valda kontrollstrategin har utvärderats för både tvådimensionella och tredimensionella gränsskiktsinstabiliteter. Fokus har varit på metodernas robusthet, där vi visar att statiska metoder som linjär-kvadratiska regulatorer (LQG) är mycket känsliga för avvikelser från den nominella modellen. Detta beror främst på att regulatorer agerar i förkompenseringsläge (”feed-foward”) på grund av strömningens karaktär och placeringen av givare och aktuatorer. För att minska känsligheten mot avvikelser och därmed öka robustheten har en givare införts nedströms och en adaptiv fXLMS algoritm (filtered-x least-mean-squares) har tillämpats. Vidare har modelleringen av fXLMS-algoritmen förenklats genom att ersätta överföringsfunktionen mellan aktuatorer och givare med en lämplig tidsfördröjning. Denna metod som kallas för dxLMS (delayed-x least-mean-squares) kräver att ytterligare en givare införs långt uppströms för att kunna uppskatta hastigheten på de propagerande instabilitetsvågorna. Denna teknik har tillämpats framgångsrikt för reglering av gränsskiktet på vingen av ett segelflygplan. Slutligen har de reglertekniska metoderna testas för komplexa slumpmässiga tredimensionella störningar som genererats uppströms lokalt i gränsskiktet. Vi visar att en signifikant försening av laminärt-turbulentomslag äger rum med hjälp av en fXLMS algoritm. En analys av energibudgeten visar att för ideala aktuatorer och givare kan den sparade energiåtgången på grund av minskad väggfriktion vara upp till 1000 gånger större än den energi som använts för reglering.
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Balla, Joseph V. "Pressure-Sensitive Paint for Detection of Boundary Layer Transition." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1345552359.
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Khapko, Taras. "Transition to turbulence in the asymptotic suction boundary layer." Licentiate thesis, KTH, Stabilitet, Transition, Kontroll, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-141344.
Full textAbstract:
The focus of this thesis is on the numerical study of subcritical transition to turbulence in the asymptotic suction boundary layer (ASBL). Applying constant homogeneous suction prevents the spatial growth of the boundary layer, granting access to the asymptotic dynamics. This enables research approaches which are not feasible in the spatially growing case. In a first part, the laminar–turbulent separatrix of the ASBL is investigated numerically by means of an edge-tracking algorithm. The consideration of spanwise-extended domains allows for the robust localisation of the attracting flow structures on this separatrix. The active part of the identified edge states consists of a pair of low- and high-speed streaks, which experience calm phases followed by high energy bursts. During these bursts the structure is destroyed and re-created with a shift in the spanwise direction. Depending on the streamwise extent of the domain, these shifts are either regular in direction and distance, and periodic in time, or irregular in space and erratic in time. In all cases, the same clear regeneration mechanism of streaks and vor- tices is identified, bearing strong similarities with the classical self-sustaining cycle in near-wall turbulence. Bifurcations from periodic to chaotic regimes are studied by varying the streamwise length of the (periodic) domain. The resulting bifurcation diagram contains a number of phenomena, e.g. multistability, intermittency and period doubling, usually investigated in the context of low-dimensional systems. The second part is concerned with spatio–temporal aspects of turbulent ASBL in large domains near the onset of sustained turbulence. Adiabatically decreasing the Reynolds number, starting from a fully turbulent state, we study low-Re turbulence and events leading to laminarisation. Furthermore, a robust quantitative estimate for the lowest Reynolds number at which turbulence is sustained is obtained at Re QC 20140213
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Ovchinnikov, Victor. "Numerical simulation of boundary layer transition due to external disturbances." College Park, Md. : University of Maryland, 2006. http://hdl.handle.net/1903/3310.
Full textAbstract:
Thesis (Ph. D.) -- University of Maryland, College Park, 2006.Thesis research directed by: Dept. of Mechanical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
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Brandt, Luca. "Numerical studies of bypass transition in the Blasius boundary layer." Doctoral thesis, KTH, Mechanics, 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3517.
Full textAbstract:
Experimental findings show that transition from laminar toturbulent ow may occur also if the exponentially growingperturbations, eigensolutions to the linearised disturbanceequations, are damped. An alternative non-modal growthmechanism has been recently identi fied, also based on thelinear approximation. This consists of the transient growth ofstreamwise elongated disturbances, with regions of positive andnegative streamwise velocity alternating in the spanwisedirection, called streaks. These perturbation are seen toappear in boundary layers exposed to signi ficant levels offree-stream turbulence. The effect of the streaks on thestability and transition of the Blasius boundary layer isinvestigated in this thesis. The analysis considers the steadyspanwise-periodic streaks arising from the nonlinear evolutionof the initial disturbances leading to the maximum transientenergy growth. In the absence of streaks, the Blasius pro filesupports the viscous exponential growth of theTollmien-Schlichting waves. It is found that increasing thestreak amplitude these two-dimensional unstable waves evolveinto three-dimensional spanwiseperiodic waves which are lessunstable. The latter can be completely stabilised above athreshold amplitude. Further increasing the streak amplitude,the boundary layer is again unstable. The new instability is ofdifferent character, being driven by the inectional pro filesassociated with the spanwise modulated ow. In particular, it isshown that, for the particular class of steady streaksconsidered, the most ampli fied modes are antisymmetric andlead to spanwise oscillations of the low-speed streak (sinuousscenario). The transition of the streak is then characterisedby the appearance of quasi-streamwise vorticesfollowing themeandering of the streak.
Simulations of a boundary layer subjected to high levels offree-stream turbulence have been performed. The receptivity ofthe boundary layer to the external perturbation is studied indetail. It is shown that two mechanisms are active, a linearand a nonlinear one, and their relative importance isdiscussed. The breakdown of the unsteady asymmetric streaksforming in the boundary layer under free-stream turbulence isshown to be characterised by structures similar to thoseobserved both in the sinuous breakdown of steady streaks and inthe varicose scenario, with the former being the mostfrequently observed.
Keywords:Fluid mechanics, laminar-turbulent transition,boundary layer ow, transient growth, streamwise streaks,lift-up effect, receptivity, free-stream turbulence, secondaryinstability, Direct Numerical Simulation.
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Anthony, Richard James. "Boundary layer transition under freestream turbulence, pressure gradient and crossflow." Thesis, University of Oxford, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.249487.
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Turner, M. R. "Numerical and asymptotic approaches to boundary-layer receptivity and transition." Thesis, University of East Anglia, 2005. http://eprints.brighton.ac.uk/5351/.
Full textAbstract:
We consider the interaction of a uniformly pulsating free-stream with the leading edge of a body, and consider its effect on transition. The free-stream is assumed to be incompressible, high Reynolds number flow parallel to the chord of the body, with a small, unsteady, perturbation of a single harmonic frequency. We present a method which calculates Tollmien-Schlichting (T-S) wave amplitudes downstream of the leading edge, by a combination of an asymptotic receptivity approach in the leading edge region and a numerical method which marches through the Orr-Sommerfeld region. The asymptotic receptivity analysis produces a three deck eigenmode which, in its far downstream limiting form, produces an upstream initial condition for our numerical Parabolized Stability Equation (PSE). Downstream T-S wave amplitudes are calculated for the flat plate, and good comparisons are found with the Orr-Sommerfeld asymptotics available in this region. The importance of the O(Re^{−1/2} ) term of the asymptotics is discussed, and, due to the complexity in calculating this term, we show the importance of numerical methods in the Orr-Sommerfeld region to give accurate results. We also discuss the initial transients present for certain parameter ranges, and show that their presence appears to be due to the existence of higher T-S modes in the initial upstream boundary condition. Extensions of the receptivity/PSE method to the parabola and the Rankine body are considered, and a drop in T-S wave amplitudes at lower branch is observed for both bodies, as the nose radius increases. The only exception to this trend occurs for the Rankine body at very large Reynolds numbers, which are not accessible in experiments, where a double maximum of the T-S wave amplitude at lower branch is observed. The extension of the receptivity/PSE method to experimentally realistic bodies is also considered, by using slender body theory to model the inviscid flow around a modified super ellipse to compare with numerical studies.
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Dris, Antonis. "Boundary layer transition on a flat plate and concave surfaces." Thesis, University of Liverpool, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.399183.
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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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Araújo, Tiago Barbosa de. "Finite bluff body wake control using boundary layer triggered transition." Instituto Tecnológico de Aeronáutica, 2012. http://www.bd.bibl.ita.br/tde_busca/arquivo.php?codArquivo=2177.
Full textAbstract:
The bluff body flow is often related to engeneering problems and because of it this kind of flow is largely studied. Most of the laboratory works are done with two-dimensinal (2D) or infinite flow, however the flows of industrial applications are normally three-dimensional (3D) and includes complex interactions among its shear layers. In this work, the flow over two finite circular cylinders mounted on a flat plate is studied. Passive flow control is applied on the finite cylinders, using single tripwire located at different azimuthal positions, is experimentally investigated. The influence of this disturbance on the near wake flow structures, vortex shedding characteristics and surface pressure are presented for two different aspect ratios, 3 and 6. Abrupt variations on mean measured parameters are observed for both aspect ratios and a critical angular position is found. Furthermore the changes due to aspect ratio variation are pointed out, specially on the near wake flow fields obtained by PIV measurements which are influenced by the inflow induced by the tip vortices. Drag and pressure fluctuations reduction as well as mean lift are observed. Further results suggest that the near wake characteristics are close to those of the infinite circular cylinder (2D) for the larger aspect ratio and that there exist a bi-stable regime of the flow in the presence of the tripwire.
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Robey, H. F. Liepmann H. W. Liepmann H. W. "The nature of oblique instability waves in boundary layer transition /." Diss., Pasadena, Calif. : California Institute of Technology, 1986. http://resolver.caltech.edu/CaltechETD:etd-05242007-150746.
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Schneider, Steven Philip Liepmann H. W. Liepmann H. W. "Effects of controlled three-dimensional perturbations on boundary layer transition /." Diss., Pasadena, Calif. : California Institute of Technology, 1989. http://resolver.caltech.edu/CaltechETD:etd-02142007-094214.
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Kalfas, A. "Transition to turbulence in the boundary layer of turbomachinery blading." Thesis, Cranfield University, 1994. http://dspace.lib.cranfield.ac.uk/handle/1826/8296.
Full textAbstract:
This thesis is concerned with the investigation of boundary layer transition phenomena in relation to turbomachinery flows. The present study involved the experimental investigation of the flow field around flat plates with various leading edge configurations. This study has also included some theoretical predictions using computational fluid dynamics.
The experimental investigation involved a variety of free stream environments which have attempted to simulate typical turbomachinery flows. Three flat plate configurations with a sharp, a semi-circular and a C4 leading edge shapes have been employed. The effect of free stream turbulence intensity over a range of free stream Reynolds numbers has been examined. Surface flow visualisation techniques have been applied to a total of 36 different experimental conditions in order to define the transitional characteristics of the boundary layer. This flow visualisation method was found to be appropriate for the large range of test cases involved and especially for the flat plate with the cylindrical leading edge. For the cylindrical leading edge configuration, a separation bubble has been detected in the vicinity of the leading edge, under all tested conditions.
Hot-wire investigation of the boundary layer has been undertaken over the flat plate with the C4 leading edge, which has been regarded as the most relevant configuration for turbomachinery applications. This method provided high frequency velocity data which have been analysed in order to obtain information about the spatial distribution of integral parameters of the boundary layer, such as the mean velocity, the turbulence intensity and the skewness and flatness factors. Boundary layer spectra have also been acquired. Statistical analysis of this data has been employed in order to obtain the intermittency distribution. The present results have been found to be in good agreement with existing transition correlations.
Calculations of the flow over flat plates with leading edge shapes similar to the experimental configuration have been undertaken using a two-dimensional elliptic Navier-Stokes solver. A solution of the flow field around a semi-circular and an elliptical leading edge has been obtained. Low Reynolds number k-Ɛ modelling has been applied in order to model the transitional characteristics of the boundary layer flow very near the wall. The use of the Nagano-Hishida version of the low-Reynolds number turbulence model led to predictions of an early start and end of transition.
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Coull, John David. "Wake induced transition in low pressure turbines." Thesis, University of Cambridge, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.608541.
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Bowles, Robert Ian. "Applications of nonlinear viscous-inviscid interactions in liquid layer flows and transonic boundary layer transition." Thesis, University College London (University of London), 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.489006.
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Garrett, S. J. "The stability and transition of the boundary layer on rotating bodies." Thesis, University of Cambridge, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.599330.
Full textAbstract:
The majority of this work is concerned with the local-linear stability of the incompressible boundary-layer flows over rotating spheres and rotating cones; convective and absolute instabilities are investigated and the effects of viscosity and streamline-curvature are included in each analysis. Preliminary investigations into the linear global-mode behaviour of the rotating-disk, rotating-cone and rotating-sphere boundary layers are also presented. The local rotating-sphere analyses are conducted at various latitudes from the axis of rotation (q), and the local rotating-cone analyses are conducted at points along cones of various half-angles (ψ), in each case convective and absolute instabilities are found within specific parameter spaces. The predictions of the Reynolds number, vortex angle and vortex speed at the onset of convective instability are consistent with existing experimental measurements for both boundary-layer types. Axial flow is found to stabilize each boundary layer with respect to convective and absolute instabilities. The global behaviour of the boundary-layer flows over rotating disks, cones and spheres is considered by taking into account the slowly varying basic state along each body surface. The locations of saddle points in the absolute frequency are determined which give the leading-order estimate of the global frequency. For the rotating disk and rotating cones, the global frequency indicates the disturbances in the boundary-layer flow are globally damped; and for rotating spheres, the global frequency indicates the boundary layer may support neutrally stable global modes when a region of absolute instability exists.
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Starling, Iain. "The use of riblets for delaying boundary layer transition to turbulence." Thesis, University of Nottingham, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.263407.
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Lingwood, R. J. "Stability and transition of the boundary layer on a rotating disk." Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.307947.
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Hosseini, Seyed Mohammd. "On stability, transition and turbulence in three-dimensional boundary-layer flows." Doctoral thesis, KTH, Stabilitet, Transition, Kontroll, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-177617.
Full textAbstract:
A lot has changed since that day on December 17, 1903 when the Wright brothers made the first powered manned flight. Even though the concepts behind flying are unaltered, appearance of stat-of-the-art modern aircrafts has undergone a massive evolution. This is mainly owed to our deeper understanding of how to harness and optimize the interaction between fluid flows and aircraft bodies. Flow passing over wings and different junctions on an aircraft faces numerous local features, for instance, acceleration or deceleration, laminar or turbulent state, and interacting boundary layers. In our study we aim to characterize some of these flow features and their physical roles. Primarily, stability characteristics of flow over a wing subject to a negative pressure gradient are studied. This is a common condition for flows over swept wings. Part of the current numerical study conforms to existing experimental studies where a passive control mechanism has been tested to delay laminarturbulent transition. The same flow type has also been considered to study the receptivity of three-dimensional boundary layers to freestream turbulence. The work entails investigation of effects of low-level freestream turbulence on crossflow instability, as well as interaction with micron-sized surface roughness elements. Another common three-dimensional flow feature arises as a resultof stream-lines passing through a junction, the so-calledcorner-flow. For instance, thisflow can be formed in the junction between the wing and fuselage on aplane.A series of direct numerical simulations using linear Navier-Stokes equationshave been performed to determine the optimal initial perturbation. Optimalrefers to perturbations which can gain the maximum energy from the flow overa period of time. In other words this method seeks to determine theworst-casescenario in terms of perturbation growth. Here, power-iterationtechnique hasbeen applied to the Navier-Stokes equations and their adjoint to determine theoptimal initial perturbation. Recent advances in super-computers have enabled advance computational methods to increasingly contribute to design of aircrafts, in particular for turbulent flows with regions of separation. In this work we investigate theturbulentflow on an infinite wing at a moderate chord Reynolds number of Re= 400,000 using a well resolved direct numerical simulation. A conventional NACA4412 has been chosen for this work. The turbulent flow is characterizedusing statistical analysis and following time history data in regions with interesting flow features. In the later part of this work, direct numerical simulation has been chosen as a tool to mainly investigate the effect of freestream turbulence on the transition mechanism of flow from laminar to turbulent around a turbine blade.QC 20151125
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Hill, J. L. "Development of a boundary layer transition model for helicopter rotor CFD." Thesis, Cranfield University, 2005. http://dspace.lib.cranfield.ac.uk/handle/1826/10711.
Full textAbstract:
A novel transition model has been developed for use in CFD simulations of helicopter
rotor aerodynamics. The model includes significantly improved physical modelling
of the transition
processes occurring in the steady and unsteady flows found on
helicopter rotors. The model has been coupled with the k-co and k-co SST two
equation turbulence models using a novel adaptation of the technique developed by
Wilcox for the low Reynolds number k-oa model.
The method has been employed to calculate transitional flows occurring in three key
ow
regimes found in helicopter aerodynamics; that around steady and unsteady
aerofoils and that around a hovering helicopter rotor. The performance of the k-co and
the k-w SST turbulence models have been
investigated for transitional flow
simulations and the k-w SST shown to
provide substantial improvements for
transitional flows
containing separations. Dramatic improvements in the computed
pressure and skin friction distributions for several aerofoil flows have been observed
over those computed using a conventional fully turbulent simulation. Corresponding
improvements are observed in the computed lift and drag polars and transition on set is
well predicted for both low and high Reynolds number flows.
A novel structured/unstructured a priori adapted grid generation strategy has been
developed for hovering rotor flows that provides improved rotor solutions for
transitional flow analysis. The method offers vast improvements in the preservation
of vorticity in the solution at greatly reduced computational expense. Tip vortices
have been maintained to a Wake
age of 1170 degrees with just 2 million cells per
blade. The transition model has then been applied to the high quality rotor solutions
and
good agreement obtained between computed and experimental results,
highlighting that three-dimensional effects have a relatively small effect on hovering
rotor transition in-board of the blade tip. I addition, the first known verification of a
Navier-Stokes rotor code against the Fogarty semi-analytical rotating at plate case
was presented and excellent agreement obtained.
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Imayama, Shintaro. "Studies of the rotating-disk boundary-layer flow." Doctoral thesis, KTH, Strömningsfysik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-158973.
Full textAbstract:
The rotating-disk boundary layer is not only a simpler model for the study of cross-flow instability than swept-wing boundary layers but also a useful simplification of many industrial-flow applications where rotating configurations are present. For the rotating disk, it has been suggested that a local absolute instability, leading to a global instability, is responsible for the small variation in the observed laminar-turbulent transition Reynolds number however the exact nature of the transition is still not fully understood. This thesis aims to clarify certain aspects of the transition process. Furthermore, the thesis considers the turbulent rotating-disk boundary layer, as an example of a class of three-dimensional turbulent boundary-layer flows. The rotating-disk boundary layer has been investigated in an experimental apparatus designed for low vibration levels and with a polished glass disk that gave a smooth surface. The apparatus provided a low-disturbance environment and velocity measurements of the azimuthal component were made with a single hot-wire probe. A new way to present data in the form of a probability density function (PDF) map of the azimuthal fluctuation velocity, which gives clear insights into the laminar-turbulent transition region, has been proposed. Measurements performed with various disk-edge conditions and edge Reynolds numbers showed that neither of these conditions a↵ect the transition process significantly, and the Reynolds number for the onset of transition was observed to be highly reproducible. Laminar-turbulent transition for a ‘clean’ disk was compared with that for a disk with roughness elements located upstream of the critical Reynolds number for absolute instability. This showed that, even with minute surface roughness elements, strong convectively unstable stationary disturbances were excited. In this case, breakdown of the flow occurred before reaching the absolutely unstable region, i.e. through a convectively unstable route. For the rough disk, the breakdown location was shown to depend on the amplitude of individual stationary vortices. In contrast, for the smooth (clean-disk) condition, the amplitude of the stationary vortices did not fix the breakdown location, which instead was fixed by a well-defined Reynolds number. Furthermore, for the clean-disk case, travelling disturbances have been observed at the onset of nonlinearity, and the associated disturbance profile is in good agreement with the eigenfunction of the critical absolute instability. Finally, the turbulent boundary layer on the rotating disk has been investigated. The azimuthal friction velocity was directly measured from the azimuthal velocity profile in the viscous sublayer and the velocity statistics, normalized by the inner scale, are presented. The characteristics of this three-dimensional turbulent boundary-layer flow have been compared with those for the two-dimensional flow over a flat plate and close to the wall they are found to be quite similar but with rather large differences in the outer region.QC 20150119
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Riley, Zachary Bryce Riley. "Interaction Between Aerothermally Compliant Structures and Boundary-Layer Transition in Hypersonic Flow." The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1471618528.
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Mersinligil, Mehmet. "Airfoil Boundary Layer Calculations Using Interactive Method And En Transition Prediction Technique." Master's thesis, METU, 2006. http://etd.lib.metu.edu.tr/upload/12607677/index.pdf.
Full textAbstract:
Boundary layer calculations are performed around an airfoil and its wake. Smith-van Ingen
transition prediction method is employed to find the transition from laminar to turbulent
flow. First, potential flow around the airfoil is solved with the Hess-Smith panel method.
The resulting velocity distribution is input to the boundary layer equations in order to find
a so called blowing velocity distribution. The output of the boundary layer equations are
also used to compute the location of onset of transition using the Smith-van Ingen en
transition prediction method. The obtained blowing velocity distribution is fed back to the
panel method to find a velocity distribution which includes the effects of viscosity. The
procedure described is repeated until convergence is observed. A computer program is
developed using the theory. Results obtained are in good accord with measurements
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Davidsson, Niklas. "Stability and transition in the suction boundary layer and other shear flows /." Luleå : Luleå University of Technology, 2007. http://epubl.ltu.se/1402-1544/2007/04/.
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Wang, Yaxing. "Instability and transition of boundary layer flows disturbed by steps and bumps." Thesis, Queen Mary, University of London, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.404889.
Full textAbstract:
An investigation of the influence of various wall perturbations on the development of instability waves has been carried out through a series of experiments to study different aspects of the stability/transition problem. The first experiment was devoted to an investigation of the effects of an isolated step on the transition of the boundary layer over a flat plate. Steps of various heights, both positive and negative, produced an upstream movement of the transition point, which was interpreted as arising from an increase in the amplification of the instability waves. The transition Reynolds number and the resulting change in the so-called 'N-factor', used for transition prediction, has been correlated with the step height to provide a useful empirical engineering design tool. The second experiment concerned the effect of a two-dimensional (2-D) sharpedged short bump on instability waves (Tollmien-Schlichting (TS) waves). The experiment was designed to study the mechanisms by which a sharp-edged surface roughness element, in the form of a 2-D bump, modified the evolution of TS wave and hence affected the process of transition. This investigation demonstrated that the interaction of a 2-D TS wave with a 2-D bump was a many-sided complex problem. The bump played multiple roles as a disturbance generator, an amplifier and a promoter of three-dimensionality. The crucial parameter was the height. Nonlinearity was the main factor that caused the enhancement of boundary layer transition and manifested itself in the appearance of higher harmonics in the spectra. In the last experiment the bump was three-dimensional (3-D) and in the form of a cylinder. The experimental results revealed that the effect of a 3-D bump was confined to a wedge-shaped region downstream of the bump. The mean velocity distortion had a distinct spanwise distribution from which a pair of streamwise vortex structures could be inferred. The interaction of a 2-D TS wave with a 3-D bump generated oblique waves with a dominant band of spanwise wavenumbers as predicted by theoretical analysis. Three thresholds were found that defined bump height ranges where there was no-effect, a linear behaviour and a nonlinear one.
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Marshall, Thomas James. "A study of three-dimensional effects in end-stage boundary-layer transition." Thesis, University College London (University of London), 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.412731.
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Langari, Mostafa. "Large eddy simulation of separated boundary layer transition under free-stream turbulence." Thesis, University of Sussex, 2014. http://sro.sussex.ac.uk/id/eprint/48940/.
Full textAbstract:
Physics of laminar-to-turbulent transition in a separated-reattached flow subjected to two free-stream turbulence levels have been explored using Large-Eddy Simulation (LES). Separation of the laminar boundary layer occurs at a curvature change over a flat plate with a semi-circular leading edge. A numerical trip has been used to generate the targeted free-stream turbulence levels. A dynamic Sub-grid-scale (SGS) model has been employed and excellent agreement has been achieved between the LES results and the experimental data. Detailed investigation of the LES data has been carried out to explore the primary instability mechanism at low (< 0.2%) and high free-stream turbulence (5.6%). The flow visualisations and spectral analysis of the separated shear layer reveal that the two-dimensional Kelvin-Helmholtz instability mode, well known to occur at low free-stream turbulence levels, is bypassed at a higher level leading to earlier breakdown to turbulence. The whole transition process leading to breakdown to turbulence has been revealed clearly by the flow visualisations and the differences between the low and high free-stream turbulence cases are clearly evident. Coherent structures are also visualised using iso-surfaces of the Q-criterion and for the high free-stream turbulence case the spanwise oriented two-dimensional rolls, which are clearly apparent in the low free-stream turbulence case, are not visible anymore. Detailed quantitative comparisons between the present LES results against experimental data and the previous LES results at low free-stream turbulence using a staggered grid have been done and a good agreement has been obtained, indicating that the current LES using a co-located grid with pressure smoothing can predict transitional flows accurately. Comprehensive spectral analysis of the separated shear layer at two free-stream turbulence levels has been performed. Under very low free-stream turbulence condition, a distinct regular vortex shedding and trace of the low-frequency flapping phenomena were detected. Under the higher free-stream turbulence however, a mild high-frequency activity was observed. No low frequency oscillations could be detected.