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1
Wang, A., et H. Lai. « Control of separated flow at low Reynolds number around NACA0012 airfoil by boundary layer suction ». Journal of Physics : Conference Series 2707, no 1 (1 février 2024) : 012122. http://dx.doi.org/10.1088/1742-6596/2707/1/012122.
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Abstract The separated flow at low Reynolds number around the NACA0012 airfoil is numerically studied by large-eddy simulation. Strategies of boundary layer suction to control flow separation are investigated. A method of using two-zone suctions, near the leading edge and near the trailing edge, are calculated. Based on verification with direct numerical simulation (DNS) and experimental data, the results of the lift and the drag, the vortices, and the strength of near-field pressure fluctuations, are checked. The results show that the two-zone suctions can supress flow transition and separation, thereby increase the lift and reduce the drag. The shedding of vortices is weakened, and the near-field pressure fluctuations are attenuated. For comparison with the two-zone suctions, the strategies of suction near the leading edge only and suction near the trailing edge only are also studied. It is found that suction near the leading edge only may suppress transition and delay separation when the suction zone is large enough, but the flow property deteriorates due to shedding vortices in the wake. The suction near the trailing edge only may improve the flow performance by reducing the size of the vortices in the rear section of the airfoil and in the wake region, but it has little effect on the separation bubble and transition.
2
Balakumar, P., et P. Hall. « Optimum Suction Distribution for Transition Control ». Theoretical and Computational Fluid Dynamics 13, no 1 (1 avril 1999) : 1–19. http://dx.doi.org/10.1007/s001620050109.
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Liu, Yuanqiang, Yan Liu, Zubi Ji, Yutian Wang et Jiakuan Xu. « Receptivity and Stability Theory Analysis of a Transonic Swept Wing Experiment ». Aerospace 10, no 10 (23 octobre 2023) : 903. http://dx.doi.org/10.3390/aerospace10100903.
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Surface suction provides an efficient way to delay boundary layer transitions. In order to verify the suction effects and determine the mechanism of suction control in transonic swept wing boundary layers, wind tunnel transition measurements in a hybrid laminar flow control (HLFC) wind tunnel model uses an infrared thermography technique in the Aircraft Research Association (ARA) 2.74 m × 2.44 m low turbulence level transonic wind tunnel. Based on the experimental data of stationary crossflow dominant transitions without and with surface suction in transonic swept wing boundary layers, in this paper, the effects on the receptivity and linear and nonlinear evolution of stationary crossflow vortices have been analyzed with the consideration of curvature. Theoretical analysis agreed with the experimental observations in regard to the transition delay caused by boundary layer suction near the leading-edge region.
4
Ma, Dongli, Guanxiong Li, Muqing Yang et Shaoqi Wang. « Research of the suction flow control on wings at low Reynolds numbers ». Proceedings of the Institution of Mechanical Engineers, Part G : Journal of Aerospace Engineering 232, no 8 (21 février 2017) : 1515–28. http://dx.doi.org/10.1177/0954410017694057.
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Laminar separation and transition have significant effects on aerodynamic characteristics of the wing under the condition of low Reynolds numbers. Using the flow control methods to delay and eliminate laminar separation has great significance. This study uses the method combined with water tunnel test and numerical calculation to research the effects of suction flow control on the flow state and aerodynamic force of the wing at low Reynolds numbers. The effects of suction flow rate and suction location on laminar separation, transition and aerodynamic performance of the wing are further researched. The results of the research show that, the suction can control laminar separation and transition effectively, when the suction holes are in the interior of the separation bubble, and close to the separation point, the suction has the best control effect. When the Reynolds number is Re = 3.0 × 105, the suction flow control can make the lift-to-drag ratio of the wing increase by 8.62%, and the aerodynamic characteristics of the wing are improved effectively.
5
Wong, P. W. C., M. Maina et A. M. Cobbin. « Transition and separation control in the leading edge region ». Aeronautical Journal 105, no 1049 (juillet 2001) : 371–78. http://dx.doi.org/10.1017/s0001924000012288.
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Abstract This paper describes an investigation of methods of controlling transition and separation in the leading edge region of military aircraft wings. For wings with the high leading edge sweep relevant to some military aircraft, if attachment line contamination can be prevented then transition is predominantly caused by crossflow instability close to the leading edge. The use of surface suction or cooling for suppressing these instabilities in order to delay transition, has been investigated in a parametric study. The placing of a short suction panel close to the leading edge has been found to be an effective means of controlling instability. Conversely, the level of cooling required to suppress crossflow instability may be too high for practical aircraft applications. The use of suction for preventing laminar separation for pressure distributions with a leading edge suction peak has also been included in the parametric study. The suction quantity required is strongly dependent on the peak height. The suction quantity that can be achieved in practice will limit the maximum peak height that can be attained without laminar separation. An investigation of leading edge stall and control has also been carried out. The analysis suggests that it is important to be able to identify whether the stall is due to laminar bubble bursting or turbulent re-separation, since different methods of controlling the stall may be required.
6
Lei, Juanmian, Qingyang Liu et Tao Li. « Suction control of laminar separation bubble over an airfoil at low Reynolds number ». Proceedings of the Institution of Mechanical Engineers, Part G : Journal of Aerospace Engineering 233, no 1 (24 août 2017) : 81–90. http://dx.doi.org/10.1177/0954410017727025.
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A laminar separation bubble appears generally on the NACA2415 airfoil at low Reynolds number. In this paper, suction control of the laminar separation bubble over the NACA2415 airfoil at low Reynolds number are simulated. The effects of suction control on the flow field and the aerodynamic characteristics of the airfoil are focused on at different angles of attack. Numerical simulations show that employing the γ-Reθt transition model coupling the k–ω shear stress transport turbulence model can predict the laminar separation bubbles accurately. The results indicate that suction control can delay the transition, decrease the velocity gradient of the boundary layer and inhibit the production of the separation bubble. The effect of the suction control becomes better with the suction location getting closer to the separation bubble and the suction speed (the suction gas speed of suction hole) getting faster. The figure of merit is introduced to evaluate energy consumption of the suction control. In consideration of the economic effects, the suction control is suitable for the larger angle of attack situation at low Reynolds number.
7
Ebrahimisadr, Hesam, et Bertrand Francois. « Water retention curves and tensile strength for studying desiccation cracking of compacted clay soils ». E3S Web of Conferences 382 (2023) : 09003. http://dx.doi.org/10.1051/e3sconf/202338209003.
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Desiccation cracking is a natural phenomenon caused by drying in near-surface earth soils subjected to constrained shrinkage. In this research, the water retention curve of two clayey soils, prepared by compaction under standard proctor conditions, are determined to study the desiccation cracking. Two techniques of suction control are applied to control the drying process and to reach the water retention curve of the soils. For the suction values higher than 4 MPa, the saline-solution method was used to impose variousrelative humidity and so, various suctions. In addition, the osmotic method was applied for the suction values between 0.5 MPa and 2 MPa. Additionally, the dynamic vapor sorption (DVS) technique is used to corroborate the water retention curves obtained with the two other techniques. The volume changes is also tracked showing significant shrinkage upon drying. To reach the tensile strength of the soil, the Braziliantest is performed on samples prepared by compaction and submitted to various suctions. According to the results, for the two tested soils, the soil with the higher plasticity index shows consistently the higher retention capacity and the larger shrinkage upon drying. Also, the obtained water retention curves exhibit a smooth transition when the suction control technique change (at suctions between 2 and 4 MPa).
8
Yang, Peng, Chiye Zhang, Hongyeyu Yan, Yifan Ren, Changliang Ye, Yaguang Heng et Yuan Zheng. « Numerical Investigation on Suction Flow Control Technology for a Blunt Trailing Edge Hydrofoil ». Mathematics 11, no 16 (21 août 2023) : 3618. http://dx.doi.org/10.3390/math11163618.
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The generation of hydro-mechanical resonance is related to the transition of the boundary layer and the development of vortex shedding. The application effect of suction control in hydrodynamics is equally deserving of consideration as an active control technique in aerodynamics. This study examines how suction control affects the flow field of the NACA0009 blunt trailing edge hydrofoil using the γ transition model. Firstly, the accuracy of the numerical method is checked by performing a three-dimensional hydrofoil numerical simulation. Based on this, three-dimensional hydrofoil suction control research is conducted. According to the results, the suction control increases the velocity gradient in the boundary layer and delays the position of transition. The frequency of vortex shedding in the wake region lowers, and the peak value of velocity fluctuation declines. The hydrofoil hydrodynamic performance may be successfully improved with a proper selection of the suction coefficient via research of the suction coefficient and suction position on the flow field around the hydrofoil. The lift/drag ratio goes up as the suction coefficient goes up. The boundary layer displacement thickness and momentum thickness are at their lowest points, and the velocity fluctuation amplitude in the wake region is at its lowest point as the suction coefficient Cμ = 0.003. When the suction slots are at the leading edge, the momentum loss in the boundary layer is minimal and the velocity fluctuation in the wake zone is negligible.
9
Biringen, S., W. E. Nutt et M. J. Caruso. « Numerical study of transition control by periodic suction blowing ». AIAA Journal 25, no 2 (février 1987) : 239–44. http://dx.doi.org/10.2514/3.9613.
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Ahmadi-Baloutaki, M., A. Sedaghat, M. Saghafian et M. Badri. « Control of Transition over Aerofoil Surfaces using Active Suction ». International Journal of Flow Control 5, no 3-4 (septembre 2013) : 187–200. http://dx.doi.org/10.1260/1756-8250.5.3-4.187.
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Bar-Haim, B., et D. Weihs. « Boundary-Layer Control as a Means of Reducing Drag on Fully Submerged Bodies of Revolution ». Journal of Fluids Engineering 107, no 3 (1 septembre 1985) : 342–47. http://dx.doi.org/10.1115/1.3242489.
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The drag of axisymmetric bodies can be reduced by boundary-layer suction, which delays transition and can control separation. In this study, boundary-layer transition is delayed by applying a distributed suction technique. Optimization calculations were performed to define the minimal drag bodies at Reynolds numbers of 107 and 108. The saving in drag relative to optimal bodies with non-controlled boundary layers is shown to be 18 and 78 percent, at Reynolds numbers of 107 and 108, respectively.
12
Friederich, Tillmann, et Markus J. Kloker. « Control of the secondary cross-flow instability using localized suction ». Journal of Fluid Mechanics 706 (13 juillet 2012) : 470–95. http://dx.doi.org/10.1017/jfm.2012.269.
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AbstractTransition control by suction in a three-dimensional boundary-layer flow subject to cross-flow instability is investigated using direct numerical simulation. Whereas the classical application of (homogeneous) suction at the wall is aimed at modifying the quasi-two-dimensional base flow to weaken primary cross-flow instability, here the three-dimensional nonlinear disturbance state with large-amplitude steady cross-flow vortices (CFVs) is controlled. Strong, localized ‘pinpoint’ suction is shown to be suitable for altering the CFVs and the associated flow field such that secondary instability is weakened or even completely suppressed. Thus significant delay of transition to turbulence can be achieved.
13
Grek, G. R., M. M. Katasonov, V. V. Kozlov, V. I. Kornilov, A. V. Kryukov et I. A. Sadovskiy. « Control of the Laminar-Turbulent Transition on the Wing Profile by Distributed Suction through a Finely Perforated Surface ». Siberian Journal of Physics 14, no 4 (2019) : 28–54. http://dx.doi.org/10.25205/2541-9447-2019-14-4-28-54.
Texte intégralRésumé :
The results of experimental investigations of the influence of distributed suction through a finely perforated section of a symmetric airfoil on the spatial development of disturbances in the boundary layer are presented. It was found that distributed suction reduces by 10 times the intensity of natural disturbances of the boundary layer and by 20 times the intensity of artificial disturbances generated by an external acoustic field. A spectral analysis of disturbances showed that suction reduces the intensity of high-frequency fluctuations for both natural and forced disturbances. It was found that the distributed suction affects the average flow – when the suction is on, the separation of the boundary layer near the trailing edge of the wing is eliminated. It was found that distributed suction significantly affects the mean flow, up to eliminating the boundary-layer separation near the trailing edge of the wing.
14
Poll, D. I. A., S. A. Walsh et M. C. Gallagher. « On the effect of uniform suction on stability and transition in zero pressure gradient viscous incompressible flow ». Aeronautical Journal 100, no 995 (mai 1996) : 143–50. http://dx.doi.org/10.1017/s0001924000027445.
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AbstractThe classic problem of stability and transition to turbulence of an incompressible flow over a flat plate subjected to uniform suction through the surface is considered. This is an important “model” problem for laminar flow control and the results have direct application to the design of low drag aerofoils. Detailed mean boundary layer calculations are presented and the flow stability is determined by obtaining numerical solutions of the Orr-Somerfield equation. Transition conditions are estimated by using the semi-empirical en method. Accurate values have been obtained for the levels of suction necessary to delay instability and transition indefinitely. Consideration has also been given to the effect of surface suction upon the plate drag.
15
Volino, Ralph J. « Passive Flow Control on Low-Pressure Turbine Airfoils ». Journal of Turbomachinery 125, no 4 (1 octobre 2003) : 754–64. http://dx.doi.org/10.1115/1.1626685.
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Two-dimensional rectangular bars have been used in an experimental study to control boundary layer transition and reattachment under low-pressure turbine conditions. Cases with Reynolds numbers (Re) ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) have been considered at low (0.5%) and high (8.5% inlet) free-stream turbulence levels. Three different bars were considered, with heights ranging from 0.2% to 0.7% of suction surface length. Mean and fluctuating velocity and intermittency profiles are presented and compared to results of baseline cases from a previous study. Bar performance depends on the bar height and the location of the bar trailing edge. Bars located near the suction surface velocity maximum are most effective. Large bars trip the boundary layer to turbulent and prevent separation, but create unnecessarily high losses. Somewhat smaller bars had no immediate detectable effect on the boundary layer, but introduced small disturbances that caused transition and reattachment to move upstream from their locations in the corresponding baseline case. The smaller bars were effective under both high and low free-stream turbulence conditions, indicating that the high free-stream turbulence transition is not simply a bypass transition induced by the free stream. Losses appear to be minimized when a small separation bubble is present, so long as reattachment begins far enough upstream for the boundary layer to recover from the separation. Correlations for determining optimal bar height are presented. The bars appear to provide a simple and effective means of passive flow control. Bars that are large enough to induce reattachment at low Re, however, cause higher losses at the highest Re. Some compromise would, therefore, be needed when choosing a bar height for best overall performance.
16
NOTO, Kazuo, et Katsumi HIRAOKA. « Control of Boundary Layer Transition on a Swept Wing by Suction. » Journal of the Japan Society for Aeronautical and Space Sciences 48, no 563 (2000) : 393–98. http://dx.doi.org/10.2322/jjsass.48.393.
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Katasonov, Mikhail M., Genrich R. Grek, Viktor V. Kozlov, Vladimir I. Kornilov, Alexey V. Kryukov et Ivan A. Sadovsky. « Control of Nonlinear Stage of the Laminar-Turbulent Transition on an Airfoil by Distributed Suction through a Finely Perforated Surface ». Siberian Journal of Physics 15, no 2 (2020) : 25–49. http://dx.doi.org/10.25205/2541-9447-2020-15-2-25-49.
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The results of experimental investigations of the influence of distributed suction through a finely perforated surface on the spatial development of perturbations of the straight wing boundary layer at the nonlinear stage of its evolution are presented in this article. It was found that distributed suction reduces the intensity of integral pulsations for natural disturbances by 90 times. A spectral analysis of disturbances showed a decrease in the intensity of high-frequency fluctuations in a narrow frequency band by two orders of magnitude for natural and forced disturbances generated by an external acoustic field. It was found that the distributed suction affects the average flow, namely, when the suction is on, the turbulent state of the boundary layer is eliminated, its separation near the trailing edge of the wing and the laminar flow is defined in the boundary layer.
18
Zhang, Weihao, et Zhengping Zou. « Large eddy simulations of periodic wake effects on boundary-layer transition of low-pressure turbine cascades ». AIP Advances 13, no 2 (1 février 2023) : 025128. http://dx.doi.org/10.1063/5.0139787.
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The periodic wake effect is one of the most important sources of unsteady disturbance in turbines. Its influence on the boundary layer transition process of the downstream blade suction surface is an important factor determining the turbine loss and aerodynamic performance, and also an effective potential approach of turbine loss control. In this paper, the high-load low-pressure turbine (LPT) cascade is taken as the research object, and the large eddy simulation based on the inhouse coed Multiblock Parallel Large-eddy Simulation is used to study the periodic influence of upstream wake. The unsteady transition process of the boundary layer on the suction surface of the turbine cascade and the spatial–temporal evolution of the vortex are discussed in detail. It is shown that there are three modes of boundary layer transition on the suction surface of the LPT cascade under the effect of wake, occurring alternately during the wake passing period. Each mode of transition has different characteristics in vortex structures, as well as boundary-layer separation and reattachment, thereby makes different losses. Although the transition mechanism and evolution process of the three modes are different, the calming regions exist in all three modes, which is important for the control of the boundary layer. This study gives an important reference for reducing the flow loss in high-load turbines by means of periodic wakes.
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Wang, Hongbiao, Lei Tan, Ming Liu, Xiang Liu et Baoshan Zhu. « Numerical Investigation on the Transition Flow around NLF Airfoil ». Energies 16, no 4 (12 février 2023) : 1826. http://dx.doi.org/10.3390/en16041826.
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A natural laminar flow (NLF) airfoil is designed to reduce drag by expanding laminar flow areas. In-depth knowledge of transition performance is essential for its aerodynamic design. The k-ω-γ-Reθ framework, which consists of the SST k-ω turbulence model and γ-Reθ transition model, is employed to simulate transitional flows around an NLF wing RAE5243 airfoil. The transition performances of the RAE5243 airfoil under various values of turbulent intensity, temperature, angle of attack, and Mach number are simulated and compared. The results show that the rise of inflow turbulent intensity will promote an earlier transition on both the suction and pressure sides. The influence of wall temperature on transition is limited. The rise of angle of attack will lead to an earlier transition on the pressure side but a later transition on the suction side. With the rise of Mach number, the transition happens earlier under a zero and positive angle of attack but later under a negative angle of attack. In addition, the correlation of transition onset locations with respect to turbulent intensity, surface temperature, angle of attack, and Mach number is established based on numerical results.
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LUNDELL, FREDRIK. « Reactive control of transition induced by free-stream turbulence : an experimental demonstration ». Journal of Fluid Mechanics 585 (7 août 2007) : 41–71. http://dx.doi.org/10.1017/s0022112007006490.
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The present wind-tunnel experiment demonstrates that a reactive control system is able to decrease the amplitude of random disturbances in a flat-plate boundary layer. The disturbances were induced in a laminar boundary layer by a turbulent free stream. The control system consisted of upstream wall-shear-stress sensors (wall wires) and downstream actuators (suction through holes). An ad hoc threshold-and-delay control algorithm is evaluated and parameter variations were performed in order to find a suitable working point of the control system. Detailed measurements of the flow field show how the control influences the disturbances in the boundary layer, whereas the effect on the mean flow owing to the control is minute. The control system manages to inhibit the growth of the fluctuations of the streamwise velocity component for a considerable distance downstream of the two actuator positions. Further downstream, however, the amplitudes of the fluctuations grow again. The flow rate used to obtain the control effect is one sixth of that necessary if continuous distributed suction is used to reach the same control objective. Finally, correlations and spectra show that the elongation of the structures in the streamwise direction is eliminated in the regions where the control has the largest effect. The spanwise scale of the disturbances is not affected by the control.
21
Laurien, E., et L. Kleiser. « Numerical simulation of boundary-layer transition and transition control ». Journal of Fluid Mechanics 199 (février 1989) : 403–40. http://dx.doi.org/10.1017/s002211208900042x.
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The laminar-turbulent transition process in a parallel boundary-layer with Blasius profile is simulated by numerical integration of the three-dimensional incompressible Navier-Stokes equations using a spectral method. The model of spatially periodic disturbances developing in time is used. Both the classical Klebanoff-type and the subharmonic type of transition are simulated. Maps of the three-dimensional velocity and vorticity fields and visualizations by integrated fluid markers are obtained. The numerical results are compared with experimental measurements and flow visualizations by other authors. Good qualitative and quantitative agreement is found at corresponding stages of development up to the one-spike stage. After the appearance of two-dimensional Tollmien-Schlichting waves of sufficiently large amplitude an increasing three-dimensionality is observed. In particular, a peak-valley structure of the velocity fluctuations, mean longitudinal vortices and sharp spike-like instantaneous velocity signals are formed. The flow field is dominated by a three-dimensional horseshoe vortex system connected with free high-shear layers. Visualizations by time-lines show the formation of A-structures. Our numerical results connect various observations obtained with different experimental techniques. The initial three-dimensional steps of the transition process are consistent with the linear theory of secondary instability. In the later stages nonlinear interactions of the disturbance modes and the production of higher harmonics are essential.We also study the control of transition by local two-dimensional suction and blowing at the wall. It is shown that transition can be delayed or accelerated by superposing disturbances which are out of phase or in phase with oncoming Tollmien-Schlichting instability waves, respectively. Control is only effective if applied at an early, two-dimensional stage of transition. Mean longitudinal vortices remain even after successful control of the fluctuations.
22
Guo, Zhengfei, et Markus J. Kloker. « Control of crossflow-vortex-induced transition by unsteady control vortices ». Journal of Fluid Mechanics 871 (22 mai 2019) : 427–49. http://dx.doi.org/10.1017/jfm.2019.288.
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The fundamental mechanisms of a hitherto unstudied approach to control the crossflow-induced transition in a three-dimensional boundary layer employing unsteady control vortices are investigated by means of direct numerical simulations. Using a spanwise row of blowing/suction or volume-force actuators, subcritical travelling crossflow vortex modes are excited to impose a stabilizing (upstream) flow deformation (UFD). Volume forcing mimics the effects of alternating current plasma actuators driven by a low-frequency sinusoidal signal. In this case the axes of the actuators are aligned with the wave crests of the desired travelling mode to maximize receptivity and abate the influence of other unwanted, misaligned modes. The resulting travelling crossflow vortices generate a beneficial mean-flow distortion reducing the amplification rate of naturally occurring steady or unsteady crossflow modes without invoking significant secondary instabilities. It is found that the stabilizing effect achieved by travelling control modes is somewhat weaker than that achieved by the steady modes in the classical UFD method. However, the energy requirements for unsteady-UFD plasma actuators would be significantly lower than for steady UFD because the approach makes full use of the inherent unsteadiness of the plasma-induced volume force with alternating-current-driven actuators. Also, the input control amplitude can be lower since unsteady crossflow vortex modes grow stronger in the flow.
23
Kostov, Mihail, Todor Todorov, Rosen Mitrev, Konstantin Kamberov et Rumen Nikolov. « A Study of a Bistable Reciprocating Piston Pump Driven by Shape Memory Alloys and Recuperative Springs ». Actuators 12, no 2 (17 février 2023) : 90. http://dx.doi.org/10.3390/act12020090.
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This paper presents and examines a new design concept for a bistable reciprocating piston pump. The bistable pump mechanism belongs to the bistable mechanisms, which have two stable positions at the end of the suction and discharge strokes. The transition between the stable positions is achieved by using triggering force at each beginning of suction and discharge and subsequent movement using a recuperative spring. In this mechanism, the triggering forces are created by two Shape Memory Alloy (SMA) wires. Geometric and force expressions for the pump suction and discharge strokes are derived. Additional equations are obtained for the balance of moments for the two stable equilibrium positions and the unstable position in the middle of the stroke. Numerical studies have been conducted for the suction and discharge strokes, considering the force exerted by the gas on the piston, which is modelled by an indicator diagram assuming a polytropic process. It was found that the load on the mechanism has significant non-uniformity. The diagrams illustrating the distribution of total moments showed that the cold SMA wire shifted the point of instability. The numerical example shows how to choose the right spring stiffness to obtain energy recovery. In this way, the triggering SMA forces act only at the beginning of the two strokes and, after that, the recuperative forces substitute the action of the SMA forces. The theoretical relationships and methods presented here are suitable for synthesizing new pumps or analyzing similar mechanisms.
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Abbaszadeh, Mohammad M., et Sandra L. Houston. « Influence of Soil Cracking on the Soil-Water Characteristic Curve of Clay Soil ». Soils and Rocks 38, no 1 (1 janvier 2015) : 49–58. http://dx.doi.org/10.28927/sr.381049.
Texte intégralRésumé :
The hydraulic conductivity for unsaturated soil conditions is more difficult to estimate than for the saturated condition. In addition, as the soil transitions from intact to cracked, the difficulty in estimating the unsaturated hydraulic conductivity increases. One critical step in the determination of unsaturated flow hydraulic conductivity is the evaluation of the Soil-Water Characteristic Curve (SWCC). In this paper, a series of laboratory studies of direct measurements of cracked soil SWCCs is presented, including challenges associated with the control of very low suction levels associated with crack dewatering. An oedometer-type SWCC apparatus, capable of suction and net normal stress control, and volume change measurement, was used in these experimental studies. It is common that SWCCs are comprised of matric suction values below about 1500 kPa, and total suction values for suctions higher than about 1500 kPa (Fredlund et al., 2012). In this study, all measured or controlled suction values were less than 1500 kPa and obtained using the axis translation method, and the curve in the higher suction range was projected by forcing the SWCC through 106 kPa for completely dried conditions (Fredlund et al., 2012). Volume change corrections were made to the reported volumetric water contents, which is of particular importance when the soil under consideration undergoes volume change in response to wetting or drying. A technique for the determination of the SWCC for cracked clay soils is presented. Test results validated the fact that the SWCC of a cracked soil can be represented by a bimodal function due to the Air Entry Value (AEV) of the cracks being much lower than the AEV of the soil matrix. It was also found that differences between the SWCC for cracked and intact soil appears only in the very low suction range.
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Cherubini, S., J. C. Robinet et P. De Palma. « Nonlinear control of unsteady finite-amplitude perturbations in the Blasius boundary-layer flow ». Journal of Fluid Mechanics 737 (26 novembre 2013) : 440–65. http://dx.doi.org/10.1017/jfm.2013.576.
Texte intégralRésumé :
AbstractThe present work provides an optimal control strategy, based on the nonlinear Navier–Stokes equations, aimed at hampering the rapid growth of unsteady finite-amplitude perturbations in a Blasius boundary-layer flow. A variational procedure is used to find the blowing and suction control law at the wall providing the maximum damping of the energy of a given perturbation at a given target time, with the final aim of leading the flow back to the laminar state. Two optimally growing finite-amplitude initial perturbations capable of leading very rapidly to transition have been used to initialize the flow. The nonlinear control procedure has been found able to drive such perturbations back to the laminar state, provided that the target time of the minimization and the region in which the blowing and suction is applied have been suitably chosen. On the other hand, an equivalent control procedure based on the linearized Navier–Stokes equations has been found much less effective, being not able to lead the flow to the laminar state when finite-amplitude disturbances are considered. Regions of strong sensitivity to blowing and suction have been also identified for the given initial perturbations: when the control is actuated in such regions, laminarization is also observed for a shorter extent of the actuation region. The nonlinear optimal blowing and suction law consists of alternating wall-normal velocity perturbations, which appear to modify the core flow structures by means of two distinct mechanisms: (i) a wall-normal velocity compensation at small times; (ii) a rotation-counterbalancing effect al larger times. Similar control laws have been observed for different target times, values of the cost parameter, and streamwise extents of the blowing and suction zone, meaning that these two mechanisms are robust features of the optimal control strategy, provided that the nonlinear effects are taken into account.
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Lebon, Benoît, Jorge Peixinho, Shun Ishizaka et Yuji Tasaka. « Subcritical transition to turbulence in a sudden circular pipe expansion ». Journal of Fluid Mechanics 849 (18 juin 2018) : 340–54. http://dx.doi.org/10.1017/jfm.2018.421.
Texte intégralRésumé :
The results of experiments on the flow through a circular sudden expansion pipe at moderate Reynolds numbers are presented. At five diameters upstream of the expansion, laminar flow was disturbed by a (constant) cross-flow jet, a suction or a (periodic in–out) synthetic jet from a hole in the wall. When the disturbance exceeded a critical value of the control parameter depending on the Reynolds number, localised turbulent patches formed downstream of the expansion at fixed axial positions. For the cross-flow jet, the onset of turbulent patches is related to the velocity ratio of the mean jet velocity to the mean pipe velocity. At low velocity ratio, turbulent patches formed intermittently. For the suction disturbance, the flow experienced a strong asymmetry of the recirculation region and required a larger velocity ratio before the turbulent patch formed. For the synthetic jet, the amplification of wavy disturbances into turbulent patches and their axial positions are controlled by the driving frequency. Overall, these results suggest the existence of different mechanisms for the development of localised turbulent patches.
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Mouyon, Philippe, Grégoire Casalis, Alain Séraudie et Sylvain Prudhomme. « Feedback control of the laminar-turbulent transition onset in a boundary layer by suction ». Experimental Thermal and Fluid Science 16, no 1-2 (janvier 1998) : 22–31. http://dx.doi.org/10.1016/s0894-1777(97)10004-8.
Texte intégral
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Thompson, Alice B., Dmitri Tseluiko et Demetrios T. Papageorgiou. « Falling liquid films with blowing and suction ». Journal of Fluid Mechanics 787 (15 décembre 2015) : 292–330. http://dx.doi.org/10.1017/jfm.2015.683.
Texte intégralRésumé :
Flow of a thin viscous film down a flat inclined plane becomes unstable to long-wave interfacial fluctuations when the Reynolds number based on the mean film thickness becomes larger than a critical value (this value decreases as the angle of inclination to the horizontal increases, and in particular becomes zero when the plate is vertical). Control of these interfacial instabilities is relevant to a wide range of industrial applications including coating processes and heat or mass transfer systems. This study considers the effect of blowing and suction through the substrate in order to construct from first principles physically realistic models that can be used for detailed passive and active control studies of direct relevance to possible experiments. Two different long-wave, thin-film equations are derived to describe this system; these include the imposed blowing/suction as well as inertia, surface tension, gravity and viscosity. The case of spatially periodic blowing and suction is considered in detail and the bifurcation structure of forced steady states is explored numerically to predict that steady states cease to exist for sufficiently large suction speeds since the film locally thins to zero thickness, giving way to dry patches on the substrate. The linear stability of the resulting non-uniform steady states is investigated for perturbations of arbitrary wavelength, and any instabilities are followed into the fully nonlinear regime using time-dependent computations. The case of small amplitude blowing/suction is studied analytically both for steady states and their stability. Finally, the transition between travelling waves and non-uniform steady states is explored as the amplitude of blowing and suction is increased.
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Ford, R. W., et D. I. A. Poll. « A Parallel Processing Approach to Transition Prediction for Laminar Flow Control System Design ». Scientific Programming 4, no 3 (1995) : 203–17. http://dx.doi.org/10.1155/1995/573945.
Texte intégralRésumé :
The performance of transport aircraft can be considerably improved if the process by which the wing boundary layer becomes turbulent can be controlled and extensive areas of laminar flow maintained. In order to design laminar flow control systems, it is necessary to be able to predict the movement of the transition location in response to changes in control variables, e.g., surface suction. At present, the technique which is available to industry requires excessively long computational time – so long that it is not suitable for use in the "design process." Therefore, there is a clear need to produce a system which delivers results in near realtime, i.e., in seconds rather than hours. This article details how parallel computing techniques on a KSR-1 produce these performance improvements.
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Dou, Chunpeng, Xinjiang Yang, Changqing Tian et Xianting Li. « Numerical Analysis on the Performance of Control Valve in Variable Displacement Wobble Plate Compressor ». Journal of Mechanical Design 127, no 2 (1 mars 2005) : 326–33. http://dx.doi.org/10.1115/1.1829725.
Texte intégralRésumé :
The performance of control valve is analyzed in this paper in order to study characteristics of the variable displacement wobble plate compressor. The forces acting on the control valve are analyzed and the mathematical model to analyze its performance is developed. A test bench is set up to get the steady and dynamic performance of control valve. The steady-state performances predicted by the mathematical model agree well with the experimental data while the predicted dynamic performances agree with the experimental results qualitatively. With the mathematical model, the steady-state performances of the control valve with four different preset control points and three different effective areas of the evacuated bellows have been analyzed, and the dynamic behavior of one type control valve has been studied. It is shown that: (1) the suction pressure at preset control point is inversely proportional to the discharge pressure; (2) the larger the starting force of spring in control valve or the smaller the effective area of evacuated bellows, the larger the suction pressure at preset control point; and the larger the starting force of spring in control valve and the smaller the maximum opening of ball valve, the more quickly the crankcase pressure changes and the smaller mass flow rate of control valve; the slope ratio of crankcase pressure to suction pressure cannot be influenced by the starting force of spring in control valve but can be influenced by the effective area of evacuated bellows; (3) the transition time of the crankcase pressure is about 1 s and can be neglected when working condition changes.
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Zhang, Xue Feng, Maria Vera, Howard Hodson et Neil Harvey. « Separation and Transition Control on an Aft-Loaded Ultra-High-Lift LP Turbine Blade at Low Reynolds Numbers : Low-Speed Investigation ». Journal of Turbomachinery 128, no 3 (1 février 2005) : 517–27. http://dx.doi.org/10.1115/1.2187524.
Texte intégralRésumé :
An experimental study was conducted to improve the performance of an aft-loaded ultra-high-lift low-pressure turbine blade known as U2 at low Reynolds numbers. This was achieved by manipulation of the laminar-turbulent transition process on the suction surface. The U2 profile was designed to meet the targets of reduced cost, weight and fuel burn of aircraft engines. The studies were conducted on both low-speed and high-speed experimental facilities under the unsteady flow conditions with upstream passing wakes. The current paper presents the low-speed investigation results. On the smooth suction surface, the incoming wakes are not strong enough to suppress the separation bubble due to the strong adverse pressure gradient on the suction surface and the low wake passing frequency, which allows the separation between the wakes more time to re-establish. Therefore, the profile losses of this ultra-high-lift blade are not as low as conventional or high-lift blades at low Reynolds numbers even in unsteady flows. Two different types of passive separation control devices, i.e., surface trips and air jets, were investigated to further improve the blade performance. The measurement results show that the profile losses can be further reduced to the levels similar to those of the high-lift and conventional blades due to the aft-loaded nature of this ultra-high-lift blade. Detailed surveys of the blade surface boundary layer developments showed that the loss reduction was due to the suppression of the separation underneath the wakes, the effect of the strengthened calmed region and the smaller separation bubble between wakes.
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Sun, Yiyang, et Maziar S. Hemati. « Feedback Control for Transition Suppression in Direct Numerical Simulations of Channel Flow ». Energies 12, no 21 (29 octobre 2019) : 4127. http://dx.doi.org/10.3390/en12214127.
Texte intégralRésumé :
For channel flow at subcritical Reynolds numbers ( R e < 5772 ), a laminar-to-turbulent transition can emerge due to a large transient amplification in the kinetic energy of small perturbations, resulting in an increase in drag at the walls. The objectives of the present study are three-fold: (1) to study the nonlinear effects on transient energy growth, (2) to design a feedback control strategy to prevent this subcritical transition, and (3) to examine the control mechanisms that enable transition suppression. We investigate transient energy growth of linear optimal disturbance in plane Poiseuille flow at a subcritical Reynolds number of R e = 3000 using linear analysis and nonlinear simulation. Consistent with previous studies, we observe that the amplification of the given initial perturbation is reduced when the nonlinear effect is substantial, with larger perturbations being less amplified in general. Moreover, we design linear quadratic optimal controllers to delay transition via wall-normal blowing and suction actuation at the channel walls. We demonstrate that these feedback controllers are capable of reducing transient energy growth in the linear setting. The performance of the same controllers is evaluated for nonlinear flows where a laminar-to-turbulent transition emerges without control. Nonlinear simulations reveal that the controllers can reduce transient energy growth and suppress transition. Further, we identify and characterize the underlying physical mechanisms that enable feedback control to suppress and delay laminar-to-turbulent transition.
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Zafar, Mehwish, Muhammad Afzal Rana et Atifa Latif. « Three-Dimensional Fluctuating Flow of a Second-Grade Fluid along an Infinite Horizontal Plate with Periodic Suction ». Mathematical Problems in Engineering 2022 (29 octobre 2022) : 1–17. http://dx.doi.org/10.1155/2022/3509721.
Texte intégralRésumé :
Laminar flow control plays a vital role in reducing drag resulting enhancement in the vehicle power by a significant amount. Theoretical and experimental studies have proven that the transition from laminar to turbulent flow (causing the drag coefficient to enhance) may be delayed/prevented by the suction of the fluid from the boundary layer to the wall. The purpose of this work is to study the effects of periodic suction on the unsteady 3-dimensional fluctuating flow of a second grade incompressible fluid flowing laminarly over a horizontal porous infinite plate. The plate is subjected to a sinusoidal transverse suction velocity while the free stream velocity oscillates in time about a constant mean. The flow turns out to be 3-dimensional because of transverse direction of the periodic suction velocity. By the series expansion method, analytic expressions for transient velocity, skin friction components, and pressure are attained. Impact of nondimensional parameters evolving in the mathematical model of the problem on these physical quantities are visualized graphically and discussed analytically. The velocity component u is found to be rising with a growth in suction parameter α. The pressure is noted to be growing with a growth in Reynolds number R e . Further, due to suction at the plate and transient free stream velocity, the pressure increases near the plate and then reaches to a steady value far-off the plate. The skin friction along the main flow is noticed to be decreasing with the rise in α for 0.5 ≤ R e ≤ 1.4 , however, an exponential rise is observed for R e > 1.4. The skin friction along the cross flow is noted to be enhancing for the rising values of suction parameter and elastic parameter. The present results have excellent agreement with previous published results in the limiting sense. This study is beneficial in designing and manufacturing laminar flow control system to enhance the vehicle power requirement.
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Deutsch, S., et W. C. Zierke. « The Measurement of Boundary Layers on a Compressor Blade in Cascade : Part 1—A Unique Experimental Facility ». Journal of Turbomachinery 109, no 4 (1 octobre 1987) : 520–26. http://dx.doi.org/10.1115/1.3262142.
Texte intégralRésumé :
A unique cascade facility is described which permits the use of laser-Doppler velocimetry (LDV) to measure blade boundary layer profiles. Because of the need for a laser access window, the facility cannot reply on continuous blade pack suction to achieve two-dimensional, periodic flow. Instead, a strong suction upstream of the blade pack is used in combination with tailboards to control the flow field. The distribution of the upstream suction is controlled through a complex baffling system. A periodic, two–dimensional flow field is achieved at a chord Reynolds number of 500,000 and an incidence angle of 5 deg on a highly loaded, double circular arc, compressor blade. Inlet and outlet flow profiles, taken using five-hole probes, and the blade static-pressure distribution are used to document the flow field for use with the LDV measurements (see Parts 2 and 3). Inlet turbulence intensity is measured, using a hot wire, to be 0.18 percent. The static-pressure distribution suggests both separated flow near the trailing edge of the suction surface and an initially laminar boundary layer profile near the leading edge of the pressure surface. Probe measurements are supplemented by sublimation surface visualization studies. The sublimation studies place boundary layer transition at 64.2 ± 3.9 percent chord on the pressure surface, and indicate separation on the suction surface at 65.6 percent ± 3.5 percent chord.
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Huang, Ganglei, Xi Chen, Jianqiang Chen, Xianxu Yuan et Guohua Tu. « The stabilizing effect of grooves on Görtler instability-induced boundary layer transition in hypersonic flow ». Physics of Fluids 35, no 4 (avril 2023) : 041701. http://dx.doi.org/10.1063/5.0146348.
Texte intégralRésumé :
Görtler vortex-induced hypersonic boundary layer transition controlled by grooves is investigated using direct numerical simulations and spatial bi-global stability analysis. In the simulations, Görtler vortices are excited by wall steady blowing and suction with spanwise wavelengths of 3 mm. It is found that when the wall is covered with grooves, the Görtler streaks keep more regular even at the end of the model. In addition, the skin friction coefficient is reduced efficiently. Furthermore, the wall-normal and spanwise velocity shear are both reduced, suppressing growths of secondary instabilities. In conclusion, grooves can delay Görtler vortex-induced transition by modifying the Görtler streaks structure and instability, which would shed light on hypersonic boundary layer transition control.
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Sarwar, Wasim, Fernando Mellibovsky, Md Mahbub Alam et Farhan Zafar. « Transition to Periodic Behaviour of Flow Past a Circular Cylinder under the Action of Fluidic Actuation in the Transitional Regime ». Energies 14, no 16 (18 août 2021) : 5069. http://dx.doi.org/10.3390/en14165069.
Texte intégralRésumé :
This study focuses on the numerical investigation of the underlying mechanism of transition from chaotic to periodic dynamics of circular cylinder wake under the action of time-dependent fluidic actuation at the Reynolds number = 2000. The forcing is realized by blowing and suction from the slits located at ±90∘ on the top and bottom surfaces of the cylinder. The inverse period-doubling cascade is the underlying physical mechanism underpinning the wake transition from mild chaos to perfectly periodic dynamics in the spanwise-independent, time-dependent forcing at twice the natural vortex-shedding frequency.
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Feng, Li-Hao, Kwing-So Choi et Jin-Jun Wang. « Flow control over an airfoil using virtual Gurney flaps ». Journal of Fluid Mechanics 767 (20 février 2015) : 595–626. http://dx.doi.org/10.1017/jfm.2015.22.
Texte intégralRésumé :
AbstractFlow control over a NACA 0012 airfoil is carried out using a dielectric barrier discharge (DBD) plasma actuator at the Reynolds number of 20 000. Here, the plasma actuator is placed over the pressure (lower) side of the airfoil near the trailing edge, which produces a wall jet against the free stream. This reverse flow creates a quasi-steady recirculation region, reducing the velocity over the pressure side of the airfoil. On the other hand, the air over the suction (upper) side of the airfoil is drawn by the recirculation, increasing its velocity. Measured phase-averaged vorticity and velocity fields also indicate that the recirculation region created by the plasma actuator over the pressure surface modifies the near-wake dynamics. These flow modifications around the airfoil lead to an increase in the lift coefficient, which is similar to the effect of a mechanical Gurney flap. This configuration of DBD plasma actuators, which is investigated for the first time in this study, is therefore called a virtual Gurney flap. The purpose of this investigation is to understand the mechanism of lift enhancement by virtual Gurney flaps by carefully studying the global flow behaviour over the airfoil. First, the recirculation region draws the air from the suction surface around the trailing edge. The upper shear layer then interacts with the opposite-signed shear layer from the pressure surface, creating a stronger vortex shedding from the airfoil. Secondly, the recirculation region created by a DBD plasma actuator over the pressure surface displaces the positive shear layer away from the airfoil, thereby shifting the near-wake region downwards. The virtual Gurney flap also changes the dynamics of laminar separation bubbles and associated vortical structures by accelerating laminar-to-turbulent transition through the Kelvin–Helmholtz instability mechanism. In particular, the separation point and the start of transition are advanced. The reattachment point also moves upstream with plasma control, although it is slightly delayed at a large angle of attack.
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HOTHAZIE, Mihai-Vladut, et Sterian DANAILA. « Effects of the boundary layer control methods on stability and separation point ». INCAS BULLETIN 13, no 1 (5 mars 2021) : 77–87. http://dx.doi.org/10.13111/2066-8201.2021.13.1.8.
Texte intégralRésumé :
This paper concerns the benefits of the active boundary layer control methods. The main focus was studying the effectiveness of suction control for a laminar flow over an airfoil. However, injection normal to or along the wall was also approached using two numerical methods. For different values and distributions of the velocity control magnitude, a systematic comparison was done. Having the results of the laminar flow, a linear stability analysis based on the small disturbance theory was carried out obtaining both the neutral stability curves and the transition point. In the end, for each case, results were presented with the corresponding observations. Additionally, a study on the dependency of the separation point with respect to the injection velocity magnitude was done.
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Ma, C. V., et H. Y. Xu. « Parameter-Based Design and Analysis of Wind Turbine Airfoils with Conformal Slot Co-Flow Jet ». Journal of Applied Fluid Mechanics 16, no 2 (1 février 2023) : 269–83. http://dx.doi.org/10.47176/jafm.16.02.1318.
Texte intégralRésumé :
A co-flow jet (CFJ), an active flow control method combining blowing and suction control, can effectively suppress the stall of airfoils. However, the streamwise jet channel along the upper surface of a conventional CFJ airfoil reduces the thickness and camber of the baseline, degrading the aerodynamic performance when the jet is inactive. The conformal slot CFJ airfoil was proposed to address this problem, but the design method is still absent. This paper proposed a general design method based on parameters including the slot angle, transition shape and distance of the injection and suction slot. The mechanism of the best parameter was analyzed. The designed conformal slot CFJ airfoil was checked under different jet intensities, and the turbine power curve was predicted when substituting CFJ airfoils for the baseline. Compared with the conventional CFJ airfoil, the designed conformal slot CFJ airfoil has three advantages: eliminating the performance loss when the jet is off, saving jet energy when suppressing the flow separation, and improving the power generation of wind turbines at low wind speeds.
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MacMANUS, D. G., et J. A. EATON. « Flow physics of discrete boundary layer suction – measurements and predictions ». Journal of Fluid Mechanics 417 (25 août 2000) : 47–75. http://dx.doi.org/10.1017/s0022112000001026.
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The primary objective of this work is to determine the detailed characteristics of the flow features induced in a boundary layer by suction through laminar flow control (LFC) perforations. An additional goal is to validate a predictive method for generic LFC suction surfaces and to apply this technique to typical flight condition configurations. Fundamental insights into the flow physics of LFC suction surfaces are obtained from a unique series of high-resolution three-component laser Doppler velocimetry (LDV) flow field measurements. The flow fields induced by isolated super-scale perforations under low-speed conditions are mapped and found to be strongly three-dimensional and profoundly different from the idealized concept of continuously distributed suction. Over a range of sub- and super-critical suction flow rates a variety of suction-dependent complex flow features are identified, including a pair of contra-rotating streamwise vortices, multiple co-rotating streamwise vortices, spanwise variations of the mean flow and inherently unstable boundary layer profiles. The measurements reveal that suction-induced transition commences with an instability of these attached vortices, resulting in the development of a pair of turbulent wedges downstream of the perforation. A finite-volume Navier–Stokes method is validated by simulating a variety of low-speed experiments and comparisons are made between the LDV measurements and the predicted flow field. The computational technique reproduces all of the observed flow features, although it slightly under-predicts their magnitude and extent. By analysing the predicted flow fields the mechanism for the formation of the trailing vortex pair is established. Earlier flow visualization experiments, which exhibited vortex shedding, are also simulated by solving the time-dependent governing equations and it is found that the principal unsteady flow features are captured. Despite the challenge posed to the computational method by the diverse range of flow phenomena induced by discrete suction, the predictions provide good agreement with the measurements and observations. The computational tool is subsequently applied to predict the flow fields of single and multiple rows of actual-scale micro-perforations under low-speed and typical transonic flight conditions. A range of suction-induced flow features are predicted and a variety of distinct flow modes are identified. The low-speed critical suction limits are also measured and a design criterion, based on the sucked streamtube characteristics, is established. The basis of this critical suction criterion is also validated for transonic flight configurations.
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Shi, Yayun, Xiayu Lan et Tihao Yang. « Hybrid Laminar Flow Control Optimizations for Infinite Swept Wings ». International Journal of Aerospace Engineering 2023 (8 mai 2023) : 1–33. http://dx.doi.org/10.1155/2023/3455238.
Texte intégralRésumé :
Maintaining the laminar flow on surfaces through active control is a significantly promising technique for reducing fuel burn and alleviating environmental concerns in commercial aviation. However, there is a lack of systematic parameter studies for the hybrid laminar flow control (HLFC) together with natural laminar flow (NLF). To address this need, we optimize the infinite swept wings with different sweep angles and at various conditions, including different Mach numbers, Reynolds numbers, and lift coefficients. The Reynolds-averaged Navier-Stokes (RANS) solver coupled with the linear stability theory is applied for the laminar-turbulent transition prediction, and the traditional optimization method based on evolutionary algorithms is applied for laminar flow wing optimization. The optimization results found that HLFC is required when the NLF fails at a larger sweep angle (35°) and Reynolds number ( 20 × 10 6 ). The lower pressure peak with boundary-layer suction is found to delay the transition of the regional aviation condition. Besides, the pressure distribution of HLFC is similar to NLF results at the lower Reynolds number ( 10 × 10 6 ) or sweep angle (25°), i.e., a gentle negative pressure gradient near the leading edge and a small favorable pressure gradient behind it. Clarifying the characteristics of laminar flow wings will advance the application of the laminar flow technique within its field.
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Dentel, S. K., et M. M. Abu-Orf. « Application of the Streaming Current Detector in Sludge Conditioner Selection and Control ». Water Science and Technology 28, no 1 (1 juillet 1993) : 169–79. http://dx.doi.org/10.2166/wst.1993.0040.
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The streaming current detector (SCD) can be utilized to measure charge characteristics of a suspension or sludge which can be related to zeta potential. In this study, the SCD was used as a tool for polymer selection and dose control toward improved sludge conditioning prior to centrifugation. In laboratory studies, polymer addition to sludges was followed by controlled mixing then measurement of streaming current (SC) and capillary suction time (CST). Plant studies employed facilities at an 18.8 MGD wastewater treatment facility in Warminster, PA, USA. Anaerobically digested sludge was conditioned with varying doses of cationic polymer and dewatered using two parallel solid-bowl centrifuges. The polymer caused a transition from negative to positive charge in both conditioned sludge and centrate. The polymer dose range for optimum dewaterability, as indicated by CST and solids recovery, corresponded to near-zero SC in all cases. Even when greater mixing intensities or times increased the polymer demand, the charge transition was similarly related. This correspondence was used to successfully confirm a calculation method establishing velocity gradient and mixing time similitude between jar tests and in-line mixing prior to the centrifuge.
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Ma, Jianlong, Qiuyan Li, Ming Zhao, Yanan Chen et Xiaohui Chai. « A Passive Flow Control Technique of a Small-Scale HAWT and TED Analysis Under Yaw Condition Based on Airfoil Concavity ». Energies 17, no 24 (21 décembre 2024) : 6448. https://doi.org/10.3390/en17246448.
Texte intégralRésumé :
To mitigate the energy loss caused by flow separation of a 300 W small wind turbine, a passive flow control technique based on the airfoil concavity was proposed. The suction surface of the blade was modified with eight different types of concavity, the results showed that the b1 elliptical concavity, with B-spline curves front-and-rear transition, significantly affected the airflow of the airfoil’s suction surface, improving the wind turbine’s aerodynamic performance by 3.26% at maximum. Then, the flow field characteristics of b1, c1, and c4 concave airfoils with typical geometric features under axial flow conditions demonstrated that the b1 airfoil concavity had the greatest impact on flow separation. Moreover, yaw angle was induced, and the wind turbine’s turbulent kinetic energy (TKE) and turbulent energy dissipation (TED) were investigated from the aspects of energy loss. The variation rule of the TED difference between the concave bottom and edge with yaw angle was summarized into an equation that quantitatively explained why the 10° yaw angle was the turning point of the power output, as well as the potential mechanism of concave airfoil-induced power enhancement. These findings provide a foundation for enhancing the aerodynamic performance of large megawatt-class wind turbines.
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Sharma, Sushank, Mostafa S. Shadloo, Abdellah Hadjadj et Markus J. Kloker. « Control of oblique-type breakdown in a supersonic boundary layer employing streaks ». Journal of Fluid Mechanics 873 (1 juillet 2019) : 1072–89. http://dx.doi.org/10.1017/jfm.2019.435.
Texte intégralRésumé :
The effectiveness of streak modes in controlling the oblique-type breakdown in a supersonic boundary-layer at Mach 2.0 is investigated using direct numerical simulations. Investigations in the literature have shown the effectiveness of streak modes in delaying the onset of transition dominated by two-dimensional waves, but in oblique breakdown, three-dimensional waves and a strong streak mode dominate the transition process. Paredes et al. (J. Fluid Mech., vol. 831, 2017, pp. 524–553) discussed the possible stabilization of supersonic boundary layers by optimally growing streaks using parabolized stability equations. However, no study has as yet been reported regarding direct nonlinear control of oblique breakdown. This study deals with the effects of large-amplitude decaying streak modes generated by a blowing–suction strip at the wall to control full breakdown in a reference case. Modes with four to five times the fundamental wavenumber are found to be beneficial for controlling the transition. In the first region after the control-mode forcing, the beneficial mean-flow distortion (MFD), generated by inducing the control mode, is solely responsible for hampering the growth of the fundamental-mode. On the whole, the MFD and the three-dimensional part of the control contribute equally towards controlling the oblique breakdown. The results show significant suppression of transition, and substantial improvements have been observed in the levels of the skin-friction coefficient and wall-temperature in comparison to the uncontrolled case. Moreover, refreshing the control using an additional downstream control strip increases the gain. However, the forcing amplitude must be carefully chosen in order not to introduce a generalized inflection point in the spanwise averaged mean flow invoking enhanced disturbance growth.
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Mun, Woongju, et John S. McCartney. « Compression mechanisms of unsaturated clay under high stresses ». Canadian Geotechnical Journal 52, no 12 (décembre 2015) : 2099–112. http://dx.doi.org/10.1139/cgj-2014-0438.
Texte intégralRésumé :
This paper investigates the compression behavior of unsaturated clay under mean stresses up to 160 MPa and different drainage conditions. A new isotropic pressure cell was developed that incorporates matric suction control using the axis-translation technique, and a high-pressure syringe pump operated in displacement-control mode was used to control the total stress and track specimen volume changes. In addition to presenting results from characterization tests on the cell, results from a series of isotropic compression tests performed on compacted clay specimens under drained and undrained conditions are presented. These results permit evaluation of the hardening mechanisms and transition points in the compression curve with increasing effective stress. As expected, specimens tested under undrained conditions were much stiffer than those tested under drained conditions. In the drained tests, the rate of compression was sufficient to permit steady-state dissipation of excess pore-water pressure except under the highest stress ranges. Suction-induced hardening was observed when comparing saturated and unsaturated specimens tested in the drained compression tests. In both the drained and undrained compression tests, the range of applied stresses was sufficient to cause collapse or dissolution of the air voids (pressurized saturation) and convergence of the virgin compression lines for unsaturated specimens with that measured for saturated specimens. A gradual transition to full-void closure was observed at high stresses when the compression curves were plotted on a natural scale, but the shapes of the compression curves at high stresses were not consistent with conventional soil mechanics models when plotted on a semilogarithmic scale. The results from this study provide insight into how constitutive models for unsaturated soils can be extended to high stress conditions for drained and undrained conditions.
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Cimpeanu, Radu, Susana N. Gomes et Demetrios T. Papageorgiou. « Active control of liquid film flows : beyond reduced-order models ». Nonlinear Dynamics 104, no 1 (19 février 2021) : 267–87. http://dx.doi.org/10.1007/s11071-021-06287-5.
Texte intégralRésumé :
AbstractThe ability to robustly and efficiently control the dynamics of nonlinear systems lies at the heart of many current technological challenges, ranging from drug delivery systems to ensuring flight safety. Most such scenarios are too complex to tackle directly, and reduced-order modelling is used in order to create viable representations of the target systems. The simplified setting allows for the development of rigorous control theoretical approaches, but the propagation of their effects back up the hierarchy and into real-world systems remains a significant challenge. Using the canonical set-up of a liquid film falling down an inclined plane under the action of active feedback controls in the form of blowing and suction, we develop a multi-level modelling framework containing both analytical models and direct numerical simulations acting as an in silico experimental platform. Constructing strategies at the inexpensive lower levels in the hierarchy, we find that offline control transfer is not viable; however, analytically informed feedback strategies show excellent potential, even far beyond the anticipated range of applicability of the models. The detailed effects of the controls in terms of stability and treatment of nonlinearity are examined in detail in order to gain understanding of the information transfer inside the flows, which can aid transition towards other control-rich frameworks and applications.
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Prabhu, L., et J. Srinivas. « A PARAMETRIC STUDY ON CONTROL OF FLOW SEPARATION OVER AN AIRFOIL IN INCOMPRESSIBLE REGIME ». IIUM Engineering Journal 19, no 1 (1 juin 2018) : 270–88. http://dx.doi.org/10.31436/iiumej.v19i1.784.
Texte intégralRésumé :
This paper presents the effects of airfoil geometry on flow separation behavior and obtains the transition patterns at a specific angle of attack. A strong adverse pressure gradient field is observed at the leading edge of the airfoil, and it results in a flow detachment. Leading edge flow separation is studied along with the variation of skin friction coefficient over the airfoil. Novelty in the approach is the development of a hybrid control scheme to delay the flow separation with blowing/suction of air (termed active control) over the airfoil together with the tapping of flow from the pressure side as in a classical passive control procedure. The active controller delays the flow separation, while the passive controller is used to reduce the drag coefficient significantly and increases the total performance of an airfoil. The effectiveness of these controls is examined by varying the control parameters including blowing/suction velocity, the position of the slot in terms of percentage of chord and size of the slot. All the numerical simulations are carried out using ANSYS-Fluent software. A surrogate model is also developed to predict the aerodynamic characteristics conveniently without much computational effort. The outcome of this study reveals that the blowing/suction velocity has a higher influence in delaying the flow separation. ABSTRAK: Kertas ini membentangkan tentang kesan geometri aerofoil pada perubahan pemisah aliran udara dan memperoleh bentuk peralihan pada darjah yang tepat. Terdapat tekanan kuat yang tidak sesuai pada kawasan kecerunan di hujung hadapan permukaan aerofoil, dan ini menyebabkan aliran udara terpisah. Pemisah aliran udara pada hujung hadapan ini dikaji bersama koefisien geseran pada permukaan aerofoil. Pendekatan baru pada kaedah ini adalah berkaitan pembangunan skim kawalan hibrid bagi melengahkan aliran pemisah udara melalui tiupan/sedutan udara (kawalan aktif) ke atas aerofoil bersama ketukan pada aliran dari tepi tekanan seperti mana prosedur klasik kawalan pasif. Kawalan aktif ini melengahkan aliran pemisah udara, sebaliknya kawalan pasif telah digunakan bagi mengurangkan koefisien penangguhan dengan ketara dan menambahkan jumlah prestasi aerofoil. Keberhasilan kawalan-kawalan ini dikaji dengan mengubah parameter kawalan termasuk kelajuan tiupan/sedutan udara, posisi slot berdasarkan peratusan garis temu dan saiz slot. Semua simulasi-simulasi numerikal ini dijalankan menggunakan perisian Ansys-Fluent. Model pengganti turut dibangunkan untuk menjangka ciri-ciri aero-dinamik dengan mudah tanpa usaha pengiraan yang banyak. Keputusan kajian ini mendedahkan tentang kelajuan tiupan/sedutan udara berpengaruh besar dalam melambatkan pemisahan aliran udara.
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Zhang, Xue Feng, et Howard Hodson. « Combined Effects of Surface Trips and Unsteady Wakes on the Boundary Layer Development of an Ultra-High-Lift LP Turbine Blade ». Journal of Turbomachinery 127, no 3 (1 mars 2004) : 479–88. http://dx.doi.org/10.1115/1.1860571.
Texte intégralRésumé :
An experimental investigation of the combined effects of upstream unsteady wakes and surface trips on the boundary layer development on an ultra-high-lift low-pressure turbine blade, known as T106C, is described. Due to the large adverse pressure gradient, the incoming wakes are not strong enough to periodically suppress the large separation bubble on the smooth suction surface of the T106C blade. Therefore, the profile loss is not reduced as much as might be possible. The first part of this paper concerns the parametric study of the effect of surface trips on the profile losses to optimize the surface trip parameters. The parametric study included the effects of size, type, and location of the surface trips under unsteady flow conditions. The surface trips were straight cylindrical wires, straight rectangular steps, wavy rectangular steps, or wavy cylindrical wires. The second part studies the boundary layer development on the suction surface of the T106C linear cascade blade with and without the recommended surface trips to investigate the loss reduction mechanism. It is found that the selected surface trip does not induce transition immediately, but hastens the transition process in the separated shear layer underneath the wakes and between them. In this way, the combined effects of the surface trip and unsteady wakes further reduce the profile losses. This passive flow control method can be used over a relatively wide range of Reynolds numbers.
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Xiao, Dandan, et George Papadakis. « Nonlinear optimal control of transition due to a pair of vortical perturbations using a receding horizon approach ». Journal of Fluid Mechanics 861 (27 décembre 2018) : 524–55. http://dx.doi.org/10.1017/jfm.2018.919.
Texte intégralRésumé :
This paper considers the nonlinear optimal control of transition in a boundary layer flow subjected to a pair of free stream vortical perturbations using a receding horizon approach. The optimal control problem is solved using the Lagrange variational technique that results in a set of linearized adjoint equations, which are used to obtain the optimal wall actuation (blowing and suction from a control slot located in the transition region). The receding horizon approach enables the application of control action over a longer time period, and this allows the extraction of time-averaged statistics as well as investigation of the control effect downstream of the control slot. The results show that the controlled flow energy is initially reduced in the streamwise direction and then increased because transition still occurs. The distribution of the optimal control velocity responds to the flow activity above and upstream of the control slot. The control effect propagates downstream of the slot and the flow energy is reduced up to the exit of the computational domain. The mean drag reduction is $55\,\%$ and $10\,\%$ in the control region and downstream of the slot, respectively. The control mechanism is investigated by examining the second-order statistics and the two-point correlations. It is found that in the upstream (left) side of the slot, the controller counteracts the near-wall high-speed streaks and reduces the turbulent shear stress; this is akin to opposition control in channel flow, and because the time-average control velocity is positive, it is more similar to blowing-only opposition control. In the downstream (right) side of the slot, the controller reacts to the impingement of turbulent spots that have been produced upstream and inside the boundary layer (top–bottom mechanism). The control velocity is positive and increases in the streamwise direction, and the flow behaviour is similar to that of uniform blowing.
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FUNAZAKI, Ken-ichi, Kazutoyo YAMADA, Nobuaki AKAHIRA et Takahiro OTSUKI. « Studies of Boundary Layer Bypass Transition Induced by Periodic External Disturbances : Effects of Boundary Layer Control using Wall Suction ». Proceedings of the JSME annual meeting 2004.2 (2004) : 309–10. http://dx.doi.org/10.1299/jsmemecjo.2004.2.0_309.
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