Articles de revues : « Wake-Induced Transition »

1

Mayle, R. E., et K. Dullenkopf. « A Theory for Wake-Induced Transition ». Journal of Turbomachinery 112, n^o 2 (1 avril 1990) : 188–95. http://dx.doi.org/10.1115/1.2927632.

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A theory for transition from laminar to turbulent flow as the result of unsteady, periodic passing of turbulent wakes in the free stream is developed using Emmons’ transition model. Comparisons made to flat plate boundary layer measurements and airfoil heat transfer measurements confirm the theory.

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2

Mandal, A. C., et J. Dey. « An experimental study of boundary layer transition induced by a cylinder wake ». Journal of Fluid Mechanics 684 (1 septembre 2011) : 60–84. http://dx.doi.org/10.1017/jfm.2011.270.

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AbstractBoundary layer transition induced by the wake of a circular cylinder in the free stream has been investigated using the particle image velocimetry technique. Some differences between simulation and experimental studies have been reported in the literature, and these have motivated the present study. The appearance of spanwise vortices in the early stage is further confirmed here. A spanwise vortex appears to evolve into a $ \mrm{\Lambda} $/hairpin vortex; the flow statistics also confirm such vortices. With increasing Reynolds number, based on the cylinder diameter, and with decreasing cylinder height from the plate, the physical size of these hairpin-like structures is found to decrease. Some mean flow characteristics, including the streamwise growth of the disturbance energy, in a wake-induced transition resemble those in bypass transition induced by free stream turbulence. Streamwise velocity streaks that are eventually generated in the late stage often undergo sinuous-type oscillations. Similar to other transitional flows, an inclined shear layer in the wall-normal plane is often seen to oscillate and shed vortices. The normalized shedding frequency of these vortices, estimated from the spatial spacing and the convection velocity of these vortices, is found to be independent of the Reynolds number, similar to that in ribbon-induced transition. Although the nature of free stream disturbance in a wake-induced transition and that in a bypass transition are different, the late-stage features including the flow breakdown characteristics of these two transitions appear to be similar.

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Kyriakides, N. K., E. G. Kastrinakis, S. G. Nychas et A. Goulas. « Boundary Layer Transition Induced by a Von Karman Vortex Street Wake ». Proceedings of the Institution of Mechanical Engineers, Part G : Journal of Aerospace Engineering 210, n^o 2 (avril 1996) : 167–79. http://dx.doi.org/10.1243/pime_proc_1996_210_358_02.

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A study has been made of the process of laminar to turbulent transition induced by a von Karman vortex street wake, in the boundary layer on a flat plate. The boundary layer developed under zero pressure gradient conditions while the vortex street was generated by a cylinder positioned in the free stream. Hot-wire measurements over a range of Strouhal frequencies and free stream velocities were used for the identification of the transition onset. From the analysis of the experimental data, two different transition mechanisms known in the literature as ‘strong’ wake and ‘weak’ wake induced transition, could be identified. It was established that, the onset of the strong von Karman wake induced transition process was a function of the free stream velocity, the position of the cylinder with respect to the plate, the cylinder diameter, the drag coefficient and the minimum velocity in the developing wake at the streamwise position of the onset of the boundary layer transition. The end of the strong wake induced transition, was defined at the streamwise distance, where the wake of the cylinder met the wall. A correlation for the prediction of the onset of the transition was developed

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Kyriakides, N. K., E. G. Kastrinakis, S. G. Nychas et A. Goulas. « A bypass wake induced laminar/turbulent transition ». European Journal of Mechanics - B/Fluids 18, n^o 6 (novembre 1999) : 1049–65. http://dx.doi.org/10.1016/s0997-7546(99)00140-5.

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5

Schulte, V., et H. P. Hodson. « Unsteady Wake-Induced Boundary Layer Transition in High Lift LP Turbines ». Journal of Turbomachinery 120, n^o 1 (1 janvier 1998) : 28–35. http://dx.doi.org/10.1115/1.2841384.

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The development of the unsteady suction side boundary layer of a highly loaded LP turbine blade has been investigated in a rectilinear cascade experiment. Upstream rotor wakes were simulated with a moving-bar wake generator. A variety of cases with different wake-passing frequencies, different wake strength, and different Reynolds numbers were tested. Boundary layer surveys have been obtained with a single hotwire probe. Wall shear stress has been investigated with surface-mounted hot-film gages. Losses have been measured. The suction surface boundary layer development of a modern highly loaded LP turbine blade is shown to be dominated by effects associated with unsteady wake-passing. Whereas without wakes the boundary layer features a large separation bubble at a typical cruise Reynolds number, the bubble was largely suppressed if subjected to unsteady wake-passing at a typical frequency and wake strength. Transitional patches and becalmed regions, induced by the wake, dominated the boundary layer development. The becalmed regions inhibited transition and separation and are shown to reduce the loss of the wake-affected boundary layer. An optimum wake-passing frequency exists at cruise Reynolds numbers. For a selected wake-passing frequency and wake strength, the profile loss is almost independent of Reynolds number. This demonstrates a potential to design highly loaded LP turbine profiles without suffering large losses at low Reynolds numbers.

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6

Funazaki, K. « Unsteady Boundary Layers on a Flat Plate Disturbed by Periodic Wakes : Part I—Measurement of Wake-Affected Heat Transfer and Wake-Induced Transition Model ». Journal of Turbomachinery 118, n^o 2 (1 avril 1996) : 327–36. http://dx.doi.org/10.1115/1.2836643.

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Measurements of wake-affected heat transfer distributions on a flat plate are made by use of a wake generator that consists of a rotating disk and several types of circular cylinder. The main purpose of this study is to construct a wake-induced transition model in terms of an intermittency factor, considering the evolution of the wake-induced turbulent region, a so-called turbulent patch in a distance–time diagram. A comparison between the proposed transition model and the measured heat transfer data reveals that the transition model yields good agreement with the measured data of all test conditions in this study.

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Funazaki, K. « Unsteady Boundary Layers on a Flat Plate Disturbed by Periodic Wakes : Part II—Measurements of Unsteady Boundary Layers and Discussion ». Journal of Turbomachinery 118, n^o 2 (1 avril 1996) : 337–44. http://dx.doi.org/10.1115/1.2836644.

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As the second part of the study, detailed hot-wire anemometry measurements of wake-affected boundary layers on the flat plate are made. These measurements are organized in order, first, to check the standpoint of the modeling of the wake-induced transition proposed in Part I, and second, to observe wake–boundary layer interaction in detail from a viewpoint of direct and indirect effect of the wake passage upon turbulent spot generation within the boundary layer, as described by Walker (1993). The validity of the presumed state of the wake-affected boundary layer in the distance–time domain, which constitutes the basis of the transition model, is confirmed to great extent. However, it is also found that the criterion for the onset of the wake-induced transition adopted in Part I should be reconsidered. Some successful attempts are therefore made to specify the transition onset.

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Mayle, R. E., et K. Dullenkopf. « More on the Turbulent-Strip Theory for Wake-Induced Transition ». Journal of Turbomachinery 113, n^o 3 (1 juillet 1991) : 428–32. http://dx.doi.org/10.1115/1.2927892.

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A theory for transition from laminar to turbulent flow with an unsteady, periodic passing of turbulent wakes in the free stream has recently been presented by the authors. The theory considers a time-averaged transitional flow caused by the formation and propagation of turbulent strips along the surface. To apply the theory, however, both the origin and a quantity related to the production rate of these turbulent strips must be known. In this paper, after a brief review of the theory, a dimensional analysis of the problem is presented and data from experiments reexamined in light of the result. From this, an expression for the time-averaged intermittency is obtained, which may be used to calculate the time-averaged distributions of various boundary layer quantities for wake-induced transitional flow.

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9

Wright, L., et M. T. Schobeiri. « The Effect of Periodic Unsteady Flow on Aerodynamics and Heat Transfer on a Curved Surface ». Journal of Heat Transfer 121, n^o 1 (1 février 1999) : 22–33. http://dx.doi.org/10.1115/1.2825954.

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Aerodynamic and heat transfer investigations were performed on a constant curvature curved plate in a subsonic wind tunnel facility for various wake passing frequencies under zero pressure gradient conditions. Steady and unsteady boundary layer transition measurements were taken on the concave surface at different wake passing frequencies in which a rotating squirrel cage was used to generate the unsteady wake flow. The data were analyzed using time-averaged and ensemble averaged techniques to provide insight into the growth of the boundary layer and transition. Ensemble averaged turbulence intensity contours in the temporal spatial domain showed that transition was induced for increasing wake passing frequency and structure. The local heat transfer coefficient distributions for the concave and convex surfaces were determined for each wake passing frequency using a liquid crystal heat transfer measurement technique. Aerodynamic and heat transfer investigations showed that higher wake passing frequencies caused earlier transition on the concave surface. Local Stanton numbers were calculated on the concave surface and compared to Stanton numbers predicted using a boundary layer and heat transfer calculation method. On the convex side, no effect of wake passing on heat transfer was observed, due to a separation bubble that induced transition.

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10

Schobeiri, M. T., L. Wright et P. Chakka. « Periodic Unsteady Flow Aerodynamics and Heat Transfer : Studies on a Curved Surface, Combined Part I and II ». International Journal of Rotating Machinery 6, n^o 6 (2000) : 393–416. http://dx.doi.org/10.1155/s1023621x00000373.

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Aerodynamic and heat transfer investigations were done on a constant curvature curved plate in a subsonic wind tunnel facility for various wake passing frequencies and zero pressure gradient conditions. Steady and unsteady boundary layer transition measurements were taken on the concave surface of the curved plate at different wake passing frequencies where a rotating squirrel-cage generated the unsteady wake flow. The data were analyzed using timeaveraged and ensemble-averaged techniques to provide insight into the growth of the boundary layer and transition. Ensemble-averaged turbulence intensity contours in the temporal spatial domain showed that transition was induced for increasing wake passing frequency and structure. The local heat transfer coefficient distribution for the concave and convex surface was determined at those wake passing frequencies using a liquid crystal heat transfer measurement technique. Detailed aerodynamic and heat transfer investigations showed that higher wake passing frequency caused transition to occur earlier on the concave surface. Local Stanton numbers were also calculated on the concave surface and compared with Stanton numbers predicted using a differential boundary layer and heat transfer calculation method. On the convex side, no effect of wake passing frequency on heat transfer was observed due to a separation bubble that induced transition.

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11

PAN, CHONG, JIN JUN WANG, PAN FENG ZHANG et LI HAO FENG. « Coherent structures in bypass transition induced by a cylinder wake ». Journal of Fluid Mechanics 603 (30 avril 2008) : 367–89. http://dx.doi.org/10.1017/s0022112008001018.

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Flat-plate boundary layer transition induced by the wake vortex of a two-dimensional circular cylinder is experimentally investigated. Combined visualization and velocity measurements show a different transition route from the Klebanoff mode in free-stream turbulence-induced transition. This transition scenario is mainly characterized as: (i) generation of secondary transverse vortical structures near the flat plate surface in response to the von Kármán vortex street of the cylinder; (ii) formation of hairpin vortices due to the secondary instability of secondary vortical structures; (iii) growth of hairpins which is accelerated by wake-vortex induction; (iv) formation of hairpin packets and the associated streaky structures. Detailed investigation shows that during transition the evolution dynamics and self-sustaining mechanisms of hairpins, hairpin packets and streaks are consistent with those in a turbulent boundary layer. The wake vortex mainly plays the role of generating and destabilizing secondary transverse vortices. After that, the internal mechanisms become dominant and lead to the setting up of a self-sustained turbulent boundary layer.

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12

Hodson, H. P., J. S. Addison et C. A. Shepherson. « Models for unsteady wake-induced transition in axial turbomachines ». Journal de Physique III 2, n^o 4 (avril 1992) : 545–74. http://dx.doi.org/10.1051/jp3:1992147.

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13

Kim, K., et M. E. Crawford. « Prediction of Transitional Heat Transfer Characteristics of Wake-Affected Boundary Layers ». Journal of Turbomachinery 122, n^o 1 (1 février 1999) : 78–87. http://dx.doi.org/10.1115/1.555430.

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The presence of wake-passing in the gas turbine environment significantly modifies the heat transfer characteristics on the downstream blade surface by causing wake-induced transition. In this study, time-dependent boundary layer calculations were carried out using a model for wake-induced transition based on a prescribed time-dependent intermittent function. The model is determined from the well-known turbulent spot propagation theory in a time-space diagram and from experimental evidence in the ensemble-averaged sense. Time-averaged heat transfer distributions are evaluated and compared with experimental results for different flow and wake-generating conditions over a flat plate. Comparison showed that the present time-dependent calculations yield more accurate results than existing steady superposition models. [S0889-504X(00)00901-6]

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14

Kubacki, Slawomir, et Erik Dick. « An algebraic intermittency model for bypass, separation-induced and wake-induced transition ». International Journal of Heat and Fluid Flow 62 (décembre 2016) : 344–61. http://dx.doi.org/10.1016/j.ijheatfluidflow.2016.09.013.

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15

Walters, D. Keith, et James H. Leylek. « Computational Fluid Dynamics Study of Wake-Induced Transition on a Compressor-Like Flat Plate ». Journal of Turbomachinery 127, n^o 1 (1 janvier 2005) : 52–63. http://dx.doi.org/10.1115/1.1791650.

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Recent experimental work has documented the importance of wake passing on the behavior of transitional boundary layers on the suction surface of axial compressor blades. This paper documents computational fluid dynamics (CFD) simulations using a commercially available general-purpose CFD solver, performed on a representative case with unsteady transitional behavior. The study implements an advanced version of a three-equation eddy-viscosity model previously developed and documented by the authors, which is capable of resolving boundary layer transition. It is applied to the test cases of steady and unsteady boundary layer transition on a two-dimensional flat plate geometry with a freestream velocity distribution representative of the suction side of a compressor airfoil. The CFD results are analyzed and compared to a similar experimental test case from the open literature. Results with the model show a dramatic improvement over more typical Reynolds-averaged Navier–Stokes (RANS)-based modeling approaches, and highlight the importance of resolving transition in both steady and unsteady compressor aerosimulations.

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Stieger, R. D., et H. P. Hodson. « The Transition Mechanism of Highly Loaded Low-Pressure Turbine Blades ». Journal of Turbomachinery 126, n^o 4 (1 octobre 2004) : 536–43. http://dx.doi.org/10.1115/1.1773850.

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A detailed experimental investigation was conducted into the interaction of a convected wake and a separation bubble on the rear suction surface of a highly loaded low-pressure (LP) turbine blade. Boundary layer measurements, made with 2D LDA, revealed a new transition mechanism resulting from this interaction. Prior to the arrival of the wake, the boundary layer profiles in the separation region are inflexional. The perturbation of the separated shear layer caused by the convecting wake causes an inviscid Kelvin-Helmholtz rollup of the shear layer. This results in the breakdown of the laminar shear layer and a rapid wake-induced transition in the separated shear layer.

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Funazaki, K., et E. Koyabu. « Effects of Periodic Wake Passing Upon Flat-Plate Boundary Layers Experiencing Favorable and Adverse Pressure Gradients ». Journal of Turbomachinery 121, n^o 2 (1 avril 1999) : 333–40. http://dx.doi.org/10.1115/1.2841319.

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This paper deals with the investigation of wake-disturbed boundary layer on a flat-plate model with an elliptic leading edge. The wakes are generated by the transversely moving bars in front of the test model. The main focus of this paper is how the wake passage affects the transitional behavior of the boundary layer under the influence of favorable and adverse pressure gradients over the test surface. Detailed measurements of the boundary layer are conducted by the use of hot-wire anemometry. An ensemble-averaging technique is also employed in order to extract the periodic events associated with the wake passage from the acquired data. The previously observed dependence of wake-induced transition on the movement of the wake generating bar is confirmed. It is also found that the wake passage induces a significant change in the flow structure downstream of the flow acceleration region.

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Funazaki, Ken-ichi, Takashi Kitazawa et Takashi Watanabe. « Boundary Layer Transition Induced by Periodic Wake Passage. Effect of Velocity Fluctuation Caused by Wake Passage. » Transactions of the Japan Society of Mechanical Engineers Series B 61, n^o 583 (1995) : 874–81. http://dx.doi.org/10.1299/kikaib.61.874.

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De Tullio, Nicola, et Neil D. Sandham. « Influence of boundary-layer disturbances on the instability of a roughness wake in a high-speed boundary layer ». Journal of Fluid Mechanics 763 (11 décembre 2014) : 136–65. http://dx.doi.org/10.1017/jfm.2014.663.

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AbstractThe excitation of instability modes in the wake generated behind a discrete roughness element in a boundary layer at Mach 6 is analysed through numerical simulations of the compressible Navier–Stokes equations. Recent experimental observations show that transition to turbulence in high-speed boundary layers during re-entry flight is dominated by wall roughness effects. Therefore, understanding the roughness-induced transition to turbulence in this flow regime is of primary importance. Our results show that a discrete roughness element with a height of about half the local boundary-layer thickness generates an unstable wake able to sustain the growth of a number of modes. The most unstable of these modes are a sinuous mode (mode SL) and two varicose modes (modes VL and VC). The varicose modes grow approximately 17 % faster than the most unstable Mack mode and their growth persists over a longer streamwise distance, thereby leading to a notable acceleration of the laminar–turbulent transition process. Two main mechanisms are identified for the excitation of wake modes: the first is based on the interaction between the external disturbances and the reverse flow regions induced by the roughness element and the second is due to the interaction between the boundary-layer modes (first modes and Mack modes) and the non-parallel roughness wake. An important finding of the present study is that, while being less unstable, mode SL is the preferred instability for the first of the above excitation mechanisms, which drives the wake modes excitation in the absence of boundary-layer modes. Modes VL and VC are excited through the second mechanism and, hence, become important when first modes and Mack modes come into interaction with the roughness wake. The new mode VC presents similarities with the Mack mode instability, including the tuning between its most unstable wavelength and the local boundary-layer thickness, and it is believed to play a fundamental role in the roughness-induced transition of high-speed boundary layers. In contrast to the smooth-wall case, wall cooling is stabilising for all the roughness-wake modes.

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Wu, Xiaohua, et Paul A. Durbin. « Boundary Layer Transition Induced by Periodic Wakes ». Journal of Turbomachinery 122, n^o 3 (1 novembre 1998) : 442–49. http://dx.doi.org/10.1115/1.1303076.

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Turbulent wakes swept across a flat plate boundary layer simulate the phenomenon of wake-induced bypass transition. Benchmark data from a direct numerical simulation of this process are presented and compared to Reynolds-averaged predictions. The data are phase-averaged skin friction and mean velocities. The predictions and data are found to agree in many important respects. One discrepancy is a failure to reproduce the skin friction overshoot following transition. [S0889-504X(00)00503-1]

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COULL, JOHN D., et HOWARD P. HODSON. « Unsteady boundary-layer transition in low-pressure turbines ». Journal of Fluid Mechanics 681 (1 juillet 2011) : 370–410. http://dx.doi.org/10.1017/jfm.2011.204.

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This paper examines the transition process in a boundary layer similar to that present over the suction surfaces of aero-engine low-pressure (LP) turbine blades. This transition process is of significant practical interest since the behaviour of this boundary layer largely determines the overall efficiency of the LP turbine. Modern ‘high-lift’ blade designs typically feature a closed laminar separation bubble on the aft portion of the suction surface. The size of this bubble and hence the inefficiency it generates is controlled by the transition between laminar and turbulent flow in the boundary layer and separated shear layer. The transition process is complicated by the inherent unsteadiness of the multi-stage machine: the wakes shed by one blade row convect through the downstream blade passages, periodically disturbing the boundary layers. As a consequence, the transition to turbulence is multi-modal by nature, being promoted by periodic and turbulent fluctuations in the free stream and the inherent instabilities of the boundary layer. Despite many studies examining the flow behaviour, the detailed physics of the unsteady transition phenomena are not yet fully understood. The boundary-layer transition process has been studied experimentally on a flat plate. The opposing test-section wall was curved to impose a streamwise pressure distribution typical of modern high-lift LP turbines over the flat plate. The presence of an upstream blade row has been simulated by a set of moving bars, which shed wakes across the test section inlet. Further upstream, a grid has been installed to elevate the free-stream turbulence to a level believed to be representative of multi-stage LP turbines. Extensive particle imaging velocimetry (PIV) measurements have been performed on the flat-plate boundary layer to examine the flow behaviour. In the absence of the incoming bar wakes, the grid-generated free-stream turbulence induces relatively weak Klebanoff streaks in the boundary layer which are evident as streamwise streaks of low-velocity fluid. Transition is promoted by the streaks and by the inherent inflectional (Kelvin–Helmholtz (KH)) instability of the separation bubble. In unsteady flow, the incoming bar wakes generate stronger Klebanoff streaks as they pass over the leading edge, which convect downstream at a fraction of the free-stream velocity and spread in the streamwise direction. The region of amplified streaks convects in a similar manner to a classical turbulent spot: the leading and trailing edges travel at around 88% and 50% of the free-stream velocity, respectively. The strongest disturbances travel at around 70% of the free-stream velocity. The wakes induce a second type of disturbance as they pass over the separation bubble, in the form of short-span KH structures. Both the streaks and the KH structures contribute to the early wake-induced transition. The KH structures are similar to those observed in the simulation of separated flow transition with high free-stream turbulence by McAuliffe & Yaras (ASME J. Turbomach., vol. 132, no. 1, 2010, 011004), who observed that these structures originated from localised instabilities of the shear layer induced by Klebanoff streaks. In the current measurements, KH structures are frequently observed directly under the path of the wake. The wake-amplified Klebanoff streaks cannot affect the generation of these structures since they do not arrive at the bubble until later in the wake cycle. Rather, the KH structures arise from an interaction between the flow disturbances in the wake and localised instabilities in the shear layer, which are caused by the weak Klebanoff streaks induced by the grid turbulence. The breakdown of the KH structures to small-scale turbulence occurs a short time after the wake has passed over the bubble, and is largely driven by the arrival of the wake-amplified Klebanoff streaks from the leading edge. During this process, the re-attachment location moves rapidly upstream. The minimum length of the bubble occurs when the strongest wake-amplified Klebanoff streaks arrive from the leading edge; these structures travel at around 70% of the free-stream velocity. The bubble remains shorter than its steady-flow length until the trailing edge of the wake-amplified Klebanoff streaks, travelling at 50% of the free-stream velocity, convect past. After this time, the reattachment location moves aft on the surface as a consequence of a calmed flow region which follows behind the wake-induced turbulence.

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Solomon, W. J., et G. J. Walker. « Incidence Effects on Wake-Induced Transition on an Axial Compressor Blade ». Journal of Propulsion and Power 16, n^o 3 (mai 2000) : 397–405. http://dx.doi.org/10.2514/2.5603.

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Lardeau, S., et M. A. Leschziner. « Modeling of Wake-Induced Transition in Linear Low-Pressure Turbine Cascades ». AIAA Journal 44, n^o 8 (août 2006) : 1854–65. http://dx.doi.org/10.2514/1.16470.

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OVCHINNIKOV, VICTOR, UGO PIOMELLI et MEELAN M. CHOUDHARI. « Numerical simulations of boundary-layer transition induced by a cylinder wake ». Journal of Fluid Mechanics 547, n^o -1 (11 janvier 2006) : 413. http://dx.doi.org/10.1017/s0022112005007342.

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Kubacki, S., K. Lodefier, R. Zarzycki, W. Elsner et E. Dick. « Further Development of a Dynamic Intermittency Model For Wake-Induced Transition ». Flow, Turbulence and Combustion 83, n^o 4 (15 mars 2009) : 539–68. http://dx.doi.org/10.1007/s10494-009-9206-2.

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Pan, Chong, Jin-Jun Wang et Guo-Sheng He. « Experimental Investigation of Wake-Induced Bypass Transition Control by Surface Roughness ». Chinese Physics Letters 29, n^o 10 (octobre 2012) : 104704. http://dx.doi.org/10.1088/0256-307x/29/10/104704.

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Stieger, R. D., et H. P. Hodson. « Unsteady dissipation measurements on a flat plate subject to wake passing ». Proceedings of the Institution of Mechanical Engineers, Part A : Journal of Power and Energy 217, n^o 4 (1 janvier 2003) : 413–19. http://dx.doi.org/10.1243/095765003322315478.

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Boundary layer measurements were performed on a flat plate with an imposed pressure gradient typical of a high-lift low-pressure (LP) turbine blade and subject to incoming turbulent wakes shed from a moving bar wake generator. A multiple-orientation one-dimensional laser doppler anemometry (LDA) technique was used to measure the ensemble-average mean flow and Reynolds stresses. These ensembleaverage measurements were used to calculate the boundary layer dissipation, thereby providing unprecedented experimental evidence of the loss-reducing mechanisms associated with wake-induced transition. The benign character of the calmed zone was confirmed and the early stages of boundary layer separation were found to have laminar levels of dissipation. A deterministic natural transition phenomenon was identified between wake passing events, highlighting the existence of natural transition phenomena in LP turbine style pressure distributions.

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Wu, Xiaohua, et Kyle D. Squires. « Three-Dimensional Boundary Layers Over an Infinite Swept Bump and Free Wing ». Journal of Fluids Engineering 117, n^o 4 (1 décembre 1995) : 605–11. http://dx.doi.org/10.1115/1.2817310.

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Three-dimensional laminar boundary layers past an infinite swept bump and free wing were investigated numerically using the fractional step method. The objective of the work was to study the effect of surface curvature induced changes in pressure gradient and changes in the freestream flow on boundary layer skewness and growth. Simulation results demonstrate that for flows over the bump the first transition from adverse to favorable pressure gradient occurs at the front concave/convex inflexion and the second transition from favorable to adverse pressure gradient occurs at the summit. For flows past a free wing, the only transition from favorable to adverse pressure gradient occurs in front of the summit and the subsequent adverse pressure gradient is larger than the corresponding value for the bump. For both the bump and wing, the increase of initial skewing angle from 0 to 30 deg causes a 10 percent reduction in the length of the wake; the wake behind the wing is about 12 percent longer in streamwise extent than the corresponding wake behind the bump. Integral parameters in the flows over the bump display a wavy trend due to the two transitions of the pressure gradient. On the other hand, the single transition from favorable to adverse pressure gradient brings about a monotonic increase of the integral parameters for flows past the wing. Near separation and reattachment, surface-streamlines are skewed strongly in the spanwise direction. Conditions of flow detachment for the bump and wing are in good agreement with correlations for laminar separating flows with power-law velocity profiles as well as correlations for wall-curvature-induced turbulent separating flows.

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Halstead, D. E., D. C. Wisler, T. H. Okiishi, G. J. Walker, H. P. Hodson et H. W. Shin. « Boundary Layer Development in Axial Compressors and Turbines : Part 1 of 4—Composite Picture ». Journal of Turbomachinery 119, n^o 1 (1 janvier 1997) : 114–27. http://dx.doi.org/10.1115/1.2841000.

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Comprehensive experiments and computational analyses were conducted to understand boundary layer development on airfoil surfaces in multistage, axial-flow compressors and LP turbines. The tests were run over a broad range of Reynolds numbers and loading levels in large, low-speed research facilities which simulate the relevant aerodynamic features of modern engine components. Measurements of boundary layer characteristics were obtained by using arrays of densely packed, hot-film gauges mounted on airfoil surfaces and by making boundary layer surveys with hot wire probes. Computational predictions were made using both steady flow codes and an unsteady flow code. This is the first time that time-resolved boundary layer measurements and detailed comparisons of measured data with predictions of boundary layer codes have been reported for multistage compressor and turbine blading. Part 1 of this paper summarizes all of our experimental findings by using sketches to show how boundary layers develop on compressor and turbine blading. Parts 2 and 3 present the detailed experimental results for the compressor and turbine, respectively. Part 4 presents computational analyses and discusses comparisons with experimental data. Readers not interested in experimental detail can go directly from Part 1 to Part 4. For both compressor and turbine blading, the experimental results show large extents of laminar and transitional flow on the suction surface of embedded stages, with the boundary layer generally developing along two distinct but coupled paths. One path lies approximately under the wake trajectory while the other lies between wakes. Along both paths the boundary layer clearly goes from laminar to transitional to turbulent. The wake path and the non-wake path are coupled by a calmed region, which, being generated by turbulent spots produced in the wake path, is effective in suppressing flow separation and delaying transition in the non-wake path. The location and strength of the various regions within the paths, such as wake-induced transitional and turbulent strips, vary with Reynolds number, loading level, and turbulence intensity. On the pressure surface, transition takes place near the leading edge for the blading tested. For both surfaces, bypass transition and separated-flow transition were observed. Classical Tollmien–Schlichting transition did not play a significant role. Comparisons of embedded and first-stage results were also made to assess the relevance of applying single-stage and cascade studies to the multistage environment. Although doing well under certain conditions, the codes in general could not adequately predict the onset and extent of transition in regions affected by calming. However, assessments are made to guide designers in using current predictive schemes to compute boundary layer features and obtain reasonable loss predictions.

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Ye, Qingqing, Ferry F. J. Schrijer et Fulvio Scarano. « Boundary layer transition mechanisms behind a micro-ramp ». Journal of Fluid Mechanics 793 (14 mars 2016) : 132–61. http://dx.doi.org/10.1017/jfm.2016.120.

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The early stage of three-dimensional laminar-to-turbulent transition behind a micro-ramp is studied in the incompressible regime using tomographic particle image velocimetry. Experiments are conducted at supercritical micro-ramp height $h$ based Reynolds number $Re_{h}=1170$. The measurement domain encompasses 6 ramp widths spanwise and 73 ramp heights streamwise. The mean flow topology reveals the underlying vortex structure of the wake flow with multiple pairs of streamwise counter-rotating vortices visualized by streamwise vorticity. The primary pair generates a vigorous upwash motion in the symmetry plane with a pronounced momentum deficit. A secondary vortex pair is induced closer to the wall. The tertiary and even further vortices maintain a streamwise orientation, but are produced progressively outwards of the secondary pair and follow a wedge-type pattern. The instantaneous flow pattern reveals that the earliest unstable mode of the wake features arc-like Kelvin–Helmholtz (K–H) vortices in the separated shear layer. Under the influence of the K–H vortices, the wake exhibits a high level of fluctuations with a pulsatile mode for the streamwise momentum deficit. The K–H vortices are lifted up due to the upwash induced by the quasi-streamwise vortex pair, while they appear to undergo pairing, distortion and finally breakdown. Immediately downstream, a streamwise interval of relatively low vortical activity separates the end of the K–H region from the formation of new hairpin vortices close to the wall. The latter vortex structures originate from the region of maximum wall shear, induced by the secondary vortex pair causing strong ejection events which transport low-speed flow upwards. The whole pattern features a cascade of hairpin vortices along a turbulent/non-turbulent interface. The wedge-shaped cascade signifies the formation of a turbulent wedge. The turbulent properties of the wake are inspected with the spatial distribution of the velocity fluctuations and turbulence production in the developing boundary layer. Inside the wedge region, the velocity fluctuations approach quasi-spanwise homogeneity, indicating the development towards a turbulent boundary layer. The wedge interface is characterized by a localized higher level of velocity fluctuations and turbulence production, associated to the deflection of the shear layer close to the wall and the onset of coherent hairpin vortices inducing localized large-scale ejections.

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LO JACONO, DAVID, JUSTIN S. LEONTINI, MARK C. THOMPSON et JOHN SHERIDAN. « Modification of three-dimensional transition in the wake of a rotationally oscillating cylinder ». Journal of Fluid Mechanics 643 (24 décembre 2009) : 349–62. http://dx.doi.org/10.1017/s0022112009992370.

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A study of the flow past an oscillatory rotating cylinder has been conducted, where the frequency of oscillation has been matched to the natural frequency of the vortex street generated in the wake of a stationary cylinder, at Reynolds number 300. The focus is on the wake transition to three-dimensional flow and, in particular, the changes induced in this transition by the addition of the oscillatory rotation. Using Floquet stability analysis, it is found that the fine-scale three-dimensional mode that typically dominates the wake at a Reynolds number beyond that at the second transition to three-dimensional flow (referred to as mode B) is suppressed for amplitudes of rotation beyond a critical amplitude, in agreement with past studies. However, the rotation does not suppress the development of three-dimensionality completely, as other modes are discovered that would lead to three-dimensional flow. In particular, the longer-wavelength mode that leads the three-dimensional transition in the wake of a stationary cylinder (referred to as mode A) is left essentially unaffected at low amplitudes of rotation. At higher amplitudes of oscillation, mode A is also suppressed as the two-dimensional near wake changes in character from a single- to a double-row wake; however, another mode is predicted to render the flow three-dimensional, dubbed mode D (for double row). This mode has the same spatio-temporal symmetries as mode A.

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Walker, G. J., J. D. Hughes et W. J. Solomon. « Periodic Transition on an Axial Compressor Stator : Incidence and Clocking Effects : Part I—Experimental Data ». Journal of Turbomachinery 121, n^o 3 (1 juillet 1999) : 398–407. http://dx.doi.org/10.1115/1.2841332.

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Periodic wake-induced transition on the outlet stator of a 1.5-stage axial compressor is examined using hot-film arrays on both the suction and pressure surfaces. The time-mean surface pressure distribution is varied by changing the blade incidence, while the free-stream disturbance field is altered by clocking of the stator relative to an inlet guide vane row. Ensemble-averaged plots of turbulent intermittency and relaxation factor (extent of calmed flow following the passage of a turbulent spot) are presented. These show the strength of periodic wake-induced transition phenomena to be significantly influenced by both incidence and clocking effects. The nature and extent of transition by other modes (natural, bypass, and separated flow transition) are altered accordingly. Leading edge and midchord separation bubbles are affected in a characteristically different manner by changing free-stream periodicity. There are noticeable differences between suction and pressure surface transition behavior, particularly as regards the strength and extent of calming. In Part II of this paper, the transition onset observations from the compressor stator are used to evaluate the quasi-steady application of conventional transition correlations to predict unsteady transition onset on the blading of an embedded axial compressor stage.

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Cho, N. H., X. Liu, W. Rodi et B. Scho¨nung. « Calculation of Wake-Induced Unsteady Flow in a Turbine Cascade ». Journal of Turbomachinery 115, n^o 4 (1 octobre 1993) : 675–86. http://dx.doi.org/10.1115/1.2929302.

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Numerical predictions are reported for two-dimensional unsteady flow in a linear turbine cascade, where the unsteadiness is caused by passing wakes generated by the preceding row of blades. In particular, an experiment is simulated in which the passing wakes were generated by cylinders moving on a rotating squirrel cage. Blade-to-blade calculations were carried out by solving the unsteady two dimensional flow equations with an accurate finite-volume procedure, thereby resolving the periodic unsteady motion. The effects of stochastic turbulent fluctuations are simulated with a two-layer turbulence model, in which the standard k–ε model is applied in the bulk of the flow and a one-equation model in the near-wall region. This involves also a transition model based on an empirical formula from Abu-Ghannam and Shaw (1980), which was adapted for the unsteady situation by applying it in a Lagrangian way, following fluid parcels in the boundary layer under disturbed and undisturbed free streams on their travel downstream. The calculations are compared with experiments for various wake-passing frequencies. On the whole, the complex unsteady flow behavior is simulated realistically, including the moving forward of transition when the wake-passing frequency increases, but not all details can be reproduced.

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Kyriakides, N. K., E. G. Kastrinakis, S. G. Nychas et A. Goulas. « Aspects of Flow Structure During a Cylinder Wake-Induced Laminar/Turbulent Transition ». AIAA Journal 37, n^o 10 (octobre 1999) : 1197–205. http://dx.doi.org/10.2514/2.613.

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KOYABU, Eitaro, Takashi HONMA, Mitsuki FUJIWARA, Ayumi MITOH et Eiji SOBU. « J0530204 Studies on Wake-Induced Bypass Transition over Flat Plate Boundary Layer ». Proceedings of Mechanical Engineering Congress, Japan 2014 (2014) : _J0530204——_J0530204—. http://dx.doi.org/10.1299/jsmemecj.2014._j0530204-.

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Nychas, S. G., A. Goulas, N. K. Kyriakides et E. G. Kastrinakis. « Aspects of flow structure during a cylinder wake-induced laminar/turbulent transition ». AIAA Journal 37 (janvier 1999) : 1197–205. http://dx.doi.org/10.2514/3.14309.

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Tang, Zhan-Qi, et Nan Jiang. « TR PIV Experimental Investigation on Bypass Transition Induced by a Cylinder Wake ». Chinese Physics Letters 28, n^o 5 (mai 2011) : 054702. http://dx.doi.org/10.1088/0256-307x/28/5/054702.

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Loiseau, Jean-Christophe, Jean-Christophe Robinet, Stefania Cherubini et Emmanuel Leriche. « Investigation of the roughness-induced transition : global stability analyses and direct numerical simulations ». Journal of Fluid Mechanics 760 (4 novembre 2014) : 175–211. http://dx.doi.org/10.1017/jfm.2014.589.

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AbstractThe linear global instability and resulting transition to turbulence induced by an isolated cylindrical roughness element of height $h$ and diameter $d$ immersed within an incompressible boundary layer flow along a flat plate is investigated using the joint application of direct numerical simulations and fully three-dimensional global stability analyses. For the range of parameters investigated, base flow computations show that the roughness element induces a wake composed of a central low-speed region surrounded by a three-dimensional shear layer and a pair of low- and high-speed streaks on each of its sides. Results from the global stability analyses highlight the unstable nature of the central low-speed region and its crucial importance in the laminar–turbulent transition process. It is able to sustain two different global instabilities: a sinuous and a varicose one. Each of these globally unstable modes is related to a different physical mechanism. While the varicose mode has its root in the instability of the whole three-dimensional shear layer surrounding the central low-speed region, the sinuous instability turns out to be similar to the von Kármán instability in the two-dimensional cylinder wake and has its root in the lateral shear layers of the separated zone. The aspect ratio of the roughness element plays a key role on the selection of the dominant instability: whereas the flow over thin cylindrical roughness elements transitions due to a sinuous instability of the near-wake region, for larger roughness elements the varicose instability of the central low-speed region turns out to be the dominant one. Direct numerical simulations of the flow past an aspect ratio ${\it\eta}=1$ (with ${\it\eta}=d/h$) roughness element sustaining only the sinuous instability have revealed that the bifurcation occurring in this particular case is supercritical. Finally, comparison of the transition thresholds predicted by global linear stability analyses with the von Doenhoff–Braslow transition diagram provides qualitatively good agreement.

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Liu, X., et W. Rodi. « Experiments on transitional boundary layers with wake-induced unsteadiness ». Journal of Fluid Mechanics 231 (octobre 1991) : 229–56. http://dx.doi.org/10.1017/s0022112091003385.

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Hot-wire measurements were carried out in boundary layers developing along a flat plate over which wakes passed periodically. The wakes were generated by cylinders moving on a squirrel cage in front of the plate leading edge. The flow situation studied is an idealization of that occurring on turbomachinery blades where unsteady wakes are generated by the preceding row of blades. The influence of wake-passing frequency on the boundary-layer development and in particular on the transition processes was examined. The hot-wire signals were processed to yield ensemble-average values and the fluctuations could be separated into periodic and stochastic turbulent components. Hot-wire traces are reported as well as time variations of periodic and ensemble-averaged turbulent fluctuations and of the boundary-layer integral parameters, yielding a detailed picture of the flow development. The Reynolds number was relatively low so that in the limiting case of a boundary layer undisturbed by wakes this remained laminar over the full length of the test plate. When wakes passed over the plate, the boundary layer was found to be turbulent quite early underneath the free-stream disturbances due to the wakes, while it remained initially laminar underneath the undisturbed free-stream regions in between. The turbulent boundary-layer stripes underneath the disturbed free stream travel downstream and grow together so that the embedded laminar regions disappear and the boundary layer becomes fully turbulent. The streamwise location where this happens moves upstream with increasing wake-passing frequency, and a clear correlation could be determined in the experiments. The results are also reported in a mean Lagrangian frame by following fluid parcels underneath the disturbed and undisturbed free stream, respectively, as they travel downstream.

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Lee, H., K. Hourigan et M. C. Thompson. « Vortex-induced vibration of a neutrally buoyant tethered sphere ». Journal of Fluid Mechanics 719 (19 février 2013) : 97–128. http://dx.doi.org/10.1017/jfm.2012.634.

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AbstractA combined numerical and experimental study examining vortex-induced vibration (VIV) of a neutrally buoyant tethered sphere has been undertaken. The study covered the Reynolds-number range $50\leq \mathit{Re}\lesssim 12\hspace{0.167em} 000$, with the numerical ($50\leq \mathit{Re}\leq 800$) and experimental ($370\leqslant \mathit{Re}\lesssim 12\hspace{0.167em} 000$) ranges overlapping. Neutral buoyancy was chosen to eliminate one parameter, i.e. the influence of gravity, on the VIV behaviour, although, of course, the effect of added mass remains. The tether length was also chosen to be sufficiently long so that, to a good approximation, the sphere was constrained to move within a plane. Seven broad but relatively distinct sphere oscillation and wake states could be distinguished. For regime I, the wake is steady and axisymmetric, and it undergoes transition to a steady two-tailed wake in regime II at $\mathit{Re}= 210$. Those regimes are directly analogous to those of a fixed sphere. Once the sphere begins to vibrate at $\mathit{Re}\simeq 270$ in regime III, the wake behaviour is distinct from the fixed-sphere wake. Initially the vibration frequency of the sphere is half the shedding frequency in the wake, with the latter consistent with the fixed-sphere wake frequency. The sphere vibration is not purely periodic but modulated over several base periods. However, at slightly higher Reynolds numbers ($\mathit{Re}\simeq 280$), planar symmetry is broken, and the vibration shifts to the planar normal (or azimuthal) direction, and becomes completely azimuthal at the start of regime IV at $\mathit{Re}= 300$. In comparison, for a fixed sphere, planar symmetry is broken at a much higher Reynolds number of $\mathit{Re}\simeq 375$. Interestingly, planar symmetry returns to the wake for $\mathit{Re}\gt 330$, in regime V, for which the oscillations are again radial, and is maintained until $\mathit{Re}= 450$ or higher. At the same time, the characteristic vortex loops in the wake become symmetrical, i.e. two-sided. For $\mathit{Re}\gt 500$, in regime VI, the trajectory of the sphere becomes irregular, possibly chaotic. That state is maintained over the remaining Reynolds-number range simulated numerically ($\mathit{Re}\leq 800$). Experiments overlapping this Reynolds-number range confirm the amplitude radial oscillations in regime V and the chaotic wandering for regime VI. At still higher Reynolds numbers of $\mathit{Re}\gt 3000$, in regime VII, the trajectories evolve to quasi-circular orbits about the neutral point, with the orbital radius increasing as the Reynolds number is increased. At $\mathit{Re}= 12\hspace{0.167em} 000$, the orbital diameter reaches approximately one sphere diameter. Of interest, this transition sequence is distinct from that for a vertically tethered heavy sphere, which undergoes transition to quasi-circular orbits beyond $\mathit{Re}= 500$.

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Park, Tae-Choon, Shin-Hyoung Kang et Woo-Pyung Jeon. « Wake-Induced Boundary Layer Transition on an Airfoil at Moderate Free-Stream Turbulence ». Transactions of the Korean Society of Mechanical Engineers B 30, n^o 9 (1 septembre 2006) : 921–28. http://dx.doi.org/10.3795/ksme-b.2006.30.9.921.

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Zhong, S., C. Kittichaikan, H. P. Hodson et P. T. Ireland. « A study of unsteady wake-induced boundary-layer transition with thermochromic liquid crystals ». Proceedings of the Institution of Mechanical Engineers, Part G : Journal of Aerospace Engineering 213, n^o 3 (mars 1999) : 163–71. http://dx.doi.org/10.1243/0954410991532927.

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Wang, J. J., C. Zhang et C. Pan. « Effects of roughness elements on bypass transition induced by a circular cylinder wake ». Journal of Visualization 14, n^o 1 (30 novembre 2010) : 53–61. http://dx.doi.org/10.1007/s12650-010-0063-9.

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ROWLEY, CLARENCE W., TIM COLONIUS et AMIT J. BASU. « On self-sustained oscillations in two-dimensional compressible flow over rectangular cavities ». Journal of Fluid Mechanics 455 (25 mars 2002) : 315–46. http://dx.doi.org/10.1017/s0022112001007534.

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Numerical simulations are used to investigate the resonant instabilities in two-dimensional flow past an open cavity. The compressible Navier–Stokes equations are solved directly (no turbulence model) for cavities with laminar boundary layers upstream. The computational domain is large enough to directly resolve a portion of the radiated acoustic field, which is shown to be in good visual agreement with schlieren photographs from experiments at several different Mach numbers. The results show a transition from a shear-layer mode, primarily for shorter cavities and lower Mach numbers, to a wake mode for longer cavities and higher Mach numbers. The shear-layer mode is characterized well by the acoustic feedback process described by Rossiter (1964), and disturbances in the shear layer compare well with predictions based on linear stability analysis of the Kelvin–Helmholtz mode. The wake mode is characterized instead by a large-scale vortex shedding with Strouhal number independent of Mach number. The wake mode oscillation is similar in many ways to that reported by Gharib & Roshko (1987) for incompressible flow with a laminar upstream boundary layer. Transition to wake mode occurs as the length and/or depth of the cavity becomes large compared to the upstream boundary-layer thickness, or as the Mach and/or Reynolds numbers are raised. Under these conditions, it is shown that the Kelvin–Helmholtz instability grows to sufficient strength that a strong recirculating flow is induced in the cavity. The resulting mean flow is similar to wake profiles that are absolutely unstable, and absolute instability may provide an explanation of the hydrodynamic feedback mechanism that leads to wake mode. Predictive criteria for the onset of shear-layer oscillations (from steady flow) and for the transition to wake mode are developed based on linear theory for amplification rates in the shear layer, and a simple model for the acoustic efficiency of edge scattering.

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Shuang, Sun, Li Wei, Lu Xin’gen, Zhang Yanfeng, Zhu Junqiang et Tong Guoxiang. « A Comparison of the Wake Effects Generated by the Biased Triangle Bar and Traditional Cylinder Bar to the Boundary Layer on Suction Surface of LPT Blade ». International Journal of Turbo & ; Jet-Engines 37, n^o 2 (25 septembre 2020) : 153–66. http://dx.doi.org/10.1515/tjj-2017-0017.

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AbstractConsidering the asymmetry of the low pressure turbine blade (LPT) wake at a low Reynolds number, the influence of asymmetric wakes which are similar to LPT wakes on the boundary layer of downstream blade rows in the near field is studied in the present paper, in order to increase wake flow prediction accuracy of the downstream blade without increasing the difficulty of the experiment or calculation load. Packb high-lift LPT airfoil was studied with CFX software. Following the analysis of the similarities between the wake generated by the cylinder bar and the triangle bar and the LPT blade wake in the near-field, the boundary layer flow characteristics on the suction surface under the different wakes were compared. In this research, it was found that the wakes of biased triangle bar shared more similarities with the LPT blade wake in the near field than the cylinder bar. Furthermore, the biased triangle bar wake was asymmetrical in terms of its centerline, and the separation bubble was suppressed while the calming effect was reduced after the wake-induced transition due to the asymmetry. And the time-averaged momentum thickness decreased by 7 % compared to the cylinder wake.

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Lu, Yuhan, Zaijie Liu, Teng Zhou et Chao Yan. « Stability analysis of roughness-disturbed boundary layer controlled by wall-blowing ». Physics of Fluids 34, n^o 10 (octobre 2022) : 104114. http://dx.doi.org/10.1063/5.0117405.

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Roughness-induced transition control is of considerable importance for high-speed vehicles. In this paper, the instability of a roughness-disturbed boundary layer controlled by spanwise-uniform wall-blowing is investigated through BiGlobal and three-dimensional parabolized stability equation (PSE-3D) analysis. Without wall-blowing, symmetric and antisymmetric unstable modes are observed when using BiGlobal analysis, with PSE-3D analysis suggesting that the symmetric mode is the dominant instability. Both modes are associated with the instability of the entire separated shear layer behind the roughness region rather than the components in certain directions, as both the wall-normal shear and the spanwise component resulting from the bending shear layer contribute to the growth of the disturbance. Upstream wall-blowing delays the roughness-induced transition by modifying the wake instability. The antisymmetric mode is the first to disappear as the blowing intensity increases while the symmetric instability is also suppressed. Upstream wall-blowing also reduces both the strength and bending of the shear layer by affecting the inflow boundary layer. This leads to a decrease in the wall-normal and spanwise contributions to the disturbance energy. Downstream wall-blowing achieves a control effect by decelerating the development of the dominant symmetric mode through the direct interaction between wall-blowing and the wake. Although the reduction in shear strength is not as strong as with upstream wall-blowing, downstream wall-blowing still relaxes the bending of the shear layer, which is related to the production of disturbance energy. In conclusion, two-dimensional wall-blowing can delay the roughness-induced transition by modifying the wake structure and instability.

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Fan, S., et B. Lakshminarayana. « Computation and Simulation of Wake-Generated Unsteady Pressure and Boundary Layers in Cascades : Part 1—Description of the Approach and Validation ». Journal of Turbomachinery 118, n^o 1 (1 janvier 1996) : 96–108. http://dx.doi.org/10.1115/1.2836612.

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The unsteady pressure and boundary layers on a turbomachinery blade row arising from periodic wakes due to upstream blade rows are investigated in this paper. A time-accurate Euler solver has been developed using an explicit four-stage Runge–Kutta scheme. Two-dimensional unsteady nonreflecting boundary conditions are used at the inlet and the outlet of the computational domain. The unsteady Euler solver captures the wake propagation and the resulting unsteady pressure field, which is then used as the input for a two-dimensional unsteady boundary layer procedure to predict the unsteady response of blade boundary layers. The boundary layer code includes an advanced k–ε model developed for unsteady turbulent boundary layers. The present computational procedure has been validated against analytic solutions and experimental measurements. The validation cases include unsteady inviscid flows in a flat-plate cascade and a compressor exit guide vane (EGV) cascade, unsteady turbulent boundary layer on a flat plate subject to a traveling wave, unsteady transitional boundary layer due to wake passing, and unsteady flow at the midspan section of an axial compressor stator. The present numerical procedure is both efficient and accurate in predicting the unsteady flow physics resulting from wake/blade-row interaction, including wake-induced unsteady transition of blade boundary layers.

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Himeoka, Yusuke, et Namiko Mitarai. « When to wake up ? The optimal waking-up strategies for starvation-induced persistence ». PLOS Computational Biology 17, n^o 2 (11 février 2021) : e1008655. http://dx.doi.org/10.1371/journal.pcbi.1008655.

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Prolonged lag time can be induced by starvation contributing to the antibiotic tolerance of bacteria. We analyze the optimal lag time to survive and grow the iterative and stochastic application of antibiotics. A simple model shows that the optimal lag time can exhibit a discontinuous transition when the severeness of the antibiotic application, such as the probability to be exposed the antibiotic, the death rate under the exposure, and the duration of the exposure, is increased. This suggests the possibility of reducing tolerant bacteria by controlled usage of antibiotics application. When the bacterial populations are able to have two phenotypes with different lag times, the fraction of the second phenotype that has different lag time shows a continuous transition. We then present a generic framework to investigate the optimal lag time distribution for total population fitness for a given distribution of the antibiotic application duration. The obtained optimal distributions have multiple peaks for a wide range of the antibiotic application duration distributions, including the case where the latter is monotonically decreasing. The analysis supports the advantage in evolving multiple, possibly discrete phenotypes in lag time for bacterial long-term fitness.

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Koyabu, Eitaro, et Tetsuhiro Tsukiji. « Wake-Induced Bypass Transition over a Flat Plate under Favorable and Adverse Pressure Gradients ». Journal of Flow Control, Measurement & ; Visualization 01, n^o 01 (2013) : 13–19. http://dx.doi.org/10.4236/jfcmv.2013.11003.

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He, Guo-Sheng, et Jin-Jun Wang. « Flat plate boundary layer transition induced by a controlled near-wall circular cylinder wake ». Physics of Fluids 27, n^o 2 (février 2015) : 024106. http://dx.doi.org/10.1063/1.4907744.

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