Un modèle pour les instationnarités basses fréquences dans les décollements compressibles.

Un modèle pour expliquer les instationnarités basses fréquences dans les décollements de couche limite induits par onde de chocs est proposé, dans les cas où l'écoulement recolle en aval. Ce modèle est basé sur les propriétés d'entraînement de la couche de mélange générée en aval du choc réfléchi, dont les mouvements basses fréquences sont liés aux contractions/dilatations successives du bulbe décollé. Ce modèle sera appliqué à différentes interactions, pour des nombres de Mach allant de 0 à 5

Instationnarités dans les décollements compressibles : cas des couches limites soumises à ondes de choc

An interaction between an oblique shock wave and a fully developed turbulent boundary layer on a flat plate, at Mach number 2.3, has been studied experimentally.This kind of interaction, for strong enough shock waves, induces separation and reattachment of the boundary layer, and is characterized by a strong unsteadiness at low frequencies. The origin of this low frequency motions is not well known. It has been partly characterized experimentally in previous studies, and common features have been observed between the present interaction, others configurations (compression ramps, blunt fins, etc...) and subsonic separation, suggesting that the governing mechanisms of the unsteadiness are of same nature.In this work, Particle Image Velocimetry (PIV) has been used in order to describe the spatial organization of this interaction. A strong statistical link has been shown between the low frequency unsteadiness of the separation shock and the successive contractions/dilatations of the detached bubble. The proposed interpretation is that the large scale movements of the separation shock is linked to the low frequency pulsations of the recirculating bubble, associated with an intermittent fluid realimentation.An aerodynamic model is proposed and determines the main parameters which influence the time scale of the phenomenon. In particular, this model gives the typical frequency for the unsteady reflected shock. This model has been applied for several experimental configurations, for Mach numbers from 0 to 5, and shows a good agreement with measurements.Une interaction entre une onde de choc oblique et une couche limite turbulente sur plaque plane, à un nombre de Mach de 2.3 a été étudiée expérimentalement.Ces interactions, pour des ondes de choc assez fortes, engendrent le décollement et le recollement de la couche limite, et sont le siège d'instationnarités basses fréquences dont les origines sont mal connues. Ces instationnarités ont été caractérisées expérimentalement en partie dans des travaux précédents, et des similarités entre l'interaction étudiée ici et d'autres configurations d'interactions ainsi qu'avec les décollements de couche limite subsonique ont été mis en évidence, suggérant que les mécanismes responsables des instationnarités sont de même nature.Pour ces travaux, la Vélocimétrie par Imagerie de Particules (PIV) a été utilisée afin de décrire spatialement l'organisation longitudinale et transversale de cette interaction. L'exploitation des mesures a mis en évidence un lien statistique fort entre les mouvements basses fréquences du choc réfléchi et les contractions/dilatations successives du bulbe décollé. L'interprétation proposée est que les grands mouvements du choc sont liés aux pulsations basses fréquences du décollement, associées à sa réalimentation intermittente en air frais.Un modèle aérodynamique en a été déduit et permet de préciser les principaux paramètres contrôlant l'échelle de temps du phénomène. En particulier, il permet de déterminer la fréquence des battements du choc. Ce modèle a été appliqué aux interactions sur plaques planes ainsi que pour d'autres configurations expérimentales, pour un éventail de nombres de Mach allant de M=0 à 5, et montre un bon accord avec les mesures

Instationnarités dans les décollements compressibles (cas des couches limites soumises à ondes de choc)

Une interaction entre une onde de choc oblique et une couche limite turbulente sur plaque plane, à un nombre de Mach de 2.3 a été étudiée expérimentalement. Ces interactions, pour des ondes de choc assez fortes, engendrent le décollement et le recollement de la couche limite, et sont le siège d'instationnarités basses fréquences dont les origines sont mal connues. Ces instationnarités ont été caractérisées expérimentalement en partie dans des travaux précédents, et des similarités entre l'interaction étudiée ici et d'autres configurations d'interactions ainsi qu'avec les décollements de couche limite subsonique ont été mis en évidence, suggérant que les mécanismes responsables des instationnarités sont de même nature. Pour ces travaux, la Vélocimétrie par Imagerie de Particules (PIV) a été utilisée afin de décrire spatialement l'organisation longitudinale et transversale de cette interaction. L'exploitation des mesures a mis en évidence un lien statistique fort entre les mouvements basses fréquences du choc réfléchi et les contractions/dilatations successives du bulbe décollé. L interprétation proposée est que les grands mouvements du choc sont liés aux pulsations basses fréquences du décollement, associées à sa réalimentation intermittente en air frais. Un modèle aérodynamique en a été déduit et permet de préciser les principaux paramètres contrôlant l échelle de temps du phénomène. En particulier, il permet de déterminer la fréquence des battements du choc. Ce modèle a été appliqué aux interactions sur plaques planes ainsi que pour d'autres configurations expérimentales, pour un éventail de nombres de Mach allant de M=0 à 5, et montre un bon accord avec les mesures.An interaction between an oblique shock wave and a fully developed turbulent boundary layer on a flat plate, at Mach number 2.3, has been studied experimentally. This kind of interaction, for strong enough shock waves, induces separation and reattachment of the boundary layer, and is characterized by a strong unsteadiness at low frequencies. The origin of this low frequency motions is not well known. It has been partly characterized experimentally in previous studies, and common features have been observed between the present interaction, others configurations (compression ramps, blunt fins, etc...) and subsonic separation, suggesting that the governing mechanisms of the unsteadiness are of same nature. In this work, Particle Image Velocimetry (PIV) has been used in order to describe the spatial organization of this interaction. A strong statistical link has been shown between the low frequency unsteadiness of the separation shock and the successive contractions/dilatations of the detached bubble. The proposed interpretation is that the large scale movements of the separation shock is linked to the low frequency pulsations of the recirculating bubble, associated with an intermittent fluid realimentation. An aerodynamic model is proposed and determines the main parameters which influence the time scale of the phenomenon. In particular, this model gives the typical frequency for the unsteady reflected shock. This model has been applied for several experimental configurations, for Mach numbers from 0 to 5, and shows a good agreement with measurements.AIX-MARSEILLE1-BU Sci.St Charles (130552104) / SudocAIX-MARSEILLE1-Inst.Médit.tech (130552107) / SudocSudocFranceF

Pressure-velocity correlations in a shock wave boundary layer interaction

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High resolution LDA measurements in transitional oblique shock wave boundary layer interaction

International audienceSpatial development of a transitional Oblique Shock Wave Interaction at Mach 1.68 is presented. This type of flow is characterised by very small length scales (boundary layer thickness is smaller than 1mm), high velocities, reverse flows and a wide range of velocity fluctuations along the transition process. Unsteady velocity fields have been obtained using a high spatial resolution Laser Doppler Anemometry system, allowing quantitative measurements of the velocity fluctuations down to y/δ = 0.1. A model to take into account the finite size of the probe volume on the mean and RMS velocity measurements is used and applied to the present measurements. Finally, the amplification of the velocity fluctuations along the transitional separated shear layer is described

mécanismes de transition dans une interaction onde de choc couche limite

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Effects of Shock-Induced Separation on Boundary Layer Transitional Mechanisms

The transitional mechanisms of a natural boundary layer as well as one interacting with shock-waves are investigated at supersonic speeds. Experiments are performed over a flat plate and a 6° compression ramp at a Mach number of 1.65, using Pitot probes and hot-wire anemometry. Comparisons are drawn between the two configurations focusing on the evolution of unstable modes of the boundary layer. Measurements are made inside the laminar boundary layer to study the primary and secondary instabilities of the natural transition process. While the flat plate exhibits the classical mechanisms of transition for a zero pressure gradient boundary layer, the compression ramp shows an altered transition process where the secondary instabilities play a less important role

Unsteadiness in supersonic Free Shock Separation in overexpanded flows

International audienceWhile operating in an overexpanded condition, nozzles exhibit side loads. These loads are due to the presence of unsteadiness in the shock wave boundary layer interactions which are developing inside the nozzle. This paper deals with an interaction, which compares with Free Shock Separation regime, and occurs when the boundary layer doesn't reattach to the floor. We present the first results on an original experimental setup of a 2D Free Shock Separation, at Mach number of 2.1. Pitot probe, static pressure probe, hot wire anemometry and Particle Image Velocimetry are used to describe the spatial organization of the flow, as well as the frequency scales obtained in various configurations. No evidence of intrinsic low time scales of the shock has been observed and the dominant frequencies of the shock spectrum are found similar to that of the subsonic external flow. Nevertheless, the amplitude of the low frequency motions are found to increase when the flow separates

TRANSITIONAL SHOCK-WAVE BOUNDARY LAYER INTERACTION OVER A COMPRESSION RAMP

Experiments are performed to study the interaction between shock-waves and a laminar boundary layer on a compression ramp at Mach 1.66. Pitot probe measurements and hot-wire anemometry are complemented with Schlieren visualisation, while Reynolds number is varied by a factor of two. The results suggest a "fully laminar" type of interaction where transition of the boundary layer occurs downstream of reattachment. On the contrary, oblique shock-wave laminar boundary layer interactions resulted in a "transitional" type of interaction with transition occurring at reattachment, for similar freestream conditions and equivalent shock strength. This difference in the type of interaction resulted in subtle deviations in the relative length and time scales of the shockwave boundary layer interaction. The non-dimensional lengths of interactions, based on mass balance across the interaction, were similar for the two geometrical configurations, but the evolution of these length scales with increasing Reynolds number were found to be different. The Strouhal number associated with low frequency unsteadiness matched across both the configurations, with typical time scales associated with transitional mechanisms in oblique shock reflection, "delayed" in the compression ramp measurements

Compressible mixing layer in shock-induced separation

International audienceUnsteadiness in separated shock/boundary layer interactions have been previously analysed in order to propose a scenario of entrainment-discharge as the origin of unsteadiness. It was assumed that the fluid in the separated zone is entrained by the free shear layer formed at its edge, and that this layer follows the properties of the canonical mixing layer. This last point is addressed by re-analysing the velocity measurements in an oblique shock reflection at a nominal Mach number of 2.3 and for two cases of flow deviation (8° and 9.5°). The rate of spatial growth of this layer is evaluated from the spatial growth of the turbulent stress profiles. Moreover, the entrainment velocity at the edge of the layer is determined from the mean velocity profiles. It is shown that the values of turbulent shear stress, spreading rate and entrainment velocity are consistent, and that they follow the classical laws for turbulent transport in compressible shear layers. Moreover, the measurements suggest that the vertical normal stress is sensitive to compressibility, so that the anisotropy of turbulence is affected by high Mach numbers. Dimensional considerations proposed by Brown & Roshko (1974) are reformulated to explain this observed trend