Numerical simulation of the compressible mixing layer past an axisymmetric trailing edge

2007 ◽  
Vol 591 ◽  
pp. 215-253 ◽  
Author(s):  
FRANCK SIMON ◽  
SEBASTIEN DECK ◽  
PHILIPPE GUILLEN ◽  
PIERRE SAGAUT ◽  
ALAIN MERLEN
Keyword(s):  
Numerical Simulation ◽  
Shear Layer ◽  
Large Scale ◽  
Free Stream ◽  
Mixing Layer ◽  
Adverse Pressure Gradient ◽  
Layer Growth ◽  
Trailing Edge ◽  
Azimuthal Mode ◽  
Compressible Mixing Layer

Numerical simulation of a compressible mixing layer past an axisymmetric trailing edge is carried out for a Reynolds number based on the diameter of the trailing edge approximately equal to 2.9 × 106. The free-stream Mach number at separation is equal to 2.46, which corresponds to experiments and leads to high levels of compressibility. The present work focuses on the evolution of the turbulence field through extra strain rates and on the unsteady features of the annular shear layer. Both time-averaged and instantaneous data are used to obtain further insight into the dynamics of the flow. An investigation of the time-averaged flow field reveals an important shear-layer growth rate in its initial stage and a strong anisotropy of the turbulent field. The convection velocity of the vortices is found to be somewhat higher than the estimated isentropic value. This corroborates findings on the domination of the supersonic mode in planar supersonic/subsonic mixing layers. The development of the shear layer leads to a rapid decrease of the anisotropy until the onset of streamline realignment with the axis. Due to the increase of the axisymmetric constraints, an adverse pressure gradient originates from the change in streamline curvature. This recompression is found to slow down the eddy convection. The foot shock pattern features several convected shocks emanating from the upper side of the vortices, which merge into a recompression shock in the free stream. Then, the flow accelerates and the compressibility levels quickly drop in the turbulent developing wake. Some evidence of the existence of large-scale structures in the near wake is found through the domination of the azimuthal mode m = 1 for a Strouhal number based on trailing-edge diameter equal to 0.26.

Effect of a mesh on boundary layer transitions induced by free-stream turbulence and an isolated roughness element

2015 ◽  
Vol 772 ◽  
pp. 445-477 ◽  
Author(s):  
P. Phani Kumar ◽  
A. C. Mandal ◽  
J. Dey
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Large Scale ◽  
Free Stream ◽  
Roughness Element ◽  
Mixing Layer ◽  
Stream Velocity ◽  
Bypass Transition ◽  
Free Stream Turbulence ◽  
Transition Delay

Streamwise streaks, their lift-up and streak instability are integral to the bypass transition process. An experimental study has been carried out to find the effect of a mesh placed normal to the flow and at different wall-normal locations in the late stages of two transitional flows induced by free-stream turbulence (FST) and an isolated roughness element. The mesh causes an approximately 30 % reduction in the free-stream velocity, and mild acceleration, irrespective of its wall-normal location. Interestingly, when located near the wall, the mesh suppresses several transitional events leading to transition delay over a large downstream distance. The transition delay is found to be mainly caused by suppression of the lift-up of the high-shear layer and its distortion, along with modification of the spanwise streaky structure to an orderly one. However, with the mesh well away from the wall, the lifted-up shear layer remains largely unaffected, and the downstream boundary layer velocity profile develops an overshoot which is found to follow a plane mixing layer type profile up to the free stream. Reynolds stresses, and the size and strength of vortices increase in this mixing layer region. This high-intensity disturbance can possibly enhance transition of the accelerated flow far downstream, although a reduction in streamwise turbulence intensity occurs over a short distance downstream of the mesh. However, the shape of the large-scale streamwise structure in the wall-normal plane is found to be more or less the same as that without the mesh.

scholarly journals Scaling of separated shear layers: an investigation of mass entrainment

2017 ◽  
Vol 826 ◽  
pp. 851-887 ◽  
Author(s):  
Francesco Stella ◽  
Nicolas Mazellier ◽  
Azeddine Kourta
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Separated Flow ◽  
Layer Growth ◽  
Upper And Lower Bounds ◽  
Stream Velocity ◽  
Physical Parameters ◽  
Separated Shear Layer ◽  
Mass Entrainment ◽  
Recirculation Length

We report an experimental investigation of the separating/reattaching flow over a descending ramp with a $25^{\circ }$ expansion angle. Emphasis is given to mass entrainment through the boundaries of the separated shear layer emanating from the upper edge of the ramp. For this purpose, the turbulent/non-turbulent interface and the separation line inferred from image-based analysis are used respectively to mark the upper and lower bounds of the separated shear layer. The main objective of this study is to identify the physical parameters that scale the development of the separated shear layer, by giving a specific emphasis to the investigation of mass entrainment. Our results emphasise the multiscale nature of mass entrainment through the separated shear layer. The recirculation length $L_{R}$, step height $h$ and free-stream velocity $U_{\infty }$ are the dominant scales that organise the separated flow (and related large-scale quantities as pressure distribution or shear layer growth rate) and set mean mass fluxes. However, local viscous mechanisms seem to be responsible for most of local mass entrainment. Furthermore, it is shown that large-scale mass entrainment is driven by incoming boundary layer properties, since $L_{R}$ scales with $Re_{\unicode[STIX]{x1D703}}$, and in particular by its turbulent state. Surprisingly, the relationships evidenced in this study suggest that these dependencies are established over a large distance upstream of separation and that they might also extend to small scales, at which viscous entrainment is dominant. If confirmed by additional studies, our findings would open new perspectives for designing effective separation control systems.

An experimental investigation of the effects of compressibility on a turbulent reacting mixing layer

1998 ◽  
Vol 356 ◽  
pp. 25-64 ◽  
Author(s):  
M. F. MILLER ◽  
C. T. BOWMAN ◽  
M. G. MUNGAL
Keyword(s):  
High Speed ◽  
Large Scale ◽  
Structural Changes ◽  
Density Ratio ◽  
Mixing Layer ◽  
Mixing Layers ◽  
Unexpected Result ◽  
Entrainment Ratio ◽  
Plan View ◽  
Compressible Mixing Layer

Experiments were conducted to investigate the effect of compressibility on turbulent reacting mixing layers with moderate heat release. Side- and plan-view visualizations of the reacting mixing layers, which were formed between a high-speed high-temperature vitiated-air stream and a low-speed ambient-temperature hydrogen stream, were obtained using a combined OH/acetone planar laser-induced fluorescence imaging technique. The instantaneous images of OH provide two-dimensional maps of the regions of combustion, and similar images of acetone, which was seeded into the fuel stream, provide maps of the regions of unburned fuel. Two low-compressibility (Mc=0.32, 0.35) reacting mixing layers with differing density ratios and one high-compressibility (Mc=0.70) reacting mixing layer were studied. Higher average acetone signals were measured in the compressible mixing layer than in its low-compressibility counterpart (i.e. same density ratio), indicating a lower entrainment ratio. Additionally, the compressible mixing layer had slightly wider regions of OH and 50% higher OH signals, which was an unexpected result since lowering the entrainment ratio had the opposite effect at low compressibilities. The large-scale structural changes induced by compressibility are believed to be primarily responsible for the difference in the behaviour of the high- and low-compressibility reacting mixing layers. It is proposed that the coexistence of broad regions of OH and high acetone signals is a manifestation of a more biased distribution of mixture compositions in the compressible mixing layer. Other mechanisms through which compressibility can affect the combustion are discussed.

scholarly journals Turbulent shear-layer mixing: growth-rate compressibility scaling

2000 ◽  
Vol 414 ◽  
pp. 35-45 ◽  
Author(s):  
M. D. SLESSOR ◽  
M. ZHUANG ◽  
P. E. DIMOTAKIS
Keyword(s):  
Growth Rate ◽  
Shear Layer ◽  
Free Stream ◽  
Layer Growth ◽  
Turbulent Shear ◽  
Turbulent Shear Layer ◽  
Thermal Energy Conversion ◽  
Layer Growth Rate ◽  
Stream Density ◽  
Convective Mach Number

A new shear-layer growth-rate compressibility-scaling parameter is proposed as an alternative to the total convective Mach number, Mc. This parameter derives from considerations of compressibility as a means of kinetic-to-thermal-energy conversion and can be significantly different from Mc for flows with far-from-unity free-stream-density and speed-of-sound ratios. Experimentally observed growth rates are well-represented by the new scaling.

Shear Layer Driven Acoustic Modes in a Cylindrical Cavity

2014 ◽  
Vol 136 (5) ◽  
Author(s):  
David B. Stephens ◽  
Francisco R. Verdugo ◽  
Gareth J. Bennett
Keyword(s):  
Shear Layer ◽  
Cylindrical Cavity ◽  
Stream Flow ◽  
Acoustic Pressure ◽  
Free Stream ◽  
Mode Switching ◽  
Stream Velocity ◽  
Azimuthal Mode ◽  
Mode Oscillation ◽  
Cavity Opening

This paper describes the interior acoustic pressure of a cylindrical cavity driven by a shear layer. Existing cavity flow literature is generally focused on rectangular cavities, where the resonance is either longitudinal or the result of excited depth modes inside the cavity. The design of the present circular cavity is such that azimuthal duct modes can be excited in various combinations with depth modes depending on free stream velocity. An acoustic simulation of the system was used to identify the modes as a function of frequency when the system is driven by an acoustic point source. With appropriate manipulation of the free stream flow, abrupt mode switching and mode oscillation were both observed, and a condition with a dominant azimuthal mode was found. The strength of the lock-on was documented for the various resonance conditions, and the effects of the cavity opening size and location were studied.

EXPERIMENTAL INVESTIGATION OF VELOCITY AUTOCORRELATION FUNCTIONS IN TURBULENT PLANAR MIXING LAYER

2010 ◽  
Vol 24 (13) ◽  
pp. 1361-1364
Author(s):  
KEH-CHIN CHANG ◽  
CHIUAN-TING LI ◽  
HSUAN-JUNG CHEN
Keyword(s):  
Shear Layer ◽  
Free Stream ◽  
Mixing Layer ◽  
Autocorrelation Functions ◽  
Measured Velocity ◽  
Proper Form ◽  
Velocity Autocorrelation ◽  
Shear Turbulence ◽  
The Time Domain ◽  
Velocity Autocorrelation Functions

The velocity autocorrelation coefficient correlates the velocity in the time domain but at the same spatial position. Turbulent planar mixing layer consists of two types of turbulence, that is, shear turbulence in the central shear layer and nearly homogeneous turbulence in both the high- and low-speed free stream sides. It is interesting to know what kind of function forms can be used to represent faithfully the experimental observations of the velocity autocorrelation coefficients in the mixing layer. Various velocity autocorrelation functions are tested with the measured data. It is found that the Frenkiel function family is the most proper form to represent the measured velocity autocorrelation coefficients in both the shear layer and free stream regimes.

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