Odile Labbé

Large-eddy simulation of a compressible flow past a deep cavity

Large-eddy simulations of the flow over a deep cavity are performed. The computations reproduce identically all the parameters of the experiment by Forestier and co-workers [J. Fluid Mech. (to be published)], including the high Reynolds number ReL=8.6×105. Spectra show an accurate prediction of the peak levels of the fundamental frequency and its first harmonics. Results are also analyzed both in terms of Reynolds and phase averages, the procedure used to compute phase averages being identical to the one used during the experiment. Agreement with the experimental data is found to be excellent. The expansion rate of the shear layer is accurately described, and the temporal physics of the flow, including the dynamics of the coherent structures, is fully recovered. By comparison with an auxiliary computation wherein the wind-tunnel upper wall is not taken into account, the cavity is found to oscillate in a flow-acoustic resonance mode. New values for the γ constant of Rossiter’s model are then proposed for a...

Large-Eddy Simulation of a Subsonic Cavity Flow Including Asymmetric Three-Dimensional Effects

Large-eddy simulations of a cavity configuration yielding a mean flow that exhibits spanwise asymmetry are carried out. Results from the computations reveal that the asymmetry is due to a bifurcation of the whole flow field inside the cavity. It is demonstrated that the bifurcation originates in an inviscid confinement effect induced by the lateral walls. The branch of the bifurcation can be selected by slightly altering the incoming mean flow. Further investigations show that underlying steady spanwise modulations of velocity are amplified under the influence of the lateral walls. The modulation of the streamwise velocity component has the largest energy content and its dominant wavelength contaminates both vertical velocity and pressure. Complementary to these linear interactions, nonlinear energy transfers from streamwise velocity to pressure are also found. A transient analysis highlights the stiff transition from a symmetrical two-structure non-bifurcated flow to a stable unsymmetrical one-and-a-half-structure bifurcated flow. The switch to the bifurcated flow induces an alteration of the Rossiter aero-acoustic loop yielding a change in the dominant Rossiter mode and the appearance of a nonlinear harmonic of the first mode

LARGE-EDDY SIMULATION OF HEAT TRANSFER OVER A BACKWARD-FACING STEP

A large-eddy simulation (LES) of the flow over a backward-facing step was conducted to investigate the heat transfer phenomena in the reattachment zone. The Navier-Stokes equations for an incompressible fluid with the temperature field considered as a passive scalar are solved using a second-order accurate scheme in space and time. An original coupling is used with a previous simulation to impose a fully turbulent flow at the entrance of the domain. The mixed scale subgrid model is used for the momentum equations while a scalar subgrid diffusivity model is employed for the temperature equation. The Reynolds number based on the velocity at the entrance of the domain and the step height is 7,432. The mean velocity field shows clearly that the shear layer issued from the step impacts the wall defining a recirculation zone, in which the reversed flow spreads into the original shear layer. A second recirculation region is found behind the step and acted like an obstacle for the first reversal flow. The mean reattachment length is well correlated to the maximum Nusselt number. The examination of the mean temperature field proves that the mixing behind the step is weak. The temperature fluctuations are important in the shear layer and in a zone issued from the reattachment point and stretched toward the region of reversed flow. Two main frequencies are identified: a high one due to the vortex shedding of the shear layer and a low one corresponding to the drift of the reattachment point.

Unsteady wake vortices simulation behind a A300 model

The development of wake vortex behind a A300 Airbus model in high-lift configuration is computed by solving the three-dimensional unsteady Navier-Stokes equations for an incompressible fluid with a second order accurate scheme in time and space. The vortices are liable to a global unsteadiness, in erratic displacement form around their mean position, called meandering. The aim of this study is to reproduce numerically the unsteady properties of the vortices and to compare the different frequencies with those measured experimentally and to analyze the squared coherence spectra to exhibit the combined movement of the main vortex.