Lionel Larchevêque

Large-eddy simulation of a compressible flow in a three-dimensional open cavity at high Reynolds number

Large-eddy simulations of a subsonic three-dimensional cavity flow with self-sustaining oscillations are carried out for a Reynolds number based on the length of the cavity equal to

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

7\,{\times}\,10^6

Turbulent Inflow Conditions for Large-Eddy-Simulation of Compressible Wall-Bounded Flows

. Meticulous comparisons with available experimental data corresponding to the same configuration demonstrate a high level of accuracy. Special attention is paid to the mixing layer that develops over the cavity and two different zones are identified. The first one is dominated by Kelvin–Helmholtz instability, and the linear as well as quadratic energy transfers leading to the filling of velocity spectra are described. The Kelvin–Helmholtz instability also appears to be forced near the origin of the layer, and it is postulated that the small recirculation bubble located in this area is responsible for the forcing. Downstream of the first zone and up to the vicinity of the aft wall, the layer behaves very similarly to a free mixing layer by exhibiting a linear spreading. An influence of the recirculating flow inside the cavity upon the growth of the layer is nevertheless observed at downstream stations. Analysis of the pressure on the floor of the cavity reveals that the self-sustaining oscillation-related pressure modes (Rossiter modes) are independent of their spanwise location inside the cavity. On the contrary, Rossiter modes exhibit streamwise modulations and it is demonstrated that a very simple two-wave model is able to reproduce the spatial shape of the modes. Nonlinear interactions between Rossiter modes are encountered, as well as nonlinear interactions with low-frequency components. A joint time–frequency analysis shows a temporal modulation of the Rossiter mode levels at similar low frequencies, resulting in a special form of intermittency with competitive energy exchanges between modes.

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

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...

Zones of Influence and Shock Motion in a Shock/Boundary-Layer Interaction

The issue of turbulent inflow conditions for large-eddy simulation (LES) is addressed through three representative examples recently treated at ONERA. First, the performance of an extension to compressible flows of the rescaling method of Lund et al. is assessed. The second example, the flow above a deep cavity shows that the imposition of turbulent fluctuations has nearly no influence on the accuracy of the simulation on this acoustically driven flow. The third case demonstrates that particular attention must he paid to the response of the inflow condition to acoustic perturbations when local hybrid Reynolds-averaged Navier-Stokes/LES approaches are considered

Mechanism of shock unsteadiness in separated shock/boundary-layer interactions

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

Numerical simulations of the unsteady flow and radiated noise over a cylindrical cavity

This paper aims at describing the main features of a shock reflection on a turbulent boundary layer. The data used for this analysis are the results of large-eddy simulations of the interaction carried out with three different shock intensities: from incipient to fully separated cases. Computational results are validated vs experiments obtained for the same interaction geometries. The main space-time properties of the leading shock motions are described together with their links with the other regions of the flow. In particular, information about the origin of the shock motion is derived from the correlations between shock motion and unsteady pressure fields. It is shown that the shock motion reveals the flow unsteadiness found in the interaction region.

Method for Spectra Estimation from High-Speed Experimental Data

A large-eddy simulation-based study is presented and focuses on different unsteadiness-source features in a Mach 2.3 shock reflection with separation. The sources of unsteadiness are localized and the path taken by disturbance as it spreads out to the whole field is defined. It is shown that the phenomena arising inside the recirculation bubble govern the whole interaction, at both low and intermediate frequencies. Indeed, the shock motion appears to mirror phenomena found in the separated zone. Moreover, features of separated-flow unsteadiness bear some resemblance to those occurring in incompressible flows. An equivalent inviscid scheme of the unsteady interaction is established in order to describe the whole shock-system unsteadiness at low and intermediate frequencies and the downstream unsteady-pressure field.

Numerical simulations of the sound generation by fl ow over surface mounted cylindrical cavities including wind tunnel installation effects

,The purpose of this paper is to investigate the acoustical field generated by the flow over a cylindrical cavity on a flat plane, similar to those located on wings and fuselage of transport aircraft, using a hybrid numerical method. Firstly, a CFD computation was carried out, using a Large-Eddy Simulation method, to compute the turbulent structures and the noise generating mechanisms. Secondly, the near-field results were injected in a Kirchhoff integral method code, in order to compute the far-field noise radiation. The retained case for this study is represented by a cavity with a unitary depth/diameter ratio, in the presence of a subsonic flow with a Reynolds number based on the diameter of the cavity equal to 4.6x10 5 . As inflow condition, a fluctuation-free velocity profile was considered. Computational results were compared to experimental data measured in an anechoic wind tunnel for the same geometry configuration, but in the presence of a turbulent boundary layer. As expected, a symmetrical flow pattern was observed joined with a fluid resonant mechanism. The computation shows that the acoustical field is less influenced by the inflow condition and results are in good agreement with the experimental measurements despite a slight under-estimation of the main tone amplitude.

Numerical simulation of spectral broadening of an acoustic wave by a spatially growing turbulent mixing layer

The present paper presents a technique to estimate spectra from experimental data sampled at discrete points in time. The interest of the current paper is to investigate spatiotemporal dependencies in supersonic turbulent flows like boundary layers and shock wave/turbulent boundary-layer interactions. The experimental data are measured by means of a dual-particle-image-velocimetry system that gives access to a certain amount of temporal information that is also in high-speed flows. The cross-correlation information in space made available by the dual-particle-image-velocimetry system can be used to reconstruct the autocorrelation for delay times ranging between the discrete points measured with the dual-particle-image-velocimetry system. This can be achieved for convection-dominated regions of the flow by moving into the convected frame of reference: similar to Taylor’s hypothesis. The principle and suitability of the suggested technique is demonstrated in a combined numerical–experimental approach. The l...