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International audience; The scattering of an acoustic wave by a 3D spatially-developing plane turbulent mixing layer is investigated by means of a Large Eddy Simulation (LES). First, a plane turbulent mixing layer of initial Reynolds number Reω, 0 ≈ 1200 and convective Mach number Mc ≈ 0.12 is computed and its characteristics are validated by comparisons to experimental and numerical studies. Then, an acoustic source is introduced in the computational domain and the LES of both the turbulent mixing layer and the acoustic field is carried out, allowing the direct computation of the scattered pressure field. Computational Aero Acoustics (CAA) methods have been implemented to minimize spurious pressure fluctuations created while the turbulent structures leave the computational domain. The side-lobes of the scattered pressure power spectral density are recovered and a parametric study, involving the convection velocity of the large turbulent structures, the amplitude and the tonal frequency of the source is carried out. The part of scattered energy, the frequency shift of the side-lobes and the spectra decrease are found to be correctly estimated. Analysis of directivity at the side-lobes frequencies show a progressive re-distribution of the acoustic energy, as the acoustic wave propagates through the turbulent shear layer.

This study presents hybrid Reynolds-averaged Navier–Stokes/large-eddy simulations of the unsteady flow and noise-generation phenomena in the slat cove of a high-lift wing profile. These computations are part of a joint numerical/experimental aeroacoustics collaborative program dedicated to slat-flow analysis. A dedicated two-element wing profile (slat plus main body) has been designed to isolate slat noise from other possible sources (e.g., the flap), while minimizing mean flow deflection effects, to improve the fidelity of open-jet wind-tunnel measurements. The design of this two-element airfoil has been performed numerically, using an optimization process based on steady Reynolds-averaged Navier–Stokes calculations. This airfoil has been investigated experimentally at the Ecole Centrale de Lyon open jet facility. Unsteady zonal hybrid Reynolds-averaged Navier–Stokes/large-eddy simulations have been performed to provide a comprehensive description of the unsteady flow inside the slat cove, focusing on th...

The scattering of an acoustic wave by a fully turbulent mixing layer is investigated by means of a Large Eddy Simulation. The turbulent characteristics of the mixing layer have been validated by comparison to experimental and numerical studies. Computational Aero Acoustics (CAA) methods have been used to minimize spurious pression fluctuations created while the turbulent structures leave the computational domain. Then, an acoustic source has been placed inside a uniform flow and the acoustic propagation has been validated. Finally, both the turbulent mixing layer and the acoustic field are calculated and the scattering phenomenon is investigated. The ’double-humped’ structures of the far field power spectral density is recovered and a parametrical study, involving the tonal frequency of the source is carried out. The comparison to previous experimental works have been enabled thanks to the definition of a relevant scattering parameter. The spectra are in good agreement with experimental works on acoustic scattering. The frequency shift of the sidelobes and the spectra’s decrease are well predicted and in a good agreement with the ones found in the literature.

This paper describes some significant steps made towards the numerical simulation of the noise radiated by the high-lift devices of a plane. Since the full numerical simulation of such configuration is still out of reach for present supercomputers, some hybrid strategies have been developed to reduce the overall cost of such simulations. The proposed strategy relies on the coupling of an unsteady nearfield CFD with an acoustic propagation solver based on the resolution of the Euler equations for midfield propagation in an inhomogeneous field, and the use of an integral solver for farfield acoustic predictions.

In this paper we present a new dynamic methodology to compute the value of the numerical coefficient present in numbers of subgrid models, by mean of a multilevel approach. It is based on the assumption of a power law for the spectral density of kinetic energy in the range of the highest resolved wave numbers. It is shown that this assumption also allows us to define an equivalent law for the subgrid dissipation, and to obtain a reliable estimation for it through the introduction of a three-level flow decomposition. The model coefficient is then simply tuned dynamically during the simulation to ensure the proper amount of subgrid dissipation. This new dynamic procedure has been assessed here in inviscid homogeneous isotropic turbulence and plane channel flow simulations (with skin-friction Reynolds numbers up to 2000).

An extension of the multilevel method applied to LES proposed in Terracol et al. [J. Comput. Phys. 167 (2001) 439] is introduced here to reduce the CPU times in unsteady simulation of turbulent flows. Flow variables are decomposed into several wavenumber bands, each band being associated to a computational grid in physical space. The general framework associated to such a decomposition is presented, and a new adapted closure is proposed for the subgrid terms which appear at each filtering level, while the closure at the finest level is performed with a classical LES model. CPU time saving is obtained by the use of V-cycles, as in the multigrid terminology. The main part of the simulation is thus performed on the coarse levels, while the smallest resolved scales are kept frozen (quasi-static approximation [Comput. Methods Appl. Mech. Engrg. 159 (1998) 123]). This allows to reduce significantly the CPU times in comparison with classical LES, while the accuracy of the simulation is preserved by the use of a fine discretization level. To ensure the validity of the quasi-static approximation, a dynamic evaluation of the time during which it remains valid is performed at each level through an a priori error estimation of the small-scales time variation. This leads to a totally self-adaptive method in which both the number of levels and the integration times on each grid level are evaluated dynamically. The method is assessed on a fully unsteady time-developing compressible mixing layer at a low-Reynolds number for which a DNS has also been performed, and in the inviscid case. Finally, a plane channel flow configuration has been considered. In all cases, the results obtained are in good agreement with classical LES performed on a fine grid, with CPU time reduction factors of up to five.

This study presents a highly-resolved Large Eddy Simulation of a three-element highlift airfoil with deployed slat and flap, which involves a large amount of CPU resources (2.6 billions of grid cells and 6 millions of CPU hours). The aim of this simulation is double: the first objective is to perform a detailed flow analysis and identify the physical events responsible for noise generation as well as the implied flow mechanisms; the second objective is to provide an important numerical database that may be used in the future to assess more affordable numerical approaches based for instance on hybrid RANS/LES methods. The assessment of the present LES relies on a recent experimental campaign performed in Onera F2 wind tunnel in the framework of a joint Onera/DLR project named LEISA2.

A multilevel method for large-eddy simulation of turbulent compressible flows is proposed. The method relies on the splitting of the turbulent flowfield into several frequency bands in space and time, each band being associated to a specific computational grid in physical space. This allows to take into account in a deterministic way the information contained on finer grids. A subgrid model adapted to such a decomposition—based on a generalization of the Germano's identity to multilevel decomposition—is also introduced. The approach is validated by several multilevel simulations in a subsonic plane channel flow configuration for a low and a high value of the Reynolds number, while reductions of the CPU times up to 80% are obtained.

This paper presents the development of an explicit filtering method for Large Eddy simulation in the framework of unstructured grids. The proposed method relies on the approximate deconvolution model, complemented with a modified Smagorinsky term. This term aims at accountin g for long-range interactions in the wavenumber space between resolved and subgrid scales. Both a classical and a multiscale formulation of this term are considered. An analytical evaluation procedure is described in order to adjust the model coefficient to our explicit filtering framework. The method relies on the introduction o f a discrete filtering operator. For that purpose, a simple volume averaging operator has been chosen in order to perform explicit filtering because of its ease of implementation on unstructured meshes. A discrete characterization procedure of this operator is also proposed in order to estimate locally in space the shape of the filter wh ich is required for the analytical evaluation of the Smagorinsky coefficient. The method is assessed using both h exahedral and tetrahedral grids on a classical homogneneous isotropic turbulence test case. First results obtained for the turbulent flow around a circular cylinder, at a diameter-based Reynolds number of 3,900 are also presented. I. Introduction Large eddy simulation is a tridimensional method that allows a fine unsteady analysis of turbulent flows. This method is based on a separation between large and small scales of the flow, only large scales being resolved while the small one’s influence is represented by a subgrid scale model. This separ ation between large and small scales is generally assumed in practice as being due to a high-pass spatial filtering of th e solution. Unfortunately, on unstructured meshes, many parameters prevent from exactly characterizing the effect ive filtering operator and therefore to develop adapted subg rid models to account for the unresolved scales. Besides, it sho uld be necessary to account for the numerical scheme’s influence, which can act as an additionnal filtering step 1 . Indeed, the use of dissipative numerical scheme to perform large eddy simulations on unstructured meshes is not well suitable because of their interaction with the subgrid scale model. Therefore, the use of more accurate numerical schemes seems fundamentally more suitable. However, the development of high-order schemes on unstructured meshes remains a significant challenge today and such schemes are moreover generally observed to be unstable in realistic configuratio ns, thus requiring an additional artificial stabilization. In this study, we will investigate a possible alternative to perform LES on unstructured grids, which relies on the use of explicit filtering methods. In such methods, a discrete fil tering operator is applied explicitely during the simulati on. In this specific context, the effective filter then becomes dire ctly known and is no longer assumed to be due implicitely to the grid and to the numerical scheme. Indeed, the introduction of this explicit filter allows to control the effective filt er. This technique seems also very interesting by its potential stabilizing effects that could make the use of low dissipative numerical schemes possible. In this paper, different strategies for explicit filtering w ill be discussed and an approximate deconvolution method based on Stolz and Adams 2 original approach will be presented. Subgrid scale modelin g associated to this method will also be considered. Since this approach is based on the intro duction of a specific discrete filter, a method to construct a discrete filtering operator on unstructured meshes will the refore be proposed. The explicit filtering method will first be detailed in sectio n II, where two possible implementations of the Approximate Deconvolution Method (ADM) will be considered. Since ADM only makes it possible to account for local interactions in the wavenumber space, an additional closure based on the Smagorinsky model will be proposed in order to

This paper presents a numerical investigation by mean of Large Eddy Simulation of the turbulent flow around a regular circular cylinder, and around one which is truncated in its rear part. Such a configuration was inspired by the experimental works of Kopiev et al., who suggested that performing a truncation of the cylinder should lead to a significant reduction of the noise emitted by the turbulent flow past the cylinder. In our simulations, it is observed – as already done in experiments – that a significant reduction (-2dB) of the noise radiation intensity is obtained in the truncated geometry case. A careful analysis of both the acoustic and aerodynamic results is then performed in order to highlight specific flow modifications that may be responsible for this noise reduction.