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This study deals with the development of a numerical wire mesh screen model for the unsteady simulation of aeroacoustic configurations. Such devices are indeed of growing interest for practical app...

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...

A methodology to mix Chimera and Immersed Boundary approaches in the same CFD computation is presented in order to deal with complex geometries. Where accuracy is required in a vicinity of an obstacle, a body-fitted mesh is used to accurately simulate the boundary layer. Otherwise, an immersed boundary method (IBM) is applied to deal with this obstacle. Each body-fitted mesh is generated independently in an overset grid fashion. A set of Cartesian grids that are adapted to the spatial resolution of body-fitted grids and immersed boundaries is generated automatically and overlaps the body-fitted grids. In this paper, we describe the overall procedure to mix the overset grid and immersed boundary approaches, then we detail how the present IBM enables to deal with geometrically complex configurations. Finally, preliminary results of the simulation of a turbulent flow past two cylinders in tandem are presented.

This paper presents the first numerical investigation via direct numerical simulation of the tone noise phenomenon occurring in the flow past laminar airfoils. This phenomenon corresponds to the radiation of discrete acoustic tones in some specific flow conditions, and has received much attention since the 1970s, and several experimental studies have been carried out to identify and understand the underlying physical mechanisms. However, several points remain to be clarified in order to provide a complete explanation of its origin. The flow around a two-dimensional NACA0012 airfoil is considered in order to have a deeper understanding of the tone noise phenomenon. Consistently with previous experimental studies, it is shown that depending on the Reynolds number and angle of attack, two different types of acoustic spectrum are observed: one which exhibits a broadband contribution with a dominant frequency together with a sequence of regularly spaced discrete frequencies, while the other one is only characterized by a simple broadband contribution. The first configuration is typical of the tone noise phenomenon. The present work shows that in this case, the mean flow on the pressure side of the airfoil exhibits a separation bubble near the trailing edge and the main tone frequency is close to the most amplified frequency of the boundary layer. The mechanism proposed in previous works for the main tone generation – which implies the existence of a separation bubble at the pressure side – is therefore validated by numerical simulation. On the other hand, the analysis of the suction side boundary layer reveals that there is no separation and that the most amplified frequency is different from the main tonal one. However, the suction side boundary layer is highly receptive to the tone frequency. Finally, an original explanation for the existence of the secondary discrete frequencies observed in the radiated pressure spectrum is given. They are associated to a bifurcation of the airfoil wake from a symmetric to a non-symmetric vortex pattern. A possible explanation for the existence of this bifurcation is the interaction between the disturbances which are the most amplified by the suction side boundary layer and those originating in the forcing of the suction side flow by the main tone noise mechanism.

This paper presents an investigation of the possible commutation errors arising in Large Eddy Simulations performed with a filter width which varies in time, e.g. when the grid/spectral resolution is modified dynamically during the computations. Some theoretical developments provide an explicit expression for this error, which is shown to depend explicitly on the filter kernel properties, and on the time derivative of the cut-off length. From this analysis, some constraints on the filter kernel are derived in order to minimize the error. The particular case of a Gaussian filter is considered, which yields to a first error term. Finally, the dependency of the temporal commutation error to the variations of the filter width is investigated by performing a priori tests in the case of isotropic homogeneous turbulence with a varying cut-off length.

This study presents several unsteady computations of the 30P30N three-element high-lift airfoil, focused on slat noise prediction. These simulations rely on several RANS/LES approaches: two global hybrid approaches, DDES (Delayed Detached Eddy Simulation) and ZDES (Zonal Detached Eddy Simulation) and one zonal approach, the NLDE (Non-Linear Disturbance Equations). Global approaches compute all the geometry, while zonal simulations only consider an LES domain clustered around the slat to save computational resources. Two grid resolutions are considered, in order to perform a critical comparison of both the modeling and grid resolution effects for this kind of configuration. The obtained results are favorably compared to near-field results measurements obtained in JAXA low-speed Wind Tunnel (JAXA-LWT2): narrow band peaks present in the pressure spectra are well recovered by all the simulations, with a very good agreement with the measurements for the fine grid results, while the medium grid simulations tend to over-estimate the magnitude of these peaks. It is also observed that the DDES approach seems to delay the development of instabilities in the slat cusp shear layer, while other approaches lead to a faster break-up of the shear layer. The zonal NLDE approach leads to significant CPU time savings due to the reduced number of grid points, as well as to shorter transient times. However, such simulations do not account for the whole diffraction and reflection effects, as observed on the far-field acoustic field.

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 presents a theoretical analysis and two applications of a high-order overlapping grid method for coupling Cartesian and curvilinear grids, developed in order to simulate aerodynamic noise. First, the overlapping grid method based on Lagrange interpolating polynomials is described and a theoretical analysis of the interpolation operator is then carried out. It shows that the interpolation generates spurious modes that depend on the wavenumbers of the signal. Besides it also gives the optimal conditions in which interpolation can be applied. Then an application to the simulation of the aeroacoustic noise generated by the vortex shedding behind a cylinder is presented. During this simulation, it appears that interpolation can create some spurious acoustic modes in regions where hydrodynamic fluctuations are significant, as predicted by the theoretical analysis. It is shown that these spurious modes disappear when a refined Cartesian grid is used (26 points per wavelength of the vortex shedding were found to be adequate in this study). At last, the simulation of the aerodynamic noise of a three element high-lift wing profile has then been carried out. For this application, the main acoustic source at the slat trailing edge is represented analytically. The propagation of the generated acoustic wave is simulated with a mean flow at rest and with a steady turbulent mean flow computed by RANS. The first application allows us to assess the method by comparing the results to a reference solution. The second one shows that the influence of a non-uniform mean flow on the directivity of an acoustic source can be observed in complex geometries. This application therefore shows that the proposed coupling method is well adapted to complex geometries that are usually met in industrial applications.

Hybrid CFD/CAA methods have generally to be used for the numerical simulation of trailing-edge noise (see [9, 20] for instance). This study focuses on the first step of such hybrid methods, which is to predict the unsteady aerodynamic sources by the mean of a 3D unsteady simulation of the flow. Such a simulation is however generally still away from the numerical capabilities of ‘usual’ supercomputers. This paper investigates the use of a zonal LES method (based on the NLDE – Non-Linear Disturbance Equations – technique) for the numerical prediction of the aerodynamic noise sources. This method makes it possible to perform only zonal LES close to the main elements responsible of sound generation, while the overall configuration is only treated by a RANS approach. Attention will be paid to the specific boundary treatment at the interface between the RANS and LES regions. More precisely, the problem of the generation of turbulent inflow conditions for the LES region will be carefully addressed. The method is first assessed in the simulation of a flat plate ended by a blunted trailing-edge, and then applied to the simulation of the flow over a NACA0012 airfoil with blunted trailing-edge.

A source-to-far-field computation procedure aimed at predicting the noise generated by rotor-stator fan interactions is presented here. Interaction noise sources are assessed using a three-dimensional Reynolds-averaged Navier-Stokes (RANS) code. A modal expansion is applied to the computational fluid dynamics (CFD) data, and a source model is used to link the CFD to an Euler code simulating the acoustic propagation. Far-field noise is classically obtained by means of a Kirchhoff integral. The source model is detailed and validated over a selected benchmark. Then the hybrid method is applied to a turbofan model, tested in the German-Dutch Wind Tunnel. Simulations are compared to boundary element method computations previously performed and used here to validate the no-flow solution. Interacting cut-on mode amplitude deduced from CFD output postprocessing is adjusted to fit in-duct measurements on the outer wall. The predicted radiation field is related to the directivity patterns measured at several axial positions upstream of the fan inlet and fairly good agreement is found using the RANS/Euler/Kirchhoff procedure.

The use of high-order centered finite difference to solve the Euler equations commonly requires a stabilization procedure. The present work is a theoretical analysis of these stabilization methods that make the whole algorithm (i) still consistent with the continuous problem and (ii) able to run long time simulations. In the present study, a theoretical analysis of the three commonly used methods resorting to the application of high-order filters is performed. An extension to non-periodic boundary conditions is studied to avoid numerical reflection and numerical instabilities due to the use of specific boundary schemes. To cite this article: R. Guenanff, M. Terracol, C. R. Mecanique 333 (2005).

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.

Resume Une methode multiniveau pour la Simulation des Grandes Echelles des ecoulements turbulents compressibles instationnaires dans l'espace physique est proposee. Elle est basee sur une decomposition du champ turbulent en plusieurs bandes frequentielles associees a des grilles de calcul emboitees, permettant une prise en compte deterministe des evenements coherents captures sur les grille les plus fines. Un modele statistique sous-maille adapte — base sur une generalisation de la relation de Germano au cadre multiniveau — est egalement introduit. L'approche est finalement validee par un calcul dans une configuration de canal plan subsonique.

This study deals with the numerical investigation of the flow around a high-lift airfoil with deployed slat and flap, with focus being made on the slat cove region. Two different techniques are used and compared to simulate the unsteady flow around such an airfoil, both being based on a hybrid RANS/LES formulation in order to minimize as much as possible the computational resources. In this paper, a comparison of the results from these two approaches (namely ZDES and NLDE) is presented, both from the statistical and unsteady points of view. The two approaches provide results which are in excellent respective agreement.

This study presents numerical simulations of the unsteady flow and noise generation phenomena in the slat cove of a highlift wing profile by mean of a zonal hybrid RANS/LES approach. These computations are part of a joint numerical/experimental aeroacoustics collaborative program, dedicated to slat flow analysis. For that purpose, a dedicated twoelement wing profile (slat+main body) has been designed in order to isolate the slat noise sources from other possible spurious sources (flap for instance), while minimizing mean flow deflection effects in order to improve the reliability of open-jet wind tunnel measurements. The design of this two-element airfoil has been done numerically using an optimization process based on steady RANS calculations. This airfoil has been investigated experimentally in Ecole Centrale de Lyon open jet facility, therefore providing unsteady wall-pressure measurements in the slat cove area and farfield acoustic measurements. Unsteady zonal hybrid RANS/LES simulations have been performed to provide a comprehensive description of the unsteady flow inside the slat cove, with focus being made on the noise generation processes. A fine physical unsteady analysis of the flow inside the slat cove is presented, as well as a comparison of numerical results with available experimental ones. The pressure spectra associated to the slat cove flow appear to be characterized by several tonal peaks emerging from a global broadband content. The existence of such peaks is discussed and attributed to a feedback loop involving the main shear layer inside the slat cove. A theoretical law is proposed and assessed at the end of the paper to predict the associated tonal frequencies.

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.

A direct numerical simulation of the o w around a swept cylinder in the presence of a micro-roughness array has been performed. The roughness elements are placed periodically in the spanwise direction and their heigth is equal to one tenth of the boundary layer thickness. The aim of the simulation is to compute the amplitude of the crosso w instability waves generated within the laminar boundary layer by the presence of the roughness array. Results show that both a steady and an unsteady instability waves are generated, even if the roughness forcing is purely steady. A linear stability analysis based on the OrrSommerfeld equations is used to characterize these waves. The unsteady one is the traveling crosso w wave which is the most amplied according to this theory. The steady one must be distinguished from the roughness-induced response. This can be done with help of the biorthogonal decomposition method developed by Tumin and extended in this work to a swept boundary layer. This method is also a way to evaluate precisely the receptivity coecien ts. This has been done for both the unsteady and the steady crosso w waves. The results obtained for the steady crosso w wave should be considered with caution, since the non-parallel and curvature eects, not taken into account yet in the decomposition method, may be signican t for this specic zero frequency.

International audience; Since the experiments of Monkewitz et al. (J. Fluid Mech. vol. 213, 1990, p. 611), sufficiently hot circular jets have been known to give rise to self-sustained synchronized oscillations induced by a locally absolutely unstable region. In the present investigation, numerical simulations are carried out in order to determine if such synchronized states correspond to a nonlinear global mode of the underlying base flow, as predicted in the framework of Ginzburg - Landau model equations. Two configurations of slowly developing base flows are considered. In the presence of a pocket of absolute instability embedded within a convectively unstable jet, global oscillations are shown to be generated by a steep nonlinear front located at the upstream station of marginal absolute instability. The global frequency is given, within 10% accuracy, by the absolute frequency at the front location and, as expected on theoretical grounds, the front displays the same slope as a k--wave. For jet flows displaying absolutely unstable inlet conditions, global instability is observed to arise if the streamwise extent of the absolutely unstable region is sufficiently large: While local absolute instability sets in for ambient-to-jet temperature ratios S = 0.453, global modes only appear for S = 0.3125. In agreement with theoretical predictions, the selected frequency near the onset of global instability coincides with the absolute frequency at the inlet. For lower S, it gradually departs from this value. © 2006 Cambridge University Press.