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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 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 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.

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.

This paper addresses the numerical simulation of the acoustic scattering and propagation through non-uniform mean flows using a high-order, finite-difference, full-conservative Euler solver and, more precisely, the specific problems raised by multi-block structured grids when complex geometries are considered. Two examples of two-dimensional computations of this type are presented. The first case, a benchmark problem from the 4 th CAA Workshop, is the simulation of the acoustic scattering from multiple rigid circular cylinders, without mean flow, of the sound generated by a spatially distributed, axisymmetric source. The proposed solution uses conformal multi-domain grids associating body-fitted grids near solid walls and quasicartesian grids elsewhere. The solutions are favorably compared to analytical data. The second example is the simulation of the acoustic scattering from a high-lift wing section with deployed slat and flap, from a source located in the slat cove region. An acoustic grid is derived from a CFD grid previously used to compute the non-uniform viscous mean flow around the wing. A first acoustic computation, performed with the fluid at rest, is favorably compared to a Boundary Element Method solution. Then a second computation is performed on the non-uniform mean flow. The comparison of both solutions highlights the influence of the flow on the directivity diagram of the sound field.