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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 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 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.

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.

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).

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.

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.