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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 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.