A rapid and low noise switch from RANS to WMLES on curvilinear grids with compressible flow solvers

Abstract

A turbulent inflow for a rapid and low noise switch from RANS to Wall-Modelled LES on curvilinear grids with compressible flow solvers is presented. It can be embedded within the computational domain in practical applications with WMLES grids around three-dimensional geometries in a flexible zonal hybrid RANS/LES modelling context. It relies on a physics-motivated combination of Zonal Detached Eddy Simulation (ZDES) as the WMLES technique together with a Dynamic Forcing method processing the fluctuations caused by a Zonal Immersed Boundary Condition describing roughness elements. The performance in generating a physically-sound turbulent flow field with the proper mean skin friction and turbulent profiles after a short relaxation length is equivalent to more common inflow methods thanks to the generation of large-scale streamwise vorticity by the roughness elements. Comparisons in a low Mach-number zeropressure-gradient flat-plate turbulent boundary layer up to Re θ = 6 100 reveal that the pressure field is dominated by the spurious noise caused by the synthetic turbulence methods (Synthetic Eddy Method and White Noise injection), contrary to the new low-noise approach which may be used to obtain the low-frequency component of wall pressure and reproduce its intermittent nature. The robustness of the method is tested in the flow around a three-element airfoil with WMLES in the upper boundary layer near the trailing edge of the main element. In spite of the very short relaxation distance allowed, self-sustainable resolved turbulence is generated in the outer layer with significantly less spurious noise than with the approach involving White Noise. The ZDES grid count for this latter test case is more than two orders of magnitude lower than the Wall-Resolved LES requirement and a unique mesh is involved, which is much simpler than some multiple-mesh strategies devised for WMLES or turbulent inflow.

Improvement of jet flow simulations using ZDES mode 3 and silent turbulence generation

“…Emphasis will also be put on the approach used to initiate turbulent fluctuations in the boundary layer for the ZDES mode 3 simulation, which is a key issue for aeroacoustic simulations. A recent approach published in Ref [9] is used in the present study. Its setup will be detailed.…”

“…The numerical setup for the ZDES mode 3 simulation involves the zonal definition of the turbulence modelling applied in the nozzle, as depicted in Figure 2. The resolved turbulent content is triggered using the IBC technique following the guidelines suggested in [9]. The tripping dots included in the nozzle with IBC are shown in Figure 3.…”

On the WMLES use of ZDES in compressible flows

“…A low Mach number (M ∞ = 0.21) flat-plate zero-pressure-gradient turbulent boundary layer case is simulated by the ZDES mode 3 technique in WMLES mode with the RANS/LES interface set at d interface w /δ = 0.125. The new approach of resolved turbulence generation with a low-noise signature recently introduced by Deck et al [1] is compared with more classical approaches, namely based on the Synthetic Eddy Method [17] and on White Noise injection [11,18,19]. The new approach relies on roughness elements described as immersed boundaries (ZIBC, Zonal Immersed Boundary Conditions as described in [20]) whose length scales are selected in order to disturb the boundary layer profile and generate large scale vorticity.…”

“…The investigation and validation of ZDES mode 3 [4][5][6]10] and of the turbulent inflow conditions required for such WMLES [1,11,12] has been mainly focused on subsonic boundary layers at high Reynolds numbers. Compressibility effects that might affect ZDES mode 3, for instance in applications at higher Mach numbers or in aeroacoustic studies, have been addressed in a limited number of studies so far.…”

“…The interaction of the resolved turbulent boundary layer with a shock wave is then considered. Finally, the new strategy by Deck et al [1] to reduce the spurious acoustic footprint of the resolved turbulence generation needed for a WMLES is discussed.…”