Compressible CFD simulations of aeroacoustics for automotive applications

Abstract

In this work, a direct noise computation method based on a low-Mach number flow solver is investigated. The new solver is implemented in the finite volume framework of the software OpenFOAM, accompanied with a new acoustic damping model for reducing spurious noise. The new solver is utilised to calculate noise generation and propagation for automotive applications. In order to validate the applicability of the low-Mach flow solver, a benchmark consisting of two-struts is calculated. The simulated aerodynamic near field as well as aeroacoustic far field are compared to wind tunnel measurements. The acoustic far field is computed using the direct method as well as a hybrid method. Both methods are evaluated based on comparing far field spectra and directivity patterns with experimental results. After validating the applicability of the low-Mach number solver, the topic of spurious noise generation in direct noise computation is addressed. Different spurious noise sources are presented and their generation mechanisms are investigated. Afterwards, two different strategies for spurious noise reduction, namely selective acoustic damping and numerical grid stretching, are discussed and validated. The acoustic damping model can substantially damp out spurious noise generated at grid interfaces without affecting the turbulence. It is also observed that the direction of grid refinement determines the direction of propagation of spurious noise. The strategies for spurious noise reduction are then applied on a side-mirror test case. For this, a new algorithm for automated and directional grid stretching is implemented. Spurious noise generation in the vicinity of the mirror’s surface as well as in the mirror’s wake could be substantially reduced and a quantitative analysis based on frequency-wavenumber spectra in the wake of the mirror is performed. Finally, the proposed flow solver, along with the strategies for spurious noise reduction, is used to directly compute noise generation on a generic vehicle model. Two different variants are calculated and the effect of the A-pillar and the side-mirror regarding their contribution to the acoustic waves on the side-window is investigated. Aerodynamic as well as aero- and vibroacoustic spectra on the side-window are calculated and compared to wind-tunnel measurements. For both variants, the results calculated using the direct method show good agreement with experimental data.