It has been shown that internal transmission of wind noise is dependent on the external aerodynamic and acoustic excitation around the automobile. Flow over the A-pillar and side-view mirror induces strongly convecting turbulence and associated acoustics which excite the side-glass. A useful tool to understand and quantify these physics is to perform temporal Fourier analysis (auto-spectra) and spatial Fourier analysis (cross-spectra and wave-number decomposition).
This study demonstrates the uses of wave-number decomposition to quantify the mechanisms associated with turbulent convection and acoustical propagation. A CFD computation using the commercial codes STAR-CCM+ is performed for the flow over a generalized side-view mirror in a freestream of 38m/s. LES-enabled turbulence is solved in a fully compressible framework so as to capture all the local acoustical propagation well beyond 3kHz. Pressures are sampled in the wake flow immediately downstream of the mirror in the region representing the side-glass. The spectral intensity, presented as a function of wave-number and frequency, is a powerful illustration of the convective and acoustical mechanisms present on various zones in the wake region. Wave number decomposition is made available directly in the commercial software, which avoids the need for data translation, mapping and data processing scripting.
The outcome from this transient CFD-analysis, though not demonstrated here, can be used for vibration and internal noise transmission using Statistical Energy Analysis (SEA) or Finite Element Analysis (FE). An associated programme provides measurements of pressure spectral, vibration and transmitted noise data against which to validate the CFD.