Computational Aeroacoustics Investigation of Automobile Sunroof Buffeting

A numerical investigation of automobile sunroof buffeting on a prototype sport utility vehicle (SUV) is presented, including experimental validation. Buffeting is an unpleasant low frequency booming caused by flow-excited Helmholtz resonance of the interior cabin. Accurate prediction of this phenomenon requires accounting for the bi-directional coupling between the transient shear layer aerodynamics (vortex shedding) and the acoustic response of the cabin. Numerical simulations were performed using the PowerFLOW code, a CFD/CAA software package from Exa Corporation based on the Lattice Boltzmann Method (LBM). The well established LBM approach provides the time-dependent solution to the compressible Navier-Stokes equations, and directly captures both turbulent and acoustic pressure fluctuations over a wide range of scales given adequate computational grid resolution. Here a case study is presented on two configurations of a real car: sunroof fully open (baseline) and addition of a deflector at the leading edge. Experimental versus predicted peak cabin sound pressure level (SPL) as a function of wind speed are compared, and the buffeting onset, peak, and offset velocity ranges are seen to be accurately predicted. Analysis and visualization of the unsteady shear layer dynamics is presented, for both buffeting and non-buffeting conditions. Changes in buffeting behavior for the different configurations are explored. It is shown that the deflector eliminates the buffeting at the peak buffeting velocity (30 mph). At 20 mph the deflector causes only a weak effect on the vortex dynamics relative to the baseline. The intended shear layer deflection appears to be thwarted by suction of the primary vortex into the opening. Further refinements (not discussed in this study) of the sunroof design for this vehicle completely removed the buffeting.