Design of Roof-Rack Crossbars for Production Automobiles to Reduce Howl Noise using a Lattice Boltzmann Scheme

A computational design study, performed in conjunction with experiments, to reduce the howl noise caused by the roof rack crossbars of a production automobile is presented. This goals were to obtain insight into the flow phenomenon causing the noise, and to do a design iteration study that would lead to a small number of cross-section recommendations for crossbars that would be tested in the wind tunnel. The flow condition for this study is 0 yaw at 30 mph inlet speed, which experimentally gives the strongest roof rack howl for the vehicle considered for this study. The numerical results have been obtained using the commercial CFD/CAA software PowerFLOW. The simulation kernel of this software is based on the numerical scheme known as the Lattice Boltzmann Method (LBM), combined with a two-equation RNG turbulence model. This scheme accurately captures time-dependent aerodynamic behavior of turbulent flows over complex detailed geometries, including the pressure fluctuations causing wind noise. Analysis was performed on twelve different configurations of crossbars and designs were ranked based on the peak of the lift spectrum on the front crossbar. The number of configurations tested in the wind tunnel was narrowed down based on these results, and trend predictions obtained from the computational analysis were consistent with experimental observations. The methodology used in this study can help streamline this type of design process.