Analysis of the interior pressure oscillations induced by flow over vehicle openings

An analytical model based on "vortex sound" theory was investigated for predicting the frequency, relative magnitude, onset, and termination of self-sustained interior pressure oscillations inside a vehicle with an open sunroof. The phenomenon was observed to be caused by a flow-excited Helmholtz resonance of the vehicle, A describing function model was developed for the flow-excitation mechanism, governed by the shedding of discrete vortices within the shear layer over the orifice, The key parameters are the cavity volume, the orifice dimensions, the how velocity, and a coefficient characterizing vortex concentration, Analytical predictions were compared with experimental results obtained for a system geometry that approximated a one-fifth scale model of a typical vehicle passenger compartment with a rectangular, open sunroof. Predicted and observed frequencies and relative interior pressure levels were in good agreement around the "critical" velocity, at which the cavity response is near resonance, The model was then applied to investigate optimal sunroof length and width for a mid-size sedan, Tradeoffs between the orifice dimensions and the critical velocity indicated that it may be difficult to avoid self-sustained oscillations using geometrical optimization alone, without the use of suppression devices such as leading edge spoilers. (C) 1997 Institute of Noise Control Engineering.