Aeroacoustic response of a slit-shaped diaphragm in a pipe at low Helmholtz number, 2: Unsteady results

In a companion paper the quasi-steady aeroacoustic behavior of diaphragms w hich prevails for low Strouhal number but Mach number of order unity was di scussed. The complementary case of a small Mach number and a Strouhal numbe r of order unity is now considered. In this case, a two-dimensional numeric al method is used to predict the Strouhal-number dependence of the coeffici ents of the scattering matrix. The numerical method solves the two-dimensio nal incompressible flow equations by means of the vortex-blob method. The q uality of the numerical results is investigated by comparing results of How e's energy formulation with results obtained by an integral formulation to calculate the pressure difference across the diaphragm. The predictions of the coefficients are compared with experimental measurements carried out at LAUM. It is shown that the low-and high-frequency behavior are quite well predicted, while for intermediate frequencies a deviation between numerical simulations and experimental measurements is observed. While this is not y et fully understood it is expected to be related to whistling induced by th e finite thickness of the diaphragm in the experiments. (C) 2001 Academic P ress.