Sound radiation from two semi-infinite dissimilar plates subject to a harmonic line force excitation in mean flow. II. Asymptotics and numerical results

Abstract

Asymptotic formulations are derived for describing far-field acoustic radiation from two semi-infinite dissimilar plates subject to an harmonic line force excitation at the joint in the presence of mean flow. Analysis shows that mean flow affects the acoustic pressure in three ways: (1) It enhances the wave number and amplitude of an acoustic wave propagating in the upstream direction, while it suppresses those of an acoustic wave propagating in the downstream direction; (2) it reduces the decay rate of the upstream propagating wave, while it increases that of the downstream propagating wave as they propagate in the plate normal direction; and (3) it rotates the radiation beam angles toward the downstream direction. The effects of mean flow are obvious when the excitation frequency is above the plate coincidence frequency, but decays significantly at low frequencies. The condition at the joint of two plates does not change the characteristics of the radiation pattern, but merely the amplitude of the radiated acoustic pressure. The looser the joint, for example, two plates being mechanically unconnected, the higher the amplitude of the resulting acoustic pressure.