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

Abstract

Asymptotic formulations are presented for describing radiated acoustic pressures from two semi-infinite dissimilar plates subject to a line force excitation at the joint in mean flow. Analysis shows that mean flow affects acoustic radiation in three ways: (1) it enhances the wave number and amplitude of an acoustic wave propagating in the upstream direction, while suppresses those of an acoustic wave propagating in the downstream direction; (2) it reduces the decay rate of the upstream propagating wave in the plate normal direction, while increases that of the downstream propagating wave; and (3) it rotates 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 reduced significantly below the coincidence frequency. The joint condition does not change the characteristics of the radiation patterns, but merely their amplitudes. The looser the joint, the higher the amplitude of the resulting acoustic pressure. The asymptotic results agree very well with those obtained by direct numerical integrations in the far field. Numerical results for the evanescent, traveling, and radiated waves in the near field from two dissimilar plates under welded, hinged, and mechanically unconnected joints are also demonstrated. [Work supported by ONR.]