Sound radiation from noncompact vibrating surfaces in motion.

Abstract

This paper presents an integral formulation for predicting sound radiation from noncompact vibrating surfaces in motion, with the effect of turbulence induced by source convection taken into account. The fluid density fluctuation is shown to be expressible in terms of volume and surface integrals. The volume integral represents the turbulence effect, while the surface integral describes the effect of the unsteady surface force and the surface displacement effect. For a class of special cases in which the Mach number M is nearly constant across the source region such as in a rectilinear motion, the ratio of the contribution of the volume integral to that of the surface integral is proportional to M/(1−MR), where MR is the component of the Mach number in the direction of wave propagating from the source to receiver. Dimensional analyses show that the turbulence effect dominates the sound radiation at high Mach numbers, especially as the Mach number approaches unity. At low Mach numbers, however, the turbulence effect is negligible and the sound field can be determined based on knowledge of acoustic quantities specified on the surface. Numerical examples of sound radiation from transversely and radially oscillating spheres moving rectilinearly at constant subsonic speeds are demonstrated. The acoustic pressures are shown to be greatly enhanced in the forward direction, while suppressed in the reverse direction. The modification of the acoustic pressure distribution also results in sound radiation in the perpendicular direction. [Work supported in part by ONR, NSF, and IMR of Wayne State Univ.]