Sound radiation into uniformly flowing fluid by compact surface vibration
Abstract
This paper describes a model problem where compact surface vibration radiates sound into a subsonically flowing fluid. There are two distinct acoustic effects. First, the radiation is increased by flow by an amount proportional to 5 times the square of the Mach number and that increase is shown by a general argument to arise from an enhanced surface damping and work done by the flow to overcome drag in the ratio 2:1. Second, the acoustic source strength is affected and resonance frequencies are significantly modified by flow. The main effect is that flow induces on the surface a force proportional to the displacement which opposes the action of natural surface elasticity. A critical velocity exists beyond which the surface is unstable; the stability limit is determined. The surface motion might be regarded as an acoustic monopole, but since aerodynamic fields are determined by the rate of change of the rate of mass outflow, the frequency dependence is more that of a quadrupole.
 Publication:

Journal of Fluid Mechanics
 Pub Date:
 October 1975
 Bibcode:
 1975JFM....71..689F
 Keywords:

 Piston Theory;
 Sound Propagation;
 Surface Properties;
 Vibration Effects;
 Inviscid Flow;
 Mathematical Models;
 Noise Generators;
 Parallel Flow;
 Structural Vibration;
 Subsonic Flow;
 Acoustics