Production of structural and acoustic waves by dipole sources adjacent to an elastic plate with a pressure-release coating

Abstract

Model problems are examined to estimate the influence of a pressure-release coating on an elastic plate on the generation of sound and vibration by hydroacoustic dipole sources. Predictions for this idealized case provide useful first approximations to the behavior of real systems in which the fluid loading is large. The dipole and plate may be assumed to characterize, respectively, the unsteady lift exerted on a blade of a ducted rotor or stator during interaction with vorticity (occurring either naturally in a turbulent stream or as discrete vortices shed from structural appendages), and the adjacent duct wall. Such longitudinal dipoles (oriented parallel to the duct axis) produce equal and opposite images in a pressure-release coating, and the efficiency of sound generation is reduced to that of a quadrupole source when the dipole is situated well within an acoustic wavelength of the wall. In practice, considerations of cost and weight require that only a finite section of the duct wall be coated. In this paper, the dependence of the structural and acoustic noise produced by a dipole on its distance from an edge of the coated region, and on its normal distance from the coating, is determined.