SOUND GENERATED BY A VORTEX INTERACTING WITH A RIB-STIFFENED ELASTIC PLATE

An analytical investigation is made of the sound produced when a line vortex translates over a parallel, nominally smooth rib-stiffener on a thin elastic plate. This is a canonical fluid-structure interaction t hat is believed to be an important source of boundary layer generated aircraft interior noise. To eliminate additional complications introdu ced by mean flow, the vortex motion is assumed to be controlled by its image in the plate. The arrival of the vortex at the rib is preceded by bending wave forerunners which are excited in the plate at that fre quency Omega at which the flexural wave phase velocity is just equal t o the vortex translational velocity nu. These waves have a group veloc ity equal to 2 nu, and extend over a length of plate ahead of the vort ex determined by the structural damping. The principal acoustic source is the interaction of this forerunner with the stiffener, and the sou nd is predominantly of frequency Omega. The amplitude of the radiation progressively increases to a maximum as the vortex approaches the rib , and subsequently decreases rapidly to zero with passage of the vorte x over the rib. Numerical results for the efficiency of sound generati on, its directivity, and the acoustic pressure signature, are given fo r clamped and simply supported conditions at the stiffener for aluminu m plates in air. (C) 1996 Academic Press Limited