INFLUENCE OF SEPARATION ON SOUND GENERATED BY VORTEX-STEP INTERACTION

An analysis of the sound produced when a line vortex interacts at low Mach number with forward or backward facing steps is made. The radiati on is dominated by an aeroacoustic dipole whose strength is equal to t he unsteady drag on the step. The drag is determined by the vorticity distribution, and a correct estimate of the sound must therefore inclu de contributions from vorticity in the separated flow induced by the v ortex. The separation is modelled by assuming that the shed vorticity rolls up into a concentrated core, fed by a connecting sheet from the edge of the step of negligible circulation. The motion everywhere is i rrotational except at the impinging vortex and the separation core, an d the trajectory of the core is governed by an emended Brown & Michael equation. For large steps it is found that estimates of the generated sound that neglect separation are typically an order of magnitude too large. The sound levels predicted for small steps with and without se paration are of comparable magnitudes, although the respective phases are different. Turbulent flow over a step frequently involves separati on and large surface pressure fluctuations at reattachment zones. The results of this paper suggest that numerical schemes for determining t he noise generated by turbulent flow over a step must take proper acco unt of ''forcing'' of the separation region by the impinging turbulenc e and of vorticity production via the no-slip condition. (C) 1997 Acad emic Press Limited.