Passive Control of Buffet Excitation

Abstract

It has recently been shown by Heenan and Morrison(1998a, b) that buffet due to separated flow downstream of a backward-facing step can be attenuated more or less completely by use of a permeable reattachment surface vented to an airtight plenum. With an impermeable surface, the global unsteadiness is driven by the feedback from the reattachment point of pressure and vorticity fluctuations towards the step. Their work also suggests that a permeable surface immediately downstream of separation also affects the pressure field. In this work therefore, the intention is to use a permeable surface to reduce buffet excitation of a fully separated flow, thereby reducing buffeting, the structural response. Buffeting is unlikely to be a serious problem with primary structures on aircraft. However, it produces deleterious effects on flaps with potentially serious problems caused by fatigue. The present work is not aimed at the extension of the buffet boundary, but rather seeks to make buffet onset less damaging to secondary structures. The initial objective of this project is to establish proof of concept, showing that a permeable surface can reduce the buffeting of a wing excited in the first bending mode. Initial positive results have been obtained and further, initial, results from a “segmented” model described below are also presented.