Unsteady separated flows and acoustic feedback effects-prediction and observation

Results are presented of computational and experimental studies being conducted at the CSIRO to investigate the vortex structures shed from plates and aerofoils of various shapes under the control of either externally applied sound or flow-induced acoustic resonances. For strongly separated flows, numerical vortex methods are used to predict the large-scale vortex structures that are shed from bluff bodies and, together with the solution of the acoustic field, predict the modification due to the feedback of the acoustic field on the separating flow. Experimentally, smoke visualisation, hot wire anemometry, microphone and LDA measurements are carried out in wind tunnels to investigate the large-scale structuresshed from bluff bodies and the excitation of a transverse acousticresonant mode. Visualisation using hydrogen bubbles in a water tunnel with oscillating side walls shows vortex shedding from plates affected by a transverse perturbation simulating the acoustic mode excited in the wind tunnel. It is possible that low levels of acoustic feedback, which produce significant modification of flow, can remain undetected by the investigator, leading to erroneous comparisons between predictions and experiments.