Flow structure in controlled and baseline subsonic cavity flows

This work presents preliminary results of an experimental investigation of the flow structure and acoustic signature in controlled and baseline subsonic cavity flows. Qualitative and quantitative data are presented and analyzed in an effort to better understand and predict the behavior of the shear layer over the cavity in response to various open-loop actuation cases. Experiments confirm that self-sustained flow-acoustic coupling occurs only at frequencies supporting an integer phase relationship around the flow-acoustic loop. Based on the results, we also speculate that only when the flow-acoustic components at frequencies supporting resonance are time-invariant can the corresponding coupling produces strong acoustic tones. Otherwise the corresponding flow-acoustic couplings produce multi-mode like resonance with tonal switching. This behavior is irrespective of the coherent structures passage frequency in the shear layer. This work is part of a larger multidisciplinary effort in the development and understanding of active feedback flow control techniques (Samimy et al. 2004).