Aero-acoustic response of flat plates at sub-sonic flow

Abstract

Natural aero-acoustic resonance of single and double flat plates was experimentally investigated. Ten single plates of six different chord-to-thickness ratios were tested for flow around single plate, and four different gap-to-thickness ratios were tested for flow around two plates in tandem. For the case of single plates, a tool for the prediction of resonant flow velocities was established using reduced velocity. For tandem plates, the acoustic response was characterized by the presence of double resonance response that is the excitation of first acoustic mode in two different ranges of flow velocities. Reduced velocity range of low-frequency resonance fits to the range of Mode1 in single plates, this is because both resonances are natural. For high-velocity resonance, a common range could hardly be identified although the increase of gap distance showed an increase in the onset reduced velocity at resonance. It is hypothesized that the first resonance of the double response is caused by normal shedding of vortices in the gap between the plates, whereas the latter resonance is due to vortices caused by the interaction of shear layers with the downstream plate. This mechanism is different than that present in case of tandem cylinders due to the difference in separation points. It was also shown that the downstream plate can serve as a duct modification that leads to the excitation of new acoustic modes that were not excited in the case of single plates.