Flow-sound interaction mechanism of a single finned cylinder in cross-flow
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The flow-sound interaction mechanism of a single finned cylinder in cross-flow is experimentally investigated and compared to that resulting from an equivalent bare cylinder. The flow field changes that result from the fins are captured by flow measurements and numerical simulations in order to be correlated to the observed acoustic resonance excitation. It is observed that the finned cylinders experience an earlier acoustic resonance and higher levels of acoustic pressure compared to their equivalent bare cylinders. This suggests that adding fins to the cylinder changes the flow field in a manner that makes it more susceptible to acoustic excitation. Flow measurements show that the vortex shedding process becomes stronger in the case of finned cylinders and the spanwise flow coherence is enhanced. The stronger vortex shedding process and coherent flow field promote the occurrence of the acoustic resonance and increase the observed noise levels. The acoustic resonance excitation is found to be significantly dependent on the fin spacing and thickness, which is explained by the impact of these fin parameters on the wake structures. Numerical simulations of the flow field downstream of the finned cylinders show that the fins lead to an organized flow structure, which is attained by concentrating the flow energy in the large-scale vortices and reducing the small-scale hair-pin vortices. This leads to the reduction of the vortex formation region in the near-wake of the cylinder, which subsequently increases the dynamic loading on the cylinder. The effect of the cylinder's aspect ratio on the acoustic resonance excitation is also investigated using the same analysis. It is shown that the finned cylinder provides a coherent flow field that is not affected by the aspect ratio, and hence, its acoustic resonance excitation levels increase with the increase of the cylinder's length. On the other hand, the bare cylinder exhibits a significant stretching in the vortex formation region with the increase in the aspect ratio. Therefore, the acoustic resonance excitation levels for the bare cylinder decrease with the increase of aspect ratio.