Influence Of Span-Wise Coherence In A Cylinder Wake On The Acoustic Radiations

In the present work the influence of the spanwise coherence of the flow structures on the acoustic emission in the case of cylinder wakes is investigated. For this purpose, the correlation between pressure fluctuations in the far field and velocity fluctuations in the wake of the cylinders of different shapes is determined experimentally by means of the so called causality correlation. 2D-2C as well as 2D-3C PIV is applied to determine the velocity fluctuations. The pressure fluctuations in the far field are measured synchronously with microphones. From the temporal and spatial distribution of the correlation results, conclusions are drawn about the dependence between the coherence of the vortex structures convecting downstream of the cylinder and the tonal components in the acoustic signature. Most interestingly, in case of a tapered cylinder geometry the temporal evolution of the correlation function shows an asymmetry, which has not been observed in previous results.