Experimental Study on Vortex Shedding and Sound Radiation from a Rectangular Cylinder at Low Mach Numbers

Abstract

Sound radiation from a rectangular cylinder, the cross-sectional aspect ratio (AR) of which varied from 0.3 to 4, was studied experimentally at Reynolds numbers of 7.5 × 103 to 1.4 × 105 and Mach numbers below 0.16. In addition to measurements of the lift-dipole sound, detailed flow fields around the rectangular cylinder were examined by means of PIV to better understand the dependence of sound radiation on the AR and Reynolds number as well as the vortex-cylinder interaction associated with sound generation. The mean square sound pressure was proportional to U6∞ St2, with almost the same factor of proportionality for aspect ratios larger than 0.5 in the Reynolds number range examined in spite of the fact that the spanwise correlation length and magnitude of the lift fluctuations were highly dependent on the AR. When vortex shedding at the trailing-edge was locked in phase with the leading-edge-generated vortices, which occurred for AR > 3, the shed vortices were quite two-dimensional, and correspondingly, the radiated sound exhibited a very sharp spectral peak like a line spectrum; the SPL was as strong as that for AR = 0.75, for which the maximum SPL was observed. Reynolds number effects on both SPL and the Strouhal number were weak except around the aspect ratio where reattachment of the separated leading-edge shear layer occurred.