Study of the Universal Strouhal Number over the Wide Reynolds Number Flow Range : Effect of Surface Roughness

Abstract

Universality of the Strouhal number in the wake flow of a cylinder over a wide range of Reynolds number, i.e., 5×10⁴<RC<10⁷, was considered for this paper. Eight cylinders with various surface roughnesses were used for the measurement. The universal Strouhal numbers considered were Roshko's number, Bearman's number, Griffin's number and what took the measured values of the width of wake streets for the reference length. These universal Strouhal numbers were calculated using the measured values of vortex frequencies, surface pressure distributions and drags. Comparisons were then made. It was made clear that Bearman's number was the most uniform of all the universal Strouhal numbers considered. It maintained a uniform value of 0.18 throughout the Reynolds number range. The reason for the nonuniformity of other Strouhal numbers was also considered.