Strouhal number for boundary shear flow past a circular cylinder in the subcritical flow regime

Strouhal number (St) is an essential dimensionless quantity for characterizing the vortex-shedding frequency of the flow past a cylinder. In this study, the Strouhal number for a near-wall circular cylinder fully immersed in a boundary shear flow in the subcritical flow regime was experimentally investigated in a water flume. The velocity fluctuations and flow fields of the lee-wake were systematically measured with an Acoustic Doppler Velocimeter (ADV) and an upward-illumination Particle Image Velocimetry (PIV) system, respectively. Based on dimensional analyses, a non-dimensional shear parameter (K) is introduced to characterize the shear flow. An explicit expression between K and e/D is obtained on the basis of the boundary theory and verified with the flume measurements. Spectral analysis of wake velocities indicates that the multi-peaks can be identified as the cylinder gets closer to the wall. The PIV measurements indicate that the multi-peaks spectra are predominantly induced by the intense interactions between the lower shear layer and wall boundary layer. By fitting the experimental results, empirical relationships for the variations of St with K or e/D are finally established for the subcritical flow regime.