Author(s): P. Liu; K. M. Lam
Keywords: Vortices; Oscillating cylinder; PIV; Keulegan-Carpenter number
Abstract: This paper investigates the vortex pattern around a circular cylinder given sinusoidal oscillation in the presence of a slow flow stream in the direction perpendicular to the line of cylinder motion. The objective is to study how the flow stream affects the development of the vortices shed from the oscillating cylinder. Hundreds of instantaneous velocity fields are obtained in one cylinder oscillation cycle with time-resolved particle-image velocimetry and over 30 cycles. The data are analysed with the phase-locked averaging technique to investigate the periodic formation and development stages of vortices at successive phases of cylinder oscillation. It is found that the number of vortices formed every cycle and the vortex patterns in the wake basically follow the same modes as a circular cylinder oscillating in otherwise quiescent water in which the non-dimensional oscillation amplitude is the governing parameter. The presence of a uniform lateral flow acts to convect the shed vortices downstream. This leads to the convection of a single vortex to form the P+S vortex mode at relatively smaller cylinder oscillation amplitudes. At larger amplitudes, when the uniform lateral flow brings the vortices formed on the upstream side of the line of cylinder motion across the line, interaction occurs between the vortices and the moving cylinder. This leads to disintegration of the vortices and disappearance of their signatures in the wake. Vortex patterns of one vortex street or double vortex streets are observed at larger cylinder oscillation amplitudes.