Wake states of a tethered cylinder

Abstract

This paper describes an experimental investigation of a buoyant,m*<1, tethered cylinder which is free to move in an arc about its pivot points. The response of the cylinder, in particular its layover angle and flow-induced motion, is considered for a range of flow velocities and mass ratios. At pertinent parameters, the flow fields were also measured using particle image velocimetry (PIV). At lower mass ratios, 0.54≤m*≤0.72, two distinct states are observed, the low-amplitude and upper states. The transition from the low-amplitude state to the upper state is characterized by abrupt jumps in the amplitude of oscillation, the mean tether angle and the drag coefficient as well as distinct changes in the cylinder's wake. At higher mass ratios, the jump does not occur; however, asm* approaches unity at low flow velocities the cylinder's motion is more periodic than that observed at lowerm*. The flow fields indicate that the low-amplitude state exhibits a 2S Kármán wake. The wake of the upper state has long shear layers extending well across the wake centreline, is not fully symmetric and is often consistent with either the 2P or P+S shedding modes. There is a collapse of the response data, in particular an excellent collapse of the mean layover angle, when the response parameters are plotted against the buoyancy Froude number,Frbuoyancy=U/((1-m*)gD)0.5. When the data collapses, the two states described above are clearly delineated.