Experimental study of vortex-induced vibrations of a tethered cylinder

Abstract

This paper presents an experimental study of the motions, forces and flow patterns of a positively buoyant tethered cylinder (m⁎<1) in uniform flow undergoing vortex-induced vibration (VIV). The flow fields have been measured using digital Particle Image Velocimetry (PIV) technique, in conjunction with a piezoelectric load cell for direct measurement of drag and lift forces acting on the tethered cylinder. The effects of varying mass ratio and Reynolds number over the range 0.61≤m⁎≤0.92 and 4000≤Re≤12 000 are examined. Results of a fixed (or stationary) cylinder at the same Reynolds numbers are provided to serve as the benchmark reference. The peak amplitude of oscillation, θmax/θD, generally increases with Re and deceases with m⁎. Similar to previous studies, the results reveal the existence of a critical mass ratio mcrit⁎≈0.7, below which large-amplitude oscillations would take place when Re is high enough, with the largest peak amplitude of θmax/θD=0.9 observed for the case of m⁎=0.61 and Re=12 000. Thus two distinct states of oscillation are categorized, namely, the small- and large-amplitude oscillation states. The distinction between the two states is also vivid in the mean and root-mean-square (r.m.s.) force coefficients (including C¯D, CD′ and CL′). The frequency of vortex shedding (fV) from the tethered cylinder is always synchronized with the cylinder's oscillation frequency (fosc), regardless of the oscillation state. A time series of instantaneous vorticity fields illustrate that vortex shedding from the tethered cylinder undergoing VIV maintains the 2S mode, but at an inclined angle to the free stream, which is most obvious in the large-amplitude oscillation state. This leads to an asymmetry in the shear layers separated from opposite sides of the cylinder, as shown by the distribution of ensemble-averaged Reynolds stress.