Vortex-Induced Vibration and Drag Coefficients of Long Cables Subjected to Sheared Flows

Abstract

The vortex-induced vibration response of long cables subjected to vertically sheared flow was investigated in two field experiments. In a typical experiment, a weight was hung over the side of the research vessel by a cable that was instrumented with accelerometers. A typical experiment measured the acceleration response of the cable, the current profile, the tension, and angle of inclination at the top of the cable. Total drag force was computed from the tension and angle measurements. Two braided Kevlar cables were tested at various lengths from 100 to 9,050 ft. As a result of these experiments, several important conclusions can be drawn: (i ) the wave propagation along the length of the cable was damped, and therefore, under most conditions the cable behaved like an infinite string; (ii ) response spectra were quite broad-band, with center frequencies determined by the flow speed in the region of the accelerometer; (iii ) single mode lock-in was not observed for long cables in the sheared current profile; (iv ) the average drag coefficient of long cables subjected to sheared flow was considerably lower than observed on short cables in uniform flows; (v ) the r.m.s. response was higher in regions of higher current speed. A new dimensionless parameter is proposed that incorporates the properties of the cable as well as the sheared flow. This parameter is useful in establishing the likelihood that lock-in may occur, as well as in estimating the number of modes likely to respond.