THE OCCURRENCE OF LOCK-IN UNDER HIGHLY SHEARED CONDITIONS

Abstract Experimental results are presented which show that two dimensionless parameters may be used to predict the likelihood of lock-in under sheared flow conditions. One parameter isNs,the number of potentially responding modes within the vortex-shedding frequency bandwidth generated by the sheared flow. The second,ΔV/Vavr, called the shear parameter in this paper, is the ratio of the change in velocity of the flow over the length of the cylinder to the spatially averaged flow velocity over the length of the cylinder. Unexpected results are shown which reveal that lock-in may occur under highly sheared conditions. The probable cause of the lock-in is that, under highly sheared conditions, the power available to one particular mode may dominate all other modes. This condition is less likely to develop in flows with lower velocity gradients.