Vortex-excited unsteady forces on resonantly vibrating, bluff structures

This report considers recent experimental findings pertaining to the vortex-excited oscillations of bluff cylinders, and compares various approaches to measuring the resultant fluid forces. As one example, measurements of the structural damping and crossflow response amplitudes were made for several systems over a representative range of wind tunnel flow speeds. The fluid dynamic damping or reaction in-phase with the cylinder's velocity also was measured for different incident flow speeds over a range which included the locking-on of the vortex shedding to the resonant cylinder vibrations. A mathematical model is proposed in connection with these measurements, and the components of the excitation or lift force on a vibrating structure can be obtained from the model once the structural damping and fluid reaction forces are known. In a second approach, the fluid forces were measured on a cylinder which was forced to oscillate in water at different frequencies and crossflow amplitudes over a range of incident flow speeds. Both the steady and unsteady forces are altered significantly in the locking-on regime, with large amplifications in both the lift and drag. These vortex-excited forces are then incorporated into the mathematical model for self-excited, resonant vibrations and the complementary nature of the two measurement schemes is discussed. The results obtained from the present study are suitable in general for application not only to the vortex-excited vibrations of rigid cylinders, but also to other flexible bluff structures and cables in air and in water.