The influence of taper ratio on vortex-induced vibration of tapered cylinders in the crossflow direction

Abstract

A series of experiments were conducted to study vortex-induced vibration of tapered cylinders free to oscillate in the crossflow direction. These experiments were performed in a re-circulating water tunnel with an open-surface test section for a Reynolds number range of Re=370–2300. Three linearly tapered cylinders with taper ratios of 29:1, 17:1 and 10:1 and a cylinder with a nonlinear taper were studied. For all tests, the mass ratio was kept constant. The maximum amplitude of the crossflow oscillations stayed almost constant for all taper ratios, at a value close to the maximum amplitude of a uniform cylinder with the same mass ratio. As the taper ratio was decreased, the lock-in range started at larger reduced velocities and sustained through higher reduced velocities. Tests were conducted for cylinders placed vertically in the test-section of the water tunnel, both with their diameter increasing downward and upward to show that the results were independent from the cylinder׳s orientation. Hybrid shedding modes (both 2S and 2P shedding over the cylinder׳s length) were observed for all the linearly tapered cases for reduced velocities up to the value corresponding to the peak amplitude of oscillations. For higher reduced velocities, only 2P shedding was observed along the length.