Fluid-elastic instability of inverted cantilevered cylinders in axial flow

Abstract

This paper describes and analyzes water-tunnel experiments involving flexible cantilevered cylinders in reverse axial flow, i.e. flow directed from the free towards the fixed end of the cylinder. It was found that the cylinder is subjected to small amplitude motion due to turbulent buffeting prior to first-mode flutter at relatively low flow velocities, followed eventually by an abrupt static divergence at higher flows. The observed flutter was quite unsteady and sometimes near-intermittent. The flutter amplitude increased with flow, prior to the onset of static divergence. The onset of static divergence displayed strong hysteresis, suggesting a subcritical bifurcation. The static divergence amplitude for very flexible cylinders could be very large, such that the free end of the cylinder would face downstream. The influence of some system parameters was investigated, such as cylinder flexural rigidity, slenderness and free-end shape. The dynamics was found to be only marginally affected by the free-end shape, in sharp contrast to the dynamics of cylinders subjected to flow directed from the clamped towards the free end. Finally, the mechanisms underlying the onset of flutter and static divergence are discussed briefly.