Experimental investigation of the effect of a downstream square plate on vortex-induced vibration and galloping of a square cylinder

Abstract

A square cylinder exposed to a fluid flow is regarded as the simplest model of the structures in sea water or air flow, and known to show the Karman vortex induced vibration (KVIV) and the galloping in separate or overlapping ranges of flow velocity. In this study, wind tunnel experiments were performed on the cross-flow vibration of a square cylinder, and effects of a downstream square plate on the vibration were investigated to develop a convenient passive technique for vibration control. The square plate has a side length w equal to the side length of the square cylinder D (= 26 mm) and set downstream with the gap ratio S/D in the range of 0.12 to 2.90 where S is the separation gap. The results show that the square plate has only little influence on KVIV and enhances the galloping, resulting in the lower critical reduced velocity of galloping Ug i.e. the onset velocity of galloping reduced to about half of the isolated counterpart at S/D = 0.98. Most remarkable effect of the downstream square plate is the enhancement of the vibration near the peak velocity of KVIV at S/D = 1.17, which is around five times larger as compared with the isolated counterpart. This may indicate occurrence of the wake-body-interference fluid elastic vibration.