Vortex-induced oscillation of bluff cylinders

A bluff cylinder elastically supported in a smooth flow may often undergo harmonic oscillation within limited ranges of the flow velocity. Wind-tunnel tests have been conducted using two types of cylinders, having separation points at the trailing or the leading edge. In the tests, two kinds of vortex-induced oscillation were found. One was a restricted oscillation having small amplitude and caused by Karman vortex-shedding, which was found in cylinders having a separation point at the trailing edge. The other, having relatively large amplitude, was found in cylinders having a separation point at the leading edge, and occurred independently of the Karman vortex. The aim of the present paper is to give an original description of the generating mechanism in the latter case. For this purpose, measurements of surface pressure were carried out in addition to flow visualization. From the results, it was concluded that a fluctuating pressure was produced due to the vortex shed from the leading edge in response to the motion, and that this pressure could be a major cause of this type of vortex-induced oscillation.