This study deals with some of the non-linear phenomena of the vortex-excited vibration of a circular cylinder in waves. Laboratory experiments have been performed to study the dynamic transverse response of a vertical circular cylinder in regular waves. The test cylinder was pivoted at its base and supported flexibly by spring at its top. The movement of the test cylinder in the direction of the inline force was restricted. The most remarkable result is the appearance of two types of peak in the amplitude of the vortex-excited vibration of the cylinder produced by perfect resonance coupled with waves and by vortex coupling. In the case of steady flow, perfect resonance appears in the range of locked-in, but in the case of wave, it appears only near to the ratio of wave frequency fw to the natural frequency fnw of the cylinder in still water fw/fnw= 1/2,1/3,1/4,…. (multi-appearance), elsewhere vortex coupling may occur for light damping in which the oscillation frequency is not simple a multiple of wave frequency.

1. Introduction

The dynamic response of offshore structures to the wave forces is one of the most important factors in their design. There have been many instances reported where the failure of a structure is believed to have been caused by its response at frequencies of wave forces acting on it. Wave forces acting on a vertical cylinder in waves are usually resolved into two components, inline force and a lift force. The in-line force acting in the direction of wave propagation is usually expressed by using Morison equation, and acts predominantly at a frequency equal to the wave frequency fw The lift force acting at fight angles to the wave propagation is caused by vortex shedding and its predominant frequency is a multiple of that of in-line force.

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