Ringing of a vertical cylinder in waves

Abstract

This paper analyses the results of two series of experiments concerned with the response of a single vertical cylinder in the inertia regime in steep non-breaking waves. We recorded first the loading on a cylinder when it was held stationary, and secondly, its response in the same waves when it was pivoted just above the floor of the wave flume, and supported at the top by springs in the horizontal plane. Spring stiffnesses were set to achieve natural frequencies (measured in still water) in the range between 3 and 11 times the dominant wave frequency. The experiments were repeated with cylinders of three different diameters.Peak loading on stationary cylinders was found to exceed the predictions of a Morison model (based on kinematics computed from a numerical model of the measured waves), though improvements were achieved through the inclusion of slender-body terms. Measured ringing responses are generally in good agreement with those computed on a quasi-static basis from the measured loading history, but in some conditions, particularly at low frequency ratios, there is clearly some feedback from the motion to the excitation. Peak accelerations in the steepest waves are found to be limited approximately to those that would occur if the maximum loading were applied as a step change. Particular attention is given to a rapid cycle of loading that occurs after the crest has passed the cylinder's axis, and to images of the flow around the cylinder at the water surface.