Nonlinear forces on a vertical truncated cylinder in Stokes 5th order waves-model test and validation of prediction model

Abstract

A series of experiments of forces on a fixed vertical truncated column due to Stokes 5th order like waves were done in a wave tank. An effort was made to generate the waves as close as possible to theoretical Stokes 5th order waves. A systematic analysis, which centers on the wave height and wave steepness, was done for the wave and wave effects on the forces. Wave elevation time series was measured in the absence of the model, at the site of the model. Horizontal and vertical forces on the model were measured using a dynamometer. Horizontal particle velocity was measured under a typical wave to find the difference between the theory and experiment. Forces on the model were calculated also using Universal Linear System Model by Kim and Wang (1999a & 199b). The measured forces (Fx, Fz) increased almost linearly with the wave steepness and with the wave period (wave height) at the given steepness. The horizontal and vertical force transfer functions Fx/(H/2) and Fz/(H/2) due to a 2.0 s period wave at H/L = 0.049 amount to 1.35 and 1.05 times the corresponding theoretical linear transfer function (LTF). The theoretical LTF was compared with the measured LTF. The theoretical LTF underestimates the measured horizontal force LTF, while it overestimates the measured vertical force LTF. The theoretical vertical linear force consistently over-predicts the nonlinear measured vertical force, whereas the theoretical linear horizontal force consistently under-predicts the nonlinear measured horizontal force. The theoretical prediction due to ULSM/L+Q (Universal Linear System Model/Linear + Quadratic) was compared with the measured horizontal and vertical forces for two typical wave forces due to low and high waves. The five harmonics of the measured and theoretical wave were the inputs. The use of the measured wave gave a more favorable comparison than the theoretical wave. However, the difference was small. Breaking of the waves at the cylinder was observed using photographs for some high waves. This trend worsened with the increasing wave height. The corresponding force time series behaves differently from the force time series due to waves that were not breaking at the cylinder.