Study on random wave force coefficients using numerical simulation of flow fields

A new approach to investigate the hydrodynamic forces acting on a vertical cylinder in random waves is presented. The time series of random wave force is obtained from numerical simulation of a two-dimensional irregular oscillating flow around a circular cylinder by numerical solution of the incompressible Navier-Stokes equation expressed in a generalized coordinate system through the use of the MAC method. The random waves are chosen from laboratory experimental data. To obtain the Morison force coefficients, two methods are used: least squares fit of total force time series and least squares fit on a wave-by-wave basis. The phenomena found in calculated results are similar to those in the experiments, and the study revealed that the so-called history effect of random wave forces is mainly due to the vortex shedding, which is confirmed by computed flow patterns.