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 NavierStokes equation expressed in a generalized coordinate system through the use of MAC method. The random waves are chosen from laboratory experimental data. 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.
One of the main difficulties in prediction of random wave forces lies in the est.imation of CD and CM for the Morison equation. It is well known that for regular waves or sinusoidal flows, CD and CM generally depend on the characteristics of the flow, such as Ke, the Keulegan-Carpenter number, and Re, the Reynolds number. But for irregular waves the villues of CD ilnd CM that should he used remains quite questionable illthough the matter has received much attention. Generally methods of analyzing the meilsured random data can be clilssified into different groups, depending on whether the basis of analysis is time domain or frequency domain, or whether or not the force coefficients CD and CM are assumed constant for a pilrticular force record. Isaacson et al. (1991) compared five different methods using a numerically generated random wave and a force record simulilted by Morison equation with constant CD ilnd CM; they concluded that the method of least squilres fit of the force times series ilnd the so-cillled wilve-by-wave method tend to give iln ilCcurate prediction of the Morison force coefficients.
