Wave forces on horizontal cylinders at low Keulegan-Carpenter and Reynolds numbers

Hydrodynamic forces on a horizontal cylinder with a circular and elongated rectangular cross-section in regular waves are studied experimentally and numerically. In the laboratory experiment, the cylinder is fixed beneath the waves with its axis parallel to the wave crests. The kinematics of the wave flow are determined from the linear wave theory using the measured wave height and period. The experiment is conducted for low Keulegan-Carpenter numbers and relatively low Reynolds numbers (KC<6 and 10,000<Re<30,000). In addition, we present a numerical simulation method for a two-dimensional flow around a cylinder undergoing orbital motion in still water. Experimental results on the horizontal cylinder in regular waves show that the trend of both drag and inertia coefficients is quite different from that obtained from a planar oscillatory flow test or results for a vertical cylinder. From the numerical simulation it is found that separation of the boundary layer begins at about KC = 1.5 for a circular cylinder in orbital motion, much smaller than that in planar oscillation. For the elongated rectangular cylinder, the force coefficients are much more complicated than for the circular cylinder because the flow always separates. It is found that the forces on an elongated rectangular cylinder fixed in waves are generally much larger than that when harmonically oscillating in still water at the same KC and Re numbers.