Wave-induced vortex generation around a slender vertical cylinder

Experimental results of wave-induced vortex generation around a slender, vertical cylinder are discussed. Coherent vortices appear in long waves (kR = 0.1) (k wavenumber, R cylinder radius) during the timing of the secondary load cycle, a strongly nonlinear component of the wave force acting on the cylinder, which is also measured. The secondary load cycle is present also in moderately long wave cases (kR = 0.3), where, however, there is no vortex formation. The measurement of vortex generation is enabled by Particle Image Velocimetry. The flow downstream the cylinder, in three horizontal planes at different depths is measured. The vortex formation that occurs in the long waves is attached to the cylinder in the form of thin vortex tubes. These appear symmetrically at angles of 40-45 degrees off the wave propagation direction. In one weak long wave case several very thin vortex tubes appear along the back side of the cylinder. Vortex diameters are 20 % of the cylinder diameter in four cases and 50 % of the cylinder diameter in one case. The measured vorticity emanates from the cylinder's boundary layer and is an order of magnitude stronger than to the vorticity caused by wave breaking. Wave breaking reduces the vortex strength. The fact that the secondary load cycle appears without and with flow separation effects indicates that the load cycle is a gravity wave phenomenon that scales with the Froude number, but that flow separation effects also contribute to the magnitude of such suction force.