One aspect of wave motions around offshore structures can be modelled by oscillating fluid past a fixed body, or conversely by oscillating a cylindrical body in a fluid. During such an oscillation, the body generates in one half-cycle of motion a vortex wake through which the body will pass in the following half-cycle. This process of flow reversal induces an effect whereby vortices from successive half-cycles pair up with one another and, as osillation amplitude increases, so we more from one periodic flow pattern to another, with each one being associated with an increasing number of vortices (n) shedding and pairing up per cycle. The importance of such vortex dynamics lies in the fact that large periodic lift force fluctuations are induced, which are a multiple (n+1) of the flow oscillation frequency. In the present case for n=1, a circular cylinder is oscillated horizontally with an amplitude or around 2 diameters, and a new vortex pair forms from each complete cycle of motion. The pair of vortices convects upwards (in this case) at almost 90 degrees to the flow direction. This vortex pattern, and others, are described by Williamson (1985). As an aside, it is interesting to note that at a certain point during each cycle, the visualization (using surface particles) takes on a rather familiar "friendly" form suggesting that one could perhaps rename the title of this contribution as "Formation of Vortex Bears in Oscillatory Flow."

Williamson, C. H. K. (1985) Sinusoidal flow relative to cylinders. J. Fluid Mech. 155, 141.