Cylinder oscillations beneath a free-surface

Abstract

This investigation considers the wake states of a cylinder oscillating beneath a free-surface. The primary considerations are the variation of the forces on the cylinder and the structure of the near wake with the frequency of oscillation and the depth of submergence of the cylinder. The fully submerged oscillating cylinder exhibits two distinctly different wake states: low- and high-frequency. As the cylinder approaches the free-surface many of the characteristics of these wake states persist. Notably, for both the free-surface and fully submerged cases, the transition between the two states corresponds to a jump in the phase of the lift force, a shift in the phase of vortex shedding and a distinct change in the structure of the near wake. The mean vorticity fields show that as the cylinder approaches the free surface the changes in the oscillating cylinder wake are generally consistent with those observed for a stationary cylinder. Close to the free-surface both the low- and high-frequency wakes become non-symmetric, as does the stationary cylinder wake; and in all cases there is a net negative lift force on the cylinder.