In contrast to the foregoing cases of oscillatory or orbital flow, or the corresponding motion of a cylinder in quiescent fluid, where presence of the free-surface has not been an issue, recent investigations have addressed the consequence of proximity of a free-surface on the patterns of vortices formed both from the cylinder and the free-surface.
Miyata et. al. (1990) studied the steady translation of a cylinder beneath a free-surface via qualitative visualization and numerical simulation. Using particle-streaks and focussing on the case where the free-surface exhibited negligible distortion, they show interesting patterns of vortices, which were interpreted with corresponding force measurements. Sheridan et. al. (1997) undertook a quantitative investigation of the nominally steady flow past a cylinder close to a free-surface, using the technique of high-image-density particle image velocimetry. The value of Froude number was sufficiently high, such that substantial distortion of the free-surface could occur. Patterns of instantaneous velocity and vorticity exhibit a variety of fundamental mechanisms of generation of vorticity layers from the cylinder and the free-surface. The development and interaction of these layers suggest important consequences for the loading of the cylinder. In the event that a cylinder penetrates the free-surface and is subjected to oscillation in the vertical direction, while the Froude number is maintained sufficiently low to avoid significant free-surface distortions, still additional classes of vorticity concentrations can develop in the near-wake of the cylinder, as characterized by Lin, Sheridan and Rockwell (1996). The foregoing features may be present, in analogous forms, for wave motion past a cylinder located close to a free-surface; this aspect has remained, however, uninvestigated.