NEAR-WAKE OF A PERTURBED, HORIZONTAL CYLINDER AT A FREE-SURFACE

A horizontal cylinder intersecting a free surface is subjected to cont rolled vertical perturbations, and the consequent vortex formation is characterized by high-image-density particle image velocimetry, which leads to instantaneous patterns of velocity, vorticity, and streamline s. For the limiting case of the stationary cylinder, the near wake doe s not exhibit rapid formation of organized vortical structures in a ma nner similar to Karman vortices. Application of perturbations, however , generates phase-locked vortex formation over a wide range of excitat ion frequencies, even at relatively low amplitudes, indicating that th e near wake in presence of a free surface is convectively, rather than absolutely, unstable. At a sufficiently high value of excitation freq uency, the formation of the initial vortex undergoes an abrupt change in timing, which is analogous to that occurring for Karman vortex form ation from a completely submerged cylinder. All of these features of t he near wake are interpreted in terms of foci, saddle points, and reat tachment points of the streamwise topology. (C) 1996 American Institut e of Physics.