Vortex formation from a cylinder moving through a free-surface.  Comparison of vortex formation at the same phase angle phi = 80° of the cylinder motion, measured from phi = 0° corresponding to the maximum depth of submergence, shows the effect of proximity of the free-surface. Depths h* = h/D of the equilibrium position of the cylinder are different, but the amplitude A* = A/D = 1.6 of its motion is maintained constant. For the limiting case where the equilibrium position is just above the free-surface at h* = -0.5, the vortices have a small value of circulation, relative to the case where the equilibrium position is immediately below the free-surface at h* = 0.3. Irrespective of the equilibrium position, however, the tendency is for formation of symmetrical layers of vorticity, in contrast to the well known case of the completely submerged cylinder, where the layers are formed in a highly asymmetrical fashion. [22] Lin, Phetkong, Sheridan, and Rockwell