Vortex rings generated by a translating disk from start to stop

As a daily life experience, when stirring a cup of tea or coffee, one may observe vortices induced by the starting and stopping motion of a spoon. This paper investigated the annular vortices induced by the unsteady motion of a disk in water from experimental measurements and numerical simulations. The circulation and core radius of the vortices are shown to scale with power laws of the disk diameter, stroke length, and time in agreement with inviscid theoretical predictions.