A note on the bathtub vortex

Abstract

Observations were made of the vortex motion which occurs when a liquid is draining from a vessel through a hole in its bottom. It was found that, for relatively quiescent initial conditions, the direction of rotation reversed as the liquid surface approached the bottom of the vessel. An explanation of this phenomenon is proposed which is based upon the vorticity associated with the circumferential component of the flow in the boundary layer on the bottom of the vessel.