Flow structure and surface shape in a stationary “bathtub vortex” in a rotating cylindrical container. In the center, the rotation is approximately 50 times quicker than at the periphery. Left: stationary bathtub vortex. Center: Flow structure: the red dye is inserted at the top and runs directly down the drain. The green dye is inserted at the bottom periphery at the left. It is sucked in and part of it spirals slowly up along the edge of the vortex core. Right: At sufficiently large rotation rates, the tip of the vortex becomes unstable and bubbles are torn off.
