Controlling vortex breakdown in swirling pipe flow —David Dennis, University of Liverpool

A vortex breakdown occurs as fluid flows from a rotating pipe into a stationary pipe at a constant rate. The decaying swirl creates an adverse pressure gradient on the pipe axis, which reverses the flow and forms a recirculation "bubble". The bubble's size is controlled by the speed of rotation.

https://downloads.sms.cam.ac.uk/2877076/2877088.mp4