When a cylindrical vessel, containing some liquid, is rotated about its vertical axis, the liquid surface is depressed down at the axis of its rotation and rises up near the walls of the vessel on all sides. This type of flow is known as vortex flow. It is of the following two types:
1. Forced vortex flow: In this type of flow, the vessel containing the liquid is forced to rotate about the fixed vertical axis with the help of some external torque.
2. Free vortex flow: In this type of flow, the liquid particles describe circular paths about a fixed vertical axis, without any external torque acting on the particles. The flow of water through the hole in the bottom of a wash basin is an example of free vortex flow.