Rotational and Irrotational Flow
What is it?
Rotational flow is when the particles of fluids are all rotating about their own axis in addition to their other movement. Irrotational flow is when the individual particles are not rotating around their axis.
Give me an example!
Water flows down a drain in a counterclockwise direction in the northern hemisphere, and in a clockwise direction in the southern hemisphere. That is because of the rotation of the individual water molecules combined with the acceleration due to gravity. By contrast, an example of irrotational flow would be the flow of water over a dam, in which the water flows straight down and rotation doesn't have an effect.
