Definition of irrotational flow

Irrotational flow

If the net rotation of the fluid element is equal to zero, then the flow is known as irrotational flow.

Or

The flow in which the element of the moving fluid suffers no net rotation from one instant to the next, with respect to the given frame of reference.

Or

The flow in which fluid particles do not rotate about their own axes and retain their original orientations, is called an irrotational flow.

• In this flow, the fluid element is subjected to much more distortion.
•  As long as the algebraic average rotation is zero, the motion is irrotational.

Example

In an irrotational flow, if a match stick is thrown on the surface of the moving fluid, it does not rotate about its axis but retains its original orientation.

Reference :

Hydraulics, fluid mechanics and hydraulic machines by R.S Khurmi

Fluid mechanics with engineering applications by Robert L. Daugherty