Ansys Employee
Imagine to have vortex and you put a floating cross shape ( X ) on the fluid surface. If the cross orientation does not change (with respect to you) while moving around the vortex then the fluid is irrotational. If it changes orientation than the fluid is rotational. The same concept can be applied to fluid elements instead of using an external object (like the X). Hence a swirling flow could be rotational or irrotational depending on the deformation of fluid elements.