I am studying fluid dynamics and while searching on the internet, found many videos explaining the mathematical aspect of rotational and irrotational flow, but i need more clarity on what actually it means, physically by irrotational and rotational flow.
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1$\begingroup$ As the name "irrotational" suggests, fluid elements in an irrotational flow don't rotate but only undergo shape change (and volume change if the flow is compressible). You may wish to make your question more specific. $\endgroup$– DeepCommented Nov 14, 2018 at 15:23
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The simplest physical interpretation is to imagine a little paddle wheel moving with the flow. If it spins, the flow is rotational. If it maintains its orientation (even if it's orbiting a core of vorticity), the flow is irrotational. Here is classic $\omega \propto 1/r$ flow:
If you move along with the highlighted region, it doesn't rotate:
This type of flow appears around a core of vorticity, such as a tornado. The air around it circulates, but is irrotational, so the wind velocity falls of with distance. Note: the core circulates really fast.
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$\begingroup$ Thanks, then Why is it necessary for a fluid to be irrotational for streamline flow? $\endgroup$ Commented Nov 14, 2018 at 22:48