In this essay I will attempt to identify the main events in the history of thought about irrotational flow of viscous fluids.
I am of the opinion that when considering irrotational solutions of the Navier–Stokes equations it is never necessary and
typically not useful to put the viscosity to zero. This observation runs counter to the idea frequently expressed that potential
flow is a topic which is useful only for inviscid fluids; many people think that the notion of a viscous potential flow is an
oxymoron. Incorrect statements like ‘‘. . . irrotational flow implies inviscid flow but not the other way around’’ can be
found in popular textbooks.
Though convenient, phrases like ‘‘inviscid potential flow’’ or ‘‘viscous potential flow’’ confuse properties of the flow
(potential or irrotational) with properties of the material (inviscid or viscous); it is better and more accurate to speak
of the irrotational flow of an inviscid or viscous fluid.
Every theorem about potential flow of perfect fluids with conservative body forces applies equally to viscous fluids in
regions of irrotational flow.