Irrotational vortices (velocity)
In fluid dynamics, a vortex is a region within a fluid where the flow is mostly a spinning motion about an imaginary axis, straight or curved. That motion pattern is called a vortical flow. In the absence of external forces, a vortex usually evolves fairly quickly toward the irrotational flow pattern, where the flow velocity v is inversely proportional to the distance r.
For an irrotational vortex, the circulation is zero along any closed contour that does not enclose the vortex axis and has a fixed value, for any contour that does enclose the axis once. (Circulation is the line integral around a closed curve of the velocity field).