In fluid dynamics and hydrodynamics, concept introduced by Lord
Kelvin (Also known as Sir William Thomson, sometimes as Kelvin of Largs or
Baron William Thomson)
The circulation is K defined as the integral of v · dl
around the closed loop l around a body in a fluid.
K= ∫v · dl
Specifically for a cylinder in a fluid, the integral would solve as K = v
· 2 · &pi · R , where v is the
surface velocity of the cylinder, and R is the
radius. For a cylinder v = R · ω, where
ω is the angular velocity, and this gives that the
circulation for a cylinder is Kcyl = 2 · π ·
ω · R 2
The concept of circulation is important for the theoretical understanding -
now that's an oxymoron - of potential flows (vorticity free flows),
to which Thomson's Theorem applies.
reference: ne.se, britannica, D.J. Tritton; Physical Fluid Dynamics