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 · d****l**
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 *K*_{cyl} = 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*