# Circulation (fluid dynamics)

In fluid dynamics, circulation is the line integral of the velocity field, around a closed curve. Circulation is normally denoted Γ (Greek uppercase gamma). Circulation was first used independently by Frederick Lanchester, Martin Kutta and Nikolai Zhukovsky.

## Definition

If V is the fluid velocity on a small element of a defined curve, and dl is a vector representing the differential length of that small element, the contribution of that differential length to circulation is dΓ:

${\displaystyle d\Gamma =\mathbf {V} \cdot \mathbf {dl} =|\mathbf {V} ||d\mathbf {l} |\cos \theta }$

where θ is the angle between the vectors V and dl.

The circulation around a closed curve C is the line integral:[1]

${\displaystyle \Gamma =\oint _{C}\mathbf {V} \cdot d\mathbf {l} }$

The dimensions of circulation are length squared, divided by time; L2⋅T−1, which is equivalent to velocity times length.

## Kutta–Joukowski theorem

The lift per unit span (L') acting on a body in a two-dimensional inviscid flow field can be expressed as the product of the circulation Γ about the body, the fluid density ρ, and the speed of the body relative to the free-stream V. Thus,

${\displaystyle L'=\rho V\Gamma \!}$

This is known as the Kutta–Joukowski theorem.[2]

This equation applies around airfoils, where the circulation is generated by airfoil action; and around spinning objects experiencing the Magnus effect where the circulation is induced mechanically. In airfoil action, the magnitude of the circulation is determined by the Kutta condition.[2]

Circulation is often used in computational fluid dynamics as an intermediate variable to calculate forces on an airfoil or other body. When an airfoil is generating lift the circulation around the airfoil is finite, and is related to the vorticity of the boundary layer. Outside the boundary layer the vorticity is zero everywhere and therefore the circulation is the same around every circuit, regardless of the length of the circumference of the circuit.

## Relation to vorticity

Circulation can be related to vorticity:

${\displaystyle \mathbf {\omega } =\nabla \times \mathbf {V} }$
${\displaystyle \Gamma =\oint _{\partial S}\mathbf {V} \cdot d\mathbf {l} =\int \!\!\!\int _{S}\mathbf {\omega } \cdot d\mathbf {S} }$

where the closed integration path (indicated by "∂S") is the boundary or perimeter of a surface S whose local perpendicular unit vector is dS. Thus vorticity is the circulation per unit area, taken around a local infinitesimal loop. Correspondingly, the flux of vorticity vectors through a surface S is equal to the circulation around its perimeter.

## References

1. Robert W. Fox; Alan T. McDonald; Philip J. Pritchard (2003). Introduction to Fluid Mechanics (6 ed.). Wiley. ISBN 978-0-471-20231-8.
2. A.M. Kuethe; J.D. Schetzer (1959). Foundations of Aerodynamics (2 ed.). John Wiley & Sons. §4.11. ISBN 978-0-471-50952-3.
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