Circulation ($\\Gamma$)
The net circulation around the airfoil cross-section. It is defined by the line integral of the velocity field: \Gamma = \oint_C \vec{v} \cdot d\vec{l}$.
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The statement of the theorem
Define the circulation around a closed contour enclosing the airfoil cross-section as the line integral of the tangential component of the velocity field : where is a point outside .