around a closed contour C enclosing the airfoil and followed in the negative (clockwise) direction. As explained below, this path must be in a region of potential flow and not in the boundary layer of the cylinder. The integrand V cos ⁡ ⁡ θ θ is the component of the local fluid velocity in the direction tangent to the curve C , and d s is an infinitesimal length on the curve C . Equation (1) is a form of the Kutta–Joukowski theorem.