Fig. 2. Kutta condition and circulation. The Kutta condition arises from a sum of the flow around an airfoil placed in an inviscid fluid (A) to additional circulation arising from the presence of viscosity (B) to yield a smooth, tangential flow from the trailing edge (C). When satisfied, the Kutta condition ensures that the vorticity generated at the trailing edge is zero. For the inviscid case, the net force on the airfoil (blue arrow) acts perpendicular to the free stream.

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