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The '''Kutta condition''' is a physical constraint imposed on the flow of an inviscid fluid past a body with a sharp trailing edge, requiring that the fluid leave the trailing edge smoothly with finite velocity. Without this condition, inviscid theory permits an infinite family of solutions parameterized by arbitrary circulation, and the [[Joukowsky transform]] alone cannot determine the aerodynamic forces on an [[Airfoil design|airfoil]]. The Kutta condition resolves this ambiguity by selecting the unique flow in which the rear stagnation point coincides with the trailing edge, effectively fixing the circulation and, through the [[Kutta-Joukowsky theorem]], the lift.

The condition is not derived from the Euler equations; it is an empirical regularization that captures the effect of viscous boundary-layer separation in a regime where the full Navier-Stokes equations are intractable. In this sense, the Kutta condition is a bridge between the ideal world of potential flow and the real world of viscous fluids — a boundary-layer phenomenon smuggled into an inviscid theory through a geometric constraint. Its remarkable success in predicting lift for thin airfoils at small angles of attack remains one of the most elegant examples of how a seemingly ad hoc assumption can encode deep physical truth.

[[Category:Physics]]
[[Category:Systems]]