Kutta condition
The Kutta condition is a principle in steady-flow fluid dynamics, especially aerodynamics, that is applicable to solid bodies with sharp corners, such as the trailing edges of airfoils.
Kuethe and Schetzer state the Kutta condition as follows:[1]: § 4.11 A body with a sharp trailing edge which is moving through a fluid will create about itself a circulation of sufficient strength to hold the rear stagnation point at the trailing edge.In fluid flow around a body with a sharp corner, the Kutta condition refers to the flow pattern in which fluid approaches the corner from above and below, meets at the corner, and then flows away from the body.
The Kutta condition is significant when using the Kutta–Joukowski theorem to calculate the lift created by an airfoil with a sharp trailing edge.
The flow over both the topside and the underside join up at the trailing edge and leave the airfoil travelling parallel to one another.
Whenever the speed or angle of attack of an airfoil changes there is a weak starting vortex which begins to form, either above or below the trailing edge.
This weak starting vortex causes the Kutta condition to be re-established for the new speed or angle of attack.
[6][5]: § 4.7-4.9 The Kutta condition gives some insight into why airfoils have sharp trailing edges,[7] even though this is undesirable from structural and manufacturing viewpoints.
[8][9] The same Kutta condition implementation method is also used for solving two dimensional subsonic (subcritical) inviscid steady compressible flows over isolated airfoils.
Once the initial transient effects have died out, the stagnation point is at the trailing edge as required by the Kutta condition.
Mathematically, the Kutta condition enforces a specific choice among the infinite allowed values of circulation.
Lower figure: Flow pattern with circulation consistent with the Kutta condition, in which both the upper and lower flows leave the trailing edge smoothly.
