Talk:Kutta condition
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[edit] Why does the Kutta condition exist ?
The flow at the trailing edge of an airfoil separates cleanly because of boundary layer separation effects.
Contrast this situation with potential flow theory which requires the flow to turn around the trailing edge, resulting in zero lift and circulation.
In reality, the flow cannot turn around the trailing edge because of an unstable boundary layer which develops at the trailing edge. Pressure differences in the upper and lower boundary layers generally result in an adverse pressure gradient developing at the trailing edge boundary layer. This instability results in the stagnation and separation of the flow off the trailing edge.
The Kutta condition represents the equilibrium balance in pressure of the upper and lower boundary layers which result in smooth flow off the trailing edge. The equilibrium is always being reset as the flow field changes, to effect the trailing edge separation.
The development of circulation and net lift around the airfoil is a normal consequence of this effect.
Cold-logic 04:52, 9 November 2007 (UTC)
== Further observations and misconceptions of the Kutta condition
1. Definition of the Kutta condition
The Kutta condition is NOT DEFINED by zero fluid velocity at the sharp trailing edge. Why .... because all points at the surface of a wing are constrained by viscosity to have zero relative velocity. The Kutta condition, in this event, would therefore satisfy every point on the wing, rendering it a useless criterion for lift analysis.
The Kutta condition is best defined as:- the condition in steady streamlined flow whereby the stagnation streamline leaves the wing at the sharp trailing edge, thereby separating the flows from both sides of the wing.
Note: Only one possible stagnation streamline can be located on the sharp trailing edge of the wing. It is uniquely determined.
2. Starting Vortex
At no time does a starting flow of a wing create a vortex flow around a sharp trailing edge. This is the whole point of the Kutta condition. Vortex flows may be shed FROM the sharp trailing edge, but they do not occur AT and around the sharp trailing edge.
During the starting flow, and subsequently, the fluid is induced to move around the wing so that a stagnation streamline occurs at the sharp trailing edge.
3. Wake Effects
In reality, the wing creates a wake of disturbed fluid so that streamlined flow does not truly occur at the sharp trailing edge as suggested by the Kutta condition. Firstly the wing boundary layer contributes to the thickness of the wake, and so does the actual physical trailing edge thickness of a real wing. The Kutta condition is essentially correct when the wake is thin. However once boundary separation effects become significant, as for stalling, the stagnation (separation) points on the respective wing surfaces diverge significantly, and the Kutta condition is no longer valid.
The separation of the flow from the windward trailing edge during stalling is indicative of boundary separation effects dominating the net wing flows, and not the Kutta condition per se. —Preceding unsigned comment added by 58.110.88.102 (talk) 12:20, 3 February 2008 (UTC)
[edit] Where does the asymmetry come from?
I still don't find the explanations for the Kutta condition satisfactory. They introduce some kind of "magic" which renders any explanation of lift useless. How can the existence of the smooth flow from the trailing edge be explained with simple physical arguments? The explanations of viscosity, boundary layer separation and high speed of the air turning around the trailing edge would apply in the same way for the leading edge. Still, the forward stagnation point does not move to the leading edge! What about the argument that the inertia of the airflow following the wing shape downwards acts against the air flowing around the trailing edge? Obviously this inertia effect would not do the same at the leading edge, thus explaining the asymmetry.
Volume Displacement and Kutta Condition.
In the far field, away from the local displacement effects of the airfoil on the fluid, the streamlines fore and aft of the airfoil must align to preserve conservation of mass or volume (if the flow is incompressible). There is no actual fluid circulating around the airfoil, despite the mathematical property of "circulation" in a lifting situation.
Because the flow on the windward side of the airfoil (lower side) cannot go around the trailing edge, as in the potential flow case, that flow must go over the leading edge of the airfoil to preserve continuity.
Therefore, additional flow is induced over the leeward side of the airfoil (upper side) than would be in the case of potential flow over the airfoil.
This induced flow generated by observance of the Kutta Condition on the airfoil, therefore directly causes a net lifting force on the airfoil.
Basically, the Kutta Condition causes more flow to be displaced over one side of the airfoil than the other (asymmetry), which then causes a net lifting force on the airfoil. —Preceding unsigned comment added by 58.106.34.73 (talk) 03:52, 17 April 2008 (UTC)
