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Returning
to the airfoil, the source of the circulation is illustrated in the top
diagram which shows the unsteady situation associated with takeoff. At
low speeds (a) the flow has not yet developed and the rear stagnation point
is located where the inviscid calculations say it should be. Pressure difference
between the top and bottom of the wing causes air to move upwards at the
trailing edge and a vortex is created at the trailing edge. As the speed increases,
the vortex is shed and conservation of angular momentum for the system
requires that a vortex of opposite rotation is formed at the trailing edge.
It is the presence of this vortex that causes the circulation required to
produce the desired lift!
The photograph shows two stages in this process.
At the top (a) the starting vortex is just forming and is rotating in
a counterclockwise direction. In (b) the starting vortex has just been detached,
and the vortex responsible for the circulation is seen at the trailing
edge of the wing. This is rotating in a clockwise direction (to conserve angular
momentum) and gives the clockwise circulation required to establish the
steady state flow that provides the required lift. |
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