lördag 1 december 2012
Incorrect Explanation of Magnus Effect
Above is the the standard explanation of the Magnus effect, giving a backspin tennis ball an upward lift force, as potential flow (left) augmented with large scale circulation (middle) to give an unsymmetric flow with lift (right). The same type of argument is used in the circulation theory of Kutta-Zhukovsky supposedly explaining the lift of a wing.
However, the above standard explanation of the Magnus effect is incorrect. The correct explanation is by unsymmetric separation as seen in the following picture of real flow:
and not by large scale circulation as in the above standard explanation. We see that in reality the incoming flow is not affected by the rotation of the ball, while in the standard explanation the incoming flow is changed from horizontal direction without circulation to a slanted upward direction with circulation.
We thus see that the standard circulation theory for the Magnus effect is unphysical, and so is Kutta-Zhukovsky circulation theory for the lift of a wing. For a correct theory see The Secret of Flight.