Detecting flow separation is also useful to check the stability properties of sails. In fact, due to the strong non-linearity of the Navier-Stokes equations, small variations of the inlet wind speed and/or direction can significantly change the behavior of the air stream, triggering abrupt flow detachments over some sail regions and reducing separations over some others. Ideal sails should remain reasonably stable under the effect of such perturbations, avoiding large deformations and flutter.

(Image courtesy of CMCS (Chair of Modeling and Scientific Computing-EPFL-Lausanne))