  We investigate flow separation in the air flow over transverse sand dunes.
CFD simulations of the air flow over differently shaped dunes are performed.
The length of the recirculation region after the brink of the dune is found to
depend strongly on the shape of the dune. A phenomenological expression for the
separation length is presented. Suitably non-dimensionalised, it depends
linearly on the angle of the dune at the slip face brink. We propose a fit for
the shape of the separating streamline, which is well approximated by an
ellipse.
