  Transverse dunes, which form under unidirectional winds and have fixed
profile in the direction perpendicular to the wind, occur on all celestial
objects of our solar system where dunes have been detected. Here we perform a
numerical study of the average turbulent wind flow over a transverse dune by
means of computational fluid dynamics simulations. We find that the length of
the zone of recirculating flow at the dune lee --- the {\em{separation bubble}}
--- displays a surprisingly strong dependence on the wind shear velocity,
$u_{\ast}$: it is nearly independent of $u_{\ast}$ for shear velocities within
the range between $0.2\,$m$$s and $0.8\,$m$$s but increases linearly with
$u_{\ast}$ for larger shear velocities. Our calculations show that transport in
the direction opposite to dune migration within the separation bubble can be
sustained if $u_{\ast}$ is larger than approximately $0.39\,$m$$s, whereas a
larger value of $u_{\ast}$ (about $0.49\,$m$$s) is required to initiate this
reverse transport.
