  Turbulent flow separation induced by a protuberance on one of the walls of an
otherwise planar channel is investigated using Direct Numerical Simulations.
Different bulge geometries and Reynolds numbers - with the highest friction
Reynolds number simulation reaching a peak of Re{\tau} = 900 - are addressed to
understand the effect of the wall curvature and of the Reynolds number on the
dynamics of the recirculating bubble behind the bump. Most of the drag is due
to the form contribution, whilst the friction contribution does not change
appreciably with respect to an equivalent planar channel flow. The most bluff
geometry has a larger recirculation region, whilst the Reynolds number increase
results in a smaller recirculation bubble and a shear layer more attached to
the bump. The energy introduced by the pressure drop follows two routes: part
of it is transferred towards the walls to be dissipated and part feeds the
turbulent production hence the velocity fluctuations in the separating shear
layer. Spatial energy fluxes transfer the kinetic energy into the recirculation
bubble and downstream near the wall where it is ultimately dissipated.
Consistently, anisotropy concentrates at small scales near the walls
irrespective of the value of the Reynolds number. In the bulk flow and in the
recirculation bubble, isotropy is restored at small scales and the isotropy
recovery rate is controlled by the Reynolds number. Anisotropy invariant maps
are presented, showing the difficulty in developing suitable turbulence models
to predict separated turbulent flow dynamics. Results shed light on the
processes of production, transfer and dissipation of energy in this relatively
complex turbulent flow where non-homogeneous effects overwhelm the classical
picture of wall bounded turbulent flows which typically exploits streamwise
homogeneity.
