  We experimentally study the turbulent flow between two coaxial and
independently rotating cylinders. We determined the scaling of the torque with
Reynolds numbers at various angular velocity ratios (Rotation numbers), and the
behaviour of the wall shear stress when varying the Rotation number at high
Reynolds numbers. We compare the curves with PIV analysis of the mean flow and
show the peculiar role of perfect counterrotation for the emergence of
organised large scale structures in the mean part of this very turbulent flow
that appear in a smooth and continuous way: the transition resembles a
supercritical bifurcation of the secondary mean flow.
