  Direct numerical simulations are performed to explore the effects of rotating
direction of the vertical asymmetric rough wall on the transport properties of
Taylor-Couette (TC) flow up to a Taylor number of $\textit{Ta} = 2.39 \times
10^7$. It is shown that compared to the smooth wall, the rough wall with
vertical asymmetric strips can enhance the dimensionless torque
\textit{Nu}$_\omega$, and more importantly, at high \textit{Ta} clockwise
rotation of the inner rough wall (the fluid is sheared by the steeper slope
side of the strips) results in a significantly bigger torque enhancement as
compared to the counter-clockwise rotation (the fluid is sheared by the smaller
slope side of the strips) due to the larger convective contribution to the
angular velocity flux, although the rotating direction has a negligible effect
on the torque at low \textit{Ta}. The larger torque enhancement caused by the
clockwise rotation of vertical asymmetric rough wall at high \textit{Ta} is
then explained by the stronger coupling between the rough wall and the bulk due
to the larger biased azimuthal velocity towards the rough wall at the mid-gap
of TC system, the increased intensity of turbulence manifesting by larger
Reynolds stress and thinner boundary layer, and the more significant
contribution of the pressure force on the surface of rough wall to the torque.
