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Direct numerical simulations are performed to explore the effects of rotatingdirection of the vertical asymmetric rough wall on the transport properties ofTaylor-Couette (TC) flow up to a Taylor number of $\textit{Ta} = 2.39 \times10^7$. It is shown that compared to the smooth wall, the rough wall withvertical asymmetric strips can enhance the dimensionless torque\textit{Nu}$_\omega$, and more importantly, at high \textit{Ta} clockwiserotation of the inner rough wall (the fluid is sheared by the steeper slopeside of the strips) results in a significantly bigger torque enhancement ascompared to the counter-clockwise rotation (the fluid is sheared by the smallerslope side of the strips) due to the larger convective contribution to theangular velocity flux, although the rotating direction has a negligible effecton the torque at low \textit{Ta}. The larger torque enhancement caused by theclockwise rotation of vertical asymmetric rough wall at high \textit{Ta} isthen explained by the stronger coupling between the rough wall and the bulk dueto the larger biased azimuthal velocity towards the rough wall at the mid-gapof TC system, the increased intensity of turbulence manifesting by largerReynolds stress and thinner boundary layer, and the more significantcontribution of the pressure force on the surface of rough wall to the torque.

Direct numerical simulation of Taylor-Couette flow with vertical asymmetric rough walls