Secondary motion impact on momentum and heat transfer in a fully developed turbulent channel flow

Surface structuring in form of streamwise-aligned triangular ridges is investigated in a DNS of a fully developed turbulent channel flow with constant wall temperatures of different values prescribed on the upper and lower walls at $\mathrm{Re}_\tau=180, 500$ and $1000$. Two arrangements of the ridges on both channel walls are considered -- a symmetrical arrangement and a staggered arrangement with a spanwise shift of the upper wall structure by a half ridge-to-ridge separation. The ridges generate a strong large-scale secondary motion and hence enhance momentum and heat transfer in the channel. The composition of skin friction coefficient and Stanton number vary depending on $Re_\tau$ and structure arrangement from $7$ to $32\%$. The component wise split-up of the Stanton number shows that staggered arrangement performs better in terms of heat transfer at higher $Re_\tau$ due to a more pronounced dispersive component.