Secondary flow and heat transfer in turbulent flow over streamwise ridges

Surface structuring in form of streamwise-aligned triangular ridges is investigated in the framework of a fully developed turbulent channel flow with constant wall temperatures of different values prescribed on the upper and lower walls at Re-b = 18000. 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 separation. The ridges generate a strong large-scale secondary motion and hence enhance momentum and heat transfer in the channel by approximately 30% relatively to the smooth channel. In spite of the fact that both arrangements translate into very similar global flow properties, the composition of skin friction coefficient and Stanton number significantly differs. The componental split-up of the friction coefficient reveals that the enhancement of momentum transfer mainly originates from the dispersive component linked to the secondary flows. For the Stanton number, however, the enhancement arises not only from the dispersive component, but also from a strong modification of the turbulent flow properties.