Sub-laminar drag over slip surfaces in turbulent channel flow

Turbulent flow control is of importance due to its potential benefits, particularly regarding drag reduction for energy savings. It was once conjectured that the minimum drag of a constant mass-flux turbulent channel flow is its corresponding laminar drag. However, several studies have shown that the sub-laminar drag can be sustainably achieved, for example, by blowing/suction, surface grooves, or curved flows. In this study, we will investigate the sub-laminar drag due to slip or hydrophobic surface control and explore its underlying mechanisms in turbulent channel flows. Direct numerical simulations are performed with the inclusion of the slip surface up to a friction Reynold number of 125. As the slip length increases, sub-laminar drag instants start to emerge, and their occurrence and duration are dramatically increased when more than 30% of drag reduction is achieved. As a result, the total fraction of time spent in the sub-laminar drag is substantially increased. A key underlying mechanism for the sub-laminar drag is the creation of negative Reynolds shear stress in the channel-center region, not in the wall region. The dependence of the Reynolds number on the sub-laminar drag will also be discussed.