High Reynolds number bulk effects induced by superhydrophobic surfaces

Drag reduction induced by superhydrophobic surfaces made of streamwise grooves is investigated through direct numerical simulation of turbulent pipe flow. When the ratio between the groove width and the viscous length exceeds 50, most of the wall slippage effect is maintained in the bulk of the flow; thus, a large drag reduction is achieved. The effect is explained in terms of a modified mean and turbulent kinetic energy balance that accounts for the slippage effect at the wall. With typical groove sizes of technological devices, values of order 50 wall units can be achieved only at high friction Reynolds number, here increased up to Reτ≃2300.