Drag-reduction effect of staggered superhydrophobic surfaces in a turbulent channel flow

In this study, direct numerical simulations of turbulent channel flow with superhydrophobic surfaces (SHS) were performed. The staggered SHS pattern alternating the free-shear and no-slip regions was designed to be robust to variations in the main flow direction. A constant-pressure gradient condition was imposed, and the Reynolds number was set to Re ⠜ = 180 and 395. The bulk mean velocity in the SHS case increased compared with the no-slip case, and the effect was enhanced by increasing the size of the free-shear region l6. The main reason for the increase in bulk mean velocity was the increase in slip velocity (or slip length). If the size of the free-shear region was small, the drag-reduction rate was found to be proportional to the slip velocity. However, the turbulent contribution to the bulk mean velocity remained almost unchanged in the no-slip case owing to the negative coherent component of the Reynolds shear stress.