Turbulent drag reduction by rotating rings and wall-distributed actuation

Turbulent channel flows altered by the combination of flush-mounted spinning rings and vertical-velocity opposition control or hydrophobic surfaces are studied through direct numerical simulations. The two types of distributed control are applied over the surface area that is not occupied by the spinning rings. The turbulent mean skin friction is reduced by 20% through the steady rotation of the rings and the effect is enhanced by the distributed controls, reaching a drag reduction of 27%. The numerically computed combined drag reduction is well predicted by an upper bound obtained by a simple idealized model. The turbulence statistics are highly nonuniform along the spanwise direction, but show a weak dependence along the streamwise direction. The wall-shear stress is highly reduced over the central region of the rings. Narrow streamwise-elongated structures forming between adjacent disks offer a detrimental global contribution to the Reynolds stresses, although locally they reduce drag. A spatially dependent form of the Fukagata-Iwamoto-Kasagi identity helps explain the different influence of the two distributed controls, although the global drag-reduction levels are similar. The opposition control is effective in altering the elongated structures between rings, but it does not contribute to enhance the drag reduction in the central-ring region. Hydrophobicity creates a more nonuniform flow and even enhances the intensity of the streamwise structures between rings, but further reduces the wall-shear stress in the central-ring region compared to the ring-only case. The mean wall-slip velocity is the additional beneficial effect offered by the hydrophobic surface between rings.