Turbulent flow and drag reduction over discontinuous slip boundaries

Direct numerical simulations are performed to investigate the effects of discontinuous slip boundaries comprising alternating slip regions and transverse no-slip stripes on turbulent channel flow and drag reduction mechanisms. Compared with a continuous slip boundary with the same effective slip length, the discontinuous configuration yields a substantially lower net drag reduction. This degradation is attributed to a diminished slip contribution resulting from the triple effects of slip area deficit, localised flow-field re-equilibration and inter-stripe interference, coupled with variations in the turbulence-modulation contribution and an intensified dispersive penalty. The no-slip stripes generate local adverse pressure gradients and induce shear-layer instabilities, triggering the development of internal boundary layers at the leading and trailing edges. This boundary-layer re-equilibration process manifests as a characteristic ‘oscillation-overshoot-decay recovery’ in the wall velocity gradient. Simultaneously, the no-slip stripes enhance turbulent kinetic energy production in the buffer layer and intensify wall-normal transport. While variations in the no-slip stripe geometries mainly affect drag reduction through the slip component and become dynamically important when the pitch lamda λ $\\lambda$ is sufficiently small, slip anisotropy offers a more robust mitigation strategy. By suppressing spanwise slip, the lateral motions amplified by no-slip stripes are effectively blocked, rendering the boundary layer insensitive to stripe-induced localised disturbances. This mechanism enables streamwise-slip-dominant discontinuous boundaries to achieve drag reduction performance that significantly surpasses that of isotropic or spanwise-slip-dominant configurations.