Turbulent flow and drag reduction over discontinuous slip boundaries

Abstract

Abstract Content of image described in text. 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.