Drag reduction mechanism in turbulent Couette flow with spanwise-aligned superhydrophobic surface texture

Direct numerical simulations are performed to explore drag reduction mechanisms in turbulent Couette flow with a transverse (spanwise) alternating slip and no-slip (SNS) boundary condition on the bottom wall, compared to the case with a no-slip (NS) condition on the bottom wall. The streamwise length ratio of the slip and no-slip phases is set to 2.5:1. The SNS condition allows for a reduction in skin-friction coefficient of approximately 31%, with over 95% of the drag reduction attributed to the turbulent term, as demonstrated by the Fukagata-Iwamoto-Kasagi identity analysis. The results of the quadrant analysis reveal that Q2 ejection and Q4 sweep motions play a dominant role, contributing 75.1% and 57.8% to drag reduction, respectively. Energy spectra analysis shows that the SNS condition redistributes energy across turbulent scales, not only reducing the wavelength of the outer peak and its energy, but extending the energy closer to the wall, compared to the NS case. Further analysis of scale-separated Reynolds shear stress demonstrates the distinct contributions from different turbulent scales, where a cutoff wavelength is set to λ x / h = 10 . The smaller-scale structures of λ x / h < 10 contribute to drag reduction mainly near the bottom wall, accounting for 55.4% of the total drag reduction, while the larger-scale structures of λ x / h > 10 contribute the reduction of 39.6%, mainly near the channel center.