Metasurface of fluid mechanics for efficient turbulence drag reduction

Drag reduction is crucial for improving aerodynamic efficiency and enhancing energy conversion, such as wind turbines. Riblets inspired by shark skin are a promising approach in drag reduction technology. In this work, further inspired by the typical wind-eroded landform yardangs in nature, a novel passive metasurface of fluid mechanics, composed of low-high-low riblets (LHLRs), was proposed to achieve enhanced drag reduction performance. The aerodynamic drag reduction performance of LHLRs is validated by comparing them with skin-inspired riblets using a flow channel with an adjustable surface yaw angle. The surface turbulent flow characteristics are obtained via direct numerical simulation, then turbulence motion analysis explains the improved drag reduction performance of LHLRs. The results indicate that the drag reduction performance of the LHLRs is improved by 91.6 % compared to the shark skin-inspired riblets, and its robustness to wind yaw angles increases from 30°to at least 45°. Turbulence motion analysis reveals that LHLRs can weaken the transport of internal turbulence, further suppress turbulence penetration, and delay the formation of secondary vortices near the wall, thereby reducing dispersive stress and enhancing drag reduction performance. Moreover, the equivalent turbulence-free region between the primary and secondary riblets of LHLRs optimizes the near-wall turbulence distribution, leading to a modification of the optimal non-dimensional square root of the groove cross-section l_g⁺ in the riblets drag-change curve.