Wall Stress Modulation by a Bio-Inspired Two-Layer Surface Microstructure

Passive flow control methods are widely employed to reduce drag in wall-bounded turbulent flows. A recent direct numerical simulation (DNS) study of separating turbulent flow over a bump by Savino and Wu (2024 J. Fluid Mech. 1000, A80) examined the role of denticle geometry on flow separation and drag reduction. The results revealed the formation of a reverse pore flow (RPF) beneath the denticle crowns when subjected to an adverse pressure gradient (APG). This RPF generates thrust and contributes to drag reduction. Two geometric features were found to be critical: staggered necks that channel the RPF, and backward-facing slits between denticle crowns that prefer APG-enabled penetration. Motivated by these findings, the present research developed a microstructure that emulates the functional mechanism of shark denticles. It consists of two functional layers: a top layer that favors APG penetration, and a bottom layer that generates thrust via channeling. DNS of turbulent channel flow over various such microstructure configurations are conducted. Pressure gradients were introduced by a symmetric insert along the channel centerline. It has a diverging front section, a flat middle part, and a converging end, creating an FPG-ZPG-APG distribution along the channel. The study revealed pressure-gradient driving pore flow, its sensitivity to microstructure geometry, and a significant drag reduction in the APG region.