Effect of Reynolds number on drag reduction in turbulent boundary layer flow over liquid-gas interface

The liquid-gas interface (LGI) on submerged microstructured surfaces has the potential to achieve a large slip effect, which is significant to the underwater applications such as drag reduction. The mechanism of drag reduction in the laminar flow over the LGI has been well recognized, while it is yet not clear for the turbulent boundary layer (TBL) flow over the LGI. In the present work, an experimental system is designed to investigate the mechanism of drag reduction in TBL flow over the LGI. In particular, the flow velocity profile near the LGI is directly measured by high-resolution particle image velocimetry by which the shear stress and the drag reduction are calculated. It is experimentally found that the drag reduction increases as the friction Reynolds number (Re-tau 0) increases. An analytical expression is derived to analyze the effect of the LGI on drag reduction, which consists of two parts, i.e., the slip property and the modifications to the turbulence structure and dynamics near the LGI. Importantly, the measured slip property also increases as Re-tau 0 increases, which is demonstrated to be the key effect on drag reduction. This has revealed the mechanism of drag reduction in TBL flow over the LGI. The present work provides physical insights for the drag reduction in TBL flow over the LGI, which is significant to the underwater applications.