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_τ0) 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_τ0 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.

中文翻译：

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雷诺数对液-气界面湍流边界层流动减阻的影响

浸入式微结构表面上的液-气界面（LGI）有可能实现较大的滑移效果，这对于水下应用（例如减阻）非常重要。LGI上层流的减阻机理已得到公认，而LGI上的湍流边界层（TBL）流动尚不清楚。在目前的工作中，设计了一个实验系统来研究LGI上TBL流的减阻机理。特别是，通过高分辨率粒子图像测速仪直接测量LGI附近的流速分布，从而计算出剪切应力和减阻。据实验发现，减阻随着摩擦雷诺数（重新_{τ 0}）增加。导出了一个解析表达式来分析LGI对减阻的影响，它由两部分组成，即滑移特性以及对LGI附近湍流结构和动力学的修改。重要的是，所测量的滑移性能也随着重新_{τ 0}增加，这被证明是减阻的关键作用。这揭示了在LGI上的TBL流中减阻的机理。本工作为降低LGI上TBL流的阻力提供了物理见解，这对水下应用很重要。