疏水微结构表面气液界面稳定性及其对流场和减阻的影响

Hydrophobic surfaces with micro- and nano-scale structures can maintain a thin air layer, and it will be a new efficient drag reduction technology under water. This technology is convenient, economical, and antifouling to a certain extent too, so it can be widely applied to the area of marine engineering and other relevant areas. Numerous researches have revealed that a hydrophobic surface patterned with micro- and nano-scale structures is able to maintain an air layer in a low flow velocity (<1.0m/s), and shows prominent drag reducing effect (>40%); when the flow is accelerated, the gas-liquid interface will be destroyed, and the efficacy of drag reduction will fail, because of the loss of the air layer above the hydrophobic surface. Therefore, it is imperative to investigate the stability of gas-liquid interface above hydrophobic surfaces. This project plans to observe the movement of the gas-liquid interface under various flow conditions using both the high speed camera and confocal microscopy, including different pressure, flow velocity, and Reynolds number etc., and establish the stability principles of gas-liquid interfaces over the hydrophobic surfaces; then study the effects of different gas-liquid interfaces on the flow field and drag reduction using the PIV with multiple scales, and summarize some gas-liquid interfaces with prominent drag reducing effect; next capture the kinematic and dynamic processes of the gas-liquid interface breaking, using the molecular dynamics-fluid dynamics coupling method which is a new scale-span simulation method, and conclude the breakup mechanism; finally, the feasible methods to maintain the particular gas-liquid interface and keep steady remarkable drag reducing effect in fully developed turbulence will be explored, by designing the micro- and nano-scale structure, controlling surface wettability, and supplying moderate gas, etc. We believe that the deep study of this project can promote the development of hydrophobic drag reduction technology and its practical engineering applications.

疏水微结构表面利于在水下封存气膜，产生壁面滑移，是一种潜在的兼具防污功能的水下减阻新方法，未来有望广泛应用于海洋工程和其它相关领域。目前该技术的瓶颈在于：低水速时（一般<1.0m/s）表面气膜稳定，减阻显著（减阻量>40%）；但提高流速后，气膜会随气液界面破坏而流失，致使减阻失效。因此，亟需开展水流作用下疏水微结构表面气液界面稳定性研究。项目拟采用共聚焦显微技术+高速摄像技术，监测典型流场条件下疏水微结构表面气液界面形态变化规律，建立气液界面稳定准则；利用变尺度PIV测试方法，研究气液界面形态变化对流场和减阻的影响，发现减阻显著的气液界面形态；基于MD-CFD跨尺度耦合模拟方法，捕捉疏水微结构上气液界面和三相接触线的演变规律，揭示其破坏的动力学机制；最后从微形貌设计、润湿性调控及气体动态补充等方面，探索湍流状态下仍能维持良好气液界面形态、减阻稳定的技术途径，为后续技术提升和工程化提供参考。

疏水微结构表面利于在水下封存气膜，是一种潜在水下减阻新方法。但水流剪切作用会造成气膜破坏，致使减阻失效。项目系统研究了疏水微结构表面气液界面稳定性及其对流场和减阻的影响，顺利完成原定研究任务和目标。取得主要成果有：1）通过测试超疏水平板表面气膜流失过程发现，存在气膜的超疏水平板表面应力明显降低，滑移显著；通过模拟超疏水纳米结构表面气液界面变形过程证明，降低气液界面两侧固液原子之间作用强度，可释放边界处剪应力，降低阻力。2）通过搭建μ-PIV系统，观测超疏水表面气液界面流场发现，超疏水环形沟槽上气液界面处滑移速度随沟槽尺寸扩大而增加，且顺流向沟槽上滑移速度远大于垂直流向沟槽；且气液界面凹凸形态会影响表面滑移速度分布。3）采用MD方法研究了超疏水纳米结构表面流场特性，并发现纳米结构表面气液界面上满足复合滑移边界条件；采用CFD方法研究了超疏水表面湍流流场，表明顺流向沟槽减阻效果明显，且提高气液界面分数能提升减阻效果。4）提出基于润湿阶跃效应的超疏水表面气膜维持技术，并在分析润湿阶跃束缚气膜原理的基础上，模拟给出气膜在润湿阶跃平板表面的变形与滑移规律；通过构造超疏水润湿阶跃转子实验，实现超过70%的减阻量。5）提出电解补气和人工微量通气两种气膜补充方法；通过超疏水沟槽表面与气膜补充方法的耦合，在低速水槽中获得超过40%的减阻量；通过润湿阶跃效应和超疏水表面气膜补充方法的耦合，在大型水洞实验（模型直径φ60mm，水速范围2~8m/s）中获得最大约30%的减阻量。项目执行期内，累计在Sci Adv、J Fluid Mech、Ocean Eng等刊物发表论文23篇（含Science子刊1篇，SCI索引15篇，EI索引19篇），申请/授权发明专利8项，撰写专著1部（2021年出版），做大会和分会场邀请报告5次，举办学术研讨会1次，培养博士3人、硕士3人。另外，项目提出的基于润湿阶跃效应的超疏水表面气膜维持技术具备在湍流状态下维持气液界面的潜力，目前正与中船713所合作开展应用验证。