超疏水沟槽表面通气减阻实验研究

减阻是解决航行体提速和增程的主要技术途径之一, 对缓解日益严峻的能源危机极为重要. 在重力式管道实验系统中, 测试给出了湍流状态下不同通气速率时减阻率随雷诺数及沟槽无量纲间距的变化规律和气膜铺展状态, 对比分析了单纯超疏水表面与超疏水沟槽表面上通气时减阻效果的差异.实验板材质为无色亚克力, 沟槽结构采用机械方法加工, 并在表面喷涂超疏水涂层. 结果表明, 持续通气能解决超疏水沟槽表面气膜层流失问题, 实现气膜层长时间稳定维持; 恒定雷诺数下, 随通气速率增大, 超疏水沟槽表面气膜铺展更趋均匀, 减阻率上升; 由于通气速率影响气膜横向扩展能力, 致使恒定通气速率下, 减阻率随雷诺数的变化呈现两种模式; 在固定雷诺数及通气速率时, 减阻率随沟槽尺寸的扩大先增后减, $S^{+}\approx 76$时减阻率最大. 分析其原因在于, 沟槽结构增大沾湿面积的同时, 显著提升了通气状态下超疏水表面气膜层的稳定性, 因而展示出与超疏水表面和沟槽表面均不相同的减阻规律, 且效果更佳. 

EXPERIMENTAL STUDY ON DRAG REDUCTION CHARACTERISTICS OF SUPERHYDROPHOBIC GROOVE SURFACES WITH VENTILATION 1)

Drag reduction is one of the main technical approaches to solve the enhancing speed and extending voyage of the vehicle under water, which is extremely crucial to alleviate the increasingly severe energy crisis all over the world. In the gravity pipeline experimental system, drag reduction characteristics with ventilation and gas film spreading state on superhydrophobic groove surfaces are tested and raised in the turbulent state. The variation laws of drag reduction rate with Reynolds number and dimensionless spacing of grooves at different ventilation rates are obtained. In addition, it is the diffierence of ventilation drag reduction that is compared and analyzed between merely superhydrophobic surfaces and superhydrophobic groove surfaces. The material of the experimental plate is colorless acrylic. The groove structure is processed via mechanical method and is sprayed by superhydrophobic coating. Results reveal that continuing ventilation can settle the issue of easy loss of gas film on superhydrophobic groove surface, and the gas film can achieve perennial stabilization. As ventilation rate adds, the gas film spreads more uniformly and drag reduction rate rises under the constant Reynolds number, which result in the notable drag reduction effect. As ventilation rate affects the capability of scaling out of gas film, drag reduction presents two modes with Reynolds number under the constant ventilation rate. When the ventilation rate and the Reynolds number are unchanging, the drag reduction rate firstly increases and then decreases with the expansion of the groove size, and the maximum reduction rate is obtained when $S^{+}\approx 76$. The inherent mechanism on drag reduction characteristics of superhydrophobic groove surfaces with ventilation is that not only the spreadability and stability of gas film layer is enhanced significantly but also the wetted area is increased obviously due to groove structures, meanwhile, the maximum value of drag reduction is larger than both the groove surface and the superhydrophobic surface.