Mechanism of gas depletion on super-hydrophobic surfaces in turbulent flows

具有纹理的超疏水（拒水）表面提供了一个令人兴奋的机会，可以潜在地减少湍流中的摩擦阻力，从而为海洋工业节省大量能源。这种减阻特性与液体流动和纹理表面之间存在的气体层有关。不幸的是，困住的气体可能会被湍流耗尽并带走，导致减阻失败。该项目的目的是为了更好地了解当超疏水表面受到湍流时气体枯竭的机制。最终，这项研究的结果将指导被动和主动方法的发展，以维持超疏水表面在高湍流中的减阻。除了培训研究生和本科生外，该教育计划还将通过马萨诸塞大学达特茅斯学院的“向上发展”项目和“聚光灯”项目为当地高中生提供工程经验。通过社会媒体和当地博物馆开展的外联活动将提高公众对开发新型减阻技术以应对能源和环境挑战的认识。由于缺乏实验数据，对湍流引起的超疏水表面气体耗竭的机理了解甚少。该项目旨在通过最先进的实验测量来填补这一知识空白。第一个项目目标是利用反射干涉对比显微镜和数字全息显微镜表征湍流中气液界面形状的时空变化。结果将说明界面如何变形，振动，并最终从表面纹理分离。第二个目标是建立壁面压力波动和气体枯竭临界条件的预测模型。根据解出的界面形状和Young-Laplace方程，估计壁面压力波动是引起界面变形的主要力量。湍流边界层内部的速度场也将用全息粒子跟踪测速仪测量。预测模型将考虑多种因素，包括纹理尺寸、纹理几何形状、雷诺数、韦伯数和初始界面形状。第三个目标是评估分层结构和注气在高雷诺数湍流中维持减阻的有效性。实验结果将验证过去计算模拟中使用的关键假设。该研究成果将促进对湍流与超疏水表面之间复杂相互作用的认识。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

A textured super-hydrophobic (water-repelling) surface provides an exciting opportunity to potentially reduce the friction drag in turbulent flows, which could lead to substantial energy-savings in the marine industry. This drag reducing property is related to the presence of a gas layer trapped between the liquid flow and the textured surface. Unfortunately, the trapped gas could be depleted and carried away by turbulent flows, causing a failure of drag reduction. The aim of this project is to better understand the mechanism of gas depletion when the super-hydrophobic surface is subject to turbulent flows. Ultimately, the results of this research will guide the development of passive and active approaches to sustain the drag reduction by super-hydrophobic surfaces in highly turbulent flows. In addition to training graduate and undergraduate students, the education plan will provide engineering experiences for local high-school students through University of Massachusetts Dartmouth’s Upward Bound program and Spotlight program. Outreach activities through social media and local museums will raise public awareness of the development of novel drag reduction technologies in addressing energy and environmental challenges. The mechanism of gas depletion on a super-hydrophobic surface caused by turbulent flows is poorly understood due to a lack of experimental data. This project aims to fill this knowledge gap through start-of-the-art experimental measurements. The first project goal is to characterize the spatial and temporal variations of the shape of gas-liquid interface in turbulent flows using Reflection Interference Contrast Microscopy and Digital Holographic Microscopy. The results will illustrate how the interface deforms, vibrates, and finally detaches from the surface textures. The second goal is to develop predictive models of the wall pressure fluctuation and the critical condition for gas depletion. The wall pressure fluctuation, which is the main force causing interface deformation, will be estimated based on the resolved interface shape and the Young-Laplace equation. The velocity field in the inner part of the turbulent boundary layer will also be measured by Holographic Particle Tracking Velocimetry. The predictive models will consider various factors including texture size, texture geometry, Reynolds number, Weber number, and initial interface shape. The third goal is to evaluate the effectiveness of hierarchical structure and gas injection for sustaining the drag reduction in high-Reynolds number turbulent flows. The experimental results will validate the key assumptions used in past computational simulations. The research outcomes will advance the knowledge of the complex interplay between turbulent flows and super-hydrophobic surface.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.