CBET-EPSRC: Surfactant impact on drag reduction of superhydrophobic surfaces in turbulent flows

超疏水表面(SHS)是一种受生物启发的工程表面或涂层，具有许多令人惊讶和有用的特性。通过将空气困在微腔内，SHS可以防止少量液体(如水滴)在表面扩散，从而产生众所周知的荷叶效应。当浸入水中时，由于被困住的空气层，SHS可以减少液体与表面之间的摩擦阻力。SHS的减阻有可能大大减少能源消耗、气体排放和海上运输成本，以及流体动力学和热传递方面的许多其他应用。在2018年国际海事组织会议之后，英国决定到2050年实现英国海运的零气体排放。SHS等减阻技术可以为实现这一重要的环境目标做出重大贡献，同时在绿色技术方面提供新的经济机会。然而，SHS在实验室或现场测试时表现出不一致的性能，无论是在层流条件下还是在湍流条件下。许多结果与理论和数值预测有很大的偏差。我们最近的实验、数值和理论工作表明，微量的表面活性剂会显著影响SHS在层流中的减阻性能。表面活性剂天然存在于海洋和河流中，以及大多数工程应用中。在湍流条件下，它们对SHS的影响目前尚不清楚。基于我们最近在层流流动方面的工作，我们假设表面活性剂也可以影响SHS在湍流中的性能，解释了在实验测试中发现的不一致以及与现有模型的不匹配，这些模型目前都忽略了表面活性剂。为了研究这一假设，我们的跨国团队，由来自美国加州大学圣巴巴拉分校的数值模拟专家和来自英国曼彻斯特大学的理论建模专家组成，将首次对表面活性剂在湍流中超疏水减阻进行基础模拟研究。我们将使用特殊的改进技术，对SHS上方含表面活性剂的湍流进行全分辨率数值模拟，以获得与海洋应用的现实条件相关的流型。此外，将开发更简单的理论模型来识别和预测关键的物理和表面活性剂过程。理论模型将使我们能够灵活地快速探索表面活性剂如何影响湍流中SHS减阻的复杂动力学。数值模拟将提供大量有关流动动力学和表面活性剂作用的详细信息，并将用于验证我们的理论模型。为了增加我们的发现的影响，来自我们的数值模拟和实现我们的模型的算法的高分辨率数据将免费在线提供。这将使研究人员能够很容易地利用我们的结果来优化SHS设计并提高其性能，即使在表面活性剂存在的情况下也是如此。我们的目标是揭示表面活性剂在现实条件下的影响，以便确定实用的缓解策略，并释放SHS在现实世界应用中的减阻潜力。

Superhydrophobic surfaces (SHS) are bio-inspired engineered surfaces or coatings with several surprising and useful properties. By trapping air inside micro cavities, SHS can prevent small amounts of liquid such as water droplets from spreading on the surface, leading to the well-known lotus-leaf effect. When immersed in water, SHS can reduce friction drag between the liquid and the surface, owing to the entrapped air layer. Drag reduction from SHS has the potential to substantially reduce energy use, gas emissions and costs in maritime transport, and numerous other applications in fluid dynamics and heat transfer. Following the 2018 meeting of the International Maritime Organisation, the UK decided to reach zero gas emissions in British maritime shipping by 2050. Drag reduction technologies such as SHS can significantly contribute towards achieving this important environmental goal, whilst providing new economic opportunities in green technologies.However, SHS have shown inconsistent performance when tested in the lab or in the field, in both laminar and turbulent flow conditions. Many results deviate significantly from theoretical and numerical predictions. Our recent experimental, numerical and theoretical work has revealed that trace amounts of surfactant can significantly impair the drag-reduction performance of SHS in laminar flows. Surfactants are naturally present in oceans and rivers, as well as most engineering applications. Their impact on SHS in turbulent flow conditions is presently unknown. Building on our recent work on laminar flows, we hypothesize that surfactant can also affect the performance of SHS in turbulent flows, explaining inconsistencies found in experimental tests and the mismatch with existing models, which currently all ignore surfactant.To investigate this hypothesis, our multi-national team, composed of experts in numerical simulation from the University of California Santa Barbara (US) and experts in theoretical modelling from the University of Manchester (UK), will perform the first ever fundamental modelling investigation of superhydrophobic drag reduction in turbulent flow with surfactant. We will implement fully-resolved numerical simulations of surfactant-inclusive turbulent flow above SHS, using special refinement techniques in order to reach flow regimes relevant to realistic conditions for maritime applications. In addition, simpler theoretical models will be developed to identify and predict key physical and surfactant processes. The theoretical models will give us the flexibility to explore rapidly the complex dynamics of how surfactant can affect SHS drag reduction in turbulent flows. The numerical simulations will provide a wealth of detailed information about the flow dynamics and the effect of surfactants, and will be used to validate our theoretical models.To increase the impact of our findings, highly resolved data from our numerical simulations and algorithms implementing our models will be made freely available online. This will allow researchers to readily exploit our results in order to optimize SHS designs and improve their performance even when surfactant is present. Our objective is to uncover the impact of surfactant in realistic conditions in order to identify practical mitigation strategies and unlock the drag-reduction potential of SHS for real-world applications.