ary currents in turbulent flows over rough wallsSecond

大多数工程和环境流动发生在粗糙度和/或地形表现出空间变化的表面上。当湍流壁流在这样的表面上演化时，它可能表现出不寻常的物理性质，这取决于表面的主要长度尺度和流动的主要长度尺度之间的关系。具体地说，当地表在横流方向上的主要长度尺度(S)变得(S)与流动的主要长度尺度(如边界层厚度或水深)相当时，流动也表现出锁定在地表非均质性上的大尺度空间非均质性。这种流动的非均质性表现为局部二次流(SCs)的形式，这些二次流通常延伸到整个流动深度，并表现为伴随着低速和高速区域的大的“时间平均”流向涡流。这种由表面引起的流动非均质性使湍流壁流的一些基本原理失效，这些基本原理是为均匀表面上的流动而发展的。因此，目前依赖于这些原理的预测工具既不能准确预测，也不能提供对包含表面诱导SC的复杂流动物理的洞察。最近，至少在两个截然不同的领域，人们认识到了表面诱导SCS的显著影响：1)工程系统的性能，如在役涡轮叶片、生物污损船体和水流控制；2)了解河流流动动力学，并将其应用于洪水管理、生态水力学和泥沙输送。近年来，南安普敦、阿伯丁、格拉斯哥和伦敦大学学院在推进这两个领域方面投入了相当大的努力。鉴于人们对这一专题的兴趣与日俱增，利用这四个领导小组之间的协同作用，全面了解地表诱导的湍流流动，并建立一个新的变革性框架来预测这种流动，将是及时的。该项目将利用上述领域四个主要小组的专业知识、领域知识和基础设施，使我们对产生二次流的非均匀表面上的流动的方法发生范式转变。将进行一系列全面的物理实验(在南安普顿和阿伯丁)和补充的数值模拟(在格拉斯哥和伦敦大学学院)，以产生前所未有的地表诱导的SCS数据。我们将首次比较和对比所有四种典型壁流(边界层、明渠、管流和闭道)中SCs的行为。所获得的数据将为确定和验证在存在表面诱导的SCS的情况下这些流动中阻力机制和动量/能量传递的潜在共性(和差异)奠定基础。综合从数据中获得的见解，将开发一个新的框架，导致物理信息的半经验和基于理论的数值模式，以预测和优化表面诱导的SCS对与工程/环境应用相关的湍流壁流的影响。

A majority of engineering and environmental flows occur over surfaces that exhibit spatial variations in roughness and/or topography. When a turbulent wall-flow evolves over such surfaces, it may exhibit unusual physical properties, depending on the relationship between the dominant length-scales of the surface and that of the flow. Specifically, when the dominant length-scale(s) of the surface in the cross-stream direction become(s) comparable to the dominant length-scale of the flow (such as boundary layer thickness or water-depth), then the flow also exhibits large-scale spatial heterogeneity that is locked-on to the surface heterogeneity. This flow heterogeneity is expressed in the form of localised secondary currents (SCs) that often extend across the entire depth of the flow and manifest themselves as large 'time-averaged' streamwise vortices accompanied by low- and high-speed regions. This surface-induced flow heterogeneity invalidates some of the fundamental tenets of turbulent wall-flows that were developed for flows over homogeneous surfaces. Therefore, current predictive tools that rely on these tenets can neither accurately predict nor offer insights into the complex physics of flows that contain surface-induced SCs. The significant effects of surface-induced SCs have recently been recognised in at least two disparate areas: 1) Performance of engineering systems such as in-service turbine blades, bio-fouled ship hulls and flow control; and 2) Understanding of the river flow dynamics with applications in flood management, eco-hydraulics and sediment transport. Over recent years, Southampton, Aberdeen, Glasgow and UCL have invested considerable efforts in advancing both these areas. Given the burgeoning interest in this topic, it would be timely to harness the synergies between these four leading groups to develop comprehensive understanding of turbulent flows in the presence of surface-induced SCs and establish a novel transformative framework to predict such flows. This project will leverage the expertise, domain knowledge and infrastructure of four leading groups in the above-mentioned areas to bring about a paradigm shift in our approach to flows over heterogeneous surfaces that generate secondary currents. A comprehensive series of physical experiments (at Southampton & Aberdeen) and complementary numerical simulations (at Glasgow & UCL) will be performed to generate unprecedented data on surface-induced SCs. We will compare and contrast the behaviour of SCs across all four canonical wall-flows (boundary layers, open-channels, pipes and closed-channels) for the first time. The obtained data will underpin identification and validation of potential universalities (and differences) in drag mechanisms and momentum/energy transfer in these flows in the presence of surface-induced SCs. Synthesising the insights obtained from the data, a new framework leading to physics-informed semi-empirical and and theoretically-based numerical models will be developed to predict and optimise the influence of surface-induced SCs on turbulent wall-flows relevant to engineering/environmental applications.