CBET-EPSRC: Turbulent flows over multiscale heterogeneous surfaces

当流体流过表面时，就形成了湍流边界层。这一边界层主要是表面产生的表面摩擦阻力的主要原因。在几乎所有的工程和环境流动应用中，这些边界层都是在非光滑或粗糙的表面上形成的，其中表面的粗糙度在设置阻力及其对流动的影响方面起着重要作用。然而，我们无法真正预测这些粗糙表面对水流的影响。这主要是因为表面粗糙度的地形在本质上通常是“多尺度”的，包含各种不同的粗糙度长度尺度。更重要的是，粗糙度长度尺度范围的变化和粗糙度特征的分布在整个地表是“非均匀的”。例如，森林或风力发电场的边缘、城市树冠、作物边界、河床、陆地-水界面、飞机上的铆钉、烧蚀的涡轮叶片、宏观生物污染的船体等。在这种不均匀的多尺度粗糙度上演变的湍流边界层经历了非均匀的表面条件，因此表现出不同于在均匀粗糙度上发展的流动的特性。因此，目前为具有均匀粗糙度的表面开发的建模和预测策略(如Moody图)既不能准确地预测，也不能提供对非均匀多尺度表面上复杂的流动物理的洞察。因此，如果我们能够预测多尺度地表非均质性对湍流边界层的影响，将在整个范围内带来相当大的进步和好处。在这项合作研究中，我们旨在应用一种系统的方法来表征非均匀多尺度表面上流动中的阻力和动量输送机制。将在英国南安普顿进行的一系列物理实验和将在美国约翰霍普金斯大学进行的大涡模拟将产生史无前例的多尺度非均匀表面上的流动数据。研究这种表面上的流动将使我们对湍流中的非线性之间的耦合及其与表面的多尺度非均质性之间的关系有一个基本的了解。这一认识将使我们在理解和预测非平衡湍流壁流以及导致其发展的多尺度相互作用方面发生范式转变。综合从数据中获得的新见解，我们将开发新的分析模型，仅基于关于表面地形的现有信息来预测阻力和动量转移的性质。其最终目标是为工程和环境流动应用开发真正可预测的模型。

A turbulent boundary layer is formed when a fluid flows past a surface. This boundary layer is primarily responsible for the skin-friction drag incurred by the surface. In almost all engineering and environmental flow applications, these boundary layers are formed over non-smooth or rough surfaces where the roughness of the surface plays a significant role in setting the drag and its repercussions on the flow. And yet, we are unable to truly predict the influence of these rough surfaces on the flow. This is primarily because the topography of surface roughness is usually "multiscale" in nature that contains a wide variety of roughness length scales. More importantly, the variation in the range of roughness length scales and the distribution of the roughness features is "heterogeneous" across the surface. Examples include edges of forests or wind-farms, urban canopies, crop boundaries, river-beds, land-water interfaces, rivets on aircraft, ablated turbine blades, macro bio-fouled ship hulls etc. The turbulent boundary layers that evolve over such heterogeneous multiscale roughness experience non-uniform surface conditions and as a result exhibit properties that are different from flows that develop over homogeneous roughness. Consequently, current modelling and prediction strategies (such as the Moody diagram) that were developed for surfaces with homogeneous roughness can neither accurately predict nor offer insights into the complex physics of flow over heterogeneous multiscale surfaces. Therefore, considerable advancements and benefits would result across a whole range of sectors if we are able to predict the effects of multiscale surface heterogeneity on turbulent boundary layers.In this collaborative research, we aim to apply a systematic approach to characterize drag and mechanisms of momentum transfers in flows over heterogeneous multiscale surfaces. A series of physical experiments - to be performed at Southampton in the UK - and Large Eddy Simulations - to be carried out at Johns Hopkins in the US - will generate unprecedented data of flows over heterogeneous multiscale surfaces. Examining flows over such surfaces will develop our fundamental understanding of the coupling between the non-linearities in the turbulent flow and its relationship with the multiscale heterogeneity of the surface. This understanding will bring about a paradigm shift in how we understand and predict non-equilibrium turbulent wall-flows and the multiscale interactions that are responsible for its development. Synthesizing the new insights obtained from the data, we will develop new analytical models to predict the drag and properties of momentum transfer based only on available information about the surface topography. The ultimate aim is to develop truly predictive models for engineering and environmental flow applications.