Understanding and exploiting non-equilibrium effects on turbulent boundary layers: Towards realisable drag reduction strategies

在交通运输和能源发电领域，即使只减少几个百分点的表面摩擦阻力，也会直接转化为燃料消耗和排放的减少，这一需求现在几乎已被普遍接受。因此，在过去的二十年里，已经提出了一系列减阻(DR)策略，但其中许多策略已经在完全发展的内部流动(即管流或槽流)中进行了测试和验证，其中没有沿流向发展的流动。因此，流动达到了与壁面条件的平衡，并报告了大幅减阻的潜力。然而，几乎所有的实际应用都涉及到外部流动，其中湍流边界层(TBL)将沿流向方向发展，从而在不断调整到规定的壁面条件时表现出非平衡行为。更重要的是，利用只有部分发展中流动受到影响的非平衡行为，可能会带来巨大的潜在好处。因此，研究非平衡效应不仅对于理解减阻策略的局限性，而且对于开发任何实际的好处都是至关重要的。在这个合作项目中，我们通过研究两种不同类型的非平衡壁面条件的影响来探索非平衡效应对壁面湍流的基本问题：(1)表面粗糙度的变化和(2)面内简谐变化表面波特性的变化。表面粗糙度是一种被动边界条件，可能会局部增加表面剪应力，而面内表面波是一种主动边界条件。这两组实验都引入了非平衡效应，根据表面变化的性质，这些效应将改变表面摩擦行为的发展。重要的是，两者都能在特定条件下降低局部表面剪应力。通过并行研究这两者，并比较湍流边界层对这些不同边界条件的响应，我们将为边界层对这些非平衡效应的尺度和调整提供新的见解。这在很大程度上符合克劳泽的“黑箱”类比的精神--未知系统的扰动和对响应的评估，在这里，还有识别阻力改变行为的额外动机。

The reduction of skin-friction drag even by a few percent in the transportation and energy generation sectors translates directly to reductions in fuel consumption and emissions, the need for which is now almost universally accepted. Consequently, in the last two decades, a whole range of Drag Reduction (DR) strategies has been proposed, but many of these have been tested and validated in fully-developed internal flows (i.e. pipe or channel flows) where there is no development of the flow along the streamwise direction. Therefore, the flow reaches an equilibrium with the wall condition and the potential for large drag reductions has been reported. However, almost all practical applications involve external flows where a turbulent boundary layer (TBL) will grow along the streamwise direction, such that it exhibits non-equilibrium behaviour as it continuously adjusts to the prescribed wall condition. More importantly, there could be significant potential benefits in exploiting non-equilibrium behaviour where only parts of the developing flow are affected. Therefore, it is of fundamental importance to examine non-equilibrium effects not only to understand the limitations of implementing a drag reduction strategy but also to exploit any practical benefits.In this collaborative project, we explore the fundamental problem of non-equilibrium effects on wall-turbulence by examining the effects of two different types of non-equilibrium wall condition: (1) change in surface roughness and (2) change in the characteristics of a harmonically-varying in-plane surface wave. The surface roughness is a passive boundary condition and may locally increase surface shear stress while the in-plane surface wave is an active boundary condition. Both sets of experiments introduce non-equilibrium effects that will alter the development of skin-friction behaviour depending on the nature of the surface change. Crucially, both can reduce the local surface shear stress under specific conditions. By examining the two in parallel and comparing the response of a turbulent boundary layer to these different boundary conditions, we will provide new insights on the scaling and the adjustment of the boundary layers to these non-equilibrium effects. This is very much within the spirit of Clauser's "black box" analogy - the perturbation of an unknown system and the assessment of the response, here, with the added motivation of identifying the drag-altering behaviour.