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

在运输和能源生产部门，表面摩擦阻力的减少，即使是百分之几，也直接转化为燃料消耗和排放的减少，现在几乎普遍接受了这一需求。因此，在过去的二十年中，已经提出了一系列的减阻（DR）策略，但其中许多已经在完全发展的内部流（即管道或通道流）中进行了测试和验证，其中没有沿着流向方向的流动发展。因此，流动达到了与壁面条件的平衡，并且已经报道了大的减阻潜力。然而，几乎所有的实际应用都涉及外部流动，其中湍流边界层（TBL）将沿着流向增长，使得其在连续调整到规定的壁条件时表现出非平衡行为。更重要的是，在只有部分发展中流量受到影响的情况下，利用非平衡行为可能会带来巨大的潜在利益。因此，研究非平衡效应不仅对了解实施减阻策略的局限性，而且对开发任何实际利益都至关重要。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条件的影响来探索非平衡效应对壁湍流的基本问题：（1）表面粗糙度的变化和（2）谐波变化的面内表面波的特性的变化。表面粗糙度是一个被动的边界条件，可能会局部增加表面剪应力，而面内表面波是一个主动的边界条件。这两组实验都引入了非平衡效应，这将改变表面摩擦行为的发展，这取决于表面变化的性质。关键是，两者都可以在特定条件下降低局部表面剪应力。通过并行检查这两个和比较湍流边界层对这些不同的边界条件的响应，我们将提供新的见解的缩放和调整的边界层，这些非平衡效应。这在很大程度上符合克劳泽的“黑箱”类比的精神--未知系统的扰动和响应的评估，在这里，增加了识别阻力改变行为的动机。

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.