How do turbulent superstructures interact with skin friction drag?

虽然已知湍流上层结构（TSS）在近壁面湍流中留下了相当大的足迹，但相对而言，人们对它们对从流体到壁面的动量传输的定量影响知之甚少，这对应于表面摩擦阻力。在第一个资助期内获得的结果表明，去除TSS并不是一种有希望的减少阻力的方法，因为流动经历了复杂的重组，并且在更小的尺度上出现了新的结构。此外，研究表明，仅根据减阻控制流的有效通道高度和雷诺数，就可以预测减阻流动中大尺度结构的变化。这表明TSS在近壁湍流循环中相当自主地存在，这与最近的文献发现一致。由于第一阶段的数值实验仅限于雷诺数范围内，其中TSS和相关尺度相互作用仅弱存在（这是该研究领域的常见挑战），我们计划在第二阶段使用TSS模型系统。该系统由湍流的沟道流组成，其中沿沟道壁放置有顺流的脊。在这些脊上形成了大规模流向涡旋形式的二次运动，这些脊已知具有较高雷诺数下自然发生的TSS的特征性质。在一项协调一致的数值和实验研究中，我们将研究这些TSS与皮肤摩擦阻力的相互作用，结合相关的尺度相互作用，以及通过引入（而不是去除）TSS来控制流动的迹象。在优先计划（SPP）第一阶段关于TSS检测的合作表明，特别是湍流中的小尺度弥散对检测相干结构具有挑战性。因此，不同的小组将在第二个筹资期内考虑若干不同的筛选办法。在本项目中，我们将考虑基于二次运动降阶模型的滤波器。这些经过过滤的数据集以及数值模拟的完整数据集将提供给SPP内主要从事有效检测TSS的其他小组。因此，本项目有助于SPP的以下目标：-量化TSS对全球运输和湍流统计的影响-了解TSS的机制和边界条件的作用-在简化模型中开发控制和有效模拟TSS的方法

Although turbulent superstructures (TSS) are known to leave a substantial footprint on near-wall turbulence, relatively little is known about their quantitative impact on the momentum transport from fluid to the wall, which corresponds to the skin friction drag. Results obtained within the first funding period indicate that the removal of TSS is not a promising approach for drag reduction as the flow undergoes complex restructuring and new structures at smaller scales emerge. In addition, changes of large scale structures in drag reduced flows were shown to be predictable based on the effective channel height and Reynolds number of drag reduced controlled flows only. This suggests that TSS exist rather autonomously from the near-wall turbulence cycle which is in agreement with recent literature findings. As the numerical experiments in the first phase are limited to a Reynolds number range in which TSS and the related scale interaction is only weakly present (which is a common challenge in this field of research), we plan to work with a model system of TSS in the second phase. This system consists of a turbulent channel flow in which streamwise ridges are placed along the channel walls. Secondary motions in the form of large-scale streamwise vortices are formed above those ridges which are known to carry the characteristic properties of naturally occurring TSS at higher Reynolds numbers. In a concerted numerical and experimental study we will investigate the interaction of those TSS with skin friction drag in combination with the related scale interaction and indications for flow control through the introduction (instead of the removal) of TSS. The collaborations within the first phase of the Priority Programme (SPP) in respect to the detection of TSS revealed that especially the small scale dispersion in turbulent flows is challenging for the detection coherent structures. Therefore, a number of different filtering approaches will be considered by different groups within the second funding period. In the present project we will consider a filter based on a reduced order model of the secondary motion. This filtered data set along with the full data set of the numerical simulation will be made available to other groups within the SPP that mainly work on the effective detection of TSS. The present project thus contributes to the following goals of the SPP: - quantification of the impact of TSS on global transport and turbulence statistics- understanding the mechanics of TSS and the role of boundary conditions - developing approaches for control and efficient modelling of TSS in reduced models