How do turbulent superstructures interact with skin friction drag?

虽然湍流超结构(TS)在近壁湍流上留下了很大的足迹，但对于它们对流体到壁面的动量传输(与壁面摩擦阻力相对应)的定量影响却知之甚少。在第一个资助期内取得的结果表明，拆除TSS并不是一种有希望的减阻方法，因为流动经历了复杂的重组，出现了规模较小的新结构。此外，仅根据减阻控制流动的有效通道高度和雷诺数，减阻流动中大尺度结构的变化是可以预测的。这表明TSS是独立于近壁湍流循环而存在的，这与最近的文献结果是一致的。由于第一阶段的数值试验仅限于TSS及其相关尺度相互作用仅存在的雷诺数范围内(这是该研究领域的共同挑战)，我们计划在第二阶段建立TSS的模式系统。该系统由湍流槽道流动组成，其中沿槽壁放置沿水流方向的脊线。在这些脊线上方形成了大尺度流向涡形式的二次运动，这些脊线具有较高雷诺数时自然形成的TSS的特征。在一项联合的数值和实验研究中，我们将结合相关的尺度相互作用和通过引入(而不是去除)TSS来控制流动的迹象，来研究这些TSS与壁面摩擦阻力的相互作用。优先方案(SPP)第一阶段在探测TSS方面的合作表明，尤其是湍流中的小尺度弥散对探测相干结构具有挑战性。因此，在第二个资助期内，不同的群体将考虑一些不同的过滤方法。在本项目中，我们将考虑基于二次运动的降阶模型的过滤器。该过滤后的数据集以及数值模拟的全部数据集将提供给SPP内主要从事TS的有效检测的其他小组。因此，本项目有助于实现战略计划的下列目标：

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