How do turbulent superstructures interact with skin friction drag?

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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