Large- vs Small-scale friction control in turbulent channel flow

Abstract: We reconsider the “large-scale” control scheme proposed by Hussain and co-workers (Phys. Fluids 10, 1049--1051 1998 and Phys. Rev. Fluids, 2, 62601 2017), using new direct numerical simulations (DNS). The DNS are performed in a turbulent channel at friction Reynolds number of up to 550 in order to account for the low-Re effects. The purpose of the present contribution is to re-assess this control method in the light of more modern developments in the field, in particular also related to the discovery of (very) large-scale motions. Moreover, we want to better characterise the physics of the control, investigate the optimal parameters and, finally, determine which aspects of this scheme actually scale in outer units. In addition to discussing the mentioned drag-reduction effects, the present contribution will also address the potential effect of the natural large-scale motions on frictional drag, and give indications on the physical processes for potential drag reduction possible at all Reynolds numbers.

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