Skin friction reduction via suppression of large scales in turbulent Couette flows

We investigate the drag-reduction achieved by suppressing the Very-Large-Scale Motion (VLSM) that is typical of turbulent Couette flows. A weak Coriolis force is used for this purpose, as pioneered by Komminaho et al. (1996). The main advantage of this strategy is that it only suppresses the VLSM without suppressing other motions of similar scale. The suppression effect is though progressive with the intensity of the Coriolis force, and direct effects of the forcing on the small scales are hard to isolate, albeit marginal. At the highest Reynolds number investigated, we observe a 13% drag reduction at constant flow rate, suggesting that significant savings can be achieved by controlling large scales when they are particularly intense (at high Reynolds numbers). Performance gains yielded by the suppression of the VLSM are partially compensated by a higher-than-expected contribution to the skin friction from smaller (yet not necessarily small) scales, as measured using the Constant Power Input (CPI) framework (Gatti et al., 2018) . The analysis of energy spectra suggests that these unexpected expenses are caused by the energisation of outer-scaled eddies residing near the centreline of the channel. We also argue that the CPI framework is better suited than the FIK identity to measure the skin friction contribution of small and large scales. Finally, we discuss the analogy between the currently used Coriolis force and other feedback control strategies.