Large-scale motions and inner/outer layer interactions in turbulent Couette-Poiseuille flows

We investigate the organization of the momentum-carrying eddies in turbulent Couette-Poiseuille flows. The study relies on a direct numerical simulation (DNS) database covering a wide range of flow configurations from pure Couette to pure Poiseuille flows, at Re-tau approximate to 250 (based on the flow properties at the stationary wall). The study highlights the occurrence of streaky patterns of alternating high and low momentum throughout the channel for all flow configurations, except near zeros of the mean shear, where streaks are suppressed. The mean streak spacing shows a relatively universal distribution in the core of the channel, where it ranges from 50 to 100 local viscous units. The validity of the local viscous scaling in collapsing flow features at different wall distances is confirmed by the analysis of the spanwise velocity spectra, which also highlights (in the case of Couette-like flows) the onset of a secondary low-wavenumber flow mode, superposed on the high-wavenumber flow mode that is responsible for the inner-layer dynamics. The effect of the former mode on the latter is studied by means of the two-point amplitude modulation coefficient, which brings to light a nonlinear modulation phenomenon. Physical mechanisms to explain the modulation effect are proposed, based on the interpretation of the conditional average events. Note that, although similar mechanisms have been previously observed in high-Reynolds-number turbulent boundary layers and channels, the modulation effect is here rather associated with the intrinsic large-scale dynamics of Couette-like flows, and takes place at DNS-accessible Reynolds numbers. We thus believe that the study of Couette-like flows may give an alternative avenue for probing inner/outer interaction effects than canonical channel flows.