“…Even though nonlinear forcings are not expected to be low rank over all frequencies and wavenumbers, especially in higher-Reynolds-number flows, the present analysis suggests that orderly structures in the forcing terms may still be found, and these can be particularly important in the dynamics of turbulent flows, such as in the self-sustained process in wall-bounded flows. A recent example is found in Morra et al (2020), where the nonlinear forcing was shown to be rank-2 for the most energetic frequencies of channel flows at moderate Reynolds numbers (Re τ = 180, 550), suggesting that, even though the forcing may not be low rank over all frequencies, it has low rank for the most dynamically important frequencies of the flow. In the present Couette flow, despite the apparent complexity of the bulk of nonlinear terms, order can still be found.…”