  Turbulence in wall-bounded shear flow results from a synergistic interaction
between linear non-normality and nonlinearity in which non-normal growth of a
subset of perturbations configured to transfer energy from the externally
forced component of the turbulent state to the perturbation component maintains
the perturbation energy, while the subset of energy-transferring perturbations
is replenished by nonlinearity. Although it is accepted that both linear
non-normality mediated energy transfer from the forced component of the mean
flow and nonlinear interactions among perturbations are required to maintain
the turbulent state, the detailed physical mechanism by which these processes
interact in maintaining turbulence has not been determined. In this work a
statistical state dynamics based analysis is performed on turbulent Couette
flow at $R=600$ and a comparison to DNS is used to demonstrate that the
perturbation component in Couette flow turbulence is replenished by a
non-normality mediated parametric growth process in which the fluctuating
streamwise mean flow has been adjusted to marginal Lyapunov stability. It is
further shown that the alternative mechanism in which the subspace of
non-normally growing perturbations is maintained directly by
perturbation-perturbation nonlinearity does not contribute to maintaining the
turbulent state. This work identifies parametric interaction between the
fluctuating streamwise mean flow and the streamwise varying perturbations to be
the mechanism of the nonlinear interaction maintaining the perturbation
component of the turbulent state, and identifies the associated Lyapunov
vectors with positive energetics as the structures of the perturbation subspace
supporting the turbulence.

Fluid Dynamics