We characterize the behavior of stochastic Navier–Stokes on \(\mathbb {T}\times [-1,1]\) with Navier boundary conditions at high Reynolds number when initialized near Couette flow subject to small additive stochastic forcing. We take additive noise of strength \(\nu ^{5/6} \Phi \textrm{d}V_t + \nu ^{2/3+\alpha } \Psi \textrm{d}W_t\) , where \(\Phi \textrm{d}V_t\) has spatial correlation in \(H_0^3\) and acts only on x-independent modes of the vorticity, while \(\Psi \textrm{d}W_t\) has spatial correlation in a lower order, anisotropic, Sobolev space \(\mathcal {H}\) and acts on x-dependent-modes. We take the initial x-independent modes in the perturbation to be small in \(H_0^3\) in a \(\nu \) -independent sense, while the non-zero x-modes are taken to be \(O(\nu ^{1/2 + \alpha })\) in \(\mathcal {H}\) . The parameter \(\alpha \) is taken to be \(\alpha > 1/12\) . Letting \(\omega \) solve the resulting perturbation equation, we split \(\omega \) into the zero x-modes \(\omega _0\) and the non-zero x-modes \(\omega _{\ne }\) . We demonstrate that an averaging principle holds wherein \(\omega _{\ne }\) is the fast variable and \(\omega _0\) is the slow variable, deriving a closed nonlinear evolution equation on \(\omega _0\) that holds over long time-scales (while the fast \(\omega _{\ne }\) modes solve a ‘pseudo-linearized’ equation to leading order with dynamics dominated by inviscid damping and enhanced dissipation). This work can also be considered the stochastic analogue of the stability threshold problem for shear flows. Furthermore, we explain the connections to the Stochastic Structural Stability Theory (S3T) in the physics literature.
