“…Such "further reduced" models are often based on modal representations of the dynamics, the modes coming from the leading part of the linearized problem ( [8]), from their Krylov subspace approximations ( [44]), from empirically determined eigenfunctions ( [43], and references therein), or from an appropriate (dissipative) part of the linear problem operator, such as the eigenfunctions of the Stokes operator in the case of the Navier-Stokes equations ( [17], [73], [74]). Beyond the qualitative similarity with singular perturbation methods that construct invariant manifolds and exploit them in controller design for finite dimensional systems ( [57]), there is an extensive interest in the use of inertial manifolds and approximate inertial manifolds for closed loop dynamics analysis and controller design (see, for example, [12,15,69,70]). Motivated by the theory of Inertial Manifolds and Approximate Inertial Manifolds, we will also explore a second (Nonlinear Galerkin, NLG) model reduction step.…”