Intrinsic expansions in large Grashof numbers for the steady states of the Navier-Stokes equations

We enable a theory of intrinsic asymptotic expansions for the steady state solutions of the full Navier-Stokes equations. Such a theory was first developed in Foias et al (2024 Commun. Pure Appl. Anal. 23, 269-303) for Galerkin approximations. To overcome the lack of local compactness in infinite dimensional spaces, we introduce the notion of asymptotic expansions in nested spaces. When the inclusion maps between these spaces are compact, we establish the existence of such an asymptotic expansion for a subsequence of any bounded sequence. This consequently yields an intrinsic asymptotic expansion in a single normed space. We apply this result to the steady states of the Navier-Stokes equations by utilizing the spectral fractional Sobolev spaces. In the case of the two-dimensional periodic boundary conditions, more properties relating the terms of the asymptotic expansion are obtained.