Lax representations for the Navier--Stokes equations

Abstract

We construct a family of Lax representations for the three-dimensional incompressible Navier--Stokes equations in vorticity form. The key idea is to introduce an auxiliary vector field, termed the \emph{vorticity-diffusion potential}, which recasts the viscous Laplacian diffusion term as a Lie-bracket advection term. On solutions of the defining equation for this potential, the viscous vorticity equation takes the form of a pure Lie-transport equation along a modified divergence-free velocity field, while the defining constraint itself is encoded as the curl-free compatibility condition for an auxiliary scalar potential. We further show that the reduction of this three-dimensional system to two spatial dimensions yields a Lax representation for the two-dimensional Navier--Stokes vorticity equation, which we generalize to a family parametrized by an arbitrary smooth function of one variable.