A Note on Non-Hydrodynamic Solutions of Kinetic Systems

We show that the one-dimensional three-component Grad system admits solutions that violate the Chapman–Enskog scaling in Knudsen number. In particular, there exist solutions that do not converge to the analogues of the Euler and Navier–Stokes equations for vanishing Knudsen number. These non-hydrodynamic solutions correspond to a fast spectral manifold in kinetic phase space.