  The equations of continuum hydrodynamics can be derived from the Boltzmann
equation, which describes rarefied gas dynamics at the kinetic level, by means
of the Chapman-Enskog expansion. This expansion assumes a small Knudsen number,
and as a consequence, the hydrodynamics equations are able to successfully
describe sound propagation when the frequency of a sound wave is much higher
than the collision frequency of the particles. When both frequencies become
comparable, these equations give a poor account of the experimental
measurements. A series of generalized hydrodynamic equations has been
introduced in the literature along the years in order to improve the continuous
description of small scale properties of fluid flow, as ultrasound propagation.
We will describe herein some of the simplified models that has been proposed so
far.
