Some Remarks on the Equations of Burnett and Grad

Both, Grad and Burnett, derived sets of equations from the Boltzmann equation, which improve the classicallaws of Navier-Stokes and Fourier for the description of rarefied gases, i.e. gases with Knudsen numbers above 0.01. Using results of other authors, it is shown that both sets of equations are closer related then is commonly thought - indeed, the Burnett equations can be derived from Grad’s equations by the so-called Maxwellian iteration. This derivation allows to identify the proper form of the Burnett equations in non-inertial frames. Moreover, Grad’s equations with more than 13 moments can describe linear boundary layers while these are not among the phenomena which can be described by Burnett’s equations.