Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Piecewise continuous distribution function method: Fluid equations and
  wave disturbances at stratified gas   Wave disturbances of a stratified gas are studied. The description is built
on a basis of the Bhatnagar -- Gross -- Krook (BGK) kinetic equation which is
reduced down the level of fluid mechanics. The double momenta set is introduced
inside a scheme of iterations of the equations operators, dividing the velocity
space along and opposite gravity field direction. At both half-spaces the local
equilibrium is supposed. As the result, the momenta system is derived. It
reproduce Navier-Stokes and Barnett equations at the first and second order in
high collision frequencies. The homogeneous background limit gives the known
results obtained by direct kinetics applications by Loyalka and Cheng as the
recent higher momentum fluid mechanics results of Chen, Rao and Spiegel. The
ground state declines from exponential at the Knudsen regime. The WKB solutions
for ultrasound in exponentially stratified medium are constructed in explicit
form, evaluated and plotted.

25
The categories of this paper are: patt-sol
5
The categories of this paper are: gr-qc
29
The categories of this paper are: funct-an
26
The categories of this paper are: bayes-an
4
The categories of this paper are: cond-mat
28
The categories of this paper are: alg-geom
14
The categories of this paper are: math-ph
23
The categories of this paper are: solv-int
20
The categories of this paper are: acc-phys
16
The categories of this paper are: stat
10
The categories of this paper are: q-bio
1
The categories of this paper are: cs
9
The categories of this paper are: nlin
33
The categories of this paper are: plasm-ph
6
The categories of this paper are: astro-ph
36
The categories of this paper are: cmp-lg