Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: The influence of near-wall density and viscosity gradients on turbulence
  in channel flows
8
The categories of this paper are: hep-ex
34
The categories of this paper are: chem-ph
28
The categories of this paper are: alg-geom
36
The categories of this paper are: cmp-lg
13
The categories of this paper are: quant-ph
4
The categories of this paper are: cond-mat
15
The categories of this paper are: nucl-ex
37
The categories of this paper are: supr-con
5
The categories of this paper are: gr-qc
10
The categories of this paper are: q-bio
24
The categories of this paper are: dg-ga
6
The categories of this paper are: astro-ph
17
The categories of this paper are: q-fin
1
The categories of this paper are: cs
7
The categories of this paper are: hep-th
9
The categories of this paper are: nlin