Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Motion and wake structure of spherical particles   This paper presents results from a flow visualization study of the wake
structures behind solid spheres rising or falling freely in liquids under the
action of gravity. These show remarkable differences to the wake structures
observed behind spheres held fixed. The two parameters controlling the rise or
fall velocity (i.e., the Reynolds number) are the density ratio between sphere
and liquid and the Galileo number.

29
The categories of this paper are: funct-an
19
The categories of this paper are: eess
5
The categories of this paper are: gr-qc
21
The categories of this paper are: adap-org
22
The categories of this paper are: chao-dyn
24
The categories of this paper are: dg-ga
6
The categories of this paper are: astro-ph
35
The categories of this paper are: mtrl-th
0
The categories of this paper are: hep-ph
37
The categories of this paper are: supr-con
4
The categories of this paper are: cond-mat
16
The categories of this paper are: stat
25
The categories of this paper are: patt-sol
14
The categories of this paper are: math-ph
28
The categories of this paper are: alg-geom
13
The categories of this paper are: quant-ph