Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: An introduction to the derivation of surface balance equations without
  the excruciating pain   Analyzing complex fluid flow problems that involve multiple coupled domains,
each with their respective set of governing equations, is not a trivial
undertaking. Even more complicated is the elaborate and tedious task of
specifying the interface and boundary conditions between various domains. This
paper provides an elegant, straightforward and universal method that considers
the nature of those shared boundaries and derives the appropriate conditions at
the interface, irrespective of the governing equations being solved. As a first
example, a well-known interface condition is derived using this method. For a
second example, the set of boundary condi- tions necessary to solve a baseline
aerothermodynamics coupled plain/porous flow problem is derived. Finally, the
method is applied to two more flow configurations, one consisting of an
impermeable adiabatic wall and the other an ablating surface.

10
The categories of this paper are: q-bio
14
The categories of this paper are: math-ph
22
The categories of this paper are: chao-dyn
9
The categories of this paper are: nlin
24
The categories of this paper are: dg-ga
26
The categories of this paper are: bayes-an
29
The categories of this paper are: funct-an
15
The categories of this paper are: nucl-ex
32
The categories of this paper are: atom-ph
2
The categories of this paper are: math
6
The categories of this paper are: astro-ph
1
The categories of this paper are: cs
8
The categories of this paper are: hep-ex
12
The categories of this paper are: nucl-th
33
The categories of this paper are: plasm-ph
21
The categories of this paper are: adap-org