Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Separation of acoustic waves in isentropic flow perturbations   The present contribution investigates the mechanisms of sound generation and
propagation in the case of highly-unsteady flows. Based on the linearisation of
the isentropic Navier-Stokes equation around a new pathline-averaged base flow,
it is demonstrated for the first time that flow perturbations of a non-uniform
flow can be split into acoustic and vorticity modes, with the acoustic modes
being independent of the vorticity modes. Therefore, we can propose this
acoustic perturbation as a general definition of sound. As a consequence of the
splitting result, we conclude that the present acoustic perturbation is
propagated by the convective wave equation and fulfils Lighthill's acoustic
analogy. Moreover, we can define the deviations of the Navier-Stokes equation
from the convective wave equation as true sound sources. In contrast to other
authors, no assumptions on a slowly varying or irrotational flow are necessary.
Using a symmetry argument for the conservation laws, an energy conservation
result and a generalisation of the sound intensity are provided.

30
The categories of this paper are: q-alg
15
The categories of this paper are: nucl-ex
0
The categories of this paper are: hep-ph
12
The categories of this paper are: nucl-th
35
The categories of this paper are: mtrl-th
3
The categories of this paper are: physics
11
The categories of this paper are: hep-lat
5
The categories of this paper are: gr-qc
9
The categories of this paper are: nlin
27
The categories of this paper are: comp-gas
23
The categories of this paper are: solv-int
13
The categories of this paper are: quant-ph
6
The categories of this paper are: astro-ph
37
The categories of this paper are: supr-con
20
The categories of this paper are: acc-phys
16
The categories of this paper are: stat