Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Designing vortices in pipe flow with topography-driven Langmuir
  circulation   We present direct numerical simulation of a mechanism for creating
longitudinal vortices in pipe flow, compared with a simple model theory. By
furnishing the pipe wall with a pattern of crossing waves secondary flow in the
form of spanwise vortex pairs is created. The mechanism `CL1' is kinematic and
known from oceanography as a driver of Langmuir circulation. CL1 is strongest
when the `wall wave' vectors make an accute angle with the axis,
$\varphi=10^\circ$ - $20^\circ$ (a `contracted eggcarton'), changes sign near
$45^\circ$ and is weak and opposite beyond this angle. A competing, dynamic
mechanism driving secondary flow in the opposite sense is also observed created
by the azimuthally varying friction. Whereas at smaller angles `CL1' prevails,
the dynamic effect dominates when $\varphi\gtrsim 45^\circ$ reversing the flow.
Curiously, circulation strength is a faster-than-linearly increasing function
of Reynolds number for the contracted case.
  We explore an analogy with Prandtl's secondary motion of the second kind in
turbulence. A transport equation for average streamwise vorticity is derived,
and we analyse it for three different crossing angles, $\varphi=18.6^\circ,
45^\circ$ and $60^\circ$. Mean-vorticity production is organised in a ring-like
structure with the two rings contributing to rotating flow in opposite senses.
For the larger $\varphi$ the inner ring decides the main swirling motion,
whereas for $\varphi=18.6^\circ$ outer-ring production dominates. For the
larger angles the outer ring is mainly driven by advection of vorticity and the
inner by deformation (stretching) whereas for $\varphi=18.6^\circ$ both
contribute approximately equally to production in the outer ring.

13
The categories of this paper are: quant-ph
15
The categories of this paper are: nucl-ex
36
The categories of this paper are: cmp-lg
32
The categories of this paper are: atom-ph
25
The categories of this paper are: patt-sol
33
The categories of this paper are: plasm-ph
4
The categories of this paper are: cond-mat
9
The categories of this paper are: nlin
6
The categories of this paper are: astro-ph
37
The categories of this paper are: supr-con
23
The categories of this paper are: solv-int
27
The categories of this paper are: comp-gas
17
The categories of this paper are: q-fin
11
The categories of this paper are: hep-lat
24
The categories of this paper are: dg-ga
1
The categories of this paper are: cs