Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Generalised quasilinear approximations of turbulent channel flow: Part
  1. Streamwise nonlinear energy transfer   A generalised quasilinear (GQL) approximation (Marston \emph{et al.},
\emph{Phys. Rev. Lett.}, vol. 116, 104502, 2016) is applied to turbulent
channel flow at $Re_\tau \simeq 1700$ ($Re_\tau$ is the friction Reynolds
number), with emphasis on the energy transfer in the streamwise wavenumber
space. The flow is decomposed into low and high streamwise wavenumber groups,
the former of which is solved by considering the full nonlinear equations
whereas the latter is obtained from the linearised equations around the former.
The performance of the GQL approximation is subsequently compared with that of
a QL model (Thomas \emph{et al.}, \emph{Phys. Fluids.}, vol. 26, no. 10,
105112, 2014), in which the low-wavenumber group only contains zero streamwise
wavenumber. It is found that the QL model exhibits a considerably reduced
multi-scale behaviour at the given moderately high Reynolds number. This is
improved significantly by the GQL approximation which incorporates only a few
more streamwise Fourier modes into the low-wavenumber group, and it reasonably
well recovers the distance-from-the-wall scaling in the turbulence statistics
and spectra. Finally, it is proposed that the energy transfer from the low to
the high-wavenumber group in the GQL approximation, referred to as the
`scattering' mechanism, depends on the neutrally stable leading Lyapunov
spectrum of the linearised equations for the high wavenumber group. In
particular, it is shown that if the threshold wavenumber distinguishing the two
groups is sufficiently high, the scattering mechanism can completely be absent
due to the linear nature of the equations for the high-wavenumber group.

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The categories of this paper are: q-fin
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The categories of this paper are: eess
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The categories of this paper are: stat
6
The categories of this paper are: astro-ph
1
The categories of this paper are: cs
30
The categories of this paper are: q-alg
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The categories of this paper are: hep-ex
10
The categories of this paper are: q-bio
34
The categories of this paper are: chem-ph
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The categories of this paper are: econ
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The categories of this paper are: acc-phys
21
The categories of this paper are: adap-org
5
The categories of this paper are: gr-qc
2
The categories of this paper are: math
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The categories of this paper are: patt-sol
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The categories of this paper are: cmp-lg