Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Non-modal transient growth of disturbances in pulsatile and oscillatory
  pipe flow   Laminar flows through pipes driven at steady, pulsatile or oscillatory rates
undergo a sub-critical transition to turbulence. We carry out an extensive
linear non-modal stability analysis of these flows and show that for
sufficiently high pulsation amplitudes the stream-wise vortices of the classic
lift-up mechanism are outperformed by helical disturbances exhibiting an
Orr-like mechanism. In oscillatory flow, the energy amplification depends
solely on the Reynolds number based on the Stokes-layer thickness and for
sufficiently high oscillation frequency and Reynolds number, axisymmetric
disturbances dominate. In the high-frequency limit, these axisymmetric
disturbances are exactly similar to those recently identified by Biau (2016)
for oscillatory flow over a flat plate. In all regimes of pulsatile and
oscillatory pipe flow, the optimal helical and axisymmetric disturbances are
triggered in the deceleration phase and reach their peaks in typically less
than a period. Their maximum energy gain scales exponentially with Reynolds
number of the oscillatory flow component. Our numerical computations unveil a
plausible mechanism for the turbulence observed experimentally in pulsatile and
oscillatory pipe flow.

5
The categories of this paper are: gr-qc
18
The categories of this paper are: econ
34
The categories of this paper are: chem-ph
29
The categories of this paper are: funct-an
16
The categories of this paper are: stat
7
The categories of this paper are: hep-th
32
The categories of this paper are: atom-ph
11
The categories of this paper are: hep-lat
31
The categories of this paper are: ao-sci
17
The categories of this paper are: q-fin
14
The categories of this paper are: math-ph
37
The categories of this paper are: supr-con
9
The categories of this paper are: nlin
33
The categories of this paper are: plasm-ph
27
The categories of this paper are: comp-gas
22
The categories of this paper are: chao-dyn