Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Near wall coherence in wall-bounded flows and implications for flow
  control   Opposition-control of the energetic cycle of near wall streaks in
wall-bounded turbulence, using numerical approaches, has shown promise for drag
reduction. For practical implementation, opposition control is only realizable
if there is a degree of coherence between the sensor--actuator pairs of the
control system (these sensors and actuators should typically be wall-based to
avoid parasitic drag). As such, we here inspect the feasibility of real-time
control of the near-wall cycle, by considering the coherence between a
measurable wall-quantity, being the wall-shear stress fluctuations, and the
streamwise and wall-normal velocity fluctuations in a turbulent boundary layer.
Synchronized spatial and temporal velocity data from numerical simulations at
$Re_\tau \approx 590$ and $ 2000$ are employed. It is shown that the spectral
energy of the streamwise velocity fluctuations that is stochastically
incoherent with wall signals is independent of Reynolds number in the near wall
region. Consequently, the streamwise energy-fraction that is stochastically
wall-coherent grows with Reynolds number due to the increasing range of
energetic large scales. This thus implies that a wall-based control system has
the ability to manipulate a larger portion of the total turbulence energy at
off-wall locations, at higher Reynolds numbers. Coherence values of 0.55 and
0.4, which are considerably lower than the maximum possible coherence 1, were
found between the streamwise and wall-normal velocity fluctuations at the near
wall peak in the energy spectrogram, respectively, and the streamwise
fluctuating friction velocity. This suggests that a closed-loop drag reduction
scheme targeting near wall cycle of streaks alone will be of limited success in
practice as the Reynolds number grows.

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The categories of this paper are: ao-sci
16
The categories of this paper are: stat
37
The categories of this paper are: supr-con
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The categories of this paper are: solv-int
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The categories of this paper are: comp-gas
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The categories of this paper are: funct-an
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The categories of this paper are: q-fin
4
The categories of this paper are: cond-mat
34
The categories of this paper are: chem-ph
20
The categories of this paper are: acc-phys
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The categories of this paper are: mtrl-th
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The categories of this paper are: atom-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: bayes-an
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The categories of this paper are: astro-ph
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The categories of this paper are: q-alg