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Low- and High-Drag Intermittencies in Turbulent Channel Flows.

Rishav Agrawal1,2, Henry C-H Ng1, Ethan A Davis3

  • 1School of Engineering, University of Liverpool, Liverpool L69 3GH, UK.

Entropy (Basel, Switzerland)
|December 8, 2020
PubMed
Summary
This summary is machine-generated.

Turbulent channel flows exhibit intermittent low-drag ("hibernating") and high-drag ("hyperactive") events. Their occurrence frequency is independent of Reynolds number when scaled by inner units, suggesting a memoryless process.

Keywords:
channel flowhibernating turbulencehot-film anemometryturbulence

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Area of Science:

  • Fluid Dynamics
  • Turbulence Research
  • Computational Fluid Dynamics

Background:

  • Turbulent channel flows display intermittent low-drag (hibernating) and high-drag (hyperactive) events.
  • These events occur in Newtonian fluids at specific Reynolds number ranges (Reτ = 70–100).

Purpose of the Study:

  • To further investigate intermittent drag events in turbulent channel flow.
  • To analyze the influence of scaling criteria on the occurrence and characteristics of these events.
  • To explore the underlying physics of drag fluctuations.

Main Methods:

  • Experimental measurements using hot-film anemometry (HFA) and laser Doppler velocimetry (LDV).
  • Direct numerical simulations (DNS) in an extended channel flow domain.
  • Conditional sampling of wall shear stress data based on magnitude and duration criteria.

Main Results:

  • Event frequency is insensitive to Reynolds number when the duration criterion is scaled in inner units.
  • An exponential distribution of event durations suggests a memoryless process.
  • Low-drag events correlate with velocities approaching Virk's maximum drag reduction asymptote near the wall.
  • Conditionally-averaged Reynolds shear stress is higher than the time-averaged value during low-drag events, except very near the wall.

Conclusions:

  • The scaling of conditional sampling criteria significantly impacts the observed frequency of intermittent events.
  • The findings provide insights into the dynamics of drag reduction and enhancement in turbulent flows.
  • The study contributes to understanding the complex nature of turbulence intermittency.