Low- and High-Drag Intermittencies in Turbulent Channel Flows

Authors

Rishav Agrawal, University of Liverpool, Liverpool & Coventry UniversityFollow
Henry C.-H. Ng, University of Liverpool, LiverpoolFollow
Ethan A. Davis, University of Nebraska-LincolnFollow
Jae Sung Park, University of Nebraska-LincolnFollow
Michael D. Graham, University of Wisconsin-MadisonFollow
David J.C. Dennis, University of Liverpool, LiverpoolFollow
Robert J. Poole, University of Liverpool, LiverpoolFollow

ORCID IDs

https://orcid.org/0000-0003-2192-2429

https://orcid.org/0000-0001-9039-483X

https://orcid.org/0000-0002-7966-1164

https://orcid.org/0000-0003-4983-4949

https://orcid.org/0000-0003-0214-7885

https://orcid.org/0000-0001-6686-4301

Date of this Version

2020

Citation

Entropy 2020, 22, 1126; doi:10.3390/e22101126

Comments

2020 by the authors.

Abstract

Recent direct numerical simulations (DNS) and experiments in turbulent channel flow have found intermittent low- and high-drag events in Newtonian fluid flows, at Reτ = uτh/ν between 70 and 100, where uτ, h and ν are the friction velocity, channel half-height and kinematic viscosity, respectively. These intervals of low-drag and high-drag have been termed “hibernating” and “hyperactive”, respectively, and in this paper, a further investigation of these intermittent events is conducted using experimental and numerical techniques. For experiments, simultaneous measurements of wall shear stress and velocity are carried out in a channel flow facility using hot-film anemometry (HFA) and laser Doppler velocimetry (LDV), respectively, for Reτ between 70 and 250. For numerical simulations, DNS of a channel flow is performed in an extended domain at Reτ = 70 and 85. These intermittent events are selected by carrying out conditional sampling of the wall shear stress data based on a combined threshold magnitude and time-duration criteria. The use of three different scalings (so-called outer, inner and mixed) for the time-duration criterion for the conditional events is explored. It is found that if the time-duration criterion is kept constant in inner units, the frequency of occurrence of these conditional events remain insensitive to Reynolds number. There exists an exponential distribution of frequency of occurrence of the conditional events with respect to their duration, implying a potentially memoryless process. An explanation for the presence of a spike (or dip) in the ensemble-averaged wall shear stress data before and after the low-drag (or high-drag) events is investigated. During the low-drag events, the conditionally-averaged streamwise velocities get closer to Virk’s maximum drag reduction (MDR) asymptote, near the wall, for all Reynolds numbers studied. Reynolds shear stress (RSS) characteristics during these conditional events are investigated for Reτ = 70 and 85. Except very close to the wall, the conditionally-averaged RSS is higher than the time-averaged value during the low-drag events.