*Article* **Low- and High-Drag Intermittencies in Turbulent Channel Flows**

**Rishav Agrawal 1,2, Henry C.-H. Ng 1, Ethan A. Davis 3, Jae Sung Park 3, Michael D. Graham 4, David J.C. Dennis <sup>1</sup> and Robert J. Poole 1,\***


Received: 7 September 2020; Accepted: 29 September 2020; Published: 4 October 2020

**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 *Re<sup>τ</sup>* = *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.

**Keywords:** hibernating turbulence; hot-film anemometry; turbulence; channel flow
