Low- and High-Drag Intermittencies in Turbulent Channel Flows

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.

On the underlying drag-reduction mechanisms of flow-control strategies in a transitional channel flow: temporal approach

“…Indeed, the hibernating intervals are approaches to lower-branch TW solutions, while the active intervals are close to an upper-branch TW solution. Very recently, this low-and high-drag intermittency is observed and quantified in experiments and in good agreement with DNS results [40]. It is worth noting that the temporal and spatial analyses on the relationship between temporal dynamics in minimal domains and spatiotemporal dynamics in extended domains also yield very similar results for the hibernating and active intervals [41] at friction Reynolds numbers ranging from 70 to 100.…”

“…In particular. a grid convergence and domain dependence have been tested in our previous studies for no-control cases [41,40] and in the current study for control cases. A numerical grid system is generated on N x × N y × N z (in x, y, and z) meshes, where a Fourier-Chebyshev-Fourier spectral spatial discretization is applied to all variables.…”

Intermittency, Moments, and Friction Coefficient during the Subcritical Transition of Channel Flow

“…( a ) Spanwise distributions of the time averaged velocity in the midplane , and ( b ) the time averaged velocity profiles at different spanwise positions. The measurements of Reference [ 33 ] are added in ( b ) as references.…”

“…Turbulence intensities at the channel center are measured and are found to increase rapidly around Re = 1050, reach a peak at Re = 1140, and then gradually decrease with increasing Re [ 29 ]. The intermittent low- and high-drag events are investigated numerically and experimentally [ 30 , 31 , 32 ], and it is found that the conditionally averaged Reynolds shear stress is higher than the mean value during the low-drag events [ 33 ]. Based on simulations of channel flows with constant pressure gradients, a linear correlation for the wall shear stress is observed between its kurtosis and its skewness squared [ 34 ].…”

On the Comparison of Flow Physics between Minimal and Extended Flow Units in Turbulent Channels

“…Embedded in a turbulent flow is inherent intermittency. The dynamics of wallbounded turbulence fluctuate between high, intermediate, and low-drag states in a stochastic fashion, which illuminates the self-sustaining process in shear flows [1][2][3][4]. The most straightforward simulation approach to identify the intermittency and self-sustaining structures is the so-called minimal flow unit (MFU) approach [5].…”