Uniform momentum zones in turbulent channel flow

The characteristics of uniform momentum zones (UMZs) and UMZ interfaces in turbulent channel flow (TCF) at friction Reynolds number ranging from 600 to 2000 are investigated. The focus is on the Reynolds number effect and the difference among the turbulent channel flow, turbulent pipe flow (TPF), and turbulent boundary layer (TBL). The UMZ is identified using the approach of probability density histograms, a discrete approximation of the probability density function. We find that the average number of UMZs increases with the Reynolds number, similar to the findings in the TBL. The first group of UMZ in the TCF, namely the quiescent core, has a thicker thickness at a higher Reynolds number. Therefore, the UMZ closer to the channel centre significantly influences high Reynolds number wall turbulence by occupying a large portion of the channel. With increasing Reynolds number, the intensity of spanwise vorticity at the UMZ interface increases, while the thickness of the local maximum decreases. The dominance of prograde vorticity at the interface also increases with the Reynolds number. Moreover, the contortion of the UMZ interface is significantly enhanced at high Reynolds numbers than at low Reynolds numbers in both the streamwise and spanwise directions. Furthermore, the number of UMZ for a given snapshot is related to the overall velocity fluctuations in the subregion, increasing with large-scale Q2 events and decreasing with large-scale Q4 events. Comparing the three flow configurations, UMZs show similarities in their trends, along with two notable differences. First, the TPF has more UMZs compared to the TCF and TBL. Second, the innermost group in the TPF is considerably thicker than that of the higher ranks, a pattern absent in the TCF and TBL. The present findings advance the understanding of UMZs in TCF and demonstrate the similarities and differences between the cases of TCF and TBL.