Structural characteristics of uniform momentum zones in turbulent boundary layers for Reynolds number below 1000

This study experimentally investigates the structural evolution of uniform momentum zones (UMZs) in a fully developed planar turbulent boundary layer. The friction Reynolds number (Reτ) in the detection region ranges up to 1000. Instantaneous velocity fields are measured using time-resolved particle image velocimetry, and the identification of UMZs is based on the probability density function (p.d.f.) of streamwise velocities. The objective of this investigation is to examine the effects of Reτ on the modal velocity, border heights, centroid heights, and thicknesses of UMZs. The peaks of the p.d.f. distribution of UMZ numbers (NUMZ) for Reτ < 1000 show a clear dependence on ln(Reτ). Both higher Reτ and NUMZ values enhance the modal velocities of UMZs, with Reτ playing a dominant role. As Reτ increases, the formation of UMZs closer to the wall displaces upper UMZs further away, resulting in a more compressed hierarchical structure in both border heights and centroid heights distributions. The variation of Reτ also influences the flow structure by altering the dominance of Q4 events to Q2 events as Reτ increases, leading to significant suppression of UMZ thicknesses due to sweeping flows at lower Reτ and increased thickness discrepancies at moderate Reτ due to ejection flows.