Uniform momentum zones in turbulent boundary layers

We examine the properties of large regions of uniform streamwise momentum in turbulent boundary layers using databases obtained from particle image velocimetry that extend over 2.3 $\delta$ (where $\delta$ denotes the boundary layer thickness) in the streamwise direction and 1.2 $\delta$ in the wall-normal direction. The investigation covers a large range of Reynolds numbers, spanning more than an order of magnitude ($Re_\tau = 10^3-10^4$), but with adequate spatial resolution to resolve most structural features. This enables accurate descriptions of the structural evolution of the uniform momentum zones (UMZs) as a function of Reynolds numbers. Our analysis reveals evidence of a hierarchical length scale distribution of structures within turbulent boundary layers, leading to zonal-like organisations. The Reynolds number dependence of these features is also investigated. Interpretation of these results is aided by employing synthetic velocity fields generated by using the attached-eddy model. Comparisons between the model and experimental results show that the widely proposed packet model would lead to a distribution of UMZs that conforms closely to those observed experimentally in this study.