Comparison of the genesis and dynamics of Reynolds shear stress within boundary layer transitional spots and near-wall developed turbulence*

In the late stages of boundary layer transition, localized packets of turbulence emerge, grow and coalesce to form the front of the turbulent boundary layer. These localized packets, or turbulent spots, have long been supposed to be more ordered versions of the structures found in developed wall-bounded turbulence. If true, it should be possible to construct a reduced-order model of the developed turbulence from information learned about the spots. Extensive examination of the spots' conditional first- and second-order statistical properties suggests that the supposition is true (Park et al. 2012, Sayadi et al. 2013, and 2014). Wu et al. 2017 later confirmed the existence of vortex clusters resembling spots deep in a developed turbulent boundary layer. The present work aims to answer this question: Does the similarity between transitional turbulent spots and developed wall-bounded turbulence extend beyond statistical and visual resemblance to their structural and temporal dynamics? We use three-dimensional structure identification and temporal tracking algorithms to identify the momentum-carrying structures within transitional turbulent spots and investigate their dynamical processes.