Data-driven decomposition of the streamwise turbulence kinetic energy in boundary layers. Part 2. Integrated energy and $A_1$

Abstract: Scalings of the streamwise velocity energy spectra in turbulent boundary layers were considered in Part 1. A spectral decomposition analysis provided a means to separate out attached and non-attached eddy contributions and was used to generate three spectral sub-components, one of which is a close representation of the spectral signature induced by self-similar, wall-attached turbulence. Since sub-components of the streamwise turbulence intensity $\overline{u^2}$ follow from an integration of the velocity energy spectra, we here focus on the scaling of the former. Wall-normal profiles and Reynolds number trends of the three individual, additive sub-components of the streamwise turbulence intensity are examined. This allows for revisiting the scaling of the turbulence intensity in more depth, in comparison to evaluating the total streamwise turbulence intensity. Based on universal trends across all Reynolds numbers considered, some evidence is given for a Townsend--Perry constant of $A_1 = 0.98$, which would describe the wall-normal logarithmic decay of the turbulence intensity per Townsend's attached-eddy hypothesis. It is also demonstrated how this constant can be consistent with the Reynolds-number increase of the streamwise turbulence intensity in the near-wall region.