On the Structure of Wall Turbulence in the Thermally Neutral Atmospheric Surface Layer

Recent experimental investigations of the canonical turbulent boundary layer at high Reynolds numbers has provided evidence on the active role of very large-scale structures, extending in the streamwise direction for several boundary layer heights, and modulating near-wall turbulence from the energy containing eddy scales down to the dissipative scales. However, the physical mechanisms governing such interactions are not completely clear yet, and the reason may be related to the fact that the structure of wall turbulence at high Reynolds numbers still deserves further investigation. In this contribution we present recent results on the structural population in wall turbulence. We compare statistical trends obtained in two very different Reynolds number experiments, one in the atmospheric surface layer at SLTEST and one in a flat plate turbulent boundary layer. While the very large scale structures of turbulence and the near-wall turbulent streaks are observed to have a well-defined location in physical space and in the energetic domain, based on the frequency or wave number spectra, the intermediate scale motions that manifest as ramplike structures still seem to suffer from Reynolds number effects. Results suggest that outer scaling may not be appropriate, implying that ramplike structures are more likely confined to the near-wall region in very high Reynolds-number flows such as the atmospheric surface layer. Spatially resolved measurements at high Reynolds numbers are needed to univocally define the correct scaling of ramplike structures and to assess Reynolds number effects in the structural description of zero pressure gradient turbulent boundary layers.