An investigation of inner and outer-scaled structures in turbulent boundary layers.

The interaction between the large outer-scaled events in the logarithmic region and the near-wall buffer region structure will be investigated. The latter consists of the now quite well defined near-wall cycle of streaks and vortices (with a dominant spanwise spacing of 100 wall units), whilst the former has been recently noted to consist of very long meandering regions of positive and negative $u$ fluctuations (frequently exceeding $20$ boundary layer thicknesses in length, and with it's own associated vortical structure). Interestingly, these large outer-scaled structures maintain a footprint on the wall, seeming to modulate the near-wall cycle. This provides a mechanism for the percolation of very low wavenumber energy from the log region into the near-wall $u$ fluctuations, explaining the Reynolds number dependence of both the streamwise energy spectra $\Phi_{uu}$ and the peak of the RMS fluctuations in the near-wall region. Recent hotwire rake measurements will be presented along with results from direct numerical simulations.