  We present experimental evidence that the superstructures in turbulent
boundary layers comprise of smaller, geometrically self-similar coherent
motions. The evidence comes from identifying and analyzing instantaneous
superstructures from large-scale particle image velocimetry datasets acquired
at high Reynolds numbers, capable of capturing streamwise elongated motions
extending up to 12 times the boundary layer thickness. Given the challenge in
identifying the constituent motions of the superstructures based on streamwise
velocity signatures, a new approach is adopted that analyzes the wall-normal
velocity fluctuations within these very long motions, which reveals the
constituent motions unambiguously. The conditional streamwise energy spectra of
the wall-normal fluctuations, corresponding exclusively to the superstructure
region, are found to exhibit the well-known distance-from-the-wall scaling in
the intermediate scale range. Similar characteristics are also exhibited by the
Reynolds shear stress co-spectra estimated for the superstructure region,
suggesting that geometrically self-similar motions are the constituent motions
of these very-large-scale structures. Investigation of the spatial organization
of the wall-normal momentum-carrying eddies also lends empirical support to the
concatenation hypothesis for the formation of the superstructures. Association
between the superstructures and self-similar motions is reaffirmed on comparing
the vertical correlations of the momentum carrying motions, which are found to
match with the mean correlations. The mean vertical coherence of these motions,
investigated for the log-region across three decades of Reynolds numbers,
exhibits a unique distance-from-the-wall scaling invariant with Reynolds
number. The findings support the prospect for modelling these dynamically
significant motions via data-driven coherent structure-based models.
