Local Eddy Structure Underlying the Linear Growth in Integral Scale in Two Different Wall-Bounded Turbulent Flows

Two classes of wall-bounded turbulent flows are considered, each of which produce surface layers characterized by linear scaling normal to the wall (z) of the horizontal correlation length of wall-normal (“vertical”) fluctuations, w': l_ww,x(z). Quantifications from two distinctly different sets of wind tunnel experiments are compared: 1) a canonical flat plate turbulent boundary layer with a shear-dominated surface layer, and 2) a class of shear-free wall-bounded surface layers created by advecting inertia-dominated grid turbulence at moderate Reynolds numbers over a flat plate. The surface layer exists both in the presence and absence of mean shear-rate and only integral length scales of w' increase linearly with distance from the surface. This indicates that the surface layer is generated from the blockage of vertical fluctuations by the impermeable surface. Analysis of instantaneous turbulence structures through conditional sampling was conducted to reveal the local structure and distribution of concentrations of vertical velocity fluctuations within the individual samples that underly the statistically identified linear increase in l_ww,x with z, both in the presence and absence of mean shear-rate. We contrast the quantified structure of the wall modulated w' “eddies” with the general form proposed by Townsend (1976) in context with “attached eddy” spectral representations (e.g., Perry, Henbest & Chong, 1986).