Mechanisms Underlying the Generation of the Surface Layer with Linear Increase in Integral Scale

The surface layer (SL) concept originated with law-of-the-wall (LOTW) phenomenology whereby a canonical turbulent boundary layer (TBL) contains an inertia-dominated sublayer outside a very thin viscous or roughness layer adjacent to a wall where key horizontal integral scales increase linearly with wall-normal distance, z. Because mean shear-rate both reflects and contributes to wall-normal turbulent momentum flux, LOTW also requires that the TBL SL be characterized by a single dominant velocity scale set by the level of turbulent momentum flux through the SL. The existence of a single velocity scale and key integral scale ~z implies a 1/z dependence for mean velocity gradient in a layer typically conflated with the SL. In the current study we explore the hypothesis that the linear variation in key SL integral scales is driven directly by blockage of vertical turbulence fluctuations at the impermeable surface unrelated to the existence of mean shear. To test this hypothesis we measured with sPIV two classes of wall-bounded turbulent flow, a flat plate TBL with shear-dominated SL, and a class of shear-less wall-bounded turbulence created by advecting inertia-dominated grid turbulence over a flat plate. The grid turbulence structure, we find, changes in response to blockage such that the vertical fluctuations within wall-modified turbulence eddies develop correlation lengths in the horizontal that increase linearly with z over an inertial shear-free SL within a larger wall-modified layer. We analyze generalizable mechanisms underlying the turbulence correlations specific to the SL.