Generalizing the Concept of a Surface Layer as Wall-Modulated Eddies

The classical description of the surface layer (SL), often called the log layer, is the inertia-dominated wall-adjacent region within the high Reynolds number turbulent boundary layer (TBL) in which integral scales scale linearly on the distance from surface z and where law-of-the-wall scaling applies. We generalize the SL concept to the wall-adjacent inertia-dominated region in any wall-bounded turbulent flow with vertical turbulence fluctuations w' directly modified by surface impermeability and causing integral scales involving w' to scale on z. We further explore the hypothesis that coherent motions can be separated into those that are directly modulated by the surface and those that are not. To do so, we compare analysis of time-resolved PIV data from two sets of wind tunnel experiments: (1) grid turbulence from three different grids interacting with an impermeable flat plate and (2) the classical TBL. In both flows we successfully identify SL regions characterized by integral scales that grow linearly with z and we apply conditional wavelet filtering to extract coherent wall-modulated eddy structures and quantify their contributions to SL statistics.