Generalizing the Concept of the Surface Layer*

The classical description of a "surface layer" is a wall-adjacent region within a high Reynolds number turbulent boundary layer in which turbulence fluctuations are directly modified by surface impermeability and where turbulence integral scales scale with z, the distance from the surface. This description includes both modulation by surface blockage and production by mean shear. To generalize the concept of a "surface layer," we consider these effects separately. Canonical boundary layer PIV data from LMFL is compared to SPIV measurements of grid turbulence blown over an impermeable flat plate to characterize the direct influence of surface impermeability on turbulence structure. We identify a region for the grid turbulence case where integral length scales of fluctuating vertical velocity grow with z and where there exists a deficit in turbulent kinetic energy. The latter is not observed in the canonical turbulent boundary layer data but is consistent with theory developed by Hunt & Graham (1978) and experiments of Thomas & Hancock (1976). By isolating surface blockage effects on grid turbulence, we aim to isolate key surface layer elements associated with the creation of attached turbulence eddies in the absence of turbulence production.