SUBGRID-SCALE ENERGY-TRANSFER AND NEAR-WALL TURBULENCE STRUCTURE

Conditional averages of the velocity field, subgrid-scale (SGS) stress es and SGS dissipation are calculated using the velocity fields obtain ed from the DNS of plane channel flow. The detection criteria isolate the coherent turbulent structures that contribute most strongly to the energy transfer between the large, resolved scales and the subgrid, u nresolved, ones. Separate averages are computed for forward and backwa rd scatter. The interscale energy transfer is found to be strongly cor related with the presence of the turbulent structures typical of wall- bounded flows: quasi-streamwise and hairpin vortices, sweeps and eject ions. In the buffer layer, strong SGS dissipation is observed near lif ted shear layers; the forward scatter is associated with ejections, th e backscatter with sweeps. Both backward and forward scatter occur in close proximity to longitudinal vortices that form a very shallow angl e to the wall. Further away from the solid boundary, in the logarithmi c region and beyond, both forward and backward energy transfer are ass ociated prevalently with ejections. Eddy viscosity models do not predi ct the three-dimensional structure of these events adequately, while s cale-similar models reproduce the correlation between the large-scale coherent structures and the SGS events more accurately. (C) 1996 Ameri can Institute of Physics.