Session Cb - Turbulence Simulation.
ORAL session, Sunday, November 23
302, Moscone Center
In large-eddy simulations (LES), the subgrid-scale (SGS) acceleration is given by the difference between the SGS stress divergence, a, and the SGS pressure gradient. For unbounded incompressible flows this difference is the solenoidal part of the SGS stress divergence. In LES of the atmospheric boundary layer (ABL), the SGS pressure field contains boundary effects through its lower boundary condition given by the normal component of a at the surface. By decomposing a into an irrotational, a solenoidal, and an irrotational solenoidal part (the gradient of a harmonic function) and the SGS pressure gradient field into an irrotational and an irrotational solenoidal part, we show that for incompressible flows the net SGS acceleration is caused by the solenoidal SGS stress divergence a_so and by the SGS pressure gradient field determined only by the normal component of a_so at the surface. To study the SGS acceleration, LES data (128^3) of the ABL are filtered at scales larger than the LES cutoff to obtain resolvable- and SGS quantities. Preliminary results for a moderately convective ABL show larger anisotropy for a_so in the surface layer than for a. The SGS pressure gradient determined by a_so is important within the height of a few LES horizontal grid sizes and decreases exponentially with height. Results from detailed analyses of the a_so and the SGS pressure gradient field will be reported.