Subgrid scale variance and dissipation of a scalar field in large eddy simulations

Subgrid scale (SGS) variance of a scalar field in large eddy simulations is only properly defined in relation to a probability density function. This solves a reported problem in the variance definition [Cook and Riley, Phys. Fluids 6, 2868 (1994); Cook, Riley, and Kosaly, Combust. Flame 109, 332 (1 997)] and allows to write a simple evolution equation for the scalar varian ce. This equation shows that a recently proposed model for scalar dissipati on in terms of the large-scale gradients [Pierce and Moin, Phys. Fluids 10, 3041 (1998)] implies dissipation and production canceling out, preventing variance decay and complete mixing at SGS level. An alternative simple mode l for dissipation in terms of a SGS mixing characteristic time is proposed and tested here. (C) 2001 American Institute of Physics.