Statistics of conditionally filtered dissipation and diffusion in large eddy simulation

In the large eddy simulation of turbulent reacting flows, thefiltered reaction terms are unclosed. It is difficult to construct thesubgrid scale (SGS) model for filtered reaction terms due to the absenceof a universal energy cascade process, which is the foundation of SGS modelsin fluid turbulence. With the conditional filter approach, theSGS models for conditionally filtered reaction terms can be closed, butconditional diffusion and dissipation have to be modeled. We use a directnumerical simulation of turbulent mixing to investigate the scale-dependenceof the conditional filtered diffusion and dissipation. The results show that theconditional filtered diffusion depends on large scales, but the conditionalfiltered dissipation depends on both large and small scales. The effectsof small scales on the conditional filtered dissipation have to be includedin its SGS models.