LEVEL-SET DYNAMICS AND MIXING EFFICIENCY OF PASSIVE AND ACTIVE SCALARS IN DNS AND LES OF TURBULENT MIXING LAYERS

The mixing efficiency in a turbulent mixing layer is quantified by monitoring the surface-area of level-sets of scalar fields. The Laplace transform is applied to numerically calculate integrals over arbitrary level-sets. The analysis includes both direct and large-eddy simulation and is used to assess the suitability of specific subgrid parameterizations in relation to predicting mixing efficiency. We incorporate several subgrid models in the comparison, e.g. the scale similarity model of Bardina, the dynamic eddy-viscosity model and the dynamic mixed model. For accurate predictions, dynamic models are favored. It is observed that the ratio between LES-filterwidth Δ and grid-spacing h has a considerable influence; a ratio of four appears suitable. Gravity driven flows can be modeled by 'active' scalar fields which couple to the momentum and energy equations. The significant increase in mixing efficiency due to buoyancy effects is directly quantified.