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turbulent mixing and combustionModeling the rate of scalar mixing in turbulent flow presents an important practical challenge. For example, in nonpremixed combustion, the heat release is almost entirely determined by the mixing rate. Rapid mixing of the initially separated fiel and oxidizer is required to maximize the heat release rate and overall efficiently of a general compact combustor. The rate of mixing in the nonpremixed combustion is expressed by the dissipation rate of the scalar variance, which is not available directly from turbulent simulations that use any kind of turbulence modeling. Similar issues arise in any reacting turbulent flow where the initial distribution of reacting species is nonuniform. Therefore the dissipation has to be modeled. All present dissipation models use algebraic relations between the dissipation and known variables, implying an instantaneous connection between the largescale flow features and the molecular level of mixing, represented by the dissipation. In the present work, an a priori study shows that the dissipationrate is instead most highly correlated with the largescale eddies at a previous time. Therefore, our modeling approach takes this "lag" into account. The new model is tested in large eddy simulation of the Sydney bluffbody flame. We show that in the region of the high dissipation latel (the "neck" region of the flame), the lagmodel gives improved predictions when compared to the state of the art algebraic model. relevant publications

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