In variable density, multi-fluid and reacting flows, a quantitative measure of the degree of molecular mixing is crucial to the development of turbulent transfer and mixing models. In a buoyancy-driven miscible mixing layer, we have found that the initial entrainment of unmixed fluid into the mixing layer causes a decrease in the measure of molecular mixedness at the centerplane of the mixing layer. Following the period of initial entrainment, the fluids within the mixing layer exhibits an increase in the degree of molecular mixing. Characterization of this and other mixing processes require scalar measurement devices with an adequate probe volume size. Spatial averaging, which occurs due to the finite probe volume size, can lead to errors in resolving the density or scalar gradients between pockets of unmixed fluids. Given a probe volume size and a priori knowledge of the functional profile of the diffusion layer being measured, we obtain an estimate for the measurement error due to spatial averaging that has occurred and make corrections accordingly. An analytical model for the measure of scalar mixing is developed as a predictor for the growth of scalar gradients in a variable scalar flow. The model is applied to a buoyancy-driven mixing layer with a Prandtl number of 7.

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