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The Effect of Structured Stirring and Mixing on Scalar Covariance of Initially Distant Scalars Ìý

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Date and time:Ìý

Thursday, September 5, 2013 - 2:00pm

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ECCR 257

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Mixing and reaction of two scalars initially separated by a third, ambient fluid is important to problems ranging from biology to combustion. This topology is fundamentally different than that where two scalars share an interface and fill the domain in that for coalescence to occur, stirring and mixing must selectively aggregate the initially distant scalars. The scalar covariance is of particular interest as it quantifies the contributions of the instantaneous processes, and is key to defining the correlation and segregation parameters used to quantify the degree of mixing.

We use a two-channel PLIF system that simultaneously and independently measures two initially isolated passive scalars advected in a variety of turbulent flows to study mixing and reaction in the low Damkohler limit. Near the source the scalar covariance is negative as filaments of each scalar largely exist independent of one another. As the scalars progress downstream attracting regions in the flow bring the two scalars together in large concentrations proportional to the local means and the scalar covariance dominates. Structures that lead to the formation and evolution of strong scalar correlations are dependent on the flow geometry. Our results demonstrate that over 80% of the downstream reaction is associated with the scalar covariance produced by instantaneous flow processes, such that the total reaction greatly exceeds that predicted my mean processes alone.

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