Role of coherent structures in an isothermally reacting mixing layer

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Abstract
The effects of coherent structures on mixing and chemical reaction in a temporally growing plane mixing layer are studied by direct numerical simulations of an incompressible flow with binary, irreversible, isothermal reactions. ‘‘Rolls’’ and ‘‘ribs’’ (counter-rotating longitudinal vortices connecting the nominally spanwise rolls) give rise to two distinctively different shapes for probability density functions (PDFs) of species concentration. It is found that roll stirring causes stationary or reverse marching PDF peaks, whereas rib stirring produces marching peaks. Deviation of the mean-mixed fluid (MMF) concentration from the mean species concentration can be caused by transverse transport of species by roll stirring. The shape of the MMF curve can be determined by the counteracting effects of the ribs and rolls. The ribs can cause significant reaction enhancement: product generation in the braid can vary by a factor of 3 compared to that in the roll for the Schmidt numbers (0.07≤Sc≤0.7) and (first) Damköhler numbers (0≤DI≤∞) studied. A high product generation rate in the braid—the case of a chemically active braid—occurs for Pe ≲ 10DI′, where Pe is a Péclet number, and DI′ is DI based on the species separation thickness. The effect of ribs on local product generation becomes more significant as DI and, more importantly, Sc increase. The reaction rate is strongly dominated by the roll dynamics when SR≲10MR, where SR is the reactive stirring rate in the ribs, and MR is the species mixing rate. The coherent structure effects found in this study help explain the experimentally observed variations in species concentration PDFs and MMF curves in both the pre- and post-mixing transition states, and suggest some additional mechanisms for reaction enhancement.