Abstract
When numerically simulating the behavior of unsteady large scales in turbulent mixing problems, subgrid diffusion of chemical species (turbulent micromixing) requires a closure. A dynamic model is proposed to estimate micromixing from the known properties of the resolved large-eddy field. The dynamic procedure also provides a direct estimation of the micromixing time. A reference scalar field is invoked in the closure, and the physical model is based on basic properties of mass transfer by diffusion. Direct numerical simulation (DNS) is utilized to validate the ability of the closure to capture the relaxation of probability density functions for three-dimensional homogeneous freely decaying turbulence and a two-dimensional mixing layer. The dynamic closure for diffusion is shown to reproduce DNS results, and then a standard test case for micromixing models is computed using large-eddy simulation.
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