Mixing characteristics of an inhomogeneous scalar in isotropic and homogeneous sheared turbulence

Turbulent mixing of an inhomogeneous passive scalar field is studied in the context of a nonpremixed reacting flow. Direct numerical simulations of an initial steplike scalar field subjected to homogeneous sheared turbulence have been performed and the results compared with those of the case of decaying isotropic turbulence. For both flow conditions, the gradient of the conserved scalar tends to align itself with the axis of the most compressive strain rate and orthogonal to the local vorticity. The magnitude of the scalar gradient is directly influenced by the local strain rate while its orientation is controlled by the local vorticity. Because of the directional features of sheared turbulence, the orientation of the scalar gradient is more ordered than in isotropic turbulence. In addition, the magnitude of vorticity indirectly affects that of the scalar gradient through strain-rate amplification by vortex stretching. In both flows, regions of high scalar-gradient magnitude or scalar dissipation (and therefore high reaction rates) tend to exist as sheets in the vicinity of sheetlike vortex structures of moderate to high vorticity. The probability density function (pdf) of the the scalar dissipation rate eF exhibits a nearly lognormal distribution with a slight negative skewness for both isotropic and sheared turbulence.