Scalar excursions in large-eddy simulations

Abstract

The range of values of scalar fields in turbulent flows is bounded by their boundary values, for passive scalars, and by a combination of boundary values, reaction rates, phase changes, etc., for active scalars. The current investigation focuses on the local conservation of passive scalar concentration fields and the ability of the large-eddy simulation (LES) method to observe the boundendness of concentrations of a passive scalar. In practice, this fundamental constraint is often violated with scalar excursions exhibiting local under- and over-shoots in their values. The present study characterizes passive-scalar excursions in LES of a shear flow and examines methods for diagnosis and mitigation of the problem. The analysis of scalar-excursion statistics provides support of the main hypothesis of the current study that the unphysical scalar excursions in LES result from dispersive oscillations of the convection-term discretization at times and locations where the subgrid-scale model provides insufficient dissipation to produce a sufficiently smooth scalar field. In the LES runs three parameters are varied: the discretization of the convection terms, the subgrid-scale (SGS) model, and the grid resolution. Scalar excursions are found to decrease as the order of accuracy of non-dissipative schemes is increased, but the improvement rate decreases with increasing order of accuracy. Two SGS models are utilized, the stretched-vortex and Smagorinsky and it is found that the more dissipative Smagorinsky model results in smaller excursions, although the magnitude of the excursions is strongly dependent on the Smagorinsky constant. The maximum excursion and volume fraction of excursions outside boundary values show opposite trends with respect to resolution. The maximum excursion increases as resolution increases, whereas the volume fraction decreases. The reason for the increase in the maximum excursion is statistical and is traceable to the number of grid points (sample size) which increases with resolution. In contrast, the volume fraction of the excursions outside the boundary limits decreases with resolution because the SGS model performs better at higher grid resolutions.

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