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Scalar excursions in large-eddy simulations

Matheou, Georgios and Dimotakis, Paul E. (2016) Scalar excursions in large-eddy simulations. Journal of Computational Physics, 327 . pp. 97-120. ISSN 0021-9991.

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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 boundedness of passive scalar concentrations. In practice, as a result of numerical artifacts, this fundamental constraint is often violated with scalars exhibiting unphysical excursions. The present study characterizes passive-scalar excursions in LES of a shear flow and examines methods for diagnosis and assesment of the problem. The analysis of scalar-excursion statistics provides support of the main hypothesis of the current study that unphysical scalar excursions in LES result from dispersive errors of the convection-term discretization where the subgrid-scale model (SGS) 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 SGS model, and grid resolution. Unphysical scalar excursions 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 examined, the stretched-vortex and a constant-coefficient Smagorinsky. Scalar excursions strongly depend on the SGS model. The excursions are significantly reduced when the characteristic SGS scale is set to double the grid spacing in runs with the stretched-vortex model. The maximum excursion and volume fraction of excursions outside boundary values show opposite trends with respect to resolution. The maximum unphysical excursions increase as resolution increases, whereas the volume fraction decreases. The reason for the increase in the maximum excursion is statistical and traceable to the number of grid points (sample size) which increases with resolution. In contrast, the volume fraction of unphysical excursions decreases with resolution because the SGS models explored perform better at higher grid resolution.

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Additional Information:© 2016 Elsevier Inc. Received 15 October 2015; Received in revised form 1 July 2016; Accepted 25 August 2016; Available online 31 August 2016. This work was supported by AFOSR Grant No. FA9550-12-1-0461, DOE Grant No. DE-NA0002382, and the John K. Northrop Chair of the California Institute of Technology. The authors would also like to acknowledge discussions and collaborations with Prof. G. Candler and his research group at the University of Minnesota, and Prof. D. Meiron and Prof. D. Pullin at Caltech. We would like to thank Dr. D. Chung (University of Melbourne) for making available the spectral code used in this study. This research was carried out at the California Institute of Technology and the Jet Propulsion Laboratory, California Institute of Technology, the latter under a contract with the National Aeronautics and Space Administration.
Funding AgencyGrant Number
Air Force Office of Scientific Research (AFOSR)FA9550-12-1-0461
Department of Energy (DOE)DE-NA0002382
Subject Keywords:Large eddy simulation; Numerical methods; Numerical model error; Scalar mixing; Turbulent mixing
Record Number:CaltechAUTHORS:20161116-160411835
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Official Citation:Georgios Matheou, Paul E. Dimotakis, Scalar excursions in large-eddy simulations, Journal of Computational Physics, Volume 327, 15 December 2016, Pages 97-120, ISSN 0021-9991, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:72076
Deposited By: Tony Diaz
Deposited On:17 Nov 2016 00:12
Last Modified:03 Oct 2019 16:14

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