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Article

An explicit algebraic model for the subgrid-scale passive scalar flux

Authors: Rasam, A., Brethouwer, G.B., Johansson, A.V.J.
Document Type: Article
Pubstate: Published
Journal: Journal of Fluid Mechanics
Volume: 721   541-577
Year: 2013

Abstract

In Marstorp et al. (2009), an explicit algebraic subgrid stress model (EASSM) for large eddy simulation (LES) was proposed, that was shown to considerably improve LES predictions of rotating and non-rotating turbulent channel flow. In this paper, we extend that work and present a new explicit algebraic subgrid scalar flux model (EASSFM) for LES, based on the modeled transport equation of the subgrid-scale (SGS) scalar flux. The new model is derived using the same kind of methodology that leads to the explicit algebraic scalar flux model of Wikstr¨om et al. (2000). The algebraic form is based on a weak equilibrium assumption and leads to a model that depends on the resolved strain rate and rotation-rate tensors, the resolved scalar-gradient vector and, importantly, the SGS stress tensor. An accurate prediction of the SGS scalar flux is consequently strongly dependent on an accurate description of the SGS stresses. The new EASSFM is therefore primarily used in connection with the EASSM, since this model can accurately predict SGS stresses. The resulting SGS scalar flux is not necessarily aligned with the resolved scalar gradient, and the inherent dependence on the resolved rotation-rate tensor makes the model suitable for LES of rotating flow applications. The new EASSFM (together with the EASSM) is validated for the case of passive scalar transport in a fully developed turbulent channel flow with and without system rotation. LES results with the new model show a good agreement with direct numerical simulation (DNS) data for both cases. The new model predictions are also compared to those of the dynamic eddy diffusivity model (DEDM) and improvements are observed in the prediction of the resolved and SGS scalar quantities. In the non rotating case, the model performance is studied at all relevant resolutions showing that its predictions of the Nusselt number are much less dependent on the grid resolution and are more accurate. In channel flow with wall-normal rotation, where all the SGS stresses and fluxes are nonzero, the new model shows significant improvements over the DEDM predictions of the resolved and SGS quantities.