Dynamic subgrid-scale model
The failings of the Smagorinsky model has lead to the formulation of more complicated models that attempt to improve upon the performance of the Smagorinsky model. Perhaps the best known of these newer models is dynamic subgrid-scale (DSGS) model of Germano et al (1991). The DSGS model may be viewed as a modification of the Smagorinsky model, as the dynamic model allows the Smagorinsky constant to vary in space and time. is calculated locally in each timestep based upon two filterings of the flow variables, which we will denote by superscript and superscript . The filters are called the grid filter and the test filter, respectively, and the test filter width is assumed to be larger the grid filter width.
Filtering with the grid filter results in the normal LES equations, with given by
Filtering again with the test filter results in a similar set of equations, but with a different subgrid-scale stress term, given by
where the superscript $rt$ indicates grid filtering followed by test filtering. The two subgrid-scale stress terms are related by the Germano identity:
is the resolved turbulent stress. The Germano identity is used to calculate dynamic local values for by applying the Smagorinsky model to both and . The anisotropic part of is the represented as
may now be computed as
In practice, the DSGS model requires stabilization. Often, this has been done by averaging in a homogeneous direction. In cases where this is not possible, local averaging has been used in place of an average in a homogenous direction.
Alternate solution (Lilly)
Lilly (1991) proposed a least squares procedure that is generally preferred to the original calculation of :
Stabilization must also be employed here as well.
- Germano, M., Piomelli, U., Moin, P. and Cabot, W. H. (1991), "A Dynamic Subgrid-Scale Eddy Viscosity Model", Physics of Fluids A, Vol. 3, No. 7, pp. 1760-1765.
- Lilly, D. K. (1991), "A Proposed Modification of the Germano Subgrid-Scale Closure Method", Physics of Fluids A, Vol. 4, No. 3, pp. 633-635.