In the Sigma model [11], the eddy viscosity is modeled by:
 | (10–13) |
where
 | (10–14) |
are the singular values of the velocity gradient tensor 
.
To compute 
, a self-contained and efficient procedure is used (see Method B in the appendix
of [11]).
The grid-scale 
is computed as 
, where V is a volume of the computational cell, and the default value of model
constant 
is 1.35.
The Sigma model ensures zero eddy-viscosity in two-dimensional and two-component flows,
including pure shear and solid rotation cases. Additionally, the model is designed to return the
correct wall asymptotic (
) behavior for wall bounded flows.
The Sigma model is available after enabling beta feature access, as described in Introduction.
The Sigma model option is available under Subgrid-Scale Model, as
shown in Figure 10.7: The Viscous Model Dialog Box with Sigma Model Enabled after selecting Large Eddy Simulation
(LES). Under Model Constants, you are able to modify the value
for 
.
You can also enable the model using the TUI command:
define/models/viscous/les-subgrid-sigma?
The model constant can be specified using the TUI command:
define/models/viscous/les-model-options/csigma