What is Large Eddy Simulation?

Large eddy simulation (LES) is an inherently transient technique in which the large scales of the turbulence are solved for, and the small-scale motions are modeled. One justification is that by modeling "less" of the turbulence and explicitly solving for more of it, the error in the turbulence modeling assumptions will not be as consequential. Furthermore, it is hypothesized that the smaller eddies are self-similar and will thus lend themselves to simpler and more universal models. The downside of the approach is the computational expense, which although less than direct numerical simulation, is still excessive.

The equations solved for large eddy simulation are obtained by a filtering rather than an averaging process. The filtered equations may be rearranged into a form that looks identical to unsteady RANS equations. However, the turbulent stress tensor, , now represents the subgrid scale stresses. This tensor is modeled using the Boussinesq approximation as follows:

(140)

where and are the subgrid scale turbulent viscosity and kinetic energy, respectively. is the strain rate tensor computed from the resolved velocity field:

(141)

A subgrid scale model is required for the subgrid scale viscosity, .

Mean-flow quantities must be obtained by gathering statistics either over a long physical time, and/or from a homogenous spatial coordinate. Field function monitors are provided in STAR-CCM+ to monitor the field statistics.

Currently, LES is only available with the Segregated Flow model in STAR-CCM+.