12.16.5 LES-Specific Solution Strategies

Large eddy simulation involves running a transient solution from some initial condition, on an appropriately fine mesh, using an appropriate time step size. The solution must be run long enough to become independent of the initial condition and to enable the statistics of the flow field to be determined.

The following are suggestions to follow when running a large eddy simulation:

1.   Start by running a steady state flow simulation using a Reynolds-averaged turbulence model such as standard $k$- $\epsilon$, $k$- $\omega$, or even RSM. Run until the flow field is reasonably converged and then use the solve/initialize/ init-instantaneous-vel text command to generate the instantaneous velocity field out of the steady-state RANS results. This command must be executed before LES is enabled. This option is available for all RANS-based models and it will create a much more realistic initial field for the LES run. Additionally, it will help in reducing the time needed for the LES simulation to reach a statistically stable mode. This step is optional.

2.   When you enable LES, ANSYS FLUENT will automatically turn on the unsteady solver option and choose the second-order implicit formulation. You will need to set the appropriate time step size and all the needed solution parameters. (See Section  26.12.1 for guidelines on setting solution parameters for transient calculations in general.) The bounded central-differencing spatial discretization scheme will be automatically enabled for momentum equations. Both the bounded central-differencing and pure central-differencing schemes are available for all equations when running LES simulations.

3.   Run LES until the flow becomes statistically steady. The best way to see if the flow is fully developed and statistically steady is to monitor forces and solution variables (e.g., velocity components or pressure) at selected locations in the flow.

4.   Zero out the initial statistics using the solve/initialize/init-flow-statistics text command. Before you restart the solution, enable Data Sampling for Time Statistics in the Run Calculation task page, as described in Section  26.12.1. With this option enabled, ANSYS FLUENT will gather data for time statistics while performing a large eddy simulation. You can set the Sampling Interval such that Data Sampling for Time Statistics can be performed at the specified frequency. When Data Sampling for Time Statistics is enabled, the statistics collected at each sampling interval can be postprocessed and you can then view both the mean and the root-mean-square (RMS) values in ANSYS FLUENT.

5.   Continue until you get statistically stable data. The duration of the simulation can be determined beforehand by estimating the mean flow residence time in the solution domain ( $L/U$, where $L$ is the characteristic length of the solution domain and $U$ is a characteristic mean flow velocity). The simulation should be run for at least a few mean flow residence times.

Instructions for setting the solution parameters for LES are provided below.

Temporal Discretization

ANSYS FLUENT provides both first-order and second-order temporal discretizations. For LES, the second-order discretization is recommended.

Solution Methods

Spatial Discretization

Overly diffusive schemes such as the first-order upwind or power law scheme should be avoided, because they may unduly damp out the energy of the resolved eddies. The central-differencing based schemes are recommended for all equations when you use the LES model. ANSYS FLUENT provides two central-differencing based schemes: pure central-differencing and bounded central-differencing. The bounded scheme is the default option when you select LES or DES.

Solution Methods