Large eddy simulation involves running a transient solution from some initial
condition, on an appropriately fine grid, 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:
Start by running a steady state flow simulation using a Reynolds-averaged turbulence model such as
, Spalart-Allmaras, 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.
When you enable LES,
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.
25.17.1 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.
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.
Zero out the initial statistics using
solve/initialize/init-flow-statistics text command. Before you restart the
Data Sampling for Time Statistics in the
Iterate panel, as described in Section
25.17.1. With this option enabled,
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
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 (
is the characteristic
length of the solution domain and
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.
FLUENT provides both first-order and second-order temporal discretizations.
For LES, the second-order discretization is recommended.
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.
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.