Monitoring and properly assessing convergence levels during a WIND run are critical in obtaining meaningful, useful results. WIND allows you to track convergence by following residuals and/or integrated forces, moments, and mass flow. For engineering applications, the recommended convergence monitoring method is the tracking of integrated quantities of interest. For example, if you are modeling a wing/body geometry to determine drag, you should monitor integrated drag and set some reasonable bounds on drag oscillations as your convergence criterion.
In steady-state (non-time-accurate) solutions, the residuals are the amounts by which the solution vector changes in a single iteration. Ideally, in approaching a steady state solution, the residuals should approach zero. However, in practice, complex geometric and flowfield features may limit the reduction in the residuals to about two orders of magnitude. WIND prints solution residuals to the list output file, in order to get a general idea of solution convergence. For each zone, WIND prints the zone number, cycle number, location of the maximum residual (i, j, and k indices), equation number for which the maximum residual occurred, the value of the maximum residual, and the L2-norm of all the residuals for all the equations over all the points in that zone. By default, the residuals are printed each iteration. The output interval may be changed, however, using the CYCLES and ITERATIONS keywords. Residuals may be plotted by first using the auxiliary program resplt to generate a GENPLOT file, then using that file as input to the plot command in the CFPOST post-processing package.
The L2-norm of the residuals will give you an idea of the overall convergence of the solution. The location of the maximum residual may give you insight into problems with a particular solution. After solution "bombs" or when convergence is unacceptably slow, the location of the maximum residual is the first place you should look for potential problems with the solution.
If the maximum solution residual decreases by four orders of magnitude
from its maximum value in a particular zone, WIND will display a
"converged" message in the list output file and stop iterating
in that zone.
Note that, if the maximum residual is high due to large initial
solution transients, WIND may decide that a zone is converged when it
may require significantly more iterations.
In the input data file, you may specify the number of orders of magnitude
required before WIND's convergence check is triggered, and you may also
set the absolute value of the maximum or L2-norm residual to be checked
for convergence.
Keywords: CONVERGE,
CYCLES,
ITERATIONS
As noted above, the optimal convergence checking criterion for a solution is an integrated quantity of interest for a particular study. For example, for an afterbody drag study to calculate pressure drag, the integrated drag is obviously the best quantity to track in monitoring convergence. This may be accomplished using WIND's flowfield integration capability. In the input data file, you may specify a number of computational surfaces for which WIND will integrate forces, moments, and/or mass flows. If you specify reference length, area, and moment center, WIND will output the forces and moments as coefficients; you may even specify output of lift and drag coefficients in place of x and y force coefficients.
You may specify zones and computational grid indices of subsets to
be integrated, and you may request that different quantities be
integrated on each subset.
You may also control the frequency of output and whether the code
outputs subset totals and/or zonal and grand totals.
The auxiliary program resplt
will produce GENPLOT plot files
of output integration quantities versus solution cycle number,
which may then be used as input to the
plot
command in the
CFPOST post-processing package.
Keywords: LOADS
In time-accurate calculations, where a constant time step is being
specified and is the same throughout the flow field, you may wish to
track a particular flow variable as the solution advances in time.
For example, you may be interested in monitoring the static pressure
on a backward-facing step as vortices roll off the back of the step.
You may request history tracking in the input data file.
Specification must include zone numbers and ranges of grid points to track,
and the frequency of the sampling.
When you request history tracking, WIND creates a
time history file containing the
sampled data.
The auxiliary program thplt
must be run to extract data from the history file into GENPLOT plot files,
which may then be used as input to the
plot
command in the
CFPOST post-processing package.
Keywords: HISTORY