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Monitoring and properly assessing convergence levels during a Wind-US run are critical in obtaining meaningful, useful results. Wind-US 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-US prints solution residuals to the list output file, in order to get a general idea of solution convergence.

For structured grids, Wind-US organizes the equations to be solved into logical "groups" that are solved together. For example, when a one- or two-equation turbulence model is used, the Navier-Stokes equations and the turbulence model equations are in two separate groups.

For each equation group in each zone, Wind-US 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.
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.

The residuals are printed by default each iteration.
The output interval may be changed, however, using the
`CYCLES` and
`ITERATIONS` keywords.
Residuals may also 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.

By default, if the maximum residual for an equation group decreases by
four orders of magnitude in a particular zone, a "converged" message is
printed in the list output file, and iterations for that group in that
zone are ended for the current cycle.
Note that, if the residuals early in the calculation are large due to
large initial solution transients, Wind-US may decide that a zone is
converged when it may require significantly more iterations.
In the input data file, with the
`CONVERGE` keyword you
can specify different convergence criteria, by setting the amount the
residual must decrease, and by telling Wind-US to use the value of the
residual rather than an order-of-magnitude reduction.
You can also specify use of the L2-norm of the residual instead of the
maximum residual using `TEST`
option 128.

*Keywords:* `CONVERGE,
CYCLES,
ITERATIONS,
TEST 128`

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-US's flowfield integration capability.
In the input data file, you may specify a number of computational
surfaces for which Wind-US will integrate forces, moments, and/or mass flows.
If you specify reference length, area, and moment center,
Wind-US 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-US 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`

Last updated 12 Dec 2005