Intelligent Solution Control is one of the keys to the robustness of the Autodesk® CFD solution. By employing elements of control theory to examine the trends of each degree of freedom, Autodesk® CFD automatically adjusts the convergence controls and the time step size to attain a solution. If the solution is changing too quickly from one iteration to the next, this algorithm automatically slows down progress in an effort to maintain stability. Alternatively, if the solution is stable and progressing too slowly, the algorithm will allow the calculation to evolve quicker, resulting in reduced solution times.
Intelligent Solution Control is enabled by default for several analysis types (as described below), but can be disabled on the Solution Controls dialog (launched from the Solve task dialog).
The convergence control values that Intelligent Solution Control chooses can be plotted on the Convergence Monitor by selecting Relax Parm from the third pull-down menu.
The convergence control values are shown for all degrees of freedom over the range of iterations.
If an analysis simply will not converge even with Intelligent Solution Control enabled, then a finer mesh may be needed. Also, inspect the applied conditions and materials to ensure that the physics are simulated correctly. If modifications to the mesh and/or physics still do not produce a converged solution with Intelligent Solution Control, please contact your technical support representative.
The manner in which Intelligent Solution Control is applied to different analysis types is described:
Intelligent Solution Control adjusts the time step size and the convergence control settings to achieve solution stability. The result is that even very physically demanding analyses that would have required manual adjustment of convergence controls will now run with virtually no manual intervention.
Steady state analyses are run internally as transient solutions when Intelligent Solution Control is enabled. Each time step consists of only one inner iteration, so solution times are not significantly longer (as is often the case in true time-varying transient analyses). Because of this, each time step is considered a single iteration. Saved results files follow this naming convention:
analysisname.res.s#
where analysisname is the name of the Autodesk® CFD analysis, and # is the number of the time step (effectively the iteration number for steady state analyses).
To run steady state analyses, ensure that Steady State is the selected Analysis Mode on the Solve dialog (it is by default). Also, when specifying how often results are saved to the disk, the default save interval is expressed in terms of iterations (instead of seconds).
Intelligent Solution Control behaves slightly differently when solving for temperature. Unlike the other solution variables in which the convergence controls and time step are adjusted, for temperature, only the time step is adjusted. Because of this, the value of the convergence control for temperature (as set in the dialog) will affect the rate of convergence of the energy equation. By varying the time step internally, Intelligent Solution Control enforces and maintains stability of the Energy Solver throughout the analysis.
For transient analyses that do not include moving objects, Intelligent Solution Control adjusts only the time step size, and does not modify any convergence control settings. This is done to prevent artificially affecting the time accuracy of the solution. (Reducing the convergence control slows down solution progression by the Solver, so it is always a good idea to use the default convergence control settings for non-Motion transient analyses.)
We have found that in some cases the time step size that Intelligent Solution Control selects can be smaller than truly necessary for convergence, which may result in significantly longer solution times. For this reason, Intelligent Solution Control is disabled by default for transient analyses. It is recommended to manually assign a time step size based on the physics of the analysis model.
For solid motion analyses, Intelligent Solution Control is disabled by default (although it can be enabled if necessary). Because it will only reduce the time step size, if the motion is user-prescribed, enabling it will generally result in longer solution times, with only a fairly small increase in stability.
For motion analyses that include flow-driven moving objects, we recommend that you enable Intelligent Solution Control. This will adjust the time step size to ensure that the object passes through only one element per time step. As the velocity of the object increases the time step will be automatically reduced to ensure stability. As the motion of the object slows, the time step size will be increased, but will not exceed the time step size manually set in the Time Step Size field on the Solve dialog.
Intelligent Solution Control has been optimized for use with transient analyses involving moving objects, and, as such, is recommended for use with flow-driven Motion.
For rotating region analyses, Intelligent Solution Control is disabled by default (although it can be enabled if necessary). Because it will only reduce the time step size, if the rotational speed is known, enabling it will generally result in longer solution times, with only a fairly small increase in stability.
We recommend that you enable Intelligent Solution Control for rotating analyses that are either free-spinning or driven by a known torque. This will automatically determine and vary the time step size throughout the analysis. The time step size will be modified to ensure that no more than three degrees of rotation pass for each time step. This criteria has been found to be quite stable for rotating analyses.