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18.3.2 Setting Integration Parameters

Under Reactions in the Species Model panel, enable Volumetric. Click on the Integration Parameters button to open the Integration Parameters panel (Figure  18.3.2).

Figure 18.3.2: The Integration Parameters Panel
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The stiff ODE integrator has two error tolerances--the Absolute Error Tolerance and the Relative Error Tolerance under ODE Parameters--that are set to default values of $10^{-8}$ and $10^{-9}$ respectively. These should be sufficient for most applications, although these tolerances may need to be decreased for some cases such as ignition. For problems in which the accuracy of the chemistry integrations is crucial, it may be useful to test the accuracy of the error tolerances in simple zero-dimensional and one-dimensional test simulations with parameters comparable to those in the full simulation.



ISAT Parameters


If you have selected ISAT under Integration Method, you will then be able to set additional ISAT parameters.

The numerical error in the ISAT table is controlled by the ISAT Error Tolerance under Integration Parameters. It may help to increase this during the initial transient solution. A larger error tolerance implies larger EOAs, greater error, but smaller tables and quicker run times. The default ISAT Error Tolerance of 0.001 may be sufficiently accurate for temperature and certain major species, but will most likely need to be decreased to get accurate minor species and pollutant predictions.

figure   

After your simulation is converged, you should always decrease the ISAT Error Tolerance and perform further iterations until the species that you are interested in are unchanged.

The Max. Storage is the maximum RAM used by the ISAT table, and has a default value is 100 MB. It is recommended that you set this parameter to a large fraction of the available memory on your computer.

You can also specify the Number of Trees and the Verbosity. The Number of Trees is the number of sub-tables within the ISAT table. For simulations with a large number of species, ISAT efficiency may be improved by increasing the number of trees from the default value of 1. The value of Verbosity allows you to monitor ISAT performance in different levels of detail. See Section  18.3.8 for details about this parameter.

To purge the ISAT table, click on Clear ISAT Table. See Section  18.3.9 for more details about using ISAT efficiently.


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