FLUENT 6 - Tips for converging the VOF free surface model with the Non Iterative Time Advancement (NITA) solution scheme on tetrahedral meshes
It has been observed that some calculations that include the VOF free surface model with surface tension and the Non-Iterative Time Advancement (NITA) solution scheme on tetrahedral meshes (complex geometries) diverge.
This solution provides some tips to improve convergence in such situations.
In general, tetrahedral meshes do not accurately resolve surface tension effects and this option should be turned off if at all possible. With the Non Iterative Time Advancement (NITA) scheme turned on, visit the Solution Controls panel and change the "Number of Updates" for all equations except Pressure from 5 (default) to 1. Also, change the under-relaxation factor for the pressure equation from the default value of 1 to 0.7.If surface tension effects are important, it is very desirable to have a hex mesh at least in areas where the interface is expected. Switch surface tension off when using a tetrahedral mesh. If NITA is used for speed-up, then make the following changes in the Solution Controls panel:
number of updates for all equations except pressure should be reduced from default 5 to 1
pressure under-relaxation should be reduced from default 1 to 0.7.