Error Message—Reciprocity Calculation Fails

Issue

During start-up processing of a radiation analysis, one (or both) of the following error messages are issued:

Error: Reciprocity calculation stalls out
Error: Reciprocity calculation fails

Solution

The radiation model enforces reciprocity to ensure a more accurate energy balance between parts with large size differences. 

To enforce reciprocity, a matrix relaxation technique is used in conjunction with the view factor calculation. After the view factors have been computed, the technique checks if the sum of all view factors for every face in the model equals 1.0. For reciprocity to work, Ai Fij = Aj Fji. If the original view factors are poor estimates then the reciprocity calculation can stall and eventually fail. If this happens, your radiation energy balance will not be as accurate.   

This message is issued when the progress towards all constraints is too slow to ever make the 1 part in 100 million accuracy.  The relaxation process gives up when the process stalls.  It may be that the reciprocity and row sum accuracy was 1.0E-04 at the time of stall.  That would mean that you will conserve energy to about 0.01% accuracy, which is acceptable in most cases. 

The error occurs if the reciprocity error is > 5%. When the error is this large, the calculation is not likely to converge, so the process is halted.

To resolve this issue: 

Often the reason for the error is that the mesh is too coarse. Adjust the mesh distribution to be finer in tight gaps and on areas of high curvature.

Another thing to consider is system resources. If there is not enough RAM to compute view factors accurately, reciprocity can be affected.