I am having trouble with some UDFs in Fluent. I am using a UDS, and after the simulation runs for 92 time steps, it consistently crashes. The strange part is that all of the residuals look fine -- all normalized residuals are 10^(-2) or lower. I even went down to the AMG solver level (because the error Fluent provided to the console was divergence at that level), but the iterations leading up to the "explosion" seem to have low residuals (again, less than 10^-2). Nothing points to divergence until it happens.
After some playing around, the source term for the UDS seems to be the culprit. When I set that term to zero or just tell Fluent not to solve that UDS equation, the solver can pass through 92 time steps without issue. But when I turn the equation/term back on, Fluent crashes. Another weird issue is that I can significantly change the source term itself (dividing it by two, ramping it up slowly, ramping it up then keeping it steady after time step 80, etc.), but the simulation consistently crashes at time step 92.
What could be causing this issue and how can I solve it?
I also attached the console output after the crash (UDS-0 is the problem).