Why are all but do-intensity residuals converging?
 

Hi all,
I am modelling steady state fluid flow and thermal transfer utilising the DO radiation model and solar loading.

My main question is what could be the reason for the do-intensity residual not converging. All other residuals (except maybe energy) seem to be converging well.

Bonus points for anyone who can dive into the theory of this residual to help me understand it better. I have tried online and not come across much useful information.

Thanks in advance. :)

(Images attached include residual plot and pathline scene to show you how the model works. Top face is set transparent allowing solar radiation into the model.)