Hi, everybody. My case is transient, 3 scalars(active), compressible problem. As I know, "Ideal f(T,P)" option is not valid in my case. And "Multicomponent" option is not valid because the background gas density is constant. So I determined using user subroutine("densit.f"). How do I code this subroutine?

If your fluid is a mixture of active scalars (and perhaps a background fluid), and each scalar behaves as an ideal gas, then the correct option for density is "Ideal f(T,p)". Look at equation (1-35) of the v3.15 Methodology Manual - "Ideal gases or mixtures thereof". You don't need to use densit.f.

Thank you very much, Richard.