I also have another question. If I wanted to set up my problem as described above using UDS transport equations instead of the species model how many equations would I need and how would I define/create the air-gas mixture under the fluid tap in materials since the mixture option only appears only when you have activated the species model? The reason I'm asking is that I've gone through all the examples in the UDF manual for setting up UDS transport equations and there only consider a single fluid, not a mixture. Also, I think if I can set up UDS transport equations then I can use C_UDSI(c, t, i) to retrieve the concentration and then use it in the source.