Scalar Plotting
 

I'm injecting N2 (Nitrogen) into air and upon plotting the N2 (getc none conc 1) I see the area which is not yet approached by N2 (filled with air in this case) as "0" in the color scheme. But since Air is 78% N2, shouldn't it be showing 0.78? In StarCD when we plot the scalar (getc none conc 1) what exactly does it plot? Is it the mass fraction?

Sounds to me you set material 1 to "air" & added a N2 scalar. In that case getc conc 1 is showing the mass fraction of scalar 1 relative to the mass fractions of the background fluid - i.e. "air".

If you want getc conc 1 to show 0.78, you need to change air to a multi-component mixture (N2, O2, etc.), initialize the domain with the correct mass fractions, and apply the appropriate scalar mass fractions to your boundary conditions.

Hope this makes sense & helps.