The downstream condition may be written: T(l,y,z)=c1T(l,y,z)+c2, where c1=1/c1 and c2=c2/c1. Thus the source terms are aW[(c11)TW+c2] at i=1 and aE[(c11)TE+c2]=aE[(1c1)TEc2]/c1 at i=nx. The latter is generally significant only at very low Reynolds numbers.