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