Volume I - Section V - CFD Methodology 
Page V - 21 
1 
effective _ molecular _ weight 
1 ~f 
/ 
concentration_molecular_weight 
+ 
(5.28) 
concentration_molecular_weight 
where / is the concentration value 
This reduces to the normal Ideal Gas Law when the molecular weight of the concentration is the 
same as that of the air. 
The harmonic average comes about by considering the volume that lKg of the mixture occupies 
namely: 
1 / | !-/ 
effective _ density concentration_density air_density 
(5.29) 
5 . 2.5 Integration of the Governing Equations 
The governing PDE’s have to be integrated, or discretized, over the solution grid so that the finite 
values of the flow variables may be predicted at each cell. The discretization process converts the 
governing PDE’s into algebraic equations. The following figure shows the cell notation that is to 
be used: 
• Storage location of scalar variables 
— ► Storage location of velocity components 
Figure 5.05 Cell notation 
In this figure we see that all scalar variables are stored at the center of each cell, or rather the 
scalar variable has the same value throughout that entire cell. The vector variables are stored on 
