Volume 1 - Section V - CFD Methodology 
Pass V - 7 
Resistances 
Resistances cover any kind of flow resistance (i.e., pressure 
drops) caused by porous items within the flow domain. 
Two options are available: 
Planar resistances: 
These provide for areas where the resistance is thin and can be 
applied in one plane. The pressure drop is given by the 
expression: 
DP = fXp(v/b ) 2 
Where 
DP Pressure drop 
f loss coefficient 
p density 
v velocity of fluid 
b geometrical free area ratio of obstruction 
Volume resistances: 
These provide for areas where the resistance occupies a 
significant thickness in the solution domain and resistance will 
occur in more than one direction. The pressure drop is the same 
as for a planar resistance, except that it is expressed as pressure 
drop per meter and the factor and free area ratio is required for 
each coordinate direction. 
Loss Coefficient and Free Area Ratio: 
The loss coefficient will depend on the actual geometry of the 
item causing the pressure drop. This will be obtained from 
experiment or empirical relationships in textbooks. Care is 
needed as it may be set with respect to an approach velocity or 
device velocity. If the latter is chosen there will be an 
associated free area ratio so the program can correctly 
calculate the pressure drop. The free area ratio is not required 
if the setting is based on the approach velocity. 
