DeltaDualCore™ Panel Awning Systems 
 
Revision 1-12 
49 
 
B.5 FDS Grid sensitivity 
A grid sensitivity check has been performed to evaluate the grid dependence of the models. FDS uses a 
Large Eddy Simulation (LES) model for computation. The grid size allows the sub-grid scale stress model 
to accurately compute the viscous stress of the flow field. The following formula, as detailed in the NIST 
Fire Dynamics Simulator (Version 5) User‘s Guide, can be used to measure how well the flow field within 
FDS modelling is resolved: 
D/δx   where, 
δx is the nominal size of a mesh cell 
D   is a characteristic fire diameter; 
 
Where:   Q* (kW) is the heat release rate 
ρ∞ (kg/m3) is the density of ambient temperature – 1 kg/m3 
Cp (kg/kg.K) is the specific heat of the gas – 1.0 kg/kg.K 
T∞ (K) is the ambient temperature – 293 K 
g (m/s2) is the acceleration due to gravity – 9.81 m/s2 
 
The grid size used in the FDS modelling is 0.125 for Models 1 & 2; and 0.25 for Model 3.  
δx = 0.125; 0.25 m 
HRR of 15 MW will be used to decide the grid size. 
Q = 15000 kW, gives D = 3.057 m. 
D/ δx = 3.057 / 0.125 
         = 24.5 
D/ δx = 3.057 / 0.25 
         = 12.2 
An acceptable value is considered to range from 4 to 16 with values higher than 16 presenting very high 
resolution. As stated in the FDS user guide (Pg. 45), the more cells spanning the characteristic fire the 
higher the resolution. These values are based on a validation study undertaken by the US Nuclear 
Regulatory Commission (McGrattan, Hill, Dreisbach, Joglar, Najafi, Peacock, Hamins, 2007).  
The nominated grid sizes are within the acceptable range, with Models 1 and 2 being of very high 
resolution. Grid sensitivity is therefore acceptable to adequately resolve the plume field. 
 
 
 

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