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FDS Mesh Size Calculator
Enter the fire size and the domain. The calculator works out the characteristic fire diameter D*, picks the cell size for the chosen D*/δx resolution, gives the cell count and the memory needed, and writes ready-to-paste &MESH lines split across MPI processes.
How it is calculated
For buoyant plumes, mesh resolution is measured by the ratio D*/δx, where δx is the cell size and D* the characteristic fire diameter:
D* = ( Q / (ρ∞ · cp · T∞ · √g) )^(2/5)
with ρ∞ = 1.204 kg/m³, cp = 1.005 kJ/(kg·K), T∞ = 293 K and g = 9.81 m/s². D*/δx is the number of cells across the characteristic diameter. The range 4–16 is often quoted (for example in the US NRC validation report NUREG-1824): about 4 is coarse, about 16 fine, and around 10 is a common compromise in projects. NIST cautions, however, that none of these values should be treated as a universal minimum — a mesh sensitivity study for the specific scenario decides.
The calculator rounds δx to 5 mm (below 10 cm) or 1 cm, fits the cell count to the domain and splits the domain along x into the chosen number of equal meshes. J and K can be matched to products of 2, 3 and 5 — the FDS pressure solver uses a fast Fourier transform in the y and z directions, although according to the FDS User Guide the gain is usually small.
Things to watch
- D*/δx is not everything. It is a plume criterion. Flow in a corridor, through a door or in a duct needs its own check — usually at least a few cells across the opening.
- Finer mesh at the fire. There is no need to refine the whole domain. Add a separate, finer mesh around the fire and keep a 2:1 cell size ratio at mesh interfaces.
- Sensitivity study. The final test is comparing results for two or three resolutions. More in the guide FDS cell size: D*/δx and the mesh sensitivity study.
- Splitting into processes. The busiest mesh sets the run time — see Multiple meshes and MPI in FDS: how to speed up a run.
Frequently asked questions
Which D*/δx should I choose?
About 4 is enough for a quick model check. Typical smoke control projects use around 10, and a mesh sensitivity study compares at least two resolutions, e.g. 10 and 16. In the end the comparison of results decides, not the number alone.
How much RAM does an FDS model need?
Roughly 1–1.5 GB per million cells plus a fixed overhead per process. The calculator uses the same estimate as the fdsrun portal; actual usage depends on the number of species, particles and outputs, among other things.
Do J and K have to be products of 2, 3 and 5?
No. The FDS User Guide recommends such numbers because the pressure solver uses a fast Fourier transform in the y and z directions, but notes that the gain is usually small since the pressure solver takes less than 10% of the run time.
How many cells per mesh (MPI process)?
A sensible range is about 0.2–1 million cells per process. Smaller meshes add communication overhead, larger ones lengthen the run. Most important is that the meshes have similar cell counts.
FDS and Smokeview are software of the National Institute of Standards and Technology; NIST is not affiliated with fdsrun.