Ashbeck assessment pipeline · FDS 6.11.1 · issued 26 July 2026
All 2 comparisons agree with their reference within 15%.
The Fire Dynamics Simulator is developed and validated by NIST. That programme is published as the FDS Validation Guide (NIST SP 1018) and covers several hundred experiments across roughly 1,500 pages. It is not reproduced here. Re-running someone else's validation adds nothing.
What NIST's work does not cover is the layer this product adds around the solver: the mesh we generate, the tenability criteria we apply, the ASET we extract from the field data and the RSET we derive from the building. That layer is our code, and it is what a chartered engineer is putting their name to. This dossier checks it against independent, published references.
Ceiling jet — Alpert (1972), Fire Technology 8, 181; SFPE Handbook 5th ed. ch. 14.
Plume — Heskestad; SFPE Handbook 5th ed. ch. 13.
Asphyxiant FED — PD 7974-6:2019 Annex D (Purser).
Visibility — PD 7974-6:2019 Annex C (Jin).
Grid convergence — ASME V&V 20-2009; Roache (1994), J. Fluids Eng. 116, 405.
A 150 kW steady fire on a 0.6 m square burner, equivalent diameter 0.677 m, characteristic diameter D* = 0.449 m. Mean flame height 1.05 m.
Two geometries are used, because the two correlations describe different situations. The ceiling-jet case is 7.2 × 7.2 m in plan with a solid ceiling 3.4 m above the fuel and all four sides open, giving H/L = 3.23; the sides are open because Alpert's correlation assumes an unconfined ceiling with no accumulated smoke layer. The plume case is 4.8 m square and 7.2 m tall with the top open, because Heskestad's correlation is valid only above the flame tip and needs height; the lowest measurement point is 0.75 m clear of the tip.
| Case | Cell size (m) | Cells | D*/δx |
|---|---|---|---|
| cj_coarse | 0.2 | 23,328 | 2.2 |
| cj_medium | 0.1 | 186,624 | 4.5 |
| cj_fine | 0.05 | 1,492,992 | 9.0 |
| plume | 0.05 | 1,327,104 | 9.0 |
| Point | Model | Reference | Difference | Agreement |
|---|---|---|---|---|
| total HRR | 150.00 | 150.0 | +0.0% | good |
| total HRR | 150.00 | 150.0 | -0.0% | good |
| Point | Model | Reference | Difference | Agreement |
|---|---|---|---|---|
| r = 0.3 m (r/H = 0.088, impingement) | — | 62.1 | — | no reference |
| r = 0.9 m (r/H = 0.265, radial) | — | 47.9 | — | no reference |
| r = 1.5 m (r/H = 0.441, radial) | — | 34.1 | — | no reference |
| r = 2.1 m (r/H = 0.618, radial) | — | 27.2 | — | no reference |
| r = 2.7 m (r/H = 0.794, radial) | — | 23.0 | — | no reference |
| r = 3.3 m (r/H = 0.971, radial) | — | 20.2 | — | no reference |
| Point | Model | Reference | Difference | Agreement |
|---|---|---|---|---|
| r = 0.3 m | — | 3.4 | — | no reference |
| r = 0.9 m | — | 2.1 | — | no reference |
| r = 1.5 m | — | 1.4 | — | no reference |
| r = 2.1 m | — | 1.0 | — | no reference |
| r = 2.7 m | — | 0.8 | — | no reference |
| r = 3.3 m | — | 0.7 | — | no reference |
| Point | Model | Reference | Difference | Agreement |
|---|---|---|---|---|
| z = 2.0 m (1.8 m above the fuel) | — | 194.2 | — | no reference |
| z = 2.8 m (2.6 m above the fuel) | — | 107.4 | — | no reference |
| z = 3.6 m (3.4 m above the fuel) | — | 69.4 | — | no reference |
| z = 4.4 m (4.2 m above the fuel) | — | 49.2 | — | no reference |
| z = 5.2 m (5.0 m above the fuel) | — | 36.9 | — | no reference |
| z = 6.0 m (5.8 m above the fuel) | — | 28.9 | — | no reference |
Ceiling-jet temperature (Alpert 1972) — r = 0.3 m (r/H = 0.088, impingement)
Ceiling-jet temperature (Alpert 1972) — r = 0.9 m (r/H = 0.265, radial)
Ceiling-jet temperature (Alpert 1972) — r = 1.5 m (r/H = 0.441, radial)
Ceiling-jet temperature (Alpert 1972) — r = 2.1 m (r/H = 0.618, radial)
Ceiling-jet temperature (Alpert 1972) — r = 2.7 m (r/H = 0.794, radial)
Ceiling-jet temperature (Alpert 1972) — r = 3.3 m (r/H = 0.971, radial)
Ceiling-jet velocity (Alpert 1972) — r = 0.3 m
Ceiling-jet velocity (Alpert 1972) — r = 0.9 m
Ceiling-jet velocity (Alpert 1972) — r = 1.5 m
Ceiling-jet velocity (Alpert 1972) — r = 2.1 m
Ceiling-jet velocity (Alpert 1972) — r = 2.7 m
Ceiling-jet velocity (Alpert 1972) — r = 3.3 m
Plume centreline temperature (Heskestad) — z = 2.0 m (1.8 m above the fuel)
Plume centreline temperature (Heskestad) — z = 2.8 m (2.6 m above the fuel)
Plume centreline temperature (Heskestad) — z = 3.6 m (3.4 m above the fuel)
Plume centreline temperature (Heskestad) — z = 4.4 m (4.2 m above the fuel)
Plume centreline temperature (Heskestad) — z = 5.2 m (5.0 m above the fuel)
Plume centreline temperature (Heskestad) — z = 6.0 m (5.8 m above the fuel)
The ceiling-jet case was run on three systematically refined grids — 0.20, 0.10 and 0.05 m, a refinement ratio of two — and Richardson extrapolation applied to each measurement point. The Grid Convergence Index is the numerical uncertainty band on the finest grid, with Roache's safety factor of 1.25 for a three-grid study.
Where the three results are not monotonic, no GCI is quoted. Extrapolating a non-monotonic series produces a confident-looking number with no meaning, and the honest report is that the quantity is not yet converged.
No convergence study available.
A convergence study that spans only well-resolved grids demonstrates nothing. The 0.20 m grid sits at D*/δx ≈ 2, far below practice, precisely so that the trend towards the refined solution is visible and measurable. It is not a recommendation.
Three parts of the pipeline are arithmetic rather than simulation, and are checked
directly against worked values from the standards rather than against a run. These are
verified on every build by firepipe/test_validation.py, which fails the
build if any coefficient drifts.
| Component | Check | Source |
|---|---|---|
| Asphyxiant FED accumulation | 1,000 ppm CO for 5 min → FED 0.177; linear in time; reaches 1.0 at 28 min | PD 7974-6:2019 Annex D |
| Visibility from obscuration | K = 0.87 m−1 → 9.2 m emitting / 3.4 m reflecting; the 10 m criterion corresponds to K = 0.80 m−1 | PD 7974-6:2019 Annex C |
| Heskestad virtual origin and flame height | 1 MW, 1.0 m pool → z0 = 0.296 m, L = 2.70 m | SFPE Handbook ch. 13 |
| Richardson extrapolation | A synthetic second-order series recovers p = 2.00 and the exact value; a non-monotonic series returns no GCI | ASME V&V 20-2009 |
Stated plainly, because a dossier that lists none is not credible.
• The comparisons are against engineering correlations, not experiments. The
correlations themselves carry scatter of order 15–25%, so agreement within that
band is the realistic expectation, not an exact match.
• A single steady fire size and one ceiling height are covered. Behaviour under
very different fire sizes, ceiling heights or confinement is not established by this
document.
• The cases are unconfined and open. They do not exercise smoke-layer
development, which is the regime most production assessments actually run in; that
regime rests on NIST's validation of FDS.
• RSET is a PD 7974-6 hand calculation derived from the building geometry. It is
not an agent-based evacuation model and does not account for occupant behaviour,
mobility impairment or route choice.
• None of this establishes that a model represents a particular building. That
judgement belongs to the engineer confirming the geometry, and the software records
who made it.
Every input deck is generated by firepipe/validation_cases.py and is
included with this dossier. To re-run the whole set:
python -m firepipe.run_validation
The comparison can be regenerated from existing output without re-running the solver
using --skip-run. The reference correlations are in
firepipe/validation.py, each stated with its source and range of
applicability.