Virtual Origin
Definition
In plume correlation models, the effective point source location below the actual fuel surface from which the plume behaves as if it originated. Corrects the correlation for finite-size fire sources.
- Model family
- Plume correlation models (e.g. Heskestad, McCaffrey)
- Effect
- Corrects plume height/temperature equations for a finite-area source
- Location
- Below the actual fuel surface, not at it
- Depends on
- Fire diameter and heat release rate
Common questions
Why not just use the actual fuel surface as the origin?+
Plume correlations were derived assuming a point source. A real fire has a finite area, so measured plume temperatures and velocities near the surface do not fit the point-source curve. Shifting the origin below the surface by an offset tied to fire diameter makes the same equations fit real data.
How is the virtual origin offset calculated?+
It is typically expressed as a function of fire diameter and heat release rate, fitted empirically from test data, and can be a negative or positive offset depending on the correlation used. Fire investigators use published correlations rather than deriving the offset from first principles.
Related terms
- Alpert Correlation
- An expression derived by Ronald Alpert at FM Global for ceiling jet temperature and velocity as functions of heat release rate, ceiling...
- CFAST
- Consolidated Fire and Smoke Transport model from NIST; a multi-room two-zone fire model used for evacuation time analysis, detector performance prediction, and...
- Fractional Effective Dose (FED)
- A physiological model that combines exposure to CO, HCN, CO2, reduced O2, and heat into a cumulative incapacitation estimate. Used with FDS...
- Heskestad Correlation
- An analytical expression derived by Gunnar Heskestad at FM Global relating fire plume centreline temperature to heat release rate and height above...
- McCaffrey Plume Model
- A three-zone empirical model for fire plume centreline temperature and velocity developed by Brian McCaffrey at NIST, with separate power-law relationships for...
- NIST FDS (Fire Dynamics Simulator)
- A freely available large-eddy simulation CFD code from NIST that models coupled fire, smoke, and heat transfer in three-dimensional geometries. Validated against...
- Soot Horizon
- The visible line on vertical surfaces marking the lowest height reached by the smoke layer during the early fire growth phase, representing...
- T-Squared Fire Growth
- A model in which heat release rate increases as the square of time from established burning (HRR = alpha × t²). The...
- Two-Zone Model
- A compartment fire model that divides the room into an upper hot gas zone and a lower cool ambient zone, computing their...