Jet fans in FDS: how to model them in a car park

fdsrun editorial team · 6 November 2026 · 7 min read

How to model a jet fan in FDS: an obstruction with HVAC vents, flow rate, thrust and mesh, switching it on and checking the flow in the results.

Jet fans are the backbone of smoke control and ventilation in underground car parks. In an FDS model they decide where the smoke goes, so how you represent them affects the result more than many other parameters. The FDS User Guide describes directly how to model a fan that is not mounted in a wall but hangs in free space and pushes gases along (section 10.2, “Jet Fans”).

The recommended way: an obstruction and a simple HVAC system

The fan is an &OBST at least one cell thick. On two opposite faces you place &VENTs with SURF_ID='HVAC' — inlet and outlet — and connect them with an HVAC duct containing a fan. Hot, smoky gas then passes through the obstruction like through a free-standing fan.

&OBST ID='JF1', XB=10.0,10.4,4.8,5.2,2.4,2.8 /
&VENT ID='JF1_IN',  XB=10.0,10.0,4.8,5.2,2.4,2.8, SURF_ID='HVAC' /
&VENT ID='JF1_OUT', XB=10.4,10.4,4.8,5.2,2.4,2.8, SURF_ID='HVAC' /
&HVAC TYPE_ID='NODE', ID='JF1 in',  VENT_ID='JF1_IN' /
&HVAC TYPE_ID='NODE', ID='JF1 out', VENT_ID='JF1_OUT' /
&HVAC TYPE_ID='DUCT', ID='JF1', NODE_ID='JF1 in','JF1 out', AREA=0.16, LOSS=0,0,
      FAN_ID='JET FAN', DEVC_ID='JF START' /
&HVAC TYPE_ID='FAN', ID='JET FAN', VOLUME_FLOW=6.0, TAU_FAN=-10 /
&DEVC ID='JF START', QUANTITY='TIME', SETPOINT=120, XYZ=0,0,0 /

Here the fan has a 0.4 × 0.4 m cross-section (4 × 4 cells with a 10 cm cell), a flow of 6 m³/s, and it starts after 120 s — for example after detection plus the delay assumed in the scenario. TAU_FAN=-10 ramps it up along a t² curve over 10 s. One &HVAC TYPE_ID='FAN' can serve many ducts, so a dozen fans need one fan definition plus their own obstructions, nodes and ducts.

• Outlet direction. Louvered fans blow at an angle — FDS represents this with a UVW vector on the outlet &VENT, e.g. UVW=-1,0,-1 (section 10.2.7; FDS example jet_fan.fds).
• Shroud. You can add a short passage of four thin plates around the fan so gas is drawn in on one side and expelled on the other; the fan obstruction then sits roughly halfway along it.
• Present all the time. A &VENT connected to HVAC and the obstruction it sits on must exist for the whole simulation — do not give them a DEVC_ID or CTRL_ID. Switching on and off is controlled on the duct (DEVC_ID/CTRL_ID on the DUCT).

Flow, velocity and thrust — don't lose the momentum

A jet fan datasheet usually gives the flow rate and the thrust (a force in newtons). Thrust is the momentum flux: T ≈ ρ · V̇ · v = ρ · V̇² / A, where V̇ is the flow rate, v the outlet velocity and A the outlet area. If the cross-section in the model is larger than the real one (because the mesh does not allow a smaller one), the same flow gives a lower velocity and less thrust, and the fan in the model is weaker than in reality.

So on a coarse mesh it is better to preserve thrust rather than flow: for a model area A_m use V̇ = √(T · Am / ρ). Example: 50 N thrust, 0.16 m² in the model, ρ = 1.2 kg/m³ → V̇ ≈ 2.6 m³/s. Write down in the report which quantity you preserved and why — it is a typical question in a model review.

A simpler way: a blowing surface

A fan can also be modelled as a surface that blows or sucks regardless of the pressure field (section 10.1): a &SURF with VEL or VOLUME_FLOW on an obstruction face. This is a good approximation for supply and exhaust fans in walls or ceilings, but a weaker one for a jet fan, because it does not draw gas in on the other side — it simply “creates” or “removes” it.

Things to watch in a car park model

• Mesh at the fan. The outlet should cover at least a few cells in each direction, otherwise the jet smears out within the first metres. With a 20 cm cell a 40 cm fan is only 2 × 2 cells — consider a finer mesh in the fan zone (FDS mesh size calculator).
• Simulation time. Jet fans push smoke over long distances quickly — set T_END and the outputs (visibility and temperature slices, &SLCF) to cover the whole path of the smoke to the exhaust.
• Run time. A jet of over ten m/s shortens the FDS time step (CFL condition), so a model with fans runs noticeably longer than the same car park without ventilation. See the estimate for your file in the .fds file check; more in the guide how long does an FDS simulation take.
• Check in the results. Add a &DEVC with QUANTITY='DUCT VOLUME FLOW' and DUCT_ID (no XYZ) to record the actual flow through each fan, and compare the velocity behind the outlet with the manufacturer's data.

Sources: FDS User Guide (NIST SP 1019), sections 10.1, 10.2.1–10.2.4 (“Jet Fans”), 10.2.7, 22.16; FDS Verification Guide, HVAC cases. FDS and Smokeview are software of the National Institute of Standards and Technology; NIST is not affiliated with fdsrun.