On the appropriateness of Fire Dynamics Simulator for the road tunnel smoke control problem

NIST has made publicly available (in the FDS Validation Guide and on GitHub) Fire Dynamics Simulator (FDS) simulation results of the Memorial Tunnel fire tests. Those FDS results as provided by NIST can be analysed to extract a critical velocity value. That is done here, with comparison to the critical velocities observed and recorded during the Memorial Tunnel tests. The conclusion is that, for the region of interest (HRR above 30 MW), for the Memorial Tunnel cross section without the ceiling, FDS overpredicts critical velocity by 40% or so. The reasons for the overpredictions remain unknown to date, and so it is unclear how the underlying issues will affect tunnel systems different to the Memorial Tunnel. Consequently, from a practitioner’s point of view, FDS cannot be relied upon to accurately assess critical or confinement velocities in road tunnels as long as the underlying problem for the discussed overpredictions is not understood and not demonstrated to be appropriately resolved. If critical velocity cannot be estimated accurately, then FDS cannot be considered a reliable tool for modelling any other deviation from the balance between forced and natural convection that is fundamental to road tunnel smoke control.