FDS: Numerical Instability (max divergence) after timed vent opening + OBST removal + BURN_AWAY #16034
Description
Discussed in #16017
Originally posted by fjzeng1999-cloud March 5, 2026
Hello respected engineers,
fire0305fds.txt
fire0305out.txt
fire0305b_devc.csv
fire0305b_hrr.csv
fire0305b_steps.csv
Revision : FDS-6.10.1-0-g12efa16-release
I have recently been simulating fire spread in a timber building composed of multiple enclosed rooms. When the fire reaches its peak intensity, the numerical simulation becomes unstable. At around 1213 s, I simulated the disappearance of a window (using a timer), which introduces fresh air and triggers flashover in the compartment. To represent structural failure caused by the high temperatures generated during flashover, I set up &DEVC timers to remove several non-combustible &OBST elements (a tiled roof) shortly after flashover. The purpose is to allow the fire to spread and trigger a larger-scale flashover. To model structural failure of timber walls and their fuel contribution, I used the BURN_AWAY logic.
The simulation runs stably until 1456 s, when it crashes with “Numerical Instability” (max divergence).
My question is: What could be causing the numerical instability in this simulation? Based on the recommendations in the FDS User Guide, I have modified many parameters and performed more than 50 runs, but the numerical instability error still occurs at apparently random times. The longest run lasted about 1700 s, and the shortest about 1250 s.
Thank you very much for your suggestions!Please! Help me.