Effects of wind and slope on the behavior of moving fire whirls

This paper numerically investigates the effects of ambient wind and sloped terrains on the behavior of moving fire whirls using Fire Dynamics Simulator (6.7.6). Five configurations were designed, including small- and large-scale L-shaped fire sources and linear fire sources on flat terrain, as well as linear fire sources on leeward slope and canyon terrain. The movement trajectories, upper and lower critical wind speeds (U_ec) for the formation, and average movement speed (V) of fire whirls were analyzed under varying wind speeds (U) and slope angles (θ = 0°, 10°, 20°, and 30°). Except for the fire whirls in the small-scale L-shaped case on flat terrain, which exhibit a transition from downwind to upwind movement, fire whirls move unidirectionally along the fire line in other configurations. The interaction between the fire source and ambient wind was revealed through the synchronized evolution of tangential velocity fields. Two scaling correlations for U_ec, incorporating heat release rate, wind direction, and slope angle, were developed and confirmed by both experimental and numerical results from this study and the literature. The dimensionless movement speed was lower than the ambient wind speed in the L-shaped fire source configurations, but exceeded it in Brazil-type and leeward slope configurations.