LES analysis on the effects of baroclinic generation of vorticity on fire-wind enhancement

Abstract

Abstract Substantial influence of fire flame on wind flow aerodynamic characteristics lead to enhancement of wind velocity that may cause considerable damages to buildings in bushfire-prone areas. Fire is associated with rotational vortical structures ranging from the small scales, sourced by baroclinic generation of vorticity ( ω B G V ), to large scale vortices arising from amalgamation mechanisms that can significantly affect wind flow characteristics during fire-wind interaction. This study aims to understand the extent to which baroclinic generation of vorticity affects wind enhanced by fire. Large eddy simulations (LES) of fire-wind interaction are conducted using fireFOAM solver of OpenFOAM platform for two different types of fuels with the aim to produce two different scenarios with similar heat release rate, but different flame temperatures. This will guarantee the production of different vortex structures in the two cases while flow expansion rate, which is proportional to the fire heat release rate, remains constant. FireFOAM solver was modified to extract fire-induced acceleration and vorticity components. The LES results show that under similar fire intensity and heat release rate conditions, wind enhancement is higher in the scenario with higher flame temperature along the centreline as well as the cross-sectional locations which are corresponding to the locations of counter-rotating vortices where the maximum wind enhancement appears. It was shown that in the case with higher flame temperature, baroclinic generation of vorticity is stronger, which causes stronger longitudinal fire-induced pressure gradient and consequently results in a higher wind enhancement in these cross-sectional locations. The distribution of maximum cross-sectional baroclinic generation of vorticity along longitudinal location are presented, confirming in both cases, baroclinic generation of vorticity reduces with an increase of distance from the fire source. However, in almost all distances, the scenario with a higher flame temperature generates stronger baroclinic generation of vorticity, so does the fire-induced pressure gradient and flow enhancement.