The Modification and Characterisation of the CSIRO Vertical Wind Tunnel for the study of untethered freefalling firebrands

Abstract: The airflow characteristics in a vertical wind tunnel at the CSIRO National Bushfire Behaviour Research Laboratory in ACT, Australia, are investigated. The facility is instrumental in examining the aerodynamic and combustion properties of tree bark, a common source of firebrands that lead to spotfires, which enable bushfires to breach containment and result in extensive house losses. The wind tunnel features a variable-speed fan, a horizontal divergence section, a turning section below a contractor, and a vertical tapered working section. The working section has dimensions of 4250 mm long and a cross-section starting at 770 x 770 mm and diverging to 1000 x 1000 mm at the exit. Air velocities in the working section up to 20 m/s, maintaining a uniform airflow across the working section with low turbulence intensity. The wind tunnel design incorporates two key elements to facilitate the study of untethered freefalling firebrands. Firstly, the vertical working section features the divergent taper, allowing a firebrand to find its terminal velocity within the velocity gradient at a particular fan speed setting. Secondly, modification of the airflow in the working section using a specifically designed screen to minimise the impact of firebrands on the walls. The screen results in higher air velocity near the walls and lower velocity in the central zone. Combined with fan speed control, these design features permit the observation of untethered burning firebrands at their terminal velocity throughout their viable lifespan. Keywords: Firebrands, Spotfire, Wind-tunnels