Performance Comparison of Solid Walls and Porous Fences in Protecting Structures from Firebrand Attack

In bushfire-prone regions, solid walls and porous fences are commonly employed as mitigation measures against windblown embers (firebrands). This computational study evaluates and compares the performance of a 2 m high solid wall and a 2 m porous fence (38% porosity) in protecting structures from firebrand showers. Using a numerical model subjected to free-stream velocities of up to 50 m/s, flow patterns and firebrand trajectories were analyzed. The findings indicate that impermeable walls offer superior protection for immediately adjacent structures by deflecting the incident flow upwards, creating a “jump board” effect. However, the deflected flow subsequently reattaches to the ground at a downstream distance, rendering structures further downwind vulnerable to ember attack. The porous fence also exhibits a similar, albeit less pronounced, upward deflection. The simulations reveal minimal flow descent downstream of the fence at lower free-stream velocities, suggesting extended downwind protection. In the immediate downstream vicinity of the porous fence, penetration by small firebrands is possible; however, prior studies have shown that the likelihood of ignition from these embers is minimal and decreases rapidly within a short downstream distance of several metres.