Firebrand deposition on cubic structures in tandem and parallel arrangements

Spot fires created by wind-carrying firebrands are common in wildland-urban interface (WUI) fires. Firebrand deposition over cubic blocks in tandem and parallel arrangements representing simplified nearby small structures in WUI was studied. The flow turbulence was modeled by large eddy simulation, and the firebrands were simulated in the Lagrangian framework. Both translational and rotational motions, and the thermal degradation of firebrands were accounted for. Various separation distances between blocks and wind speeds were considered. Simulations revealed that the blocks shielded their immediate leeward areas on the ground from firebrand deposition. These areas were distinguished by having no deposited firebrands, and were hence called safe zones, which were nearly in the shape of an isosceles trapezoid. The longer base of the trapezoid coincided with the bottom edge of the block's leeward face, and the shorter base and the legs of the trapezoid constituted the borderlines of the safe zone. The safe zone length (i.e. the trapezoid height) in the wind direction increased with the wind speeds without a significant sensitivity to the block position in either arrangement. For the blocks in the tandem arrangement with a smaller separation distance and/or higher wind speeds, the safe zone encroaches on the windward face of the downwind block. Therefore, an upwind block also shielded the lower part of the windward face of the downwind block from firebrand deposition in these cases. In none of the simulations, firebrands were deposited on the leeward faces. In all simulations, the firebrands deposited on the lateral faces that were parallel to the wind direction were much less than those on the windward faces.

Automated summary

Spot fires caused by wind-carried firebrands are common in wildland-urban interface (WUI) fires. This study focused on firebrand deposition over cubic blocks in tandem and parallel arrangements, representing nearby small structures in the WUI. Large eddy simulations were used to model flow turbulence, while firebrands were simulated in the Lagrangian framework. The simulations revealed that the blocks protected their immediate leeward areas from firebrand deposition, creating safe zones in the shape of an isosceles trapezoid. The length of the safe zone increased with wind speeds, and upwind blocks also protected the lower part of the windward face of downwind blocks from firebrand deposition in certain cases.