Experimental Investigations on the Hydrodynamics of an Idealized Wildland-Urban Interface (WUI) with Implications to Wildfire Management

The wildland-urban interface (WUI) is characterized by a sharp transition between vegetated land and non-vegetated or paved land. The change in land conditions affects the propagation of wildfires across the interface, which is known as the edge effect in the literature of vegetated canopy flows. Vegetation plays a critical role in determining the propagation speed and the direction of fire. It is well-known that fire spread (speed) positively correlates with wind speed (Linn et al. 2002 and 2005). The latter, in turn, is governed by the interactions between the approaching wind, the canopy structure, and the fire (Keeley and Syphard (2019)). Thus, any predictions of fire spread must begin with the understanding on the fluid dynamics of canopy flows. A conundrum in fire containment is that the clearing of trees to create an area void of fuel (fuel-break) is not always effective in stopping the advancement of fire (Pimont et al. (2009)). In some cases, it actually accelerates fire propagation. This conundrum can be resolved by realizing that the supply of heat is governed by canopy flows and that the clearing of trees speeds up the local wind such that heat from one location can reach another location with trees (fuel). We present a preliminary experimental study to address two hypotheses: (1) near the ground the spread of fire is mostly governed by horizontal mixing layer whereas at the top of canopy both vertical and horizontal mixing layers are active, and (2) the multiscale nature of trees enhances turbulent mixing and results in a faster fire spread. Our experiments consist of a lab-scale fractal tree canopy model placed inside a water flume and point buoyant sources are distributed within the canopy to mimic fire behavior. We considered two canopy parameters – canopy density and the presence/absence of a midstory . Planar particle image velocimetry (PIV) and laser-induced fluorescence (LIF) were used to quantify turbulent mixing of a passive tracer (heat).