Canopy mixing layers are found in various environmental systems such as forests, aquatic vegetation, coral reefs, corn fields, and building arrays. Having similar flow and turbulence structures as a mixing layer, canopy systems are characterized by coherent structures (arising from Kelvin-Helmholtz instabilities) that strongly control the flux of momentum, turbulence, and scalars through the system. Given their strong impact on the local flow, their role should be considered in studying wildfire spread.
Our flume studies offer insight into how canopy shear dynamics (CSD) impact various aspects related to wildfire spread and combative methods. Canopy gaps and edges are found in forests as either built or naturally found ‘fuel-breaks'. They are intended for prevention of convective and radiative spread of wildfires, and for fire-personnel access. Although used extensively in wildfire management efforts despite its high costs, fuel-breaks do not take fluid mechanics into consideration, particularly that of spot-fire spread. This is a mode of spread by which firebrand are transported by the plume and crosswind to start new fires upon landing far down-wind. To understand the role of CSD in the exacerbation of conditions favorable for spread, we have investigated the flow in and around canopy gaps, the interaction of buoyant plumes with CSD, and the transport of particles by buoyant plumes over and through canopy caps. Key findings indicate that in wind-driven conditions often found during rapid wildfire spread (convective Froude number, Fr > 1), CSD is strongly present and significantly impacts the trajectory and behavior of buoyant plumes. These impacts have implications for convective heat transfer to downwind fuel, as well as for transport of firebrand.