Impact of canopy-induced instability on buoyant plumes

Characterizing the interaction between buoyancy-driven plumes and ambient cross winds is crucial to understanding and modeling wildfire spread in forest canopies. However, current classifications of buoyancy or wind-dominated wildfire systems are based only on scalings using ambient crossflow velocities rather than canopy flow properties. Analogous to mixing-layers, flow through forest canopies is characterized by the presence of Kelvin-Helmholtz instabilities at the canopy top that affect momentum, turbulence, and scalar fluxes into and out of the canopies. Therefore, in this study we explore the impact that these canopy-induced instabilities have on buoyant plumes, over a wide parametric range, in order to develop a scaling that better encapsulates the behavior of convective plumes in canopy flow conditions. The study is conducted in a recirculating flume with submerged model vegetation and thermally buoyant plumes. In varying how wind or buoyancy-dominated the system is, we vary the convective Froude number (Fr_c) of the plumes. Additionally we control the momentum thickness of the canopy mixing layer to alter the strength of the canopy signal. Our results indicate that while the Fr_c may set the mean behavior of the plume, the canopy signal has a strong impact on the unsteady beavhior of plume and its trajectory. Plume visualization using Synthetic-Schlieren methods and thermocouple readings are used to analyze the behavior and trajectory of the plume under different conditions.