Investigating Buoyant Plume Dynamics Induced by Localized Fire-Simulated Heating over Plant Canopies Using LES

TL;DR

This paper investigates how a buoyant plume from a localized heat source interacts with a cross-wind over a tall, homogeneous canopy using large-eddy simulations (LES). The authors deploy the PALM solver with a plant-canopy module to compare canopy and no-canopy cases, isolating canopy aerodynamic effects on near-field structures and far-field trajectories. Key findings include the formation of counter-rotating vortex pairs near the source, plume-aligned vortex tubes with entrainment, canopy-induced recirculation on the leeward side, and altered plume tilt and oscillations; momentum-flux quadrant analysis reveals canopy-modified transport patterns consistent with some field observations. The results offer mechanistic insights and a computationally efficient framework for refining parameterizations of plume rise and fire-atmosphere interactions in canopy environments.

Abstract

The interaction of a buoyant plume with a plant canopy results in turbulent flow features distinct from those in a grassland environment. In this work, we model the turbulence dynamics of a buoyant plume in a homogeneous plant canopy with a crosswind using large-eddy simulations. As the plume interacts with the crosswind, we observe increased vorticity at the windward edge and tilted hair-pin-like vortical structures on the leeward side. Strong rotational cores, representing counter-rotating vortex pairs (CVPs), form as the flow twists and spirals into the leeward side of the buoyancy source from either side. Flow patterns aloft exhibit helical motions as the CVPs aloft propagate downstream, trailing the plume. We also simulate a no-canopy environment to facilitate comparison. The plume tilts less steeply near the source in the canopy case due to the canopy drag and its leeward side is marked by flow recirculation near the canopy top, which obstructs the upstream flow as it approaches. Moreover, the plume transition from the rise phase to the bent-over phase is delayed due to the canopy's aerodynamic effects and the oscillatory behavior of the far-field mean plume centerline is more damped. Additionally, in the canopy environment, there is downward momentum transfer primarily via ejections above the canopy and sweeps within the canopy space, upstream of the plume centerline. On the leeward side, counter-gradient motions play a significant role in transferring momentum away from the buoyancy source, with outward interactions being most dominant. Contrarily, in the no-canopy environment, counter-gradient motions near the surface are flanked upstream by an ejection-dominated region and downstream by a sweep-dominated region. Insights into the distinct plume behavior in canopy vs. no-canopy environments are vital for comparing with experiments and refining fire behavior or plume rise models.

Figures (9)

• Figure 1: A schematic of the computational setup for the simulations in a canopy setting. (a) Velocity profile at the inlet. (b) Domain description; kinks along the vertical axes represent regions with vertical grid stretching.
• Figure 2: Contours of the 1-h mean temperature plotted on the $XZ$ plane $y=500$ m (passing through the center of the heated patch) for the (a) canopy and (b) no-canopy cases. Dashed horizontal line in (a) represents $z=h_\text{c}$
• Figure 3: Color contours of the 1-h mean vertical velocity ($\overline{w}$) for the (a) canopy and (b) no-canopy cases. Grey points represent 1-h mean plume centerline ($\mu$) in the far-field (Zone 2); vertical yellow dotted lines represent $\mu\pm s_t$ at multiple locations in the far-field (Zone 2); green dots represent the plume centerline in Zones 1 and 1(B) (in the canopy case), while white slanted dotted lines represent the plume centerline inclination. In (a), the black dashed line represents the inclination of the plume centerline in the transition zone (Zone 1(B)). Trend-lines (dash-dotted) in Zone 2 are obtained using MATLAB's fminsearch() for a linear fit to the oscillating mean plume centerline
• Figure 4: 3-D Streamtubes colored by vorticity magnitude ($\text{s}^{-1}$) (a) as observed from the downstream face of the domain and (b) demonstrating the presence of CVPs (C1, C2) near the source, in the canopy case. (c) A comparison of the structure of 3-D streamtubes between the canopy and no-canopy cases. with recirculating streamtubes. A1, A2--helical patterns along the plume edges; B-- secondary helical pattern; (D) recirculating streamtubes
• Figure 5: (a) Streamtubes in the $XZ$ plane passing through the center of the surface patch and (b) isosurfaces of the Q-criterion shown for the canopy and no-canopy cases