Characterizing turbulence at a forest edge: A vorticity budget analysis around a canopy

Abstract

Vorticity is a key characteristic of flow patterns that determine wildland fire behavior, frontal evolution, and wind-canopy interaction. Investigating the role of vorticity in the flow fields around vegetation can help us better understand fire-atmosphere feedback and the influences of vegetation on this feedback. In modeling vorticity, “perhaps the greatest knowledge gap exists in understanding which terms in the vorticity equation dominate [...] (and) when one or the other might dominate” (Potter, 2012). In this study, we investigate the role of vorticity in boundary layer dynamics and canopy/forest edge effects using HIGRAD/FIRETEC, a three-dimensional, two-phase transport model that conserves mass, momentum, energy, and chemical species. A vorticity transport equation was derived and discretized. Simulations were performed over a cuboidal homogeneous canopy surrounded by surface vegetation. This derivation led to the discovery of a drag tilting and stretching term, which shows that gradients in the aerodynamic drag of the vegetation, tied to heterogeneities in surface area-to-volume ratio, play an important role in the generation of vorticity. Results from the vorticity budget analysis show that the drag tilting and stretching term contributes significantly in the areas where these gradients are present, namely the edges of the canopy.