Three-dimensional scalar microfront systems in a large-eddy simulation of vegetation canopy flow

Large-eddy simulation is used to reproduce neutrally stratified airflow inside and immediately above a vegetation canopy. A passive scalar is released from the canopy and the evolution of scalar concentration above the canopy is studied. The most significant characteristic of the scalar concentration is the repeated formation and dissipation of scalar microfronts, a phenomenon that has been observed in nature. These scalar microfronts consist of downstream-tilted regions of high scalar concentration gradients. Computer visualization tools and a conditional sampling and compositing technique are utilized to analyze these microfronts. Peaks in positive pressure perturbation exceeding an experimental threshold are found to be effective indicators of scalar microfronts. Convergence of the streamwise velocity component and divergence of the cross-stream velocity component are observed in the immediate vicinity of scalar microfronts, which helps explain their relatively long lifetimes. Many of these three-dimensional features have been observed in previous field studies of canopy flow.