A large-eddy simulation of scalar microfront systems in canopy turbulence

Large-Eddy Simulation (LES) was used to reproduce neutrally stratified airflow in a vegetation canopy. A passive scalar was released from the canopy and the evolution of scalar concentration above the canopy was studied. The most distinguished characteristic of the scalar concentration was 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 gradient. Computer visualization tools and a conditional sampling and compositing technique were utilized to analyze these scalar microfronts. Peaks in positive pressure perturbations exceeding an experimental threshold were found to be effective indicators of scalar microfronts. The convergence of the streamwise velocity component and the divergence of the cross-stream velocity component were observed in the immediate vicinity of scalar microfronts. Many vortices were identified in the flow but exhibited a weak relation with scalar microfronts. Additional to three-dimensional spatial patterns, the time series of scalar microfront systems were examined to reveal how they developed and dissipated.