The Link between Coherent Structures and Particle Transport in Canopy Flows

Particle transport in and above plant canopies by turbulence plays a role in many agricultural and forestry systems including the dispersion of airborne pollens, seeds, and spores. An important factor that influences the speed and range over which these particles disperse is the ability of particles to escape the canopy space. Above the canopy, increased wind speed and turbulent fluxes combine with the absence of deposition to provide a much more efficient avenue for long distance dispersal presumably contributing to “fat-tailed” dispersion kernels. It is well accepted that turbulent transport in dense canopies is dominated by intermittent coherent vortex structures with length scales on the order of the canopy height. These coherent structures are thought to consist of a superposed pair of head-up and head-down hairpin vortices that give large contributions to ejection and sweep events, respectively. Although it is commonly asserted that these structures dominate scalar transport near the canopy top, a strong link between these structures and scalar transport has yet to be established. In this study, a coupled Eulerian-Lagrangian approach was used to link coherent structures to scalar transport, particularly to scalar ejection and re-entry from and to the canopy, respectively. Large-eddy simulation (LES) was used in conjunction with a Lagrangian particle dispersion model (LPDM) to show that the ejection-sweep structures in fact are primarily responsible for the escape and re-entry of particles at the canopy top through utilization of a newly developed conditional sampling method in which Eulerian sampling events are triggered based on vertical particle motion at the canopy top. Results indicate that as the canopy becomes increasingly sparse, stronger yet less frequent structures increase the probability that a given particle will be ejected from the canopy, presumably resulting in a greater likelihood of long distance dispersal events.