Effect of vegetative canopy architecture on vertical transport of massless particles

Large-eddy simulations of approximately resolved heterogeneous vegetative canopies with repeating row structure were compared to 'equivalent' homogeneous simulations to explore how overall canopy density and horizontal heterogeneity influence the vertical transport of non-depositing massless fluid parcels. A Lagrangian approach was used to quantify particle dispersion. The subgrid component of particle motion was modeled with a Langevin equation that was integrated with a new semi-implicit scheme that successfully minimized rogue trajectories. With rogue trajectories controlled, the subgrid model had a negligible impact on average statistics. Analysis suggested that above the canopy top, canopy density and heterogeneity had a minor effect on mean profiles of particle concentration and vertical flux. However, increasing canopy density resulted in a linear increase in particle residence time, and increased the importance of release height on canopy escape. The average time of persistent vertical particle motions did not follow this monotonic trend. For sufficiently dense canopies, the time scale of persistent vertical motions increased with decreasing canopy density in agreement with mixing-layer scaling. As wall shear became significant, a transition was observed in which persistence decreased with decreasing canopy density. The effect of canopy heterogeneity on residence time and persistence was well correlated with the strength of dispersive fluxes. Canopy heterogeneity decreased the average time for a particle to escape the canopy, and also reduced the coherence of vertical particle motions.