A Lagrangian Investigation of Momentum and Scalar Transport in Plant Canopies

Turbulent transport in plant canopies is unique in many respects. Such unique aspects include highly efficient vertical scalar transport even in the presence of small gradients, which is commonly linked to the dominance of canopy-scale coherent structures. These interesting traits also add to the level of complexity of the flow, which in turn leads to a violation of a wide range of simplifying assumptions that aid in analysis (e.g., isotropy, homogeneity, Taylor's hypothesis). All of these complications stem from the fact that many turbulence quantities of interest were originally conceptualized in a Lagrangian framework, and are only translated into our Eulerian frame of reference for interpretation. However, investigation of canopy turbulence in a Lagrangian view can reveal unique information that cannot be directly calculated from an Eulerian viewpoint.

In this study, we investigated canopy transport in the more natural Lagrangian framework. High-resolution simulation tools were used in which we have a complete description of the trajectories of a large number of individual particles released from varying locations in canopies with a range of densities. Visualizations solidify the notion that particle trajectories are dominated by intense sweep-ejection cycles. Individual particle motions appear to be substantially more coherent than the Eulerian field. This is exemplified by the Lagrangian autocorrelation function, which shows a unique signature in the vertical velocity correlation as well as the streamwise-vertical velocity cross-correlation that indicates a dominance of roller vortex structures. We directly calculated other statistics that are commonly only estimated in an Eulerian framework, such as canopy residence time and advective velocity scales.