Plant Reconfiguration and Consequences for Turbulence and Dispersion Inside Plant Canopies

Turbulent dispersion of scalar quantities such as heat, water vapor, carbon dioxide, biogenic volatile organic compounds and pathogenic spores within plant canopies play a major role in many problems of practical interest. The non-Gaussian, strongly skewed, highly organized, and vertically heterogeneous turbulence produced by the interactions between the flow and the plant canopy and the complex spatial distribution of sources and sinks of these scalars give rise to one of the most challenging problems in atmospheric turbulence. In this work we revisit the problem of scalar dispersion within a plant canopy using a combination of experimental data obtained in a large maize field and numerical experiments using large eddy simulation. In particular, we focus on the origins of skewness in the streamwise and vertical velocity fluctuations and their consequences for dispersion. Flexible plants bend in response to flows and this reconfiguration mechanism allows plants to minimize the increase of drag force with increasing velocity, ensuring survival in flow-dominated habitats. This mechanism has been observed over a wide variety of vegetation, including flowers, leaves, grasses, reeds, trees, freshwater plants, and seaweeds. In this work we show that accounting for the effect of reconfiguration is required for large-eddy simulation (LES) models to reproduce the observed skewness of the streamwise and vertical velocity components and the distribution of sweeps and ejections. Additional LES runs are conducted for the same maize field to investigate the structure of turbulence in different reconfiguration regimes, with mean vertical momentum flux at the canopy top constrained by measurements. Reconfiguration regime has negligible effects on LES predictions of the total vertical momentum flux and the components of turbulent kinetic energy, but produces profound changes in the mechanisms of momentum transport. Higher order moments and the frequency, strength, and depth of penetration of sweeps and ejections are very sensitive to the regime of reconfiguration. This work demonstrates the necessity to model the effect of reconfiguration in LES studies of canopy flows, and highlights the impacts of reconfiguration on the structure of turbulence and on dispersion of scalars emitted inside the plant canopy.