Impacts of the Reconfiguration Mechanism on the Structure of Turbulence Inside Plant Canopies

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. The effect of reconfiguration can be described quantitatively by introducing a drag coefficient that decreases with increasing velocity. Typically, a power-law decrease is used, and the exponent is known as the Vogel number. In this work we show that accounting for the effect of reconfiguration is required for large-eddy simulation (LES) models to reproduce the skewness of the streamwise and vertical velocity components and the distribution of sweeps and ejections observed in a large maize field. 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. The regimes of reconfiguration are characterized by different values of the Vogel number: (1) a zero Vogel number represents a rigid canopy with no reconfiguration; (2) a Vogel number of -2/3 represents strong reconfiguration for one-dimensional (1-D) linear elastic bending; (3) a Vogel number of -4/3 represents strong reconfiguration for two-dimensional (2-D) linear elastic bending; (4) a Vogel number of -1 represents either strong reconfiguration with fractal dimension between 1-D and 2-D or moderate reconfiguration for 1-D linear elastic bending. The change of the Vogel number 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 such as skewness and kurtosis are also 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.