The Impact of Unsteady Reconfiguration on Turbulence Structure within a Flexible Canopy: Large-Eddy Simulation Study of a Cornfield

Thursday, 18 December 2014: 9:45 AM
Marcelo Chamecki, Pennsylvania State University, Department of Meteorology, University Park, PA, United States, Ying Pan, Pennsylvania State University Main Campus, University Park, PA, United States, Heidi Margaret Nepf, MIT, Cambridge, MA, United States and Elizabeth Follett, Massachusetts Institute of Technology, CEE, Cambridge, MA, United States
Flexible plants bend in response to fluid motion and this reconfiguration mechanism allows plants to minimize the increase of drag force with increasing velocity, ensuring survival in flow-dominated habitats. The effect of reconfiguration on the flow field can be modeled by introducing a drag coefficient that decreases with increasing velocity. Typically, a power-law decrease of the drag coefficient with increasing velocity is used, and the negative exponent is known as the Vogel number. In practice, the Vogel number is a function of canopy rigidity and flow conditions. 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 cornfield. Additional LES runs are conducted to investigate the structure of turbulence in different reconfiguration regimes, with mean vertical momentum flux constrained by measurements. 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. This work demonstrates the necessity to model the effect of reconfiguration in LES studies of canopy flows. It also highlights the impacts of reconfiguration on the structure of turbulence and the dynamics of momentum fluxes, as well as any other process that depends on velocity fluctuations above and within the canopy region.