ORGANIZED STRUCTURES IN DEVELOPING TURBULENT-FLOW WITHIN AND ABOVE A PLANT CANOPY, USING A LARGE-EDDY SIMULATION

A Large Eddy Simulation (LES) model representing the air flow within a nd above a plant canopy layer has been completed. Using this model, th e organized structures of turbulent flow in the early developmental st ages of a crop are simulated and discussed in detail. The effect of th e drag due to vegetation is expressed by a term added to the three-dim ensional Navier-Stokes equation averaged over the grid scale. For the formulation of sub-grid turbulence processes, the equations for the ti me-dependent SGS (Sub-Grid-Scale) turbulence energy equation is used, which includes the effects of dissipation (both by viscosity and leaf drag), shear production and diffusion. The organized structure of turb ulent flow at the air-plant interface, obtained numerically by the mod el, yields its contribution to momentum transfer. The three-dimensiona l large eddy structures, which are composed of spanwise vortices ('rol ls') and streamwise vortices ('ribs'), are simulated near the air-plan t interface. They are induced by the shear instability at inflection p oints of the velocity profile. The structure clearly has a life cycle. The instantaneous image of the structure is similar to those observed in the field observations of Gao et al. (1989) and in the laboratory flume experiments of Ikeda and Ota (1992). These organized structures also account for the well known fact that the sweep motion of turbulen ce dominates momentum transport within and just above a plant canopy, and the motion of ejection prevails in the higher regions.