Numerical study of scalar advection in canopies using a higher-order closure model

A two-dimensional third-order turbulence closure model has been used to investigate the scalar advection across a canopy edge. The statistics computed from the simulated flow are examined to reveal edge effects and plant density effects.; Plant density acts as an important factor for canopy turbulence. Most of turbulence statistics vary systematically with increasing plant density. The roughest canopy has maximum values of turbulence statistics.; Flow disturbance caused by the edge is complex in the transition region before the flow adjusts to approach the equilibrium state further downwind. Edge effects on turbulence are emphasized by rapid variations of wind velocities, pressure, Reynolds stress, and standard deviations of velocity components. Increased plant density intensifies the edge effect on air flow and turbulence.; The variations of concentration, streamwise/vertical eddy fluxes, and variance of passive scalar across a canopy edge are greatly dependent on wind speed and momentum flux (or Reynolds stress). The vertical eddy flux within the canopy is proportional to stress (or diffusivity, K) and inversely proportional to wind speed (u) and vice versa for horizontal advection. Most of sources are removed from the canopy by horizontal advection in the vicinity of the edge but by vertical eddy flux farther downwind. Vertical advection and divergence of streamwise eddy flux have opposite signs overall but the magnitude is much smaller than horizontal advection. The horizontal advection can be greater than the source strength when the scalar is supplied by divergence of horizontal flux and vertical advection. A denser canopy enhances the edge effects generally.; Scalar exchanges due to the local advection within the canopy and vertical turbulent eddy at the canopy top have similar magnitudes along the dimensionless fetch, x'= xK/uh2 even in atmospheric conditions which consider vertical flow and non-uniform wind speed and stress in the streamwise direction, except for the very dense canopy. But overall wind speed and diffusivity are important factors on the issue of fetch. The ratio of advection to turbulent flux has a power-law relation.; These results provide functional guidelines of the approximate relationship among horizontal advection, vertical turbulent exchange, fetch, turbulent diffusivity, wind speed, and canopy height.