5.7
A numerical simulation of airflow over and through a forest edge using large eddy simulation
Bai Yang, University of California, Davis, CA; and R. H. Shaw and K. T. Paw U
Large-eddy simulation is employed to model the turbulent flow around the roughness transition induced by a forest edge. Flow statistics and the momentum balance have been investigated and tested against field and wind tunnel observations.
Calculated mean streamlines and wind vectors exhibit patterns that are comparable to those from wind tunnel experiments. An upward distortion of the flow field at the leading edge is shown by streamlines and wind vectors in an x-z slice, with x indicating the streamwise, y the spanwise, and z the vertical directions. The distortion angle is greatest one canopy height downwind of the edge and a little below treetop height, more or less coincident with the level of maximum leaf area density. The time- and y-averaged streamwise velocity in an x-z plane reveals a rapid deceleration below 1.5 canopy heights, extending from a streamwise distance of about 4 canopy heights upwind of the edge to about 10 canopy heights downwind. In contrast, an acceleration of streamwise velocity is seen above 1.5 canopy heights in the vicinity of the leading edge. The vertical profiles of normalized streamwise velocity at the various locations along the streamwise direction closely match those obtained from field experiments.
The blockage by the forest edge creates a high-pressure zone immediately upwind. Analysis of the momentum budget shows this to be the expected consequence of the discontinuity of drag at the edge. Unlike the region far downwind of the edge, in which downward diffusion of momentum is the source of momentum to drive canopy wind, the pressure gradient force and horizontal advection of streamwise velocity are identified as the major momentum sources to balance drag in the forest immediately downwind of the edge.
Session 5, Canopy micrometeorology
Thursday, 17 August 2000, 8:45 AM-11:15 AM
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