Extended Abstract (56K)2.2
Budget of Velocity Variances Across A Forest Edge: A Comparison between Field, Wind Tunnel and Numerical Simulation Studies
Bai Yang, Univ. of California, Davis, CA; and A. P. Morse, R. H. Shaw, and K. T. Paw U
A comparison of budget terms in the prognostic equations for streamwise and vertical velocity variances in the flow across a forest edge has been performed between field and wind tunnel experiments and large eddy simulation (LES).
At canopy height the vertical shear production term is identified by all three studies as the major source for streamwise velocity variance and dominating the other terms. Starting at the forest edge shear production gradually increases in the transition zone and levels out at 12 h (h is canopy height) when equilibrium is achieved. Normalized values for this term from the three studies are of the same order of magnitude, with the largest value from wind tunnel and smallest value from the LES. Similarities also exist between the three experiments in vertical advection and horizontal convergence terms. Differences do exist in the turbulence transport term (third order moment). The pressure redistribution term, which could not be measured in either the wind tunnel or the field, appears in the LES as the predominant sink term. Like the vertical shear production term, the pressure redistribution term also picks up at the edge and gradually increases in magnitude along the flow direction.
Regardless of magnitude, the budget terms at the canopy top for vertical velocity variance available from field and wind tunnel experiments, including vertical advection, vertical convergence production and turbulence transport terms, show similar variations along the flow direction to those from the LES. These terms are enhanced in magnitude due to flow distortion immediately at the back of the forest edge and relax to close to zero after some distance from this edge. One interesting feature of the vertical convergence production from all three experiments is that it changes sign at about 6 h downwind from the edge. The only difference is that the LES exhibits a smaller magnitude than the other two. With its ability to solve the pressure perturbations, the LES shows, at canopy height, much larger values for pressure redistribution and pressure transport terms than the above mentioned terms, with the former one as a source term to balance the latter one. The pressure redistribution term does not change to a positive value until about 2 h downwind from edge, and starts to increase and adjust to the new rough surface only after the sign switch. This is a little later than is the case for the major source term, shear production term, for streamwise velocity variance, which starts to increase immediately at the edge.
Session 2, turbulence and dispersion in canopies Part 1
Monday, 20 May 2002, 1:30 PM-3:30 PM
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