Flow Structure and Turbulence in a Model Plant Canopy

The mean and turbulence flow characteristics within and above a wind tunnel model canopy were measured using Particle Image Velocimetry (PIV). The mean velocity profile of the flow was adjusted to match with that of a corn canopy using screens with variable mesh sizes. PIV measurement range extended from 0.81 to 2.13 times canopy height, with each height containing 2360 vector maps. The smallest resolved scale was 5 times the Kolmogorov length scale. The Taylor scale Reynolds numbers ranged between 360 and 800. Conditional sampling based on quadrant analysis shows substantial differences in flow structure around the canopy height, especially between sweep and ejection events. During sweeps, the downward flow generates a thin, highly turbulent, shear layer containing multiple small-scale vortices just below canopy height; while during ejections, the upward flow expands the shear layer and small scale flow structures to a broad region above the canopy. Consequently, during sweeps the turbulent kinetic energy and Reynolds shear stress have distinct peaks just below canopy height, whereas during ejections they have broad maxima above the canopy. In addition, conditional averaging shows that the differences in flow structure have significant effect on the distributions of production, turbulent diffusion, dissipation rates.