Structure of the Reynolds Stress in a Canopy Layer

The u, w velocity covariance above and within a plant canopy (Zea mays L) was examined using the technique of quadrant analysis to separate the momentum transport into events classified as sweep, ejection, and outward and inward interactions. A hyperbolic hole of variable size acted as an excluded region in the u, w domain to asses the relative importance of short-lived events of large magnitude. The results of the analysis were a reasonably close match to rough-wall wind tunnel studies but differed in some respects from a similar experiment performed elsewhere in a flexible wheat canopy. Generally, sweeps exceeded ejections in their contribution to the Reynolds stress, especially at mid-canopy, while the interaction events were of minor importance. Sweeps that were large in magnitude relative to the time-averaged stress were evident at all levels and were intermittent in character. Compared with the layers above, the effect of the canopy was to increase the dominance of sweeps over ejections and to increase the degree of intermittency of the diffusion process. A reversal of this trend occurred in the bottom layers of the canopy. The third-order conditional probability distribution was sufficient to predict the total difference between sweep and ejection fractions, but could not accurately predict the extent to which large sweeps reinforced the overall momentum transport.