A WIND-TUNNEL STUDY OF AIR-FLOW IN WAVING WHEAT - SINGLE-POINT VELOCITY STATISTICS

We analyse single-point velocity statistics obtained in a wind tunnel within and above a model of a waving wheat crop, consisting of nylon s talks 47 mm high and 0.25 mm wide in a square array with frontal area index 0.47. The variability of turbulence measurements in the wind tun nel is illustrated by using a set of 71 vertical traverses made in dif ferent locations, all in the horizontally-homogeneous (above-canopy) p art of the boundary layer. Ensemble-averaged profiles of the statistic al moments up to the fourth order and profiles of Eulerian length scal es are presented and discussed. They are consistent with other similar experiments and reveal the existence of large-scale turbulent coheren t structures in the flow. The drag coefficient in this canopy as well as in other reported experiments is shown to exhibit a characteristic height-dependency, for which we propose an interpretation. The velocit y spectra are analysed in detail; within and just above the canopy, a scaling based on fixed length and velocity scales (canopy height and m ean horizontal wind speed at canopy top) is proposed. Examination of t he turbulent kinetic energy and shear stress budgets confirms the role of turbulent transport in the region around the canopy top, and indic ates that pressure transport may be significant in both cases. The res ults obtained here show that near the top of the canopy, the turbulenc e properties are more reminiscent of a plane mixing layer than a wall boundary layer.