A wind tunnel study of momentum transport in sparse perennial agricultural canopies

Many perennial agricultural crop canopies are organized using trellis systems. The use of a trellis is advantageous for the deployment of mechanized agricultural equipment, control of light penetration, and manipulation of soil conditions. Grape vineyards are one well-known example of a trellised system. Although these canopies are an important class, they have received considerably less attention than continuous dense canopies. To study how the architecture of these canopies impacts momentum transport, a series of wind tunnel experiments was conducted. The wind tunnel canopy consisted of rows of mesh material spaced ½, 1, and 3 times the canopy height oriented perpendicular to the flow direction. The experiments used very high-resolution 29MP stereo particle imaging velocimetry (PIV) to characterize momentum transport between the rows and above the rows within 1 canopy height. In addition to examining multiple row architectures, verification experiments behind a single mesh row are compared to existing windbreak data. Preliminary analysis of the canopy structure PIV data indicates that the vertical profiles of mean velocity and turbulent shear stress derived from the PIV data are consistent with profiles observed in homogeneous canopies for all three-canopy cases. The impact of canopy heterogeneity and row spacing is most prominently observed when examining two-dimensional ensemble averaged turbulent shear stress and velocity variance between the rows. An elevated shear stress layer originates on the downwind side of each row and propagates into the canopy with increasing downwind distance. The depth of penetration of this shear layer was found to depend on the spacing between rows with smallest row spacing resulting in only limited penetration consistent with a horizontally homogeneous canopy.