Internal Foliar Air Velocity Characteristics for Soybeans Tested in a Wind Tunnel Journal of the ASABE

The objective of this study was to characterize air velocities within a group of soybean plants under controlled conditions in a University of Tennessee biosystems engineering low-speed wind tunnel. The wind tunnel had 13 module sections that were each sized 1.22 m long, 2 m high, by 2 m wide, and was powered with a 93.25-kW motor and variable frequency drive at a mean air velocity of 1.4 m s-1. Extreme high air velocities were avoided to reduce the impact and added complexities of plant structural deflections. Wind tunnel tests were conducted with soybeans grown in pots placed in the wind tunnel with pot row spacings selected as 0.25, 0.38, and 0.51 m to vary the openness of the plant canopy. These row spacings represented narrow production practices that were perceived as difficult for spray droplets to penetrate. Soybeans were grown in pots outdoors to achieve plants that resembled the hardy structural characteristics of those in field production. Plants reached the R3 growth stage when the air velocity experiments started, and mean plant height and width were 0.51 and 0.46 m, respectively. Outdoor plant canopy leaf area index (LAI) measured at heights of 0.28 and 0.43 m for pot row spacings of 0.25, 0.38, and 0.51 m were 8.27, 6.66, and 5.59 m2 m-2 and 7.04, 6.49, and 5.01 m2 m-2, respectively. Thus, increased measurement height and increased row spacing reduced LAI values as expected. Soybean pots were placed in rows oriented along the length of the wind tunnel. Air velocity component values in the foliar canopy were the greatest for longitudinal components, followed by vertical and lateral components. A complex system of three-dimensional air velocities was observed along soybean rows of various row spacings. The soybean canopy was perceived as the root cause of wind velocity blockage resulting in mean longitudinal wind velocities ranging from -0.01 to 0.61 m s-1 depending on row spacing, location between- or in-row and measurement height. Using ambient wind to reliably and consistently aid spray penetration and droplet movement into soybeans grown under conditions with dense foliage appeared to further compound the issue of spray penetration.