Particle Deposition in a Douglas-fir Canopy: A Wind Tunnel Simulation of Aerial Spraying

Abstract

ABSTRACT A wind tunnel study was performed to simulate the aerial spraying of pesticides. A Douglas-fir canopy containing more than 50 trees approximately 0.76 m in height was established in a wind tunnel having cross-sectional dimensions of 3.65 m x 2.44 m and an effective test section length of 17.42 m. Drop-lets, tagged with uranine dye and generated using two vibrating orifice atomizers, were injected into the wind tunnel through the ceiling. Sections of the foliage and actual Tussock moth insect larvae placed within the foliage were analyzed fluoroscopically to assess the flux distribution which occurred during tests. The tests were conducted using three different monodis-perse particle sizes (nominally 35 /^m, 60 ^m, 140 fim) at three different approach flow velocities (0.45, 2.24, and 4.03 m/s). Results were presented as a function of STK number based upon a characteristic length of 5.65 x 10-2 cm, which is one-half the needle width of Douglas-fir. A critical STK number of ~4 was iden-tified, which if exceeded, would not improve the over-all average flux distribution. This is not to imply that one should necessarily calculate a particle size for efficient spraying based upon an STK value >4 as there does exist a mass bias on the upstream crown of the tree for larger STK values. The results in this study apply to those nominal aerodynamic sizes quoted above which are introduced at a canopy height corres-ponding to z/H = 1.0.