ANALYSIS OF AND EXPERIMENTAL MEASUREMENTS MADE ON A MOVING AIR-ASSISTED SPRAYER WITH 2-DIMENSIONAL AIR-JETS PENETRATING A UNIFORM CROP CANOPY

Air-assisted crop sprayers use air-jets to enhance the transport and d eposition of agricultural pesticides in crops that are difficult to sp ray using conventional equipment. However, the use of air-jets can cau se excessive environmental contamination or give an ineffective treatm ent if the flow characteristics are poorly matched to the target crop. To establish a scientific basis for improving the design and control of air-assisted sprayers, this paper presents an analysis of the momen tum and turbulent kinetic energy conservation equations for a two-dime nsional air-jet penetrating a uniform crop canopy from a moving spraye r. From the analysis of these equations the velocity and turbulent kin etic energy along the jet centre-line are shown to decay exponentially with penetration distance. The decay exponents are shown to be propor tional to the inverse of the jet width, the square of the ratio of spr ayer speed to initial air-jet velocity and the crop density. Velocity and turbulence kinetic energy measurements are presented for a two-dim ensional air-jet penetrating an artificial crop canopy. The canopy was constructed from a regular array of flow blockage planes which were a djusted to give an experimental range for area density (i.e. the cross -sectional area of blockage normal to the initial air-jet how directio n per unit volume) between 0.7 m(-1) and 3.0 m(-1) for two different v alues of plane spacing of 0.23 m and 0.46 m. The experimental results verify the exponential decay form for the distributions of air flow pr operties along the jet centre-line. The values derived for the exponen tial decay coefficient for both jet centre-line velocity and turbulent kinetic energy were found to give a poor correlation with the canopy area density over the full range of experiments. Instead, this correla tion exhibited an asymptotic characteristic as the exponential decay c oefficients for mean centre-line velocity and turbulent kinetic energy become very large at finite area density during high planar blockage. A rationale for this additional effect of crop structure is presented in terms of the additional losses produced by local flow channelling within dense canopies. The asymptotic characteristic was successfully modelled by redefining the crop density as the weighted sum of the inv erse of the two orthogonal mean how gaps in the artificial crop canopy . This new form of crop density replaces area density in an otherwise conventional model for momentum and turbulent kinetic energy losses du e to small-scale volume averaging of air-jet and crop interactions. (C ) 1996 Silsoe Research Institute