Analysis of and experimental measurements made on a moving air-assisted sprayer with two-dimensional air-jets penetrating a uniform crop canopy

Air-assisted crop sprayers use air-jets to enhance the transport and deposition of agricultural pesticides in crops that are difficult to spray using conventional equipment. However, the use of air-jets can cause excessive environmental contamination or give an ineffective treatment 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 momentum and turbulent kinetic energy conservation equations for a two-dimensional air-jet penetrating a uniform crop canopy from a moving sprayer. From the analysis of these equations the velocity and turbulent kinetic energy along the jet centre-line are shown to decay exponentially with penetration distance. The decay exponents are shown to be proportional to the inverse of the jet width, the square of the ratio of sprayer speed to initial air-jet velocity and the crop density. Velocity and turbulence kinetic energy measurements are presented for a two-dimensional air-jet penetrating an artificial crop canopy. The canopy was constructed from a regular array of flow blockage planes which were adjusted to give an experimental range for area density (i.e. the cross-sectional area of blockage normal to the initial air-jet how direction per unit volume) between 0.7 m(-1) and 3.0 m(-1) for two different values 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 properties along the jet centre-line. The values derived for the exponential 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 correlation exhibited an asymptotic characteristic as the exponential decay coefficients 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 inverse 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 due to small-scale volume averaging of air-jet and crop interactions. (C) 1996 Silsoe Research Institute