Abstract
This project was aimed at providing better understanding of the interactions between the jets produced by air assisted orchard sprayers and the plant canopies to which they apply pesticide products. In addition, it sought to discover what scope there is for changing pesticide application practices in UK orchards. Laboratory scale experiments were used to gain understanding of the behaviour of two dimensional air jets impinging on a range of porous structures. Flow visualisations were used to obtain a general idea of the flow patterns produced and detailed measurements gave quantification of the jet's behaviour. Measurements of a jet impinging on a single screen showed that the jet's behaviour could be described in terms of the variation of the axial volume flow and the momentum flux across and downstream of the screen. Further to this, it was shown that the behaviour of a jet impinging on arrays of porous screens could be equated to the impingement of the jet on subsequent screens as if they were in isolation. This finding applied for screens at spacings greater than four times the repeating length scale of the screen (i.e. the hole pitch). Measurements with moving jets showed that the momentum flux was unaffected by the jet's movement but that the axial volume flow increased with jet traverse speed. A questionnaire survey of UK growers was used to gather information on practices within orchards. This showed a continuing shift in growing practices towards dwarfing rootstocks, a change in application methods towards low volume spray application, but a continuing preference for the axial fan design of orchard sprayers. Of the sprayers being used by respondents to the survey, 45% were over ten years old. The findings on jet flow through porous obstacles gives an understanding of the fundamental interactions involved in the operation of orchard sprayers. A discussion of the findings in terms of the interactions between sprayer jet's and plant structures demonstrated that the canopy density is likely to have a large influence on the spray drift production. Further work is proposed that would quantify the level of drift that would be produced by an industry standard sprayer for a given canopy density. Applying the optimum canopy density for minimal drift would then be a possible strategy for reducing spray drift. The results of the survey suggested that such a strategy could be of some appeal to growers who are reluctant to change spray machinery but have made changes to growing systems over the years.