N. Woods

The entrapment of particles by windbreaks

A theory is developed for calculating the entrapment of particles by a windbreak, with four results. (1) The fraction of particles in the oncoming flow which pass through the windbreak, or transmittance of the windbreak for particles (sigma), is related to the optical porosity (tau). The very simple approximation sigma=tau works well for most applications involving the interception of spray droplets by windbreaks. Results from a field experiment agree with the theoretical predictions. (2) A new equation for the bulk drag coefficient of a windbreak is tested against numerical, wind tunnel and field experiments. This enables the bleed velocity for the flow through the windbreak to be predicted in terms of the screen pressure coefficient (k) of the barrier. (3) The relationship between k and tau is different for a vegetative barrier than for a screen across a confined duct, implying a lower Fc for given tau. (4) The total deposition of particles to a windbreak is determined by a trade-off between particle absorption and throughflow, implying an optimum value of tau for maximum total deposition. For particles larger than 30 mum and vegetation elements smaller than 30 mm, this occurs near tau = 0.2

A simple guide to predicting aircraft spray drift

A sensitivity analysis has been performed on a simple Gaussian Diffusion Model used to predict spray drift deposition from agricultural aircraft at far wake distances. It is intended that the model be used as a training tool to demonstrate clearly the basic effects of windspeed, turbulence intensity, release height, droplet size and uniformity on far downwind spray drift deposition i.e. at distances greater than 50 m. Overlapping the model outputs has also provided a useful benchmark against which experimentally obtained spray drift values may be compared. This information has assisted in the preparation of guidelines regarding required buffer zone distances and the best ways in which aircraft pesticide spray drift can be effectively reduced and managed.

Agricultural aircraft spray performance: calibration for commercial operations

Abstract A system for testing agricultural aircraft spray systems alongside a commercial operator in Australia is described. Factors affecting the uniformity of deposit achieved on the ground are discussed, together with the need to make suitable adjustments to airborne spraying and spreading equipment. Measurements from a single flight-line are used to determine the optimum swath width for a particular aircraft configuration. The role of a microcomputer in storing, presenting and evaluating spray distribution data in routine commercial operations is demonstrated.