COMPUTER MODELING OF THE METEOROLOGICAL AND SPRAYING PARAMETERS THAT INFLUENCE THE AERIAL DISPERSION OF AGROCHEMICAL SPRAYS

An insight into the nature of prevailing meteorological conditions and the manner in which they interact with spraying parameters is an impo rtant prerequisite in the analysis of the dynamics of agrochemical spr ays. Usually, when these sprays are projected from hydraulic nozzles, their initial velocity is greater than that of the ambient wind speed. The flowfield therefore experiences changes in speed and direction wh ich are felt upstream as well as downstream of the spray droplets. The pattern of the droplet Row, i.e. the shape of the streamlines marking typical trajectories, will be determined by a balance of viscous forc es related to wind speed, inertial forces resulting from the accelerat ion of the airstream and pressure forces which can be viewed in terms of the drag forces exerted on the spray droplets themselves. At a cert ain distance in the ensuing motion, when the initial velocity of the s pray droplets has decreased sufficiently for there to be no accelerati on, their trajectories will be controlled entirely by the random effec ts of turbulence. These two transport processes in the atmosphere can be modelled mathematically using computers. This paper presents a mode l that considers the velocity of spray droplets to consist of a ballis tic velocity component superimposed by a random-walk velocity componen t. The model is used to study the influence of meteorological and spra ying parameters on the three-dimensional dynamics of spray droplets pr ojected in specified directions in neutral and unstable weather condit ions. The ballistic and random-walk velocity components are scaled by factors of (1-xi) and xi respectively, where xi is the ratio of the se dimentation velocity and the relative velocity between the spray dropl ets and the surrounding airstream. This ratio increases progressively as the initial velocity of the spray droplet decreases with air resist ance and attains a maximum when the sedimentation velocity has been re ached. As soon as this occurs, the random-walk process predominates. T he computed effects of the release height of spray droplets, atmospher ic turbulence intensity, evaporation, drop size spectrum, wind velocit y and wind direction on the transport process have been studied and an analysis of spray drift is provided.