SIMULATING THE DYNAMICS OF SPRAY DROPLETS IN THE ATMOSPHERE USING BALLISTIC AND RANDOM-WALK MODELS COMBINED

This paper presents a simulation model based on earlier work that comb ines both ballistic and random-walk models to describe the three-dimen sional dynamics of spray droplets released in a specified direction fr om ground-based appliances in various weather conditions. The velocity of spray droplets is considered as a weighted sum of their ballistic and random-walk velocities scaled by a factor (1 - beta) and beta, res pectively, where beta is defined as the ratio of the sedimentation vel ocity and the relative velocity between the spray droplets and the amb ient wind speed. The contribution of the random-walk model to the init ial velocity is seen to be negligible at first, but increases progress ively, though not proportionally, as beta increases. As soon as the sp ray droplets attain their sedimentation velocities, beta = 1, the rand om-walk velocity component predominates and beta plays no further part in the calculations. The predicted effects close to the sprayer of th e drop size, wind velocity and direction, evaporation on the transport process have been evaluated and combined to provide an analysis of sp ray drift.