The paper presents a model for describing the damage that might be done to buildings by wind-borne debris in a sustained high-speed wind such as a typhoon or hurricane, as distinct from the damage inflicted by the direct wind pressure itself. The work, carried out under the auspices of the UN International Decade for Natural Disaster Reduction programme, was concerned with the effects of high typhoon wind speeds in urban areas in the Pacific Rim region of South East Asia (J.A. Wills, T.A. Wyatt, B.E. Lee, Warnings of high winds in densely populated areas. IDNDR Flagship Programme Report 4, Thos. Telford, London, 1998).

This paper presents some of the results of a project whose aim has been to produce a full simulation model which would determine the efficacy of pesticides for use by both farmers and the bio-chemical industry. The work presented here describes how crop architecture can be mathematically modelled and how the mechanics of pesticide droplet capture can be simulated so that if a wind assisted droplet-trajectory model is assumed then droplet deposition patterns on crop surfaces can be predicted. This achievement, when combined with biological response models, will then enable the efficacy of pesticide use to be predicted.

This paper sets out a novel method of modelling the capture of pesticide spray by a cereal crop. Firstly, the transport model, based upon that of Mokeba et al. (J. Wind Eng. Ind. Aerodyn. 67 & 68 (1997) 923–933), is described. This models the effect of turbulence on each individual spray droplet by using a random-walk model incorporating the Lagrangian variances and time-scale of the air velocity. The crop model, which incorporates a realistic three-dimensional parameterization of the plant, is then detailed and the effects of the crop on the airflow are considered. Some useful applications to justify the added complexity of this new method are then outlined and a perspective offered on the question of spray penetration of a crop canopy. Finally, an attempt is made at looking at the problem of drift at the scale of the field.

A finite-difference model for the movement of pesticide through a barley leaf is described. The model considers the leaf to consist of a number of layers possessing different solubility and mobility for pesticides. Response is modelled via appropriate, experimentally determined tolerance distributions where the determining variable is the concentration of the pesticide at any particular time. The model has been compared with experimental data for the proportion of Erysiphe graminis spores germinating at various distances from a single deposit of Dinocap. A good agreement was obtained when basing the tolerance distribution on concentration. The effect of varying partition coefficients and flow speeds on the area of control for a range of threshold values and times is demonstrated. The effect of varying droplet size is also investigated. The potential of this type of model is thus shown for the design of treatments in precision crop protection. The range of values that the parameters should ideally assume for a particular application can be investigated and the results borne in mind when designing a suitable treatment.

Engineered structures such as buildings and bridges in certain regions of the world need to be designed to withstand tropical cyclone winds, otherwise known as typhoons or hurricanes. In order to carry out this design, it is necessary to be able to estimate the maximum wind speeds likely to be encountered by the structure over its expected lifetime, say 100 years. Estimation of the maximum wind involves not only the overall strength of the tropical cyclone, but the variation of wind speed with radius from the centre, circumferential position, and with height above the ground surface. In addition, not only the mean wind speed, but also the gust factor must usually be estimated as well. This paper investigates a number of recent mathematical models of tropical cyclone structure and comments on their suitability for these purposes in a variety of scenarios.

Engineered structures such as buildings and bridges in certain regions of the world need to be designed to withstand tropical cyclone winds, otherwise known as typhoons or hurricanes. In order to carry out this design, it is necessary to be able to estimate the maximum wind speeds likely to be encountered by the structure over its expected lifetime, say 100 years. Estimation of the maximum wind involves not only the overall strength of the typhoon, but the variation of wind speed with radius from the centre, circumferential position, and with height above the ground surface. In addition, not only the mean wind speed, but also the gust factor must usually be estimated as well. This paper investigates a number of recent mathematical models of typhoon structure and comments on their suitability for these purposes in a variety of scenarios.

An insight into the nature of prevailing meteorological conditions and the manner in which they interact with spraying parameters is an important prerequisite in the analysis of the dynamics of agrochemical sprays. 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 which are felt upstream as well as downstream of the spray droplets. The pattern of the droplet flow, i.e. the shape of the streamlines marking typical trajectories, will be determined by a balance of viscous forces related to wind speed, inertial forces resulting from the acceleration of the airstream and pressure forces which can be viewed in terms of the drag forces exerted on the spray droplets themselves. At a certain distance in the ensuing motion, when the initial velocity of the spray droplets has decreased sufficiently for there to be no acceleration, their trajectories will be controlled entirely by the random effects of turbulence. These two transport processes in the atmosphere can be modelled mathematically using computers. This paper presents a model that considers the velocity of spray droplets to consist of a ballistic velocity component superimposed by a random-walk velocity component. The model is used to study the influence of meteorological and spraying parameters on the three-dimensional dynamics of spray droplets projected in specified directions in neutral and unstable weather conditions. The ballistic and random-walk velocity components are scaled by factors of (1–ξ) and ξ respectively, where ξ is the ratio of the sedimentation velocity and the relative velocity between the spray droplets and the surrounding airstream. This ratio increases progressively as the initial velocity of the spray droplet decreases with air resistance and attains a maximum when the sedimentation velocity has been reached. As soon as this occurs, the random-walk process predominates. The computed effects of the release height of spray droplets, atmospheric turbulence intensity, evaporation, drop size spectrum, wind velocity and wind direction on the transport process have been studied and an analysis of spray drift is provided.

This paper presents a simulation model based on earlier work that combines both ballistic and random-walk models to describe the three-dimensional dynamics of spray droplets released in a specified direction from 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−β) and β, respectively, where β is defined as the ratio of the sedimentation velocity and the relative velocity between the spray droplets and the ambient wind speed. The contribution of the random-walk model to the initial velocity is seen to be negligible at first, but increases progressively, though not proportionally, as β increases. As soon as the spray droplets attain their sedimentation velocities, β = 1, the random-walk velocity component predominates and β plays no further part in the calculations. The predicted effects close to the sprayer of the drop size, wind velocity and direction, evaporation on the transport process have been evaluated and combined to provide an analysis of spray drift.