Heavy particle dispersion over level terrain and in windbreak flow.

摘要

This PhD thesis first re-examines and further develops an analytical solution for the deposition swath of heavy particles released in the atmosphere from an elevated source over uniform terrain, correcting the particle diffusivity for the crossing trajectory effect. Despite its neglect of the turbulent flux, the revised (approximate) analytical solution proves to be accurate within 20% provided the variable u(H s)/wg ≤ 7 (ratio of the mean horizontal wind speed at source height to the particle gravitational settling velocity). In this domain of validity, simple formulae relating the statistics of the deposition swath to u(Hs)/ wg are derived. The rest of this work investigates the impact of a windbreak on heavy particle dispersion and its numerical modelling. Setting up a windbreak is a common practice in agriculture, often aimed at controlling particle transfer as a result of filtration and distortion of the carrying windflow. New observations of heavy particle deposition within a flow disturbed by a porous thin fence, as well as measurements of concentration along vertical profiles on both sides of a thick 'shelterbelt' of corn are presented. These data are used to diagnose the capabilities of a Lagrangian Stochastic (LS) particle trajectory model, coupled to a wind model providing fields of wind statistics, to capture the impact of a windbreak on dispersion. The concentration and the deposition rates, as simulated by the model, match the observation within 30% accuracy. These results suggest that (1) the LS model handles properly the heterogeneities of the flow; (2) the heuristic adjustments made to account for the inertia of heavy particles are useful approximations; (3) the description of particle interactions with leaves provides a reasonably satisfactory account of filtration by a natural hedge. The numerical simulations show that while the windflow disturbance lowers the deposition peak, it also spreads deposition further away from the source. In the case of a thick windbreak, an important fraction of the particles entering the shelterbelt across its upstream face are lifted out of its volume by the mean updraft induced by the deceleration of the flow, these particles thereby escaping deposition.