The entrapment of particles by windbreaks

A theory is developed for calculating the entrapment of particles by a wind break, with four results. (1) The fraction of particles in the oncoming flo w which pass through the windbreak, or transmittance of the windbreak for p articles (sigma), is related to the optical porosity (tau). The very simple approximation sigma=tau works well for most applications involving the int erception of spray droplets by windbreaks. Results from a field experiment agree with the theoretical predictions. (2) A new equation for the bulk dra g coefficient of a windbreak is tested against numerical, wind tunnel and f ield experiments. This enables the bleed velocity for the flow through the windbreak to be predicted in terms of the screen pressure coefficient (k) o f the barrier. (3) The relationship between k and tau is different for a ve getative 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, impl ying an optimum value of tau for maximum total deposition. For particles la rger than 30 mum and vegetation elements smaller than 30 mm, this occurs ne ar tau = 0.2. (C) 2001 Elsevier Science Ltd. All rights reserved.