Theoretical predictions of raindrop impaction on a slab type building

The turbulent air flow around a slab type building has been measured by Wise, Sexton and Lillywhite using a scale model in a wind tunnel. Using their data procedures are developed for calculating raindrop trajectories for droplets having radius r in the range 0 ≦ r ≦ 2500 μ with gradient wind speeds up to 26·8 m/s.

Grazing trajectories, for fixed droplet size, are obtained and from these the effective collision cross-section of the building is deduced. Employing the droplet size distribution of Best, for a given rainfall intensity, raindrop impaction rates over the building are evaluated. Approximate analytical methods are developed to predict impaction rates and their dependence on the building aspect ratio.