Impingement of rain drops on a tall building

Soiling on the walls of limestone buildings can be washed off when the surface erodes due to rain impingement. In this study, the delivery of rain to the 42-story Cathedral of Learning in Pittsburgh, Pennsylvania, represented by a 30 m×30 m×160 m rectangular block, was modeled using the RNG K– model for turbulence and Lagrangian trajectory calculations for individual rain drops. Local Effect Factors (LEF) for the rectangular block compared well with earlier work in the literature. LEFs increased with wind speed, raindrop size, and height along the block. Wind speed, direction, and rain intensity were measured continuously over a seven-week period and provided input parameters for modeling rain fluxes to the Cathedral of Learning. Model results suggested that sections of the building receiving larger amounts of rain corresponded to white areas, indicating that rain fluxes have a significant effect on the soiling patterns. Intermediate wind speeds (2.5 and 5 m s−1) resulted in high rain fluxes. Although less frequent, high wind speeds also resulted in high rain fluxes. Much of the rain was delivered to the block as 1.25 and 2.5 mm drops with 5 mm drops having a smaller effect. Consideration of wind incidence angles other than 0° was shown to be important for future modeling efforts.