Wind-driven rain distributions on two buildings

Wind-driven rain is an important consideration in the hygrothermal performance of building envelope parts. Wind-driven rain (in liquid form) can increase the amount of moisture present in the structure by more than 100 times that due to vapor diffusion. To date, very little work that provides field or laboratory wind driven rain data to moisture transport models is available. This information is a definite requirement as a boundary condition by the more sophisticated hygrothermal models such as LATENITE and WUFIZ which consider both vapor and liquid moisture flows. In this paper, the wind driven rain striking the exterior facade of two buildings (one twice the size of the other) is generated using a three-dimensional computational fluid dynamics (CFD) model that solves the air flow and particle tracking of the rain droplets around these two buildings. These simulations were carried out for a city center region. Four factors which govern wind-driven rain are investigated in this work: (a) upstream unobstructed wind conditions, (b) the rainfall intensity, (c) the probability distribution of raindrop sizes, and (d) the local flow patterns around the building. All four of these governing factors make wind-driven rain on a building facade very distinct. Simulations were carried out for three wind speeds of 5, 10 and 25 m/s, three rainfall intensities of 10, 25 and 50 mm/h and three wind directions 0 degrees, 30 degrees and 45 degrees from the west face of the buildings. In this paper, only the results of the 0 degrees wind direction are discussed. The results show distinct wetting patterns on the top of the building of both the two buildings which is most concentrated at the corners when the wind was normal to the facade surface. For the tallest building a distinct wetting pattern is displayed in the mid-height of the building. This information from wind engineering is directly employed for the design of building envelope moisture control. Results on a series of simulations are presented to demonstrate the effect of wind conditions, rain intensities, the interaction between the two buildings, and the droplet sizes on the wetting patterns on the faces of the short and tall building.