Spreading, splashing and bouncing of wind-driven raindrops on building facades

Wind-driven rain (WDR) significantly affects buildings. Examples are the durability of building walls, the weathering and soiling of buildings and monuments, algae formation at building facades, mould growth at inside wall surfaces, indoor climate and energy consumption of buildings. In numerical simulation models to analyse the hygrothermal performance of building components, the WDR intensity on a building facade is traditionally implemented as a (uniform) moisture flux boundary condition. In reality however, WDR is the sum of individual raindrops, which do not only spread, but may also splash or bounce off the facade. In this paper, these phenomena, i.e. spreading, splashing and bouncing and their potential occurrence at raindrop impact on building facades are investigated. Laboratory measurements were made of water drop impact on a dry, clean and relatively smooth porous ceramic brick surface. Results show that—depending on impact angle, impact speed and diameter—raindrops impinging on porous material surfaces can show either spreading, splashing or bouncing, which results in variable shapes and sizes of wetted areas. The possibility and importance of splashing and bouncing at the windward facade of a building are investigated by combining the drop impact measurements with computational fluid dynamics (CFD) simulations of raindrop trajectories impinging on the windward building facade of a 10 × 10 × 10 m3 cubic building. It is shown that depending on the meteorological conditions, splashing and bouncing can occur at large parts of the facade. This implies that the current implementation of WDR as a boundary condition in numerical hygrothermal simulation models, which does not take into account these effects, can significantly overestimate the real moisture flux boundary condition.