Impact, runoff and drying of wind-driven rain on a window glass surface: Numerical modelling based on experimental validation

• Combination of CFD model and droplet-behaviour model.
• Simulation of impingement, coalescence, runoff and evaporation.
• Case study shows evaporation reduces runoff by 26% for cumuliform rain.
• Case study shows evaporation reduces runoff by 4% for stratiform rain.
• Important for study of self-cleaning glass and nanoparticle leaching from building facades.

This paper presents a combination of two models to study both the impingement and the contact and surface phenomena of rainwater on a glass window surface: a Computational Fluid Dynamics (CFD) model for the calculation of the distribution of the wind-driven rain (WDR) across the building facade and a semi-empirical droplet-behaviour model. The CFD model comprises the calculation of the wind-flow pattern, the raindrop trajectories and the specific catch ratio as a measure of the WDR falling onto different parts of the facade. The droplet-behaviour model uses the output of the CFD model to simulate the behaviour of individual raindrops on the window glass surface, including runoff, coalescence and drying. The models are applied for a small window glass surface of a two-storey building. It is shown that by far not all WDR that impinges on a glass surface runs off, due to evaporation of drops adhered to the surface. The reduction of runoff by evaporation is 26% for a typical cumuliform rain event and 4% for a typical stratiform rain event. These models can be used to provide the knowledge about WDR impact, runoff and evaporation that is needed for the performance assessment of self-cleaning glass or the study of the leaching of nanoparticles from building facades.