Ten questions concerning modeling of wind-driven rain in the built environment

Abstract

Wind-driven rain (WDR) in the built environment is a complex multiscale phenomenon. Wind flows in complex urban environment and rain events of various intensities may lead to very different rain deposition distributions within the city. Proper modelling of WDR is required as moisture is a main cause of material degradation in the built environment but also as understanding the water cycle in the urban environment is essential to provide solutions for the urban heat island, amongst others. What are the main aspects to be taken into account to predict wind-driven rain? How should such aspects be considered and modeled? Is it possible and relevant to predict in detail the moisture loads due to rain in complex systems as cities? This paper answers these questions from a multiscale perspective combining modeling and experimental work. The different scales relevant for accurate estimation of wind-driven rain in the built environment are presented. Rain deposition on complex geometries can be modeled by CFD, taking into account turbulent dispersion. Such modeling provides the impact velocity and angle of attack for each droplet size at any location on the urban surfaces. Using this information and the structure of the surface, the fate of the rain droplets can be predicted, namely whether it splashes, bounces or simple spreads. KeywordsWind-driven rain, built environment, droplet trajectory, computational fluid dynamics (CFD), WDR catch ratio, droplet fate, porous materials Introduction Rain in the built environment is a complex phenomenon which has impact on, amongst others, durability of the materials, vitality of urban vegetation, management of storm water and comfort, safety and health of the population. Not only does rain induce an environmental load on the surfaces it impacts, but, contrary to the other environmental loads of solar radiation and wind flow, a large part of the rain water remains present after the rain event and such presence must be accounted for despite the fleeting and stochastic nature of liquid flow. To properly capture rain, the different time scales of the phenomenon need to be considered. The first time scale is the few minutes rain droplets spend in the air before deposition. Air flow is drastically affected by the presence of buildings, influencing the distribution of rain load in the built environment. Then, the few milliseconds when droplets impact surfaces explain the different outcomes of spreading, splashing and/or rebound. The settlement of droplets on surfaces takes minutes, and the wetting of the surface and absorption in the porous media take several hours. In addition, as more and more droplets rest on a surface, they may coalesce and form a water film, resulting in rivulets and film run-off on the different surfaces. After a rain event, evaporation of the droplets and drying of the materials can take hours to days. As such rain deposition and the ensuing fate, i.e. the succession of subjected phenomena, of water is a multifaceted phenomenon which can be appropriately mo...

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