SNF – Rain

Wetting of building facades by wind driven rain in the urban context

Wetting by wind-driven rain (WDR) refers to rain droplets impinging on the building façade. The droplet impact leads to a rich pallet of surface and contact phenomena, such as (1) dynamic spreading, splashing or bouncing of the droplet depending on impact speed, angle and surface characteristics such as roughness, (2) pinning or spreading of the droplet, (3) sorption from the droplet into the porous substrate, (4) evaporation from the droplet surface, (5) film forming and run-off accompanied by instabilities leading to formation of fingerlike rivulets or sawtooth patterns. Droplet physics is of interest in a manifold of research fields, such as coating, printing, spraying technology, microchip production, microfluids applications, analysis of DNA microarrays, etc. A rich pallet of models was also developed ranging over different scales and with different complexity.

Wind-driven rain water is a main agent of deterioration of building materials, which becomes an important issue when retrofitting old or historical buildings by adding insulation, in order to make our cities more energy efficient. Not only for durability assessment, but also for the assessment of soiling of façades and leaching of harmful biocides and nanoparticles from our buildings, we need to determine accurately the amount of WDR and the duration of its presence/contact and runoff on different façade parts.

In this project we want to develop an integrated methodology, based on a combination of validated models that capture the main droplet physics of the wetting/dewetting of building façades, in a more accurate manner than existing methods currently used in building physics, but, at the same time, in a sufficiently simplified way as to be applicable in engineering.

The work of the applicants in the past have fundamentally contributed to a better understanding and modeling of the different components of the phenomenon of WDR wetting of façades of stand-alone buildings. However, given the densities of our cities and the world-wide urbanization trends, it becomes necessary to extent our research activities in predicting rain deposition and runoff on building façades in the urban context. Thriving towards an integrated methodology, the present project aims at filling the following gaps, defining the following work packages: (WP1) developing validated models to predict WDR at the urban scale, (WP2) acquiring an accurate knowledge of the physical processes occurring at the droplet scale and, by upscaling, determining more accurately the real WDR loads, (WP3) integrating WDR, sorption, evaporation, film forming and runoff into a model for evaluating the hygrothermal behavior of the building envelopes.