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Wind-driven rain (WDR) is one of the most important moisture sources with potential negative effects on the hygrothermal performance and durability of building facades. The impact of WDR on building facades can be understood in a better way by predicting the surface wetting distribution accurately. Computational fluid dynamics (CFD) simulations can be used to obtain accurate spatial and temporal information on WDR. In many previous numerical WDR studies, the turbulent dispersion of the raindrops has been neglected. However, it is not clear to what extent this assumption is justified, and to what extent the deviations between the experimental and the numerical results in previous studies can be attributed to the absence of turbulent dispersion in the model. In this paper, an implementation of turbulent dispersion into an Eulerian multiphase model for WDR assessment is proposed. First, CFD WDR simulations are performed for a simplified isolated high-rise building, with and without turbulent dispersion. It is shown that the turbulence intensity field in the vicinity of the building, and correspondingly the turbulence kinetic energy field, has a strong influence on the estimated catch ratio values when turbulent dispersion is taken into account. Next, CFD WDR simulations are made for a monumental tower building, for which experimental data are available. It is shown that taking turbulent dispersion into account reduces the average deviation between simulations and measurements from 24 to 15%.

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.

The relation between wind-driven rain (WDR) and its potential negative effects on the hygrothermal performance and durability of building facades can be better understood by the correct estimation of the spatial and temporal distribution of the WDR intensity. Computational Fluid Dynamics (CFD) simulations with Eulerian Multiphase (EM) modeling are used to obtain accurate spatial and temporal information on WDR. The EM model has the advantage of predicting the WDR intensity on all surfaces of a complex geometry within the domain at once. There is a lack of numerical studies on the WDR intensity in generic and idealized multi-building configurations. In this paper, WDR intensities on an array of 9 low-rise cubic building models for wind from three different wind directions are estimated numerically using the EM model including the turbulent dispersion of raindrops. The numerical results are validated by comparing the calculated catch ratio values with data from field measurements in Dübendorf, Switzerland after two rain events with different characteristics. The CFD simulations successfully estimate the WDR intensities at the positions of 18 WDR gauges for both rain events. The influence of turbulent dispersion is found to be lower than 3% for both rain events. It is found that, for oblique wind directions, even though the maximum WDR intensity on the facades is lower, the whole building is exposed to up to 57% more WDR.

In this report we show that synchrotron X-ray radiography is a powerful method to study liquid-air interface penetration through opaque microtextured surface roughness, leading to wetting transition. We investigate this wetting phenomenon in the context of sessile drop evaporation, and establish that liquid interface sinking into the surface texture is indeed dictated by the balance of capillary and Laplace pressures, where the intrinsically three-dimensional nature of the meniscus must be accounted for. Air bubble entrapment in the texture underneath impacting water drops is also visualized and the mechanisms of post-impact drop evaporation are discussed