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

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 selfcleaning glass or the study of the leaching of nanoparticles from building facades

On the accuracy of wind-driven rain measurements on buildings

Wind-driven rain (WDR) measurements on buildings are being conducted for many decades. They provide an indication of the WDR falling onto different parts of a building facade and are an essential tool for WDR model development and model validation. However, up to now, very few investigations concerning the accuracy of WDR measurements have been performed. No publication of WDR measurements has been found that provides an indication of the errors involved. Availability of error estimates is essential for the interpretation and the use of WDR measurement data. In this paper, the main errors associated with WDR measurements are identified and investigated. It is shown that especially the evaporation of adhesion water from the gauge catch area can be important and a method to estimate this error will be proposed. It is shown that this error can be very large (up to 100%) and that it depends not only on the gauge type but also on the type of rain event. Finally, guidelines for the design of WDR gauges and for the selection of WDR measurement data that are suitable for model development and model validation are given

Driving rain on building envelopes - I. Numerical estimation and full-scale experimental verification

An accurate method for the quantification of real-life driving rain loads on building envelopes from generally available climatic data such as wind speed, wind direction and horizontal rainfall intensity serves various purposes, from the development of design guidelines for building envelopes to the incorporation of driving rain loads as a boundary condition in Heat-Air-Moisture (HAM) transfer analysis models. In this paper, an existing numerical technique for driving rain simulation is incorporated into a practical numerical method to estimate driving rain loads on building envelopes based on the building geometry and the climatic data at the building site. This numerical method is applied for several sequences of spells around a low-rise test building and the results are experimentally verified. It is shown that the numerical method can accurately estimate the spatial and temporal distribution of driving rain loads on building envelopes

Driving rain on building envelopes - II. Representative experimental data for driving rain estimation

A practical numerical method for driving rain estimation was presented in "Driving Rain on Building Envelopes—I" (Blocken and Carmeliet, 2000). An important prerequisite in employing this method is that the climatic data used as input are representative. In this paper, the attainment of representative experimental data for driving rain estimation is analysed. The importance of a sufficiently small time step to obtain representative climatic data measurements is indicated. It is shown that representative averaged values for wind speed and rainfall intensity for longer time steps can be obtained by averaging the measured data with the rainfall amounts as weighting factors. The effects of using different averaging techniques on the accuracy of the calculated driving rain results are investigated. It is found that the presented weighted averaging technique can provide accurate representative averaged data, whereas commonly used averaging techniques can give rise to large errors

Overview of three state-of-the-art wind-driven rain assessment models and comparison based on model theory

In the past, different calculation models for wind-driven rain (WDR) have been developed and progressively improved. Today, the models that are most advanced and most frequently used are the semi-empirical model in the ISO Standard for WDR (ISO), the semi-empirical model by Straube and Burnett (SB) and the CFD model by Choi, extended by Blocken and Carmeliet. Each of these models is quite different, and to the knowledge of the authors, no comparison of these models has yet been performed. This paper first presents a detailed overview of the three models, including new insights in similarities between these models and relations with recent research results. Based on this overview, it provides a comparison focused on the extent to which the different influencing parameters of WDR are implemented in the models. It shows that the implementation of the influencing parameters is most pronounced for the CFD model, less pronounced for the ISO model and least pronounced for the SB model. It is also shown that in the two semi-empirical models, the values of the wall factor W (for ISO) and the rain admittance function RAF (for SB), which have the same definition in both models, can differ more than a factor 2 from each other. The two models can therefore provide very different results. They also require differently defined reference wind speed values as input. The overview and the comparison in this paper provide the basis for future comparison studies and future improvements of the semi-empirical models

Driving rain on building envelopes - II. Representative experimental data for driving rain estimation

A practical numerical method for driving rain estimation was presented in "Driving Rain on Building Envelopes—I" (Blocken and Carmeliet, 2000). An important prerequisite in employing this method is that the climatic data used as input are representative. In this paper, the attainment of representative experimental data for driving rain estimation is analysed. The importance of a sufficiently small time step to obtain representative climatic data measurements is indicated. It is shown that representative averaged values for wind speed and rainfall intensity for longer time steps can be obtained by averaging the measured data with the rainfall amounts as weighting factors. The effects of using different averaging techniques on the accuracy of the calculated driving rain results are investigated. It is found that the presented weighted averaging technique can provide accurate representative averaged data, whereas commonly used averaging techniques can give rise to large errors

Wind-driven rain

Wind-driven rain (WDR) is one of the most important moisture sources for building facades (Sanders 1996, Blocken and Carmeliet 2004) and it is expected to become even more important in the future (Sanders and Phillipson 2003, Phillipson and Sanders 2004). Knowledge of the WDR intensity impinging on building facades is essential as a boundary condition for numerical Heat-Air-Moisture (HAM) transfer models that are used to examine the hygrothermal behaviour and the durability of building facade components exposed to the outside climate. WDR research in building physics can be divided into two parts, namely: (1) the assessment of the direct WDR intensity impinging on the building facade and (2) the assessment of the response of the facades to these loads. While the first part focuses on the amount and the distribution of the WDR intensity across building surfaces, the latter part includes the contact and surface phenomena that occur when WDR drops hit the wall. Examples are splashing, absorption, adhesion, runoff and evaporation. In each part, different approaches can be employed: experimental, (semi-) empirical and numerical methods. The state-of-the-art of WDR research in 1996 was reported by Sanders (1996) in the framework of the International Energy Agency Annex 24. Later, a review on WDR research in building physics was composed by Blocken and Carmeliet (2004). These two documents indicated that, although important advances have been made in the past, there is still of lot of work to be done, both concerning WDR impact assessment and especially concerning WDR contact and surface phenomena. This report presents a brief overview of earlier achievements as well as the achievements made during Annex 41

Overview of three state-of-the-art wind-driven rain assessment models and comparison based on model theory

In the past, different calculation models for wind-driven rain (WDR) have been developed and progressively improved. Today, the models that are most advanced and most frequently used are the semi-empirical model in the ISO Standard for WDR (ISO), the semi-empirical model by Straube and Burnett (SB) and the CFD model by Choi, extended by Blocken and Carmeliet. Each of these models is quite different, and to the knowledge of the authors, no comparison of these models has yet been performed. This paper first presents a detailed overview of the three models, including new insights in similarities between these models and relations with recent research results. Based on this overview, it provides a comparison focused on the extent to which the different influencing parameters of WDR are implemented in the models. It shows that the implementation of the influencing parameters is most pronounced for the CFD model, less pronounced for the ISO model and least pronounced for the SB model. It is also shown that in the two semi-empirical models, the values of the wall factor W (for ISO) and the rain admittance function RAF (for SB), which have the same definition in both models, can differ more than a factor 2 from each other. The two models can therefore provide very different results. They also require differently defined reference wind speed values as input. The overview and the comparison in this paper provide the basis for future comparison studies and future improvements of the semi-empirical models

Driving rain on building envelopes - II. Representative experimental data for driving rain estimation

A practical numerical method for driving rain estimation was presented in "Driving Rain on Building Envelopes—I" (Blocken and Carmeliet, 2000). An important prerequisite in employing this method is that the climatic data used as input are representative. In this paper, the attainment of representative experimental data for driving rain estimation is analysed. The importance of a sufficiently small time step to obtain representative climatic data measurements is indicated. It is shown that representative averaged values for wind speed and rainfall intensity for longer time steps can be obtained by averaging the measured data with the rainfall amounts as weighting factors. The effects of using different averaging techniques on the accuracy of the calculated driving rain results are investigated. It is found that the presented weighted averaging technique can provide accurate representative averaged data, whereas commonly used averaging techniques can give rise to large errors

Spatial and temporal distribution of driving rain on a low-rise building

This paper presents a practical numerical method to determine both the spatial and temporal distribution of driving rain on buildings. It is based on an existing numerical simulation technique and uses the building geometry and climatic data at the building site as input. The method is applied to determine the 3D spatial and temporal distribution of wind-driven rain on the facade a low-rise building of complex geometry. Distinct wetting patterns are found. The important causes giving rise to these particular patterns are identified: (1) sweeping of raindrops towards vertical building edges, (2) sweeping of raindrops towards top edges, (3) shelter effect by various roof overhang configurations. The comparison of the numerical results with full-scale measurements in both space and time for a number of on site recorded rain events shows the numerical method to yield accurate results