Guidelines for the required time resolution of meteorological input data for wind-driven rain calculations on buildings

An important question in wind-driven rain (WDR) calculations on buildings, either with semi-empirical formulae or with Computational Fluid Dynamics (CFD), concerns the required time resolution of the meteorological input data: wind speed, wind direction and horizontal rainfall intensity. Earlier work has indicated that the use of 10 min input data can provide accurate results, while the use of arithmetically averaged hourly data can yield significant underestimations in the calculated WDR amounts. This paper builds further on this earlier work by providing a detailed investigation of the parameters that determine the required time resolution for WDR calculations on building facades: (1) the averaging technique, (2) the building geometry and the position at the building facade and (3) the type of the rain event. It is shown that all three parameters can have a large influence on the required time resolution. Depending on these parameters, hourly or even daily wind and rain input data could provide accurate results, while in other situations they can lead to very large errors. Finally, guidelines for the required time resolution as a function of the influencing parameters are provided.     

Numerical simulations of wind-driven rain on building envelopes based on Eulerian multiphase model

A numerical simulation approach for evaluation of wind-driven rain (WDR) on building envelopes is presented based on Eulerian multiphase model. Unlike existing methods, which are generally on the basis of Lagrange frame to deal with raindrop motions by trajectory-tracking techniques, the present approach considers both wind and rain motions and their interactions under Euler frame. By virtue of the Eulerian multiphase model, the present method could significantly reduce the complexity in evaluations of WDR parameters, simplify the boundary condition treatments and is more efficient to predict transient states of WDR, spatial distributions of rain intensity, impacting rain loads on building surfaces, etc. A numerical example shows that the simulation results by the present method agree well with available experimental and numerical data, verifying the accuracy and reliability of the WDR simulation approach based on the Eulerian multiphase model. It is also demonstrated through the validation example that the present method is an effective tool for numerical evaluations of WDR on building envelopes.     

Comparison of experimental and numerical results of wood-frame wall assemblies wetted by simulated wind-driven rain infiltration

A large-scale building envelope experiment was conducted to study the effect of three sheathing materials and two vapor retarders on the drying performance of walls exposed to simulated rain infiltration in springtime in Montreal. The moisture source was a pre-wetted component within the wall called the bottom plate insert. Its moisture content was monitored on a daily basis through the course of the 35-day experiment. The experimental set-up was simulated using a two-dimensional hygrothermal model, WUFI-2D, and the moisture content within the bottom plate inserts was used to study the ability of the model to predict the wall response to the initial liquid water load. The differences between the experimental results are mainly attributed to: air convection loops within the insulated space, which are not accounted for in the simulation; estimation of the initial moisture content distribution within the bottom plate inserts in the simulation; isotropic material properties for an orthotropic material like wood material properties that were taken from a variety of sources and did not cover the entire moisture content range and use of a two-dimensional domain to simulate three-dimensional wall systems.     

A review of wind-driven rain research in building science

Wind-driven rain (WDR) or driving rain is rain that is given a horizontal velocity component by the wind. WDR research is of importance in a number of research areas including earth sciences, meteorology and building science. Research methods and results are exchangeable between these domains but no exchanges could yet be noted. This paper presents the state-of-the-art of WDR research in building science. WDR is the most important moisture source affecting the performance of building facades. Hygrothermal and durability analysis of facades requires the quantification of the WDR loads. Research efforts can be classified according to the quantification methods used. Three categories are distinguished: (1) experimental methods, (2) semi-empirical methods and (3) numerical methods. The principles of each method are described and the state-of-the-art is outlined. It has been the intent of the present paper to bring together the reports, papers and books—published and unpublished—dealing with WDR research in building science to provide a database of information for researchers interested in and/or working in WDR research, independent of their field of expertise.     

A combined CFD-HAM approach for wind-driven rain on building facades

Numerical heat-air-moisture (HAM) transfer models are increasingly being used to study the hygrothermal performance and the durability of building facades. One of the most important boundary conditions for HAM simulations is wind-driven rain (WDR). Due to the complexity of WDR, however, the current HAM models generally incorporate it in a very simplified way. Recent research has shown that CFD can provide quite accurate estimates of the spatial and temporal distribution of WDR on building facades. Therefore, in this paper, a combined CFD-HAM approach is presented. It consists of implementing catch-ratio charts resulting from CFD simulations into the HAM model. Within the model, these charts are used to convert the standard meteorological input data (wind speed, wind direction and horizontal rainfall intensity) into WDR distribution records that are used as boundary condition for the actual HAM simulations. The combined approach is demonstrated for a simplified wall model. It is shown that the accuracy of the HAM-simulation results is to a large extent determined by the time resolution of the meteorological input data and by the data-averaging technique used for these data. Some important guidelines for accurate HAM analyses with WDR are provided.     

Selecting moisture reference years using a Moisture Index approach

Recent history has documented the premature failures of building envelopes in various regions—in North America most notably on the West Coast and the East Coast. The MEWS Consortium, a project undertaken by IRC and its partners, has addressed this issue in detail. The strategy for answering these questions was based on predicting the moisture management performance of wall systems as a function of climate, wall construction, and material properties through mathematical modeling. A key task was to determine what years to use as input for the simulations. Moisture Reference Years were selected using a Moisture Index approach developed for MEWS. This paper will develop the approach and compare it with other methods of selecting moisture reference years for hygrothermal simulations.     

On the validity of numerical wind-driven rain simulation on a rectangular low-rise building under various oblique winds

Wind-driven rain (WDR) is one of the most important boundary conditions for hygrothermal building envelope analysis. Although Computational Fluid Dynamics (CFD) simulation of WDR on building facades has been applied intensively in the past decade, validation is still quite limited, and most previous validation efforts have focused either on wind directions perpendicular to the facade or on buildings of complex geometry. This paper addresses CFD simulations of WDR on the west facade of a simple, rectangular low-rise test building for various oblique winds and CFD validation by comparing the simulation results with full-scale measurements. It is shown that overall, fairly accurate results can be obtained, but that the numerical simulations can significantly underestimate the WDR amounts near the downwind edge of the facade when the wind direction is increasingly oblique. These discrepancies are at least partly attributed to the very small impact angles of the raindrops at these facade positions and the resulting inaccuracies in the numerical model.     

Intercomparison of wind-driven rain deposition models based on two case studies with full-scale measurements

Three different calculation models for wind-driven rain (WDR) on buildings are compared for two case studies for which full-scale measurements are available. The models 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. This paper builds further on two generic studies in which these models were compared based on model theory and based on their application for idealized building configurations and for constant wind and rain conditions. In the present study, the models are applied to calculate WDR on the facades of a low-rise test building and a monumental tower building for actual transient rain events. The spatial and temporal distributions of WDR at the windward facade are determined and the model results are compared with each other and with the full-scale measurements. The agreement between the CFD results and the measurements is on average 20-25%, whereas the ISO and SB models show large discrepancies at many facade positions, up to a factor 2-5. The identification of the reasons for the discrepancies is based on the previous generic studies and on the detailed information provided by the validated CFD simulations. The reasons include: (1) the ISO and SB models do not take into account the wind-blocking effect; (2) they do not model the variation of shelter by roof overhang as a function of the wind speed and; (3) they only provide information for a limited number of building geometries. In spite of these deficiencies, these models provide a strong basis for further development. The deficiencies can be addressed with CFD, and it is suggested that future research should focus on improving the semi-empirical models based on the detailed results of validated CFD simulations.     

High-resolution wind-driven rain measurements on a low-rise building—experimental data for model development and model validation

With the specific intention to provide experimental data for model development and model validation, a new measurement setup for wind, rain and wind-driven rain (WDR) has been designed and installed at the Laboratory of Building Physics (Katholieke Universiteit Leuven). This paper focuses on the new measurement setup and on the obtained measurement results. The CFD-based design and the installation of the measurement setup are outlined and samples of the database containing the wind, rain and high-resolution WDR measurements are provided and discussed. This paper also provides the link to a website from which the experimental WDR database can be downloaded. Finally, the use of these data to determine WDR coefficients and their use in WDR assessment are briefly addressed.     

Proposed approach for defining climate regions for Turkey based on annual driving rain index and heating degree-days for building envelope design

Field surveys in Turkey indicate that a significant number of exterior wall assemblies in various regions of the country suffer from moisture degradation. These cases reveal that designers are in need for a climate scheme of Turkey, which indicates regions that require special provisions to prevent moisture degradation. Hence, this paper presents an approach for defining climate regions for Turkey. Initially, annual driving rain index (aDRI) based on monthly data is calculated and a driving rain map of Turkey is produced. Then, population-weighted heating degree-days (PW HDD) zones of Turkey are presented. Based on the aDRI and PW HDD zones, three climate regions of Turkey are established. Region 1 represents sheltered locations, i.e. locations which have aDRI less than 3. Region 2 represents locations (aDRI between 3 and 6 and PW HDD<1600), which are exposed to moderate driving rain between September and the end of May when the mean temperatures are above zero. Region 3 includes locations (aDRI between 3 and 6 and PW HDD between 1601 and 2500) which are moderately exposed to driving rain all throughout the year when the mean temperature is well above zero. As a conclusion, the proposed climate regions suggest that the design of wall assemblies located at Regions 2 and 3 must incorporate special provisions to prevent moisture degradation.     

A framework for using a present weather observation method to calculate a driving rain wall factor at any location

In this paper, the author presents a framework for using a present weather observation method to compute a driving rain wall factor at any location. First, an airfield index must be calculated for the closest appropriate weather observation station. The station index is then modified to an airfield index for the city, town or village in question by correcting for general location, topography and rainfall at the wall site. The derived airfield index can then be used to calculate a wall index using previously developed correction factors. An example is provided to illustrate the methodology. This type of framework is a necessary step if proposed standard methodologies for assessing the driving rain impact on a vertical surface using meteorological data are to find widespread practical application.     

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.     

Validation of CFD simulations of wind-driven rain on a low-rise building facade

For decades, the assessment of the amount and intensity of wind-driven rain (WDR) falling onto building facades has been performed either by measurements or by semi-empirical methods such as the WDR index and the WDR relationship. In the past 15 years, numerical assessment methods based on Computational Fluid Dynamics (CFD) have secured their place in WDR research. Despite the widespread use of these methods at present, very few efforts have been made towards validation of CFD simulations of WDR on buildings. This paper presents a detailed validation study for a low-rise building of complex geometry, supported by a recently published, high-resolution full-scale wind, rain and WDR measurement dataset. It is shown that the CFD simulations can provide quite accurate predictions of the amount of WDR impinging on the building facade, for a number of very different rain events, and that the main discrepancies, in this study, are due to a simplification of the upstream wind conditions.     

Wind-driven rain as a boundary condition for HAM simulations: Analysis of simplified modelling approaches

While the numerical simulation of moisture transfer inside building components is currently undergoing standardisation, the modelling of the atmospheric boundary conditions has received far less attention.     

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.     

Impact,absorption and evaporation of raindrops on building facades

In this paper, the impact, absorption and evaporation of raindrops on building facades is investigated by experimental and numerical means. Laboratory experiments were carried out to study the impact of water drops with different diameters, impact speeds and impact angles on a porous building material surface (ceramic brick). The measurements showed that large drops with high impact speeds splash, and that drops with high impact speeds and small impact angles bounce. The measurements, furthermore, allowed measuring the maximum spreading length and width of the drops as a function of drop diameter, impact speed and impact angle. Then, a numerical analysis was performed to study the distribution of impact speed and angle for raindrops hitting the facade of a 4×4×10 m³ tower building. The results demonstrated typical and important tendencies of impact angle and speed across the facade. Finally, the experimental and numerical data were used in a more precise three-dimensional simulation of impact, absorption and evaporation of random and discrete wind-driven raindrops. This was compared with the common one-dimensional simulation of absorption and evaporation at the facade considering a continuous uniform rain load as boundary condition, and significant differences between the two approaches were observed.     

The influence of the wind-blocking effect by a building on its wind-driven rain exposure

Wind-Driven Rain (WDR) is one of the most important moisture sources that affect the hygrothermal performance and the durability of building facades. The complexity of WDR has led to the use of Computational Fluid Dynamics (CFD) to predict the amount of WDR falling onto building facades. Recently, the CFD model for WDR simulation has been successfully validated for a low-rise building of complex geometry and for a range of rain events, providing confidence for further numerical studies. In this paper, the influence of the wind-blocking effect by a building on its WDR exposure is examined. Part of the latest WDR CFD validation study for the VLIET building and CFD simulations of the WDR distribution on four different single-building configurations are presented. It is shown that the wind-blocking effect is one of the main factors that govern the WDR distribution pattern. As a result, high-rise buildings do not necessarily catch more WDR than low-rise buildings.     

The mutual influence of two buildings on their wind-driven rain exposure and comments on the obstruction factor

Wind-driven rain (WDR) deposition on a two-building configuration is studied with Computational Fluid Dynamics (CFD). The configuration consists of a high-rise building screened by a low-rise building. Validation of the wind-flow simulations is performed with Particle Image Velocimetry (PIV) measurements in a wind tunnel. Raindrop motion is simulated by Lagrangian particle tracking in the mean wind-flow pattern with a reference wind speed U₁₀=10 m/s. Horizontal rainfall intensities R_h=5 and 30 mm/h are considered. Simulations of WDR are performed for the two-building configuration and for each building separately, to analyse the mutual influence of the buildings on their WDR deposition pattern. The simulation results indicate that this influence is very pronounced and that it is to some extent opposite to what might be expected. The low-rise building influences the deposition on the high-rise building (downstream disturbance), not by partly shielding it from wind and WDR, but by increasing the strength of the standing vortex between the two buildings. This locally increases WDR intensities on the high-rise building facade by more than a factor 2 for both R_h=5 and 30 mm/h. On the other hand, the high-rise building influences deposition on the low-rise building facade (upstream disturbance) by the wind-blocking effect. This effect yields a reduction in WDR deposition on the low-rise building facade by up to about 25% for both R_h=5 and 30 mm/h. In the European standard draft for WDR assessment, the mutual influence can only be taken into account by a simplified reduction factor, called the obstruction factor. It only considers downstream disturbances, and does not consider the possibility of increased WDR deposition due to neighbouring buildings. Care should therefore be exercised when using the current version of the obstruction factor to determine WDR exposure.     

A limit state design (LSD) approach for comparing relative drying performance of wood-frame envelope systems with full-scale lab testing

The capacity of drying out moisture accumulated within stud cavities, especially moisture due to rain penetration, is an important characteristic affecting the performance and durability of building envelope systems. This paper introduces a new approach for evaluating such drying performance based on the concepts and procedures of Limit State Design (LSD) used in structural engineering. For a well performing envelope, the moisture load must be less than the drying capacity of the system. The drying capacities are obtained through full-scale experiments that utilize moisture loading derived from a moisture source (water tray) placed at the bottom of the stud cavity. In moving out of the cavity, part of the moisture will be absorbed by the materials surrounding the stud cavity. When any part of the wall specimen reaches 20% MC, the cumulative evaporation from the water tray is termed as the ICEA (in-cavity evaporation allowance) of that wall system. The ICEA value is dependent on the envelope configuration and is a good indicator of the drying performance of that wall system. By comparing ICEA values of wall systems with their respective moisture loads, those envelopes having ICEA values higher than the amount of rain penetration calculated from prevailing driving rain and faults in the envelope are deemed to have adequate drying capacity.