Wind-driven Rain Measurements Research Articles

CFD modeling of Wind-Driven Rain (WDR) on a mid-rise building in an urban area

Wind-Driven Rain (WDR) loading on building facades is a crucial factor for designing sustainable and climate-resilient buildings and preserving historical buildings. WDR loading on buildings has been studied previously but results for such a multi-parameter problem are not generally conclusive. Thus, the relevant provisions of ISO semi-empirical model cannot be applied with confidence for complex building configurations, such as those in urban areas given that the estimated WDR can be more than twice of the field measurements. This paper aims to evaluate the effectiveness of two WDR techniques, namely Lagrangian Particle Tracking (LPT) and Eulerian Multiphase (EM), combined with the steady-state standard k-ω RANS turbulence model (referred to as RANS-LPT and RANS-EM). The results obtained from the RANS-EM approach are compared with the RANS-LPT results reported in the literature for a six-story mid-rise residential building located in Vancouver, Canada. The study considers 13 distinct rainfall events, including stand-alone and urban area configurations with and without overhangs. The RANS-EM and RANS-LPT approaches are evaluated by comparing modeled wind and WDR against wind-tunnel and on-site WDR measurements, respectively. The study found that the RANS-EM requires less computational time and provides more accurate results for the test building situated in an urban area compared to the RANS-LPT.

Analysis of wind-driven rain characteristics acting on building surfaces in Shanghai based on long-term measurements

Wind-driven rain (WDR) is a critical boundary condition for the thermal and moisture coupling calculations of buildings, and understanding its distribution patterns on facades helps to better design building envelope structures and reduce humidity-induced problems. Given that WDR research in China is limited to a small number of numerical simulations, with field measurement data especially lacking, long-term and high-time resolution WDR measurements on a building module were launched in Shanghai. This research aimed to explore the WDR characteristics of building facades under meteorological conditions in the Shanghai region and compare the accuracy of existing semi-empirical models of WDR through measurements. The results show that when the angle between the inflow wind direction and the vertical direction of the wall facade increased, the WDR capture ratio decreases, and the increases in rainfall amount and wind speed contributed to an increase in WDR amount. The measurement relative errors had a negative exponential relationship with the WDR amount in this research. Compared to non-rainy conditions, the temperature during rainfall was 4.0–5.6 °C lower, and relative humidity was 9.8–21.8% higher. WDR results were compared with two semi-empirical models, which indicated that the ISO model was 0.5–0.8 times that of the measured values, while the ASHRAE model was 1.3–2.4 times greater. The collected data can provide a valuable and unique reference for WDR semi-empirical models and computational fluid dynamics simulation validation in China.

Wind-driven rain on buildings: Accuracy of the ISO semi-empirical model

Previous studies have shown that the accuracy of the ISO semi-empirical wind-driven rain (WDR) model can be improved by applying more detailed wall factors determined based on high-resolution field measurements. However, discrepancies between measurements and predictions still exist. The availability of high-resolution WDR field data collected over a long period of time makes a detailed analysis possible. Field measurements of WDR on two mid-rise and one high-rise buildings in three Canadian regions over a year and a half were analyzed. Two main factors that can significantly improve the ISO WDR model were identified, i.e. the time resolution (5 ​min data vs. hourly data) and the correction of difference in wind speed and wind direction between the building site and weather station using hourly data. It is recommended to take WDR measurements at shorter-duration and calculate wall factors using both shorter-duration records and hourly averaged data. In general, the ISO model works well if all the differences in wind conditions between site and the weather station can be taken into account. The observed discrepancy can be reduced to within 2% for façades facing the prevailing wind-direction, while to about 60% for façades off the prevailing wind direction in complex urban settings.

Experiment of wind-driven-rain measurement on building walls and its in-situ validation

This study proposes an empirical equation that estimates the amount of wind-driven rain (WDR) on a building wall, meaning the amount of rainfall intercepted by a building wall. The proposed equation was formulated by analyzing the results obtained in previous studies on the vertical profile and spatial distribution of WDR on building walls. A laboratory experiment was conducted to collect WDR measurements for several building wall models that fit the proposed empirical equation. This study also observed the WDR for a miniature building located on the rooftop of the Engineering Building of Korea University. The proposed empirical equation was validated by comparing its estimates to in-situ observations. The results indicate that the estimations and the observed results are similar overall, but there are some discrepancies particularly when the WDR is small, with a correlation coefficient of 0.71 and root mean square error of 1.90 mm/hr⋅m2. This study also estimates the amount of rainwater that can be collected from a building wall. The result shows that, even for a small building with a ratio of building wall to building roof of one, the amount of rainfall intercepted by the building wall can be up to 50% of the amount of rainwater collected on the building roof. This finding supports the idea that the WDR from a building wall can be a significant source of harvested rainwater.

Improved assessment of wind-driven rain on building façade based on ISO standard with high-resolution on-site weather data

Wind-Driven Rain (WDR) is one of the major moisture sources that cause building envelope failures. The quantity and spatial distribution of WDR are important considerations for durable building envelope designs and are essential boundary conditions for hygrothermal modelling. Within a comprehensive research program of quantifying WDR exposure of buildings and the effectiveness of overhang on reducing WDR wetting, a six-storey building located in Vancouver, Canada was instrumented for field WDR measurements. One of the challenges in field WDR measurements is the validity of on-site wind measurements. The accurate measurements of on-site wind conditions are essential for correlating WDR on façade with on-site weather conditions and for generating the spatial distribution correction factor required in the semi-empirical WDR models. This paper focuses on discussing the procedure necessary for proper on-site wind measurements for quantifying WDR on façade based on field and wind-tunnel measurements. The proper procedure to calculate the spatial distribution correction factor, namely wall factor according to ISO standard, and its impact on the accuracy of the ISO model is also discussed. The accuracy of ISO model can be significantly improved by using more detailed wall factors calculated based on high-resolution on-site wind and rain measurements.

Field measurements of wind-driven rain on mid-and high-rise buildings in three Canadian regions

Wind-driven rain (WDR) is an important boundary condition for the study of hygrothermal behaviour and durability of building envelopes. Understanding the WDR characteristics is important for establishing designs that minimize moisture related issues. Given the limited data available on field measurements of WDR, especially for mid- and high-rise buildings, a number of buildings have been instrumented with weather stations and driving rain gauges for WDR measurements on building façade in three Canadian cities (Vancouver, Montreal, and Fredericton). The high-resolution WDR data collected provide a valuable and unique dataset for improving semi-empirical WDR models and validating Computational Fluid Dynamic (CFD) models. This paper presents the experimental setup, spatial distribution of WDR on façades in terms of catch ratios and wall factors, and the comparison between measurements and calculated WDR using ISO standard 15927. The results show that the spatial distribution of WDR on façade is significantly influenced by building geometry, façade details, and its surroundings. The wall factors vary with both the width and height of the building façade. The ISO semi-empirical model typically overestimates WDR on buildings studied under Canadian climates.

Analysis of time-resolved wind-driven rain on an array of low-rise cubic buildings using large eddy simulation and an Eulerian multiphase model

Time-resolved wind-driven rain (WDR) load is investigated on an array of low-rise cubic buildings using an Eulerian multiphase (EM) model together with large eddy simulation (LES). The influence of wind-flow unsteadiness on the unsteady behavior of raindrops and the WDR intensity is discussed in detail. The wind-flow field predicted with LES has been validated with wind-tunnel measurements. The mean WDR intensity values obtained using the EM model are found to be in agreement with in-situ WDR measurements. The time-resolved simulations show that the instantaneous specific catch ratio values of smaller droplets fluctuate a lot around their mean values due to higher influence of turbulence. Instantaneous specific catch ratios for the smallest raindrops are mainly dictated by local turbulent structures present in the shear layer or below the rooftop level. On the other hand, the motion of larger raindrops is mainly influenced by the larger-scale motions above the cubes. It is also shown that, running means of the specific catch ratio over a time window of 100–300 s of physical time stabilize to a constant value.

Moisture dynamics in the masonry fabric of historic buildings subjected to wind-driven rain and flooding

Current climatic projections show clearly that increasingly more extreme weather events are to be expected in the future. Historic buildings are considered to be the most vulnerable to this adverse climatic impact, via moisture induced deterioration and resulting strength decay in their construction materials. Therefore, the identification of these climatic effects is important to be able to develop suitable tools to mitigate them, both for individual buildings and on a regional scale. This paper presents the analysis of a comprehensive environmental monitoring of two historic buildings in Tewkesbury, Gloucestershire, UK, to provide thorough insight on their performances under environmental loading on a comparative basis. Firstly, the effect of wind-driven rain (WDR) and flooding is assessed by correlation with relative humidity (RH) measurements. The WDR measurements are then compared against values calculated using well established semi-empirical models and reasons behind the limited correlation are discussed. The dynamic hygrothermal response of two different historic fabrics is studied in greater detail by monitoring in-wall temperature and RH. The conclusions drawn from the analysis of the monitoring outputs are then further elaborated on by using hygrothermal characterization obtained by dynamic vapour sorption (DVS) testing of material samples extracted from the fabric of these buildings. The study concludes that the current environmental conditions pose a threat on the building envelopes unless routine maintenance is provided, and that monitoring methodology devised is clearly successful in quantifying the exposure of the two historic buildings to environmental conditions, onsetting deterioration phenomena in the envelop materials.

The Spell Definition in ISO-15927 and its Impact on the Rain Deposition on the Building Facade

Wind-driven rain (WDR) is one of the most important causes for water damage in buildings. Therefore, the first crucial step to assess the hygrothermal performance of the building envelope, is the appropriate estimation of the amount of rainwater striking the building's façade. ISO 15927 offers the annual average index –mainly to assess the moisture content of absorbent surfaces, and the spell index – more related to the likelihood of water penetration through joints. To calculate these indices, assumptions are made concerning the length of the period of ‘no rain’, called ‘spell definition’. Obviously, the choice of this spell definition will characterise the WDR-amount. In this paper, WDR measurements of a 3-storey building in Vancouver, Canada are used to investigate how this spell definition affects the rain load. Different filter criteria are used to exclude errors due to measurement equipment. By means of the catch ratio as a dimensionless parameter, the results of the analysis for different spell definitions are compared to hourly and 5-min data. It is concluded that longer spell definitions result in lower catch ratios and an underestimation of the WDR load. Hourly data turns out to be a more conservative approach for WDR-assessment of this case study, but is able to represent the spread on the catch ratio most closely to the original 5min-dataset.

Wind-driven rain on two parallel wide buildings: Field measurements and CFD simulations

Most studies of wind-driven rain (WDR) reported in the literature focus either on isolated buildings or on a particular building in geometrically complex environments. There is a need for experimental and numerical studies for more generic multi-building geometries. The present study reports detailed field measurements and numerical simulations of WDR that are conducted for an idealized geometry with two parallel wide buildings with different heights, located in Dübendorf, Switzerland. The datasets of rain events and WDR measurements with high spatial and temporal resolution are made available online to download and are intended for model development and validation. Numerical simulations are performed with computational fluid dynamics (CFD) based on the 3D steady Reynolds-averaged Navier–Stokes (RANS) equations and an Eulerian multiphase (EM) model for WDR, including the turbulent dispersion of raindrops. The numerical results are validated by comparing the calculated catch ratio values and cumulative WDR amounts with data from the field measurements. The CFD simulations accurately estimate the WDR intensities at the positions of 18 WDR gauges. The average discrepancies between the numerical and experimental results are found to be 6.9% for the rain event on February 20–21, 2014 and 4.9% for the rain event on August 2–3, 2014. In different building configurations, the influences of recirculation regions, sheltering, wind-blocking effect and acceleration of wind determine the WDR distribution on the downstream building. WDR can increase due to recirculation regions and acceleration of wind, while wind-blocking effect and sheltering decrease WDR.

High-resolution field measurements of wind-driven rain on an array of low-rise cubic buildings

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 envelopes can be understood in a better way when the WDR intensity distribution can be accurately predicted. Most field experiments of WDR reported in the literature focused on either stand-alone buildings or on buildings in geometrically complex environments. There is a need for high-resolution measurements in more generic and idealized multi-building configurations. The present study reports WDR measurements that were conducted with high spatial and temporal resolution in a test setup consisting of an array of 9 low-rise cubic building models, located in Dübendorf, Switzerland. Detailed descriptions are provided of the building models, the surroundings, the measuring instruments, the measurements of WDR, wind speed, wind direction, horizontal rainfall intensity and air temperature during three selected rain events, as well as error estimates for the WDR measurements. The datasets of rain events and WDR measurement results are made available online to download and are intended for WDR model development and validation.

Wind-driven rain on the facade of a monumental tower: Numerical simulation, full-scale validation and sensitivity analysis

Wind-driven rain (WDR) is one of the most important moisture sources that affect the hygrothermal performance and the durability of building facades. The facades of the Dutch monumental building St. Hubertus show severe deterioration caused by WDR. Assessment of the amount and intensity of WDR falling onto the facades is necessary as input for numerical heat-air-moisture (HAM) transfer models to analyse the causes of the moisture problems and the impact of remedial measures. In this study, a numerical simulation method based on Computational Fluid Dynamics (CFD) is used to predict the amount of WDR impinging on the south-west facade of the tower of the building. The paper focuses on the numerical simulation results, the validation of these results and their sensitivity to two parameters: the level of geometrical detailing of the computational building model and the upstream terrain aerodynamic roughness length. Validation is performed by comparison of the numerical results with a dataset obtained from on-site WDR measurements. It is shown that the CFD simulations provide fairly good predictions of the amount of WDR impinging on the south-west facade of the tower, except for the lower part. It is also shown that the local effects of geometrical facade details are significant and can yield differences in WDR exposure up to 40%, while their effect at other positions is negligible. Finally, the sensitivity of WDR simulations to the upstream aerodynamic roughness length is discussed.

A dataset of wind-driven rain measurements on a low-rise test building in Norway

Semi-empirical models and especially numerical simulation models based on Computational Fluid Dynamics (CFD) are increasingly being used to calculate wind-driven rain (WDR) on building facades. The development, verification and validation of these models require accurate and complete WDR measurement datasets. Although many WDR measurement campaigns have been conducted in the past, few reports of these contain enough information for model development and model validation. This paper presents a complete dataset of WDR measurements for a low-rise test building in Trondheim, Norway. It contains a detailed description of the building, its surroundings and the meteorological station, as well as WDR measurements in free-field conditions and on the building facades, measurements of wind speed, wind direction, horizontal rainfall intensity, temperature, relative humidity and error estimates for the WDR measurements. The paper also provides the link to a website from which the full set of data can be downloaded.

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.

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.