Wind-driven Rain Research Articles

Sand detachment under rains with varying angle of incidence

In the case of wind-driven rain, as the angle of rain incidence increases, greater force components act parallel to the surface and lower force components act perpendicular to the surface. Therefore, raindrop impact angle could influence the detachment process in the existence of substantial lateral jet development. This could be particularly significant for a cohesionless sand surface that has a weaker resistance to the dislodgement by a raindrop impact than a soil surface. Experiments of simulated wind-driven rain were conducted to evaluate sand detachment rates under increased lateral jetting induced by wind velocities of 6.4, 10.0, and 12.0 m s− 1 at nozzle operating pressures of 75, 100, and 150 kPa and incident on windward and leeward slopes of 4 and 9°. With these set-ups, it was possible to have varying angles of incidence and to determine the effect of the compressive stress and shear stress, evaluated by the horizontal and vertical kinetic energy fluxes (Etx and Etz, respectively), on the detachment rates from the sand surface. At the same vectorwise sum of Etz and Etx, the results showed that there was, for similar values of Etz and Etx in windward slopes, more sand detachment rate; however, for dissimilar values of Etz and Etx in leeward slopes where Etx was significantly higher than Etz, there was less sand detachment rate. Consequently, with certain values of the compressive stress and shear stress, the sand detachment rates under wind-driven rains peaked; and Etz was the limiting component since the rates significantly decreased as Etz particularly decreased in spite of very large values of Etx.

Soil slip/debris flow localized by site attributes and wind-driven rain in the San Francisco Bay region storm of January 1982

GIS analysis at 30-m resolution reveals that effectiveness of slope-destabilizing processes in the San Francisco Bay area varies with compass direction. Nearly half the soil slip/debris flows mapped after the catastrophic rainstorm of 3–5 January 1982 occurred on slopes that face S to WSW, whereas fewer than one-quarter have a northerly aspect. Azimuthal analysis of hillside properties for susceptible terrain near the city of Oakland suggests that the skewed aspect of these landslides primarily reflects vegetation type, ridge and valley alignment, and storm–wind direction. Bedrock geology, soil expansivity, and terrain height and gradient also were influential but less so; the role of surface curvature is not wholly resolved. Normalising soil-slip aspect by that of the region's NNW-striking topography shifts the modal azimuth of soil-slip aspect from SW to SE, the direction of origin of winds during the 1982 storm—but opposite that of the prevailing WNW winds. Wind from a constant direction increases rainfall on windward slopes while diminishing it on leeward slopes, generating a modelled difference in hydrologically effective rainfall of up to 2:1 on steep hillsides in the Oakland area. This contrast is consistent with numerical simulations of wind-driven rain and with rainfall thresholds for debris-flow activity. We conclude that storm winds from the SE in January 1982 raised the vulnerability of the Bay region's many S-facing hillsides, most of which are covered in shallow-rooted shrub and grass that offer minimal resistance to soil slip. Wind-driven rainfall also appears to have controlled debris-flow location in a major 1998 storm and probably others. Incorporating this overlooked influence into GIS models of debris-flow likelihood would improve predictions of the hazard in central California and elsewhere.

Comparison of Different Techniques for the Measurement of Precipitation in Tropical Montane Rain Forest Regions

Abstract Characteristics of different precipitation measurements in a tropical mountain valley in southern Ecuador are compared in this study to determine potential errors. The instruments are used for different ecological purposes like erosion studies, through fall measurements, investigation of atmospheric chemistry, and modeling of area rainfall distribution. Five recording devices (two precipitation radars, an electro-optical present weather sensor, and two tipping buckets) and three totaling gauges were operated in parallel at a designated site. Data were taken between 1998 and 2003 with different temporal resolution and different operational periods. The general agreement between the instruments is rather good; deviations are in the expected range of 10%–20% of the annual total of about 2200 mm. The remote sensing devices are superior in registering the frequent occurrence of light rain but are not capable of detecting the full range of rain intensities observed. The tipping buckets and the totaling gauges are reliable instruments, but a certain fraction of light drizzle and wind-driven rain is not detected. The present weather sensor has the widest range of sensitivity and supplies additional information on drop spectra. All datasets are affected by operational problems (interruptions, synchronization errors); hence, the redundancy given here seems reasonable for an ecosystem study.

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.

Effect of long-term changes in air pollution and climate on the decay and blackening of European stone buildings

Abstract This paper reviews the long-term effects of past, present and future air pollution and climate on the decay of stones from historic buildings. It summarizes the historical effects as well as causes and consequences of damage. The most significant change in terms of pollution has been a shift from high levels of sulphate deposition from coal burning to a blackening process dominated by diesel soot and nitrogen deposition from vehicular sources in cities. Blackening of light-coloured fabric eventually reaches a point where it becomes publicly unacceptable. Public opinion can assist the development of aesthetic thresholds and derive limit values for elemental carbon in urban air. Public perception is also affected by the pattern of blackening. This century new climate regimes will cause dramatic changes in blackening patterns by wind-driven rain. Climate changes, most particularly changes in temperature, humidity stress and time of wetness, can also affect the weathering of stone in terms of responses to frost and soluble salts. Future pollution and climate are relevant considerations in the management of historic buildings.

Significance of wind-driven rain (wind-splash) in the erosion of blanket peat

Wind-splash is a process in which wind and rain combine to cause soil erosion. In upland Britain, the conditions necessary for wind-splash erosion are relatively common and frequently occur in locations where blanket peat is an important land cover. A typical location is Moss Flats (North Pennines, northern England, UK). Wind-splash processes were monitored intensively at this site over 3 months using a circular configuration of mass flux sediment samplers, and meteorological data logged from an on-site automatic weather station. Maximum peat flux rates were measured between south-southwest and west-northwest directions in association with relatively moderate intensity, frontal rainfall, typically 4–6 mm h − 1 . Wind-splash processes operate in any direction due to changeable synoptic weather patterns. Windward peat fluxes were typically 2–13 times greater than those recorded at leeward orientations. Spatial patterns of erosion are reflected in the wider landscape through the development of small-scale, erosional landforms (peat hags), which frequently display preferred orientations within the range of maximum peat flux. It is suggested that wind-splash may be a more important process of peat erosion than hitherto reported in UK upland areas.

On the errors associated with the use of hourly data in wind-driven rain calculations on building facades

Wind-driven rain (WDR) is the one of the main moisture sources for building facades. It is an important factor in the dry and wet deposition of pollutants, facade surface soiling and facade erosion. WDR calculations require data records of wind speed, wind direction and horizontal rainfall intensity as input. Most meteorological datasets contain at best arithmetically averaged hourly wind and rain data. Their use is common practice in WDR calculations. As an example, existing WDR standards request at best hourly data. This paper however demonstrates that the use of such data can yield (very) large errors in the calculated WDR amounts and intensities. The reason is that arithmetic averaging on an hourly basis generally causes an important loss of information about the co-occurrence of wind and rain. An improved data averaging technique for wind and rain data is proposed that respects this co-occurrence by applying appropriate weighting factors in the averaging procedure. The performance of this technique is evaluated by WDR calculations on buildings in three cities with different climates. While arithmetically averaged hourly data yield large underestimation errors (Eindhoven, The Netherlands: 11%, Bloomington, USA: 45%, Grahamstown, South Africa: 31%), the improved averaging technique provides very good results (errors: 0%, 4%, 3%, respectively). In conclusion, WDR calculations should not be performed with arithmetically averaged hourly data. Instead, either high-resolution data (e.g. 10-min data) or hourly data that have been obtained with the proposed weighted averaging technique should be used.

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.

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. This article analyses the modelling of the wind-driven-rain load on building facades by partial simplification of a complex CFD-based method along the lines of the European Standard method. The results indicate that the directional dependence of the wind-driven-rain coefficient is not of substantial importance. A constant wind-driven-rain coefficient appears to be an oversimplification though: the full variability with the perpendicular wind speed and horizontal rain intensity should be preserved, where feasible, for improved estimations of the moisture transfer in building components. In the concluding section, it is moreover shown that the dependence of the surface moisture transfer coefficient on wind speed has an equally important influence on the moisture transfer in building components.

Conservative modelling of the moisture and heat transfer in building components under atmospheric excitation

While the transfer equations for moisture and heat in building components are currently undergoing standardisation, atmospheric boundary conditions, conservative modelling and numerical efficiency are not addressed. In a first part, this paper adds a comprehensive description of those boundary conditions, emphasising wind-driven rain and vapour exchange, the main moisture supply and removal mechanism, respectively. In the second part the numerical implementation is tackled, with specific attention to the monotony of the spatial discretisation, and to the mass and energy conservation of the temporal discretisation. Both issues are illustrated with exemplary hygrothermal simulations. Numerical efficiency is treated in two follow-up papers.

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.

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.

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.

A Review of Climate Loads Relevant to Assessing the Watertightness Performance of Walls, Windows, and Wall-Window Interfaces

Abstract When assessing a wall assembly's ability to manage rainwater and control rain penetration, the two key climatic elements to consider are wind speed and rainfall intensity. However, of significance to rain penetration is the effect of wind-driven rain on the building cladding — that is wind coincident with rainfall. When water is present at openings in the cladding, water is driven into the layers of the assembly by the action of wind. Paths providing a direct line from openings in the cladding to inside the assembly offer particularly vulnerable points for water entry. Performance testing helps determine the location of vulnerable locations in a wall assembly and the test loads at which penetration occurs, and it possibly relates the amount of entry to specific details and simulated climate effects. Undertaking watertightness performance tests requires knowledge of extremes in wind-driven rain or specifically the occurrence and level of extreme rainfall events for locations of interest. A review of climate information on wind-driven rain is provided, and its relevance to assessing the watertightness performance of walls, windows, and wall-window interfaces is discussed. Values of rainfall intensity, duration, and frequency or occurrence are given, emphasizing the level of significance of these variables to different North American climates.

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.

On the validity of the cosine projection in wind-driven rain calculations on buildings

Wind-driven rain (WDR) is one of the most important boundary conditions governing the hygrothermal performance and the durability of building facades. Information concerning the quantity of WDR falling onto building facades is an essential requirement as a boundary condition for Heat–Air–Moisture transfer analyses and for building facade design. The quantity of WDR can be calculated with either semi-empirical methods (such as the WDR relationship) or numerical simulation methods that are based on Computational Fluid Dynamics (CFD). The WDR relationship is most often used. It applies the cosine projection to take into account the effect of varying wind direction on the WDR quantity or intensity. Up to now, the validity of the cosine projection for WDR calculations has not yet been investigated. Its use was suggested in the past and it has been adopted for all semi-empirical WDR calculations since then. Also, in the recently developed numerical simulation methods, it is tempting to apply the cosine projection to reduce the computational expense. In the present paper, the validity of the cosine projection is investigated based on 3D numerical simulations of WDR with CFD. It will be shown that the cosine projection, although generally accepted, is not valid and that it can give rise to significant errors.

The vapor diffusion resistance and air permeance of masonry and roofing systems

Several building parts contain layers composed of separate elements that are mortared together or simply interlock or overlap. Typical examples are masonry walls, brick veneers, tiled roofing systems and slated roofing systems. The mortar joints, the interlocks and overlaps should affect the water vapor diffusion resistance and the air permeance of the composite layer, at least in comparison with the vapor and air flow properties of the pure elements. However, little information is found in literature that allows a quantification of the differences. The article describes a purpose designed test method for measuring the water vapor diffusion resistance of composite layers. It comments on the results and also gives data for the air permeances, measured with a pressure box. From the results and data, it is clear that mortared joints, interlocks and overlaps decrease the water vapor diffusion resistance and increase the air permeance of a composite layer compared to the pure material. In fact, mortared joints, interlocks and overlaps act as preferential paths for water vapor and air mitigation. As a consequence, using composite layers as outside finish in thermally insulated building parts, as done in pitched roofs and cavity walls, diminishes the interstitial condensation risk, accelerates drying of the cladding when wetted by wind-driven rain and affects the effectiveness of outside air cross ventilation below or behind the finish.

Effect of Simulated Wind-Driven Rain on Duration and Distance of Dispersal of Xanthomonas axonopodis pv. citri from Canker-Infected Citrus Trees.

Dynamics of dispersal of the bacteria that causes citrus canker (Xanthomonas axonopodis pv. citri) were assessed in simulated wind-driven rain splash. The wind/rain-splash events were simulated using electric blowers to generate turbulent wind (15 to 20 m s-1) and sprayer nozzles to produce water droplets entrained in the wind flow. The splash was blown at an inoculum source of canker-infected trees 1 m downwind. The splash downwind of the source of the infected trees was collected by vertical panel samplers and funnel samplers. The duration over which bacteria were dispersed in spray was assessed in continuous wind at intervals from 0 to 52 h after commencing the simulated rain splash event. In one experiment on 11 February 2003, a total of 1.48 × 106 bacteria were collected by panels 1 m downwind from the inoculum source during the first 10 min of dispersal, but the numbers declined to 3.60 × 105 bacteria after 1 h and ranged between 1.42 × 105 and 1.93 × 104 up to 52 h. In a more detailed study (15 July 2003) of dispersal duration over 4 h, the greatest quantity of bacteria collected by panel samplers were dispersed in the first 5-min period (1.01 × 108 bacteria collected). By 10 min after initiation of dispersal, approximately one-third (3.09 × 107 bacteria collected) of the initial number was being dispersed, and by the end of the first hour, only one-tenth (1.31 × 107 bacteria collected) of the initial quantity was dispersed. Funnel samplers placed at ground level under the trees showed a similar trend. The distance to which bacteria were dispersed in wind-blown splash was also tested under simulated conditions: on 18 September 2003, bacteria were collected by panel samplers at all distances sampled (1, 2, 4, 6, 8, 10, and 12 m) with the greatest number of bacteria deposited at 1 m (4.93 × 106 bacteria collected), while 2.22 × 103 bacteria were deposited over a 10-min period 12 m from the inoculum source. Wind speed declined from 19.5 m s-1 upwind of the trees to 2.8 m s-1 1 m downwind, and by 4 m downwind from the inoculum source, movement was similar to the surrounding air. The data on duration and distance of dispersal were best described by power law regression models compared to exponential models. Citrus canker is readily dispersed in wind-driven rain and is dispersed in large quantities immediately after the stimulus occurs, upon which wind-driven splash can disperse inoculum over a prolonged period and over a substantial distance.

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.