Guidelines for wind, rain and wind-driven rain measurements at test-building sites

Transcription

1 Guidelines for wind, rain and wind-driven rain measurements at test-building sites Bert Blocken, Dr. Ir. Laboratory of Building Physics, Department of Civil Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 40, 3001 Leuven, Belgium Jan Carmeliet, Prof. Dr. Ir. Laboratory of Building Physics, Department of Civil Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 40, 3001 Leuven, Belgium Also at: Building Physics Group, Faculty of Building and Architecture, Technical University Eindhoven, P.O. box 513, 5600 MB Eindhoven, The Netherlands KEYWORDS: wind-driven rain, driving rain, wind flow, rainfall, building facade, experiments, accuracy SUMMARY: At test-building sites of building physics laboratories and building research institutes, wind, rain and winddriven rain (WDR) measurements are commonly performed to support research on the hygrothermal performance and the durability of building components. However, up to now, very little attention has been given to the accuracy and adequacy of these measurements. This paper provides a number of guidelines to obtain accurate and adequate wind, rain and WDR measurements at test-building sites. Three sets of guidelines are distinguished: (1) guidelines for the selection of the measurement equipment, (2) guidelines for the location of the measurement equipment at the site and (3) guidelines for the selection of accurate measurement data from the total amount of data available. The violation of these guidelines can at least partly be held responsible for the discrepancies that are often observed between measurements and the corresponding calculation results. 1. Introduction Wind, rain and wind-driven rain (WDR) are important boundary conditions for the study of the hygrothermal performance and the durability of building components. WDR is one of the most important moisture sources for building facades. Three categories of methods for quantifying WDR on building facades exist: (1) measurements, (2) semi-empirical models and (3) numerical simulation models that are based on Computational Fluid Dynamics (CFD). A review of these quantification methods has recently been provided (Blocken & Carmeliet 2004). Measurements are the primary tool in WDR studies. They do not only provide a direct indication of the amount of WDR falling onto different parts of a building facade but they are also essential for the development and the validation of semi-empirical models and numerical simulation models. Test-building sites of building physics laboratories and building research institutes generally comprise a measurement set-up for wind, rain and WDR. The parameters measured are the standard meteorological variables wind speed, wind direction and horizontal rainfall intensity (i.e. the intensity of rain falling through a horizontal plain) together with measurements of the WDR that falls onto the test building facades, sometimes supplemented with measurements of the free-field WDR. There are a number of general requirements for these measurements: (1) Adequate measurement equipment should be selected to minimise errors: sufficient accuracy and time resolution and a limited influence of the gauge body on the measurements are imperative; (2) The standard wind and rain measurements as well as the free-field WDR measurements should be taken in free-field conditions. This means that they should not be significantly influenced by the presence of the building or other nearby obstructions. This has important implications on the location of the measurement equipment at the test site. (3) Furthermore, in case of violation of at least

2 on of the two above-mentioned requirements, it is important to carefully select suitable and reliable data from of the total amount of data available to conduct further analyses. Up to now, very little attention has been given to these requirements and to the consequences of their violation. The accuracy and adequacy of wind, rain and WDR measurements that are obtained with calibrated equipment for use in building physical research have almost always been taken for granted. In particular, very little attention has been given to the installation and the position of the measurement equipment at the site. This is illustrated by the large variety in configurations of meteorological stations at different research institutes. It is partly due to the fact that there is no clear standard on how and where to perform these measurements. In this paper, a number of guidelines to obtain accurate and adequate wind, rain and WDR measurements at test-building sites are provided. Section 2 presents guidelines for the selection of the measurement equipment and the time resolution for the measurements. In section 3, guidelines for the location of the measurement equipment on the site are provided. Section 4 focuses on the selection of measurement data. The application of the guidelines is briefly illustrated in section Measurement equipment: selection, accuracy and time resolution 2.1 Wind Standard measurements comprise measurements of the mean horizontal wind speed (m/s) and wind direction (degrees clockwise from north). The requirements of wind measurements and wind-measuring systems are described in detail in the Guide to meteorological instruments and methods of observation published by the World Meteorological Organisation (WMO 1996). Here, we focus on the most common types of instruments. A common combination is a cup anemometer to measure the horizontal component of the wind-velocity vector and a wind vane to measure the wind direction. An often quoted disadvantage of cup anemometers is the so-called cup overspeeding, i.e. a bias in the measured mean wind speed due to fluctuations in the longitudinal wind-speed component (turbulence). According to Kristensen (1993), overspeeding only in extreme cases exceeds about 2%. However, Kristensen has shown that the fluctuations in the other wind components give rise to other types of bias: the positive bias in the mean wind speed due to wind-direction fluctuations is much larger than the overspeeding and can be as much as 18%. A good alternative, although significantly more expensive, is an ultrasonic anemometer (Fig. 1a). It can measure the three components of the wind-velocity vector (and hence also the wind direction) by determining the effect of the wind on the transit time of acoustic pulses that are transmitted in opposite directions across known paths. Its main advantages are high accuracy (typical error of 1.5% on wind speed and 2-4 on wind direction), the absence of moving mechanical parts and the very short time constant. Wind speed should be sampled at short intervals that can range from 1 minute to less than 1 second. These samples are typically averaged to obtain the mean wind speed over a certain period of time. The choice of an appropriate averaging period is guided by the location of the spectral gap in the power spectrum of the horizontal wind speed (Van der Hoven 1957). This spectrum is based on full-scale measurements and is illustrated in Fig. 1b. It exhibits a number of peaks in which the majority of the wind energy is situated. The region with low energy in between the peaks is called the spectral gap. It stretches from periods of 10 minutes to periods of more than 1 hour. In the gap, the mean wind speed shows stationarity, which means that its value is relatively steady. Therefore, averaging wind-speed measurements over a period of 10 minutes to 1 hour provides fairly stable mean values. This is the reason why these averaging intervals are practically always used. 2.2 Rain Many types of rain gauges have been developed for the measurement of the rainfall intensity and/or the rainfall sum falling on the ground. They differ by shape, size and by the measuring principle used. Here, only the two most common types of recording rain gauges that are used in building physics are mentioned. The tipping-bucket rain gauge is well-known. Its major disadvantages are the low resolution (typically 0.1 or 0.2 mm/tip) and the fact that a signal (a tip of the bucket) is only given when the bucket is full, which causes variable shifts between the time of the actual rain collection and the time of the signal of rain collection. A more expensive alternative is a capacitance rain gauge (Fig. 1c). This type is less known. It consists of a cylindrical rainwater collector containing a probe made of a stainless steel rod covered by

3 (a) (b) (c) FIG. 1. (a) Ultrasonic anemometer. (b) Power spectrum of the horizontal wind speed (Van der Hoven 1957). The energy of the wind is mainly situated in the peaks that are situated at a period of 1 year, 4 days, 1 day and 1 minute. In the spectral gap between 10 minutes and about 1 hour, little energy is situated and the mean wind speed shows stationarity for these time periods. (c) Capacitance rain gauge. PTFE (polytetrafluoroethylene). The metal rod and the collected rainwater respectively form the inner and outer plate of a coaxial-type capacitor while the PTFE serves as the dielectric. An increase of rainwater in the collector causes the surface area of the capacitor to increase and hence the total capacitance which is measured and converted to an analogue voltage output (Nystuen et al. 1996). An important advantage over a tipping-bucket rain gauge is that an instantaneous sample of the collected water volume can be taken at any time required. The resolution is 0.02 mm. The reported accuracy is 1 mm but the real accuracy is often significantly better. Another advantage is that evaporation can be measured. Errors in rainfall measurements have been studied by many researchers in the past. The error that is considered most important is the wind error. It is due to the systematic deformation of the wind flow above the gauge orifice (and hence of the raindrop movements in this flow) by the presence of the gauge body itself (Fig. 2a). When the rain gauge is placed at a certain height above ground (between 0.5 and 1.5 m), without any precautions, the undermeasurement error is typically 2 to 10 % (WMO 1996). But it is clear that as the wind error increases with wind speed (WMO 1982), it will be much higher for rain gauges positioned on building roofs and at the top of meteorological masts. Various precautions to limit this error are all based on the same goal: to make the air flow horizontal above the gauge orifice (e.g. Sumner 1988, WMO 1996). Three recommended options to achieve this are (1) ground-level gauges (i.e. rain gauges placed in a pit with their orifice level with the ground surface), (2) to build a turf wall around the gauge as specified in Fig. 2b, or (3) to fix a shield around the rain gauge orifice. Other systematic errors usually of lesser importance are the wetting loss on the internal walls of the collector, the wetting loss in the (a) (b) FIG. 2 (a) Cross-section of a rain gauge exposed to wind. The gauge body disturbs the wind-flow pattern and raindrops (trajectories are displayed with dashed lines) are swept over the rim of the gauge due to the sudden increase in vertical wind speed near the orifice of the gauge. (b) Cross-section of a rain gauge sheltered from wind by constructing a circular turf wall around it. The wind velocity over the orifice of the rain gauge is horizontal and as a result the trajectories of the drops are not (or only slightly) deflected.

4 container when it is emptied, in- and out-splashing of water and evaporation from the container. The WMO (1996) provides the following estimates for maximum evaporation losses: up to mm per day in winter and up to 0.8 mm per day in summer. Given the extreme variability of rain, the minimum sampling interval for rain measurements should be selected with care. Clearly, hourly data are generally not appropriate, certainly not for the registration of short-duration showers (Sumner 1988). In general, the shorter the time interval, the more accurate the registration of the phenomena will be. On the other hand, even the most sophisticated rain gauges are incapable of measuring instantaneous rainfall (Sumner 1988). Jones & Sims (1978) correctly state that the nature of instrumentation forces a compromise whereby instantaneous precipitation is considered to be that which occurs over a duration of one minute. Sumner (1981, 1988) mentions that due to errors in timing, local turbulence and so on, it is probably more reasonable to settle for sampling intervals of 5, 10 or 15 minutes. Focusing on the use of meteorological data for Heat-Air-Moisture transfer analyses, Hens (1996) correctly states that hourly data may not be good enough for precipitation. He mentions that, given the fact that only hourly data are available for most weather stations and that the costs to obtain them are high, hourly data are often considered to be the best choice. But he also stresses that, as a general guideline, the time-averaging period should not induce loss of important information for the case analysed. 2.3 Wind-driven rain Wind-driven rain on buildings Measurements of WDR on buildings are performed with plate-type WDR gauges. These are all of a similar basic design (Fig. 3). They consist of a collection area, a draining tube and a reservoir. The collection area is made up of a shallow tray (collection plate or catch area) of some material, shape and size and it is fixed at the building surface. It has a raised rim around the perimeter to prevent the collection of water from outside the reservoir. As opposed to standard wind and rain measurement equipment, WDR gauges are not industrially manufactured and there exists no standard on their design. As a result, there are almost as many types of WDR gauges as there are researchers using them. Although the basic design is the same, the gauges differ by material, shape and size of the collection area, the draining tube and the reservoir. Recent research concerning the accuracy of WDR measurements has been conducted by Högberg et al. (1999), van Mook (2002) and by Blocken & Carmeliet (2005a). The latter authors listed the following five possible error sources for WDR measurements: (1) adhesion-water evaporation; (2) evaporation from the reservoir; (3) splashing of drops from the collection area; (4) condensation on the collection area and (5) wind errors. The first error refers to the fact that a certain amount of rain water is always adhered to the gauge collection area (adhesion water) and is not collected in the reservoir. After and also to a lesser extent during the rain spell, this water evaporates. This error can be very important (up to 100%). The wind error is caused by the disturbance of the wind-flow pattern near the gauge, mainly by the rim of the gauge. The following guidelines for the design of WDR gauges were provided (Blocken & Carmeliet 2005a): Limit the amount of adhesion water at the gauge collection area. For traditional plate-type gauges, plain sheet glass is preferred over PMMA and PVC. PTFE should not be used. Information for other material types or surface finish is provided in the above-mentioned paper. fixing holes gutter collection area 200 mm 10 mm rim tube to reservoir front view side view perspective view FIG. 3. Wind-driven rain gauge designed, manufactured and installed at the Laboratory of Building Physics, K.U.Leuven. The gauge is made of PMMA (polymethyl-methacrylate) with a collection area A = 0.2 x 0.2 m².

5 Limit the amount of adhesion water at the bottom part of the gauge and in the draining tube. The latter should be kept as short as possible and the material type should be selected for minimal adhesion water. Minimise evaporative losses from the reservoir (e.g. by exposing only a small water surface to the ambient air, by minimizing the ventilation rate in the reservoir and by regularly adding a few drops of light oil). If possible, direct the collected rainwater towards the inside of the building into reservoirs mounted at the inner wall surface to avoid frost damage to the reservoirs, to reduce the variability in the evaporative losses and to avoid excessive evaporative losses due to heating by solar radiation. Limit the height of the rim of the gauge to reduce wind errors. The time resolution of WDR measurements depends on the purposes for which the data will be used. Heat- Air-Moisture transfer analyses are typically conducted with data on an hourly basis and therefore WDR measurements should at least be taken every hour. Note that it is not useful to conduct WDR measurements on a very short time basis, as the time between the impact of raindrops on the collection area and the collection of this amount of water in the reservoir may be several seconds up to several minutes Free-field wind-driven rain The same guidelines hold for free-field WDR gauges. Note that the wind-error can be significantly more pronounced here, because not only the rim of the gauge but also the gauge body will cause an important disturbance of the wind field. Special free-field WDR gauges have been designed by a few researchers to specifically reduce the wind error (Korsgaard & Madsen 1964, Choi 1996, Blocken & Carmeliet 2004). 3. Location of the measurement equipment on the site 3.1 Wind The standard height for wind measurements over land is 10 m (WMO 1996). Standard free-field windspeed and wind-direction measurements should be performed outside the wind-flow pattern that is disturbed by the building. Fig. 4 illustrates the results of a CFD (Computational Fluid Dynamics) simulation: contours of the dimensionless wind speed (magnitude of the 3D velocity vector) around a simple block-type low-rise building. The values are made dimensionless by division by the free-field horizontal wind speed at 10 m, U 10 (which is the one that should be measured). The figure indicates that there are regions of important upstream and downstream disturbance. Especially the latter is very pronounced. Measurements should be taken outside these regions. Given the fact that all wind directions are possible, measurements should be taken sufficiently far away from the building to be out of the wake at all times. If this is not possible, data gathered for a certain range of wind directions should not be used for analysis purposes. The size of the recirculation region behind the building can range between 1.5 to 2.5 times the largest dimension of the building. The length of the wake however can go up to 20 times the height of the building. Note that this also implies that other buildings, far away from the test site, can have a pronounced influence on the measurements. In general, especially measurements in the recirculation region and on or above the roof of a building should be avoided. Note that CFD can be used to assess the extent of the disturbed wind-flow pattern and to select adequate locations for the measurement equipment U 10 WIND isoline U/U 10 = 1 recirculation far wake 10 m FIG. 4. Illustration of the disturbed wind-flow pattern over a low-rise building. Contours of dimensionless wind speed (U/U 10 ) are indicated. At the isoline (U/U 10 = 1) the same wind speed as the free-field wind speed U 10 is found.

6 3.2 Rain The same guidelines hold for the position of the rain gauge because the raindrop trajectories are directly influenced by the wind-flow pattern around the building. Measurements should not be taken in regions with highly perturbed flow (recirculation region behind the building, on the building roof) and also not in regions where the wind speed is high (e.g. on top of the roof, or worse, on top of a mast) because this will severely increase the wind error (up to 20%-50% and more). The best option is to conduct rain measurements in free-field conditions, close to the ground surface and with special provisions to limit the wind error (as indicated in section 2.2). 3.3 Wind-driven rain Wind-driven rain on buildings Care should only be taken to keep the collection area of the gauge as much as possible flush with the facade surface Free-field wind-driven rain For the location of free-standing WDR gauges, the guidelines mentioned in 3.1 and 3.2 apply. 4. Selection of measurement data In almost all cases, it is not possible to fully take into account all of the above-mentioned guidelines due to financial and space limitations. Correction of the measurements for the errors involved is only possible in a limited number of situations. Therefore, it is important to carefully select, from the total amount of data available, only those measurements that are accurate and adequate for the intended analyses. E.g. for standard wind measurements near buildings, data with wind directions for which the anemometer is in the wake of the building should not be selected. For WDR studies, a number of guidelines for selecting accurate wind, rain and WDR data from experimental databases have been proposed (Blocken & Carmeliet 2005b): Select measurements for which the wind and rain equipment is situated mainly outside the wind-flow pattern disturbed by the building(s). Select those measurements with large WDR sums, hence reducing the relative adhesion-waterevaporation error. This error is especially important for WDR events with small WDR sums. It should be estimated. This can be done as described in (Blocken & Carmeliet, 2004; 2005a). Measure the evaporation from the reservoir and correct the measurements for this error. Select rain events for which WDR splashing errors will be absent or small; i.e. rain events characterized by reference wind-speed values U 10 lower than 10 m/s and with horizontal rainfall intensities with a small possibility for large drops: R h < 20 mm/h. Select rain events for which the wind direction during rain is approximately perpendicular to the facade under study, hence limiting the wind error in the WDR measurements on the facade. These guidelines are very strict. Their violation can explain at least part of the discrepancies that have been found between measurements and the corresponding semi-empirical and numerical calculations of WDR. In many cases, strictly following these guidelines implies that a significant amount of the measurement data that are gathered can not be used. 5. Application: VLIET test site, Laboratory of Building Physics As an example of the application of the guidelines in this paper, the design of the new measurement set-up at the VLIET test site and the selection of data are briefly discussed. The VLIET building is illustrated in Fig. 5a. It is surrounded by other nearby buildings and obstructions except for south-west direction. A view of the south-west terrain is provided in Fig. 5b. The elements providing shielding from wind in the direct vicinity of the building are some low agricultural constructions to the south and a row of high poplars to the west side. The new measurement set-up consists of (Fig. 6): (1) a meteorological mast positioned in front of

7 (a) N-W facade Roof overhang length 0.44 m 0.44 m 0.41 m 0 m 0.32/0.30/0.32/0.32 m S-W facade 7.9 m m (b) 7.2 m 25.2 m FIG. 5. (a) VLIET test building. North-west and south-west facade. The building dimensions, including roof overhang length, and the positions and numbers of the wind-driven rain gauges (indicated by black squares) are indicated. (b) View at the terrain south-west of the VLIET building. (Photograph taken with the back against the south-west facade). new meteorological meteorological mast mast old meteorological mast rain gauge behind wind shield 12.5 m 2 m 20 m FIG. 6. Location of the old meteorological mast on top of the building roof and location of the new meteorological station (mast and horizontal rain gauge) in front of the south-west facade. SW the south-west facade that is equipped with three cup anemometers and one ultrasonic anemometer, (2) a capacitance rain gauge surrounded by a semi-circular turf wall and (3) a large number of WDR gauges (Fig. 3 and 5a). The old meteorological mast is positioned on the building roof and will therefore be situated in the wind-flow pattern that is disturbed by the building. The intention of the new mast was to provide as much as possible free-field measurements of wind and rain for south-west wind direction. The optimal position for the mast (20 m in front of the south-west facade) was selected based on CFD simulations, as a compromise between the minimizing the influence of the upstream disturbance of the wind-flow field and minimizing the influence of the row of trees and the other obstructions upstream of the mast on the measurements. The accurate ultrasonic anemometer provides the basic measurements (reference wind speed U 10 and wind direction θ), the cup anemometers provide an indication of the vertical wind-speed profile. A self-siphoning capacitance rain gauge was installed at the base of the mast, surrounded by a semi-circular turf wall to reduce the wind error for south-west wind (Fig. 6). Twenty-three new WDR gauges were manufactured at the Laboratory of Building Physics and are positioned mainly across the south-west facade (Fig. 3, 5a). Apart from the material of the collection area (PMMA), the guidelines given in section 2.3 were adhered to. The measurements have been used for the validation for CFD simulations of WDR on the south-west facade of the VLIET building (Blocken & Carmeliet 2005c). The combination of all guidelines mentioned above has resulted in the fact that for these studies, only measurement data with approximately south-west wind direction (wind directions in the interval [200, 250 ]) could be considered accurate and adequate.

8 6. Conclusions Three sets of guidelines have been provided for wind, rain and wind-driven rain measurements at testbuildings sites: (1) guidelines for the selection of the measurement equipment, (2) guidelines for the location of the measurement equipment at the site and (3) guidelines for the selection of accurate measurement data from the total amount of data available. It is almost always impossible to satisfy all of the guidelines in the second set. Therefore, the measurement data obtained should be carefully selected according to the third set of guidelines. These data-selection guidelines can sometimes be relaxed depending on the purpose for which the measurements are used. The authors believe that deviations from these guidelines can be held responsible for at least part of the discrepancies that have been found in the past when comparing wind-driven rain measurements with winddriven rain calculations by semi-empirical models such as the European Standard Draft (CEN 1997) or by numerical simulation models (CFD). 7. Acknowledgements This research is funded by the government of Flanders (Belgium). The first author is a post-doctoral research fellow of the FWO-Flanders (Fund for Scientific Research in Flanders). Their contribution is gratefully acknowledged. 8. References Blocken B, Carmeliet J. (2004). A review of wind-driven rain research in building science. Journal of Wind Engineering and Industrial Aerodynamics 92, 13: Blocken B, Carmeliet J. (2005a). On the accuracy of wind-driven rain measurements on buildings, Building and Environment. Submitted. Blocken B, Carmeliet J. (2005b). High-resolution wind-driven rain measurements on a low-rise building experimental data for model development and model validation. Journal of Wind Engineering and Industrial Aerodynamics. Submited. Blocken B, Carmeliet J. (2005c). Validation of CFD simulations of wind-driven rain on a low-rise building facade. Journal of Wind Engineering and Industrial Aerodynamics. Submitted. CEN. (1997). Hygrothermal performance of buildings Climatic data Part 3: Calculation of a driving rain index for vertical surfaces from hourly wind and rain data. Draft pren Choi ECC. (1996). Measurement of wind-driven rain. Proceedings of the 5 th Australian Wind Engineering Society Workshop. Adelaide, 1996, paper A-6. Hens H. (1996). Applied Building Physics 1, Boundary conditions and performance requirements (In Dutch; Toegepaste Bouwyfsica 1, Randvoorwaarden en prestatie-eisen), Third Edition, Acco, Leuven. Högberg AB, Kragh MKK, van Mook FJR. (1999). A comparison of driving rain measurements with different gauges, Proceedings of the 5th Symposium of Building Physics in the Nordic Countries, Gothenburg, pp Jones DMA, Sims AL. (1978). Climatology of instantaneous rainfall rates. Journal of Applied Meteorology 17: Korsgaard V, Madsen TL. (1964). A note on driving rain measurements. The Heat Insulation Laboratory, Technical University of Denmark, Copenhagen Kristensen L. (1993). The cup anemometer and other exciting instruments, Doctor Technices Thesis, Risø National Laboratory, Technical University of Denmark. Nystuen NA, Proni JR, Black PG, Wilkerson JC. (1996). A comparison of automatic rain gauges, Journal of Atmospheric and Oceanic Technology 16: Sumner G. (1988). Precipitation: process and analysis. John Wiley & Sons, 444 p. Sumner GN. (1981). The nature and development of rainstorms in coastal East Africa. Journal of Climate 1(2): Van der Hoven I. (1957). Power spectrum of horizontal wind speed in the frequency range from to 900 cycles per hour. Journal of Meteorology 14: van Mook FJR. (2002). Driving rain on building envelopes, Ph.D. thesis, Building Physics Group (FAGO), Eindhoven University of Technology, Eindhoven University Press, Eindhoven, The Netherlands, 198 p. WMO. (1982). World Meteorological Organization, Methods of correction for systematic error in point precipitation measurement for operational use (Sevruk B.), Operational Hydrology Report, WMO-No. 589, Geneva. WMO. (1996). World Meteorological Organization, Guide to meteorological instruments and methods of observation, Sixth edition, WMO-No. 8.