Prediction and evaluation method of wind environment in the early design stage using BIMbased CFD simulation

IOP Conference Series: Materials Science and Engineering Prediction and evaluation method of wind environment in the early design stage using BIMbased CFD simulation To cite this article: Sumi Lee and Doosam Song 2010 IOP Conf. Ser.: Mater. Sci. Eng. 10 012035 Related content - Reliability verification of an assessment tool for outdoor thermal environment Y S Jee, J Y Lim and D S Song - The application of computational fluid dynamics to pedestrian level wind safety problem induced by high-rise buildings Li Lei, Hu Fei, Cheng Xue-Ling et al. - Flow and heat distribution analysis of different transformer sub-stations H Hasini, N H Shuaib, S B Yogendran et al. View the article online for updates and enhancements. Recent citations - Understanding the effect of background awareness in urban wind environment visualizations to minimize information entropy Kyosuke Hiyama et al - BIM-Enabled Structural Design: Impacts and Future Developments in Structural Modelling, Analysis and Optimisation Processes Hung-Lin Chi et al This content was downloaded from IP address 148.251.232.83 on 22/12/2018 at 02:02

Prediction and Evaluation Method of Wind Environment in the Early Design Stage using BIM-based CFD simulation Sumi Lee 1, Doosam Song 2 1 Graduate School, Sungkyunkwan University, Korea 2 Professor, Department of Architectural Eng., Sungkyunkwan University, Korea Correspondence email: dssong @skku.edu Abstract. Drastic urbanization and manhattanization are causing various problems in wind environment. This study suggests a CFD simulation method to evaluate wind environment in the early design stage of high-rise buildings. The CFD simulation of this study is not a traditional in-depth simulation, but a method to immediately evaluate wind environment for each design alternative and provide guidelines for design modification. Thus, the CFD simulation of this study to evaluate wind environments uses BIM-based CFD tools to utilize building models in the design stage. This study examined previous criteria to evaluate wind environment for pedestrians around buildings and selected evaluation criteria applicable to the CFD simulation method of this study. Furthermore, proper mesh generation method and CPU time were reviewed to find a meaningful CFD simulation result for determining optimal design alternative from the perspective of wind environment in the design stage. In addition, this study is to suggest a wind environment evaluation method through a BIM-based CFD simulation. 1. Introduction Growing number of high-rise and high-density buildings due to drastic urbanization is causing a number of problems with respect to wind environment. For instance, wind may blow strong in between buildings, causing difficulty in walking of pedestrians, or to the contrary, wind may never blow because of high-rise buildings and heat and pollutants remain causing discomfort to pedestrians or people resting there. These phenomena even occur at surroundings of about ten story high buildings as well as high-rise buildings [1]. Thus, wind environment needs to be considered in the design stage of high-rise buildings. To reduce damages by wind, proper measures must be taken in the planning stage of buildings by predicting wind environment around the buildings. Open spaces and facilities around buildings must be reviewed for their usages and planned not to displease users from the perspective of wind environment. Additionally, wind environment change by newly constructed buildings must be reviewed in the planning stage to prevent negative influence to the existing building in terms of natural ventilation performance and pleasant wind environment for pedestrians. Most previous studies for evaluating wind environment outside buildings focus on air current analysis around high-rise buildings. R. Yoshie[2], Cheng-Hu Hu[3], et al. reviewed factors such as calculation condition, domain size, grid density and turbulence model which influence CFD simulation results for the air current analysis around buildings, and then they suggested appropriate values for a c 2010 Published under licence by Ltd 1

significant result. Qingyan Chen[4] conducted a research on indoor/outdoor air current and natural ventilation effect using CFD. Besides, there are a number of theses which analyzed indoor/outdoor air current. However, previous studies focus on detailed CFD methodology to analyze wind environment around buildings and there is no reported research to suggest a methodology for the prompt evaluation of wind environment for design alternatives in the early planning stage. Furthermore, Arno Schlueter[5] suggested a simple BIM-based simulation method so that a designer can easily evaluate energy performance of a design alternative and change the design. The purpose of this study is to review evaluation criteria for wind environment to predict and evaluate wind environment around buildings in the planning stage and suggest a wind environment analysis method using BIM-based CFD tools. Specifically, a simple and fast evaluation methodology was suggested in order for a designer to evaluate wind environment for a design alternative in the planning stage. 2. Review of wind environment evaluation criteria The following shows wind environment evaluation criteria defined in previous studies to evaluate environmental effect by the occurrence of strong wind around buildings. 2.1. Penwarden s evaluation criteria The original Beaufort wind force scale defined the relations between wave and wind speed. Present Beaufort wind force scale by Penwarden was modified to reflect the relation between a person and wind speed [6]. (see Table 1). Beaufort wind force scale, which only considers wind speed, has limitations because the impact of wind on a person is determined not just by wind speed as an influenced factor, but also frequency, momentary strong wind and so forth. Thereafter, Penwarden presented a new wind environment evaluation criteria, saying that wind speed is acceptable if the frequency of wind speed over 5m/s is less than 0.2% [7]. Table 1. Penwarden s Beaufort wind force scale Beaufort Number Velocity (m/s) Effects 0,1 0-1.5 Calm, no noticeable wind 2 1.6-3.3 Wind felt on face 3 3.4-5.4 Wind extends light flag, Hair is disturbed, Clothing flaps 4 5.5-7.9 Raises dust, dry soil and loose paper, Hair disarranged 5 8.0-10.7 Force of wind felt on body, Drifting snow becomes airborne, Limit of agreeable wind on land 6 10.8-13.8 Umbrellas used with difficulty, Hair blown straight, Wind noise on ears unpleasant, Windborne snow above head height(blizzard) 7 13.9-17.1 Inconvenience felt when walking 8 17.2-20.7 Generally impedes progress, Great difficulty with balance in gusts 9 20.8-24.4 People blown over by gusts 2.2. Davenport evaluation criteria [8] Davenport suggested a standard to determine whether wind environment of a region is acceptable by using two indexes (see Table 2,3) of Beaufort number and wind frequency for each target area s usage and human activity. For instance, in case of strolling at a park, Beaufort number representing tolerable is 5 and wind speed for it is 21m/s. In that case, if wind over 21m/s is less than 1/wk, wind is tolerable, but if wind over 21m/s is 1 or more/wk, it is not. Evaluated wind speed is average wind 2

speed or effective wind speed( ) at the height of 1.83m and effective wind speed is defined as following equation (1). = (1) : average wind speed for an hour [m/s], : standard deviation for turbulent wind measured for an hour Table 2. Davenfort s Beaufort wind force scale Force Description Wind Speeds (m/s) Mean Limits Specifications 2 Light breeze Gentle breeze 5 4-7 Wind felt on faces 3 Gentle breeze 10 8-12 Leaves and small twigs in constant motion 4 Moderate breeze 16 12-18 Raises dust and loose paper 5 Fresh breeze 21 19-24 Small trees in leaf begin to sway 6 Strong breeze 28 25-31 7 Moderate breeze 35 32-39 Whole trees in motion 8 Gale 42 39-46 Breaks twigs off trees Large branches in motion whistling heard in telephone wires Table 3. Davenport's wind environment evaluation criteria Activity Areas Applicable RELATIVE COMFORT Comfort Tolerable Unpleasant Dangerous Walking fast Sidewalks 5 6 7 8 Strolling, skating Parks, entrance skating rinks 4 5 6 8 Standing, sitting short exposure Parks, plaza areas 3 4 5 8 Standing, Outdoor restaurants sitting long exposure band shells, theatres 2 3 4 8 Representative criteria for acceptability < 1/wk. < 1mo. < yr. Units : Beaufort Number, Temperature > 10 2.3. Murakami's wind environment evaluation standard Murakami suggested wind environment evaluation methods using gust wind occurring frequency and wind change ratio. 2.3.1. A Method using wind change ratio [10]. The method using wind change ratio suggested by Murakami evaluates wind environment by comparing wind speeds before and after the construction of a building. The wind speeds are measured at the height of 10.6m where the National Weather Service measures wind speeds and and are defined as follows. = = Wind environment is warned if the following condition is met. (2) 3

The evaluation method using wind change ratio can evaluate the effect by building on the neighborhood and region. a newly constructed 2.3.2. A method using occurrence frequency of gust wind.[9] Damages by wind and relation to wind were investigated for two years on residents living low-rise residential housings near a fourteen story building and evaluation criteria were suggested as in table 4. Murakami used daily maximum gust wind speed as an evaluation wind speed, considering the life cycle of residents. Three types of wind were used as wind speed index, which are 10, 15, 20m/s of daily maximum gust wind speed for 2~3 seconds at the height of 1.5m. Table 4. Acceptable criteria for wind environment based on occurrence frequency of daily maximum gust wind speed a Level of assessment of strong wind and acceptable exceedance frequency class (at a height of 1.5m) Effect of Areas applicable Daily maximum gust wind speed (m/s) strong wind (example) 10 15 20 Daily maximum mean wind speed (m/s) 10/GF b 15/GF b 20/GF 1 2 3 Area used for purposes Shopping street in 10 0.9 0.08 most susceptible to wind residential area; (37days/ year) (3days/ year) (0.3days/ year) effects outdoor restaurant Areas used for purposes Residential area; 22 3.6 0.6 not too susceptible to park (80) (13) (2) wind effects Areas used for purposes 35 7 1.5 least susceptible to wind Office street (128) (26) (5) effects a When these criteria are applied, values of wind-tunnel experimental maximum mean wind speed may be used as the indicator of wind speed. Maximum mean wind speed can be given by converting the gust wind speed using gust factor. b GF = gust factor (height 1.5m, averagingg time 2-3 s). Area where wind speeds are particularly high(1.6-2.5); typical city area(2..0-3.5). 2.4. Evaluation criteria by Melbourne [11] This is an evaluation method based on annual maximum gust and uses an instantaneous value as wind speed data from the perspective of the effect of gust on a person. Melbourne is the only index using annual maximum gust as a wind speed index. The greatest disadvantage of this method is that evaluation standard was suggested based on small number of measurement data. Table 5. Melbourne wind environment evaluation criteria 23m/s Completely unacceptable 16m/s Generally acceptable 13m/s Generally acceptable for stationary short-exposure activities 10m/s Generally acceptable for stationary long-exposure activities 2.5. Other evaluation criteria Other wind environment evaluation criteria are those by Yoshida [12], Jackson [13], Lawson [14] and so forth and Jackson modified the measurement height of 10m by Beaufort criteria to 2m which is proper to evaluate wind environment near buildings. 4

Table 6. Comparison of various wind environment evaluation criteria Suggested by references Evaluation wind speed Evaluation criteria k a Measuremen t height Penwarden [6] Absolute wind speed Beaufort (1973) - 1-2m Penwarden [7] One hour average wind speed Excess portion (1975) - 1-2m Davenport [8] One hour average wind speed Beaufort + Occurring frequency 1.5 10m Murakami [9] Daily maximum gust wind speed Occurring frequency - 1.5m Murakami [10] Wind speed before and after construction Wind change ratio - - Melbourne [11] Annual maximum gust Excess portion - - Yoshida [12] Annual average wind speed locality + Excess portion - 5-10m Jackson [13] Five minute average wind Beaufort + Occurring speed frequency 3.0 2m Lawson [14] 3 second or 15 minute average wind speed Beaufort + Excess portion - - a k = The coefficient of turbulent (weighting factor) 3. BIM based CFD Simulation to analyze the wind environment around the building 3.1. BIM based simulation tool To make eco-friendly buildings possible, evaluation of buildings on energy consumption and environmental performance must be conducted from the design stage to maintenance to destruction stages. Researches on BIM technology are active these days for the analysis on buildings eco-friendly performance. BIM-based eco-friendliness analysis on building designs has the advantage in saving a lot of time for such analysis using design alternative and building information (dimension, material property and so forth) made by designers in the design stage. This study is to suggest a BIM-based performance evaluation method for wind environment to be used in the design stage. Thus, Design Builder was used for CFD simulations using BIM files. Using BIM files, Design Builder can do simple CFD analysis, building energy load calculation, energy consumption estimation and indoor/outdoor air current distribution and process BIM files in the form of gbxml processed by Revit, ArchiCAD. The program is easy-to-use, can be used to reflect the calculation results of building energy and environment to the design in the early design stage. Figure 1. BIM and wind environment evaluation process 3.2. Case study This chapter covers case study for the wind environment analysis using the BIM-based CFD simulation method suggested by this study. 5

3.2.1. Overview of the target building. The target for the analysis consists of six buildings two tower-type buildings (A) and four flat-type buildings (B, C). A, B and C buildings are respectively 101.5, 70 and 50.4m high 3.2.2. Simulation conditions. 1) Domain Size. As the first stage of the simulation, Design Builder was run and gbxml files were imported. Figure 2 shows the files imported and the drawing only shows the abstract mass of buildings. If how the topography of the area influences wind environment needs to be analyzed, modeling files having both of topography and building data have to be imported. Figure 2. Imported gbxml file After modeling files imported, target analysis area has to be set. This is called a domain setting. Domain size has influence on calculation results and thus, topography, building layout and building heights must be considered in the domain setting. Thus, proper setting of domain size is critical and Hu, et al. [15] suggests as minimum domain size five times x-axis length of the wind obstacle, three times y-axis width of it, and five times z-axis height of it. In this study, this rule was applied and the lengths of x, y and z-axis were set respectively at 683, 443, and 507m. (See Figure 3). 2) Grid generation. The computational grid is a key element in CFD because it determines the level of resolution of a flow field. If it is too big, precise calculation on air current distribution cannot be acquired and on the other hand, if it is too small, the analysis takes exponentially large amount of time. Thus, in this study, three types (10, 5 and 3m) of mesh size were simulated to find and review a proper mesh size for significant results on wind environment of design alternative in the planning stage. Using the automatic grid creation algorithm provided by Design Builder CFD, analysis grids (rectangular solid-shaped grids) were created. Mesh density is maximized at the target calculation area and for other areas, gradually increasing meshes are created in general. Thus, at the center of analysis target area, the maximum mesh size was 10m 10m 10m for Case 1, 5m 5m 5m for Case 2, and 3m 3m 3m for Case 3 and at other areas, grid size was gradually enlarged. The increasing rate of the grids was set at 1.2 to minimize the margin of error between grids [16]. 6

Figure 3. Domain size Figure 4. Grid generation 3) Boundary Condition. Boundary condition setting for computational domain has a critical impact on the accuracy of CFD results. Boundary condition and input condition were set the same for the three cases (see Table 7) and Table 8 shows the number of cells created. Table 7. Boundary condition and input condition Turbulence Model standard k- model Wind direction North Wind Profile V 1 =5m/s, Boundary layer height d(m) =370.0 Flow exponent a = 0.22 (suburban) Temperature Isothermal Table 8. Grid division data Case Case 1 Case2 Case3 (Minimum grid size) 10m 5m 3m Number X cells 69 98 151 Number Y cells 74 87 115 Number Z cells 30 57 92 Max aspect ratio 21.2 12.0 13.3 7

3.2.3. Analysis results. 1) Mesh dependence of wind speed distribution. After the CFD simulation, wind speed data for each case were compared. Wind speeds at the height of 8.4m were compared where the location common for 10m, 5m, and 3m grid division and closest to the ground was selected. Slice location for the comparison of air current analysis results was the area between buildings, comprising C1 parallel to X-axis and C2 and C3 parallel to Y-axis. (See Figure 5). Appendix A and B show the time spent and the specification of computer used. Figure 5. Wind speed data comparison for each grid type Figure 6. Investigated locations (C1, C2, C3) From the perspective of the smallest grid size of 3m, there was a big difference to the grid size of 10m, but the grid size of 5m showed similar results. (See Figure 6). Based on these results, 5m is recommended as the maximum grid size to minimize the calculation time and to get significant data in the design stage. 2) Wind environment performance evaluation. For the wind environment performance evaluation in the early design stage, wind environment at the height of pedestrians and the effect of to-beconstructed building block on nearby wind environment must be analyzed and reviewed. Stationary CFD simulation was conducted in this study to review wind environment, and among many wind environment evaluation criteria above, Beaufort wind scale by Penwarden was used for the wind environment for pedestrians whereas Murakami wind change ratio was used in case of new buildings. 8

a) Wind distribution at pedestrian level with Beaufort wind force scale. To evaluate wind environment for pedestrians, wind speed distribution at the height of 1.5m was shown in figure 7 using Beaufort scale. There are areas in the complex having over 3 Beaufort scale, showing that wind environment surrounding buildings is not good. Furthermore, over 4 Beaufort scale is distributed along the north-south axis near buildings and thus, pedestrians are likely to feel unpleasant when walking there. b) Effect of the new building block on wind environment with Wind Change Ratio suggested by Murakami. In the simulations of this study, reference air velocity ratio (U 1 ) was 3.15 m/s at the height of 1.5m when there was no building in the complex. Based on this, areas having wind change of above 1.10 after the construction of buildings need attention as shown in Figure 8. Figure 7. Wind speed distribution of the complex in Beaufort scale Figure 8. Attentive area in the complex with wind ration of above 1.10 4. Conclusions This study reviewed previous evaluation standards to evaluate wind environment surrounding buildings and selected evaluation standards applicable CFD simulation which is suggested by this study. In addition, mesh generation method, which influences wind distribution result and CPU time in CFD simulation for finding an optimal design alternative from the perspective of wind environment, was reviewed. Such review showed that maximum mesh size can be 5m to get substantially significant results in case of wind environment evaluation for a mega complex with multi-housing building block. Furthermore, when stationary CFD simulation is conducted for the wind environment evaluation in the planning stage, Beaufort wind force scale and Murakami wind change ratio are expected to lead to justifiable results. Thus, Beaufort wind force scale by Penwarden was used for the wind environment evaluation for pedestrians and wind change ratio by Murakami was used for the wind environment change by the construction of new buildings. The BIM-based wind environment evaluation method suggested by this study will be a very useful tool to find an optimal design alternative from the perspective of wind environment in the planning stage by evaluating and reviewing wind environment in the planning stage. Appendices Appendix A. CPU RAM Graphic Card Table A1. Computer Specification AMD Athlon(trn) X2 250 Processor, 3013 MHZ 3.00 GB NVIDIA GeForce 210 (512MB) 9

Appendix B. Table A2. CPU time for CFD simulation Case 1 (10m) Case2 (5m) Case3 (3m) The time required 1 hour 30 minutes 12 hours 20 hours Acknowledgements This work was supported by grant No. R33-2008-000-10027-0 from World Class University (WCU) project of the Ministry of Education, Science & Technology (MEST) and the Korea Science and Engineering Foundation (KOSEF) through SungKyunKwan University. References [1] Takahashi T 1975 Field measurement of wind environment in Meguro-ku street AIJ (in Japaness). [2] Yoshie R, Mochida A, Tominaga Y, Kataoka H, Harimoto K, Nozu T and Shirasawa T 2007 Cooperative project for CFD prediction of pedestrian wind environment in the Architectural Institute of Japan Journal of Wind Engineering and Industrial Aerodynamics 95 1551-78 [3] Cheng-Hu Hu 2005 Using a CFD approach for the study of street-level winds in a built-up area Building and Environment 40 617-31 [4] Qingyan Chen 2004 Using computational tools to factor wind into architectural environment design Energy and Buildings 36 1197-1209 [5] Arno Schlueter and Frank Thesseling 2009 Building information model based energy/exergy performance assessment in early design stages Automation in Construction 18 153-63 [6] Penwarden A D 1973 Acceptable wind speeds in towns Building Science 8 259-67 [7] Penwarden A D and Wise AFE 1975 Wind environment around buildings Building Research Establishment Report HMSO [8] Isyumov N and Davenport A G 1975 The ground level wind environment in built-up areas Prc. 4th Int. Conf. on Wind Effects on Buildings and Structures London 420-22 [9] Murakami S., et. al. 1983 Investigation on statistical characteristics of wind a ground level and criteria for assesing wind induced discomfort : Part-II Characteristics of turburence of city wind at ground level Transactions of the Architectural Institute of Japan 325 74-84 (in Japaness). [10] Murakami S., et. al. 1993 Review for renovation plan of Kyushima district in Nagoya Architetural Research Forum in Japan (in Japaness). [11] Melbourne M H 1978 Criteria for environmental wind conditions Journal of Wind Engineering and Industrial Aerodynamics 3 241-49 [12] Yoshida S., et.al. 1978 Wind environment of Shinjiku area in Tokyo 5th Symposium of Wind Resistant Performance of Structure (in Japaness). [13] Jackson P S 1978 The evaluation of windy environments Building and Environment 13 251-60 [14] Lawson T V 1980 Wind effects on building & Applied Science Publishers [15] Cheng-Hu Hu and Wang F 2005 Using a CFD approach for the study of street-level winds in a built-up area Building and Environment 40 617-31 [16] Cowan I R, Castro I P and Robins A G 1997 Numerical considerations for simulations of flow and dispersion around buildings Journal of Wind Engineering and Industrial Aerodynamics 67-68 535-45 10