The Performance-based Wind Environment Analysis in Campus -Taking University of Shanghai for Science and Technology as an Example

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1 Journal of Civil Engineering and Construction 2:2 (2013) The Performance-based Wind Environment Analysis in Campus -Taking University of Shanghai for Science and Technology as an Example Miao Ye 1,a, Li Yang 2, b, Bao-jie He 1,c 1 School of Environment and Architecture, University of Shanghai for Science and Technology, Shanghai, , P.R. China 2 College of Architecture & Urban Planning, Tongji University, Institute for Advanced Study in Architecture and Urban Planning, Key Laboratory of Ecology and Energy Saving Study of Dense Habitat (Tongji University), Ministry of Education, Shanghai, , P. R. China a @qq.com, b yangli.arch@gmail.com, c ivy_league@qq.com Abstract: In this paper, the wind environment of University of Shanghai for Science and Technology is analyzed by CFD simulation technology. With comparison between wind pressure contours, we found that there were many areas where wind pressure was negative and wind pressure in some aisle was too large due to serried buildings. Through comparing between wind velocity contours, we concluded large acreage of low vortex area existed in this campus, especially around the laboratory and bathroom. Then we put forward three measures to improve the bad wind environment, and finally verified one measures as buildings partition is feasible. Therefore, schools can know where the wind environment is adverse for students through CFD numerical analysis results and take measures to improve this wind environment. Keywords: Green school, CFD, Wind environment, Building layout, Numerical analysis 1. Introduction At present, the Green Building evaluation systems which have been mature and influential are Building Research Establishment Environment Assessment Method in Britain, Leadership in Energy and Environmental Design in US, GBTool in Canada and CASBEE in Japan. With sustainable society development, it is not only commercial and residential building that are required to be a Green Building, but also each campus is needed to become Green School. In 1997, Sweden firstly required all campus should become eco-school, and then Sweden put forward a conception Green school in Its main purpose is to apply the environment management measures in the daily management of school according to the idea of sustainable development, based on achievement of basic education [1]. After being continuous improved, all the sources and challenges in the school could be fully used to enhance the environment where teachers and students live [2]. Before long, America, Australia and Hong Kong both enacted Green School Evaluation Systems. For example, America put forward LEED for school, which the evaluation indexes are shown in Table 1. Table 1 LEED standard value proportion distribution Material Sustainable Savingwater atmosphere Energy and Program and Indoor environment location source Value Proportion 22% 8% 27% 20% 23% In the LEED for school, a good wind environment is also essential requirement for all teachers and 41

2 2:2 (2013) Journal of Civil Engineering and Construction students. Only when pedestrian feel wind comfort and safety the wind environment is beneficial to human health [3]. According to some research data, we draw a conclusion that when the wind velocity is less than 1.5m/s at the height of 1.5m there is no influence on human normal activities [4]. This is also presented in Table 2 which is criteria for wind nuisance. On the other hand, poor ventilation will seriously hinder the flow of air resulting in developing a calm zone or vortex zone in some areas [5]. Table 2 Criteria for wind comfort and safety according to NEN8100 P(V>5m/s) Grade Activity Traversing Strolling Sitting <2.5 A Good Good Good B Good Good Moderate C Good Moderate Poor D Moderate Poor Poor >20 E Poor Poor Poor Only a good school design can achieve green school, therefore we had better to conduct an adverse school environment including the wind environment which is always the focus in architecture design. Accidents caused by the wind environment are duly common all around the world [6]. For examples, due to improper design and layout of the construction monomer, pedestrians are struggling to walk because it is possible that strong winds will blow down the broken glass. With the development of cities, more and more high-rise buildings locate at the sides of streets [7]. For the sake of aesthetics, most of high-rise building adopt glass curtain wall. As a result, strong winds converge in streets and corridors, and then form a strong vortex flow. Originally solid glass curtain wall are scraped seriously so that it had a bad influence on the safety of pedestrians below. It is not only the building group that will cause undesirable regional wind climate, but also the single building will deform adverse wind conditions. Some research results show that high-rise buildings have the tend to bring high speed air flow in the concentrated buildings [8], especially, at the corner of those buildings, the speed of the air flow is stronger, forming the parked vortex formation in front of them[9]. That will deteriorate pedestrian-level wind environment around the building and endanger the safety of pedestrian. The building environment in campus is different from the normal residential area or office area. For the comprehensive properties of campus, in order to construct wonderful green school, we should arrange appropriately the layout of all buildings in campus so that the around wind environment is more comfort and safety to the passing teachers and students. 2. CFD simulations: computational model, computational domain, grid and solution parameters In this paper, we adopted CFD methods to simulate the wind environment of campus building. CFD began from the early 30s of the 20th century as a computer simulation technology, which composited physical science, numerical methods and computer graphics in a whole [10]. At present, CFD has been widely applied in all fields. Its main content is to express the original continuous in time and space physical quantity field (such as temperature field, pressure field, velocity field) with a series of finite discrete points set of variables values. Finally, we can get a field variable approximation through solving the algebraic equation set. Compared with traditional theoretical analysis method and experimental measurements, CFD has the advantage of well adaptation and widely application. At present, CFD which is widely applied consists of Fluent Phoenics 42

3 Journal of Civil Engineering and Construction 2:2 (2013) Airpak and so on. This paper utilizes Fluent as the research tool, which can simulate problems from incompressible fluid to highly compressible fluid with a variety of evaluation methods and multiple grids accelerating convergence technology to achieve very good convergence speed and precision. At the moment, Fluent has been widely applied all the field such as aerospace, transportation, automobile manufacturing, and engineering design related to fluid mechanics, owing to the popular good simulation performance. Besides, Fluent could not only accurately simulate the physical phenomena such as air flow, heat transfer and pollution of research objects, but also simulate ventilation system, air quality, heat transfer, issues of pollution and comfort of the air flow. Turbulence model is one of the most components of CFD [11]. General CFD has many various levels of turbulence models; commonly contain algebraic model, an equation model and two equation models, the turbulent stress model and large eddy simulation and so on. Wind flow in campus generally belongs to the incompressible flow, low and weak buoyancy turbulence. The commonly used mathematical models are the standard κ - ε model and large eddy simulation (LES) and so on. In comparison, the cost of standard two equation model κ - ε model calculation is low, with small fluctuations, high precision in the numerical calculation. As a result, the model is widely applied in the low-speed turbulent numeral analysis because it is easy for network adaption. Because the building in the campus is much complex, we adopt the unstructured grid namely T gird to mesh generate the around fluid. Therefore, this context takes standard κ - ε model. 3. The selection of research object This context chose University of Shanghai for Science and Technology as the research object. Figure 1 is the aerial view of the campus. The area which red line surrounds is the object we analyzed. Due to the empty surroundings, we are able to simplify the model. Therefore, when we utilize Gambit to set up the model, only choose the simple model as the research object. At the same time, choose the profile of 1.75 m high to analyze [12]. After setting up the model, then it is time to define the boundary condition. Among this process, defining the wind speed and wind direction is much more important. Fig. 1 Aerial view of University of Shanghai for Science and Technology In this paper, according to actual condition of Shanghai, we made a statistics about the season wind in Shanghai. As all we know, Shanghai is a hot summer and cold winter region, belonging to subtropical maritime monsoon climate. Moreover, Shanghai is near the east ocean so that the sea wind through Shanghai is much stronger. As a result, how to build a benign wind environment is essential necessary for all the campus in Shanghai. Based on the data of International Meteorological Energy Source, the authors made statistics of the wind frequency for a year in Shanghai. The diagram is following: 43

4 2:2 (2013) Journal of Civil Engineering and Construction Table 3 wind speed and wind direction frequency of a year in Shanghai Speed The total frequency of km/h km/h km/h km/h km/h Direction wind direction 0º º º º º º º º º º º º º º º º Frequency of wind speed From Table 3, the most occurrence frequency of wind speed in a year in Shanghai so that we could choose the wind speed of 5m/s in this case, and the angle of incidence is separately 0º,150º. As all we know, in summer at Shanghai, the wind direction is close 0º, in winter is similar with 150º. Besides, wind velocity varied with height as this function relationship which is shown below and Figure 2: v v0 H H o v Where: 0 is the speed we choose as 5m/s according to statistics. H is the height. H o is the standard reference height, set as 10m/s. is the Ground roughness coefficient, set as

5 Journal of Civil Engineering and Construction 2:2 (2013) Fig. 2 Wind velocity variation with height According to the meteorological statistical results, we began to define the boundary condition of the model. Firstly, in term of the inlet angle of 0º, we defined the east of model space as velocity inlet boundary conditions, defined the top, west, north, and south of the model space as the Outflow free discharge boundary. In the same way, when the inlet angle is 150º, we defined the north and west as velocity inlet boundary conditions, and the top, east and south of the model space as outflow free discharge boundary. In addition, assume that the flow on the surface of the stream has been fully developed and flow has been restored to normal flow without a building block, namely the export relative pressure is zero. The surfaces of buildings and ground are fixed and unmoved, so it is allowed to adopt the no slip wall condition, which is one boundary conditions can restricted areas of the fluid and solid. For a viscous fluid, it is better to use adhesion conditions. In other word, fluid velocity at the wall is approximately same to the speed at the wall, and the speed of no-slip wall and fluid velocity at the wall all are zero. The dimension of building group in the campus is 140m*140m*18m as shown in Figure 3. To satisfy the requirements for upstream and downstream domain length, different computational domains have been made for simulations with different wind different wind direction [13]. The upstream domain length is kept as short as possible to avoid the occurrence of unintended stream wise gradients. The downstream domain length is taken long enough to allow the development of the wake region behind the buildings, which is beneficial for convergence of the simulations. The domain height for all inlet angles is 90m, which is the 5 times the height of the highest building. For wind direction 150º, the domain dimension are L W H= m 3. For wind direction 0º, the domain dimension are L W H= m 3. Fig. 3 Simplified model of the research object 45

6 2:2 (2013) Journal of Civil Engineering and Construction During the process of building model, the element is Tet/Hybrid and the type is TGrid when meshing. Special care was given to the development of a quality and high-resolution grid as much as possible. As a result, for different domain dimension, the number of cells that computational grid in this case consist of is different. For wind direction 0º, the number is ; for wind direction 150º, the number is ; For wind direction 330º, the number is The results of CFD simulations In this study, we chose the pedestrian height (Z=1.75m) as the surface to research. In the following the analysis result, we selected the relative total pressure and relative tangential velocity contour figures for three directions. Fig. 4 The total wind pressure contour for direction 0º Fig. 5 The wind velocity contour for direction 0º 46

7 Journal of Civil Engineering and Construction 2:2 (2013) Fig. 6 The wind velocity vector diagram for direction 0º Fig. 7 The total wind pressure contour for direction 150º Fig. 8 The wind velocity contour for direction 150º 47

8 2:2 (2013) Journal of Civil Engineering and Construction Fig. 9 The wind velocity vector diagram for direction 150º 4.1 wind pressure comparison In figure 4,the wind pressure value range is from -7Pa to 7Pa. On the contrary, the wind pressure value range is from -6Pa to 24Pa, which is larger than figure 4. As a result, it is necessary to take some windbreak measures, such as partition walls so that cold air permeation due to large leeway and thermal consumption can be avoided. What s more, area of negative pressure in figure 7 is larger than in figure 4. Those negative will deform suction at the surface of buildings resulting in more pollution air into buildings. If wind pressure at windward side of buildings is positive and leeside is negative, it is very easy for buildings to happen deformation, especially the slab joint. When the difference between positive and negative pressure is much great, the slab joint will crack. At the same time, negative at the surface of buildings has a great influence on insulation board because too large negative pressure could lift insulation board. In figure 4, at the right bottom of research area where there is a playground, the maximum of wind pressure is 17Pa, which is harmful to students who take exercises on the playground. It is better to plant some trees to shelter from strong wind. In figure 4 and figure 7, because building A, B, C are vertically arranged side by side and A, G, H are horizontally arranged side by side, wind pressure in the aisle between buildings is larger than that around buildings. When pedestrian pass through the aisle, they can feel strong thrust owing to pressure difference so that it is inconvenience for them to act. Therefore, it is necessary to ensure enough separation between buildings to avoid discomfort due to narrow aisle. Besides, building A is a laboratory. However, there are negative pressure zone on the left of A in figure 4 and the back of A in figure 7, which is very unfavorable for pollution emissions 4.2 wind velocity comparison In figure 5, the value range of wind speed is from -2m/s to 6m /s, and in figure 8 is from -5m/s to 2m/s. These two value ranges are similar, but the main in figure 8 is negative direction. The area of wind velocity absolute value less than 1m/s in figure 8 is larger than in figure. As we know, if wind speed is less than 1m/s, it is easy to deform low vortex area which is bad for pollution emissions. On the other hand, the maximum of wind speed in figure 5 is 6m/s, larger than 5m/s as shown in Table 2. As a result, blowing dust will affect human breathe and keep out people s horizon. Therefore, school should take some shelter measure in summer while they strengthen ventilation effect in winter. In figure 5 and figure 8, there are areas where wind speed is 0 m/s at the leeside of buildings A, B, C. 48

9 Journal of Civil Engineering and Construction 2:2 (2013) Among them, A is laboratory, and B and C are dormitories, which all have a strict requirement for ventilation. It is necessary to set ventilation equipment at the leeside of those buildings. Besides, in the aisle between buildings D, E, F, wind velocity is less than 1m/s and even 0 m/s. However, this aisle is the only way for students to enter and come out from the bathroom. As a result, pollution air discharged out from bathroom through windows can t be dispersed out of this aisle. It is better to increase the height of the windows in the bathroom so that pollution air will not gather together in this aisle to harm students health. 5. Strategies to improve wind environment in school 5.1 Campus landscaping The main problem which wind environment researches is the air flow around between the surface buildings and environment surrounding. Therefore, influence of vegetation cover on the wind environment cannot be neglect. Moreover, wind shield that vegetation deformed is able to strengthen the wind resistance to decrease wind speed in the downstream. On the other hand, vegetation can reduce energy consumption for the reason that it adjusts heat and moisture balance of environment through shading and evaporation to absorb sun energy. Finally, in summer, trees shelter sunshine to reduce sun radiation. As a result, temperature of buildings surface and underlying surface is decreasing to stop heat transportation and secondary radiation of underlying surface. In winter, as a wind barrier, vegetation can effectively weaken wind velocity to provide pleasant wind environment. 5.2 Building openings Building openings have an important influence on building wind load. In general, openings can reduce wind effect on the surface of buildings. Due to openings, average wind pressure will be reduced for the decrease of loading area. After considering the building facades effect, the dimensions and layout of openings also should be taken into account especially the layout, which have an obvious weaken effect to average wind pressure at the surface of openings. However, not all opening can reduce wind pressure effectively. Researches don t show that the smaller dimension of openings is, the stronger enhancing effect of wind pressure is. Only the increase effect can be most obvious when the dimension of openings is median. Moreover, if the position of openings is too low, the wind speed in the openings will be larger than that of top surface. 5.3 Building partition Since buildings in school have been constructed, it is feasible to setting shelters at the windward side of buildings. There are two improvement projects as follows: Setting wind shield In order to weaken strong airflow around the corner of high-rise buildings, it is feasible to set wind shield at the corner of balcony. Research data show that wind velocity at the corner of building will decrease considerably after setting wind shield. Therefore, wind shield is a much effective measure to against strong wind Setting windward wall Setting some windward wall to stop strong wind into the aisle is one of the methods. The windward can not only stop strong wind blowing into buildings, but also keep dust out of buildings. When setting windward wall at the front of gap between buildings, it is obvious that wind velocity has decreased to a great extent. 49

10 2:2 (2013) Journal of Civil Engineering and Construction In this paper, we chose the measure of setting windward wall to conduct a numerical simulation. We set two windward walls with dimension of 10m*0.24m*2.5m in front of gap between building A, B, C as shown in Figure 10. After obtained the wind pressure and wind speed contour for direction 150, we took a comparison between the former and the improved as follow. Fig. 10 Simplified model of the research object after improved Fig. 11 The total wind pressure contour for direction 150ºafter improved Fig. 12 The total wind speed contour for direction 150ºafter improved 50

11 Journal of Civil Engineering and Construction 2:2 (2013) Wind pressure comparison: Comparing between Figure 7 and Figure 11, wind pressure value range is from -13Pa to 17Pa in the latter, but from -6Pa to 24Pa in the former. Therefore, due to the windward walls, wind pressure has decreased to a great extent. Especially, most wind pressure on the left of buildings A, B, C is 15Pa to 12Pa in figure 7, but is 14Pa to 11Pa in figure 11. Besides, wind pressure in the area beneath building H where the playground locates is -1Pa, less than 6 Pa before setting windward wall. Wind velocity comparison: Owing to wind speed value are the some in figure 8 and figure 12, we chose color to compare between the former and the latter. The acreage of yellow color beneath building C in figure 12 is smaller than that in figure 8, namely, wind speed around building C decreased obviously after setting windward walls. Moreover, due to the windward walls, the color area at the bottom right of building H disappears in figure 12, that is, wind velocity in this area has a great diminution and then wind environment has improved with windward walls. At the same time, there is a great difference for wind speed at the aisle between building A and B. After improvement with windward walls, a yellow area appears at the aisle between A and B, where wind speed is 2m/s lager than the before as 0m/s. As a result, pollution air in laboratory A can be discharged well. Finally, there is also a yellow area in the aisle between building D, E, F, which is beneficial to exclude vapor and pollution of bathroom. Above all, after setting two windward walls, wind environment in this school has gained great improvement because wind pressure has decreased obviously. On the other hand, when the wind speed around playground and building C has diminished so much, wind speed beside laboratory and bathroom has a great increase. In a word, setting windward walls at windward side has a good influence on wind environment in a school. 6. Conclusion By CFD computer technology, authors make an analysis of a university campus wind environment. In this study, two most common wind speeds are selected to load the building group in the schoolyard. As a consequence, we gained two wind pressure contours and two wind velocity vectors. Through comparing these two wind pressure contours, we draw a conclusion that some place where wind pressure is too high is necessary to take measure against strong wind and wind environment around laboratory is adverse to discharge pollution air. At the moment, with horizontal comparison between two wind speed vector pictures, we find out enough width between buildings is the guarantee for the comfort and safety of wind environment and. Finally, we put out three measures to improve wind environment. And chose the third to conduct a numerical simulation so that the result proved this measure really can improve wind environment. With the development of CFD technology, it will be more and more widely applied in the architectural design, especially the numerical analysis of wind environment. Besides, CFD is able to simulate pollutant s distribution and concentration. Therefore, in terms of evaluation for Green school, we are no longer limited to simple rules and criterions., but could rely on CFD to evaluate more scientifically and authority. Only achieve this can school administrator be stimulated to construct an ecological and energy saving campus environment so that students will enjoy a high-quality life. 7. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Subject 51

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