Wind Flow Effects on a 56-Story Tall Building and its Surrounding Environment Authors: Omid Esmaili, Graduate Student, University of Tehran, School of Engineering, Department of Civil Engineering, Tehran, 1517756313, [email protected]Siamak Epackachi, Graduate Student, University of Tehran, School of Engineering, Department of Civil Engineering, Tehran, [email protected]Arastoo Ardakani, Graduate Student, University of Tarbiat Modares, School of Engineering, Tehran, [email protected]Mahdi Samadzad, Graduate Student, University of Tehran, School of Engineering, Department of Civil Engineering, Tehran, [email protected]Rasoul Mirghaderi, Assisstant Professor, University of Tehran, School of Engineering, Department of Civil Engineering, Tehran, [email protected]Ali Asghar Taheri Behbahani, DynaSys Consultant Engineering Co., Tehran, [email protected]ABSTRACT In recent years, new combinations of shape, height and configuration of buildings have induced intense near ground wind flows, which can cause unacceptable human discomfort. Therefore, the wind environment in public access ways and leisure areas has become a major design consideration in new and existing building complexes. Recently, Computational Fluid Dynamics (CFD) has become a powerful tool for the study of environmental problems, as this technique is more feasible and cheaper than other methods, namely the wind tunnel technique. This paper deals with one of the most common situations of urban planning, which lead to the amplification of the ground level wind speed around a 56-story tall building with especial architectural plan which consists of three wings with identical plan dimensions of nearly 48 meters by 22 meters which are placed at 120 degree from one another in the vicinity of three other 36-story tall buildings with rectangular architectural plan. A CFD program FLUENT, is used to illustrate the effectiveness of these building arrangement in increasing the wind speed and the correspondent discomfort level. Some design practices for the avoidance of severe induced wind speeds around buildings will be inferred from the present analysis, which can be useful for engineers, architects and urban planners. KEY WORDS: Pedestrian comfort, Wind flow, CFD, Tall buildings
Transcript
Wind Flow Effects on a 56-Story Tall Building and its Surrounding Environment
Authors: Omid Esmaili, Graduate Student, University of Tehran, School of Engineering, Department of Civil Engineering, Tehran, 1517756313, [email protected] Siamak Epackachi, Graduate Student, University of Tehran, School of Engineering, Department of Civil Engineering, Tehran, [email protected] Arastoo Ardakani, Graduate Student, University of Tarbiat Modares, School of Engineering, Tehran, [email protected] Mahdi Samadzad, Graduate Student, University of Tehran, School of Engineering, Department of Civil Engineering, Tehran, [email protected] Rasoul Mirghaderi, Assisstant Professor, University of Tehran, School of Engineering, Department of Civil Engineering, Tehran, [email protected] Ali Asghar Taheri Behbahani, DynaSys Consultant Engineering Co., Tehran, [email protected] ABSTRACT In recent years, new combinations of shape, height and configuration of buildings have induced intense near ground wind flows, which can cause unacceptable human discomfort. Therefore, the wind environment in public access ways and leisure areas has become a major design consideration in new and existing building complexes.
Recently, Computational Fluid Dynamics (CFD) has become a powerful tool for the study of environmental problems, as this technique is more feasible and cheaper than other methods, namely the wind tunnel technique.
This paper deals with one of the most common situations of urban planning, which lead to the amplification of the ground level wind speed around a 56-story tall building with especial architectural plan which consists of three wings with identical plan dimensions of nearly 48 meters by 22 meters which are placed at 120 degree from one another in the vicinity of three other 36-story tall buildings with rectangular architectural plan. A CFD program FLUENT, is used to illustrate the effectiveness of these building arrangement in increasing the wind speed and the correspondent discomfort level. Some design practices for the avoidance of severe induced wind speeds around buildings will be inferred from the present analysis, which can be useful for engineers, architects and urban planners. KEY WORDS: Pedestrian comfort, Wind flow, CFD, Tall buildings
INTRODUCTION The wind-induced pedestrian discomfort in urban areas has greatly attracted research over the past few decades but the problem is far from being closed. Due to the limited land resources, together with new technologies and designers’ creativity, many buildings with new shapes, heights and spatial arrangements continue being erected with resulting wind flows not well tolerated by pedestrians in public recreational areas. Because of the frictional drag of the earth surface, wind speeds at the ground level are much lower than in free atmosphere (gradient wind) and are a function of the surface roughness − the rougher or more built up area, the lower the speed near the ground. There is, however, an exception to the rule. Tall, slab-like buildings tend to deflect wind down into previously sheltered areas. The resulting increased wind speeds near the ground are annoying and at times dangerous to pedestrians. This problem is becoming more widespread with the increasing number of tall buildings. Indeed cities with many tall buildings have become known as "windy cities."
However, in urban areas, due to the interference of buildings, the near ground flow exhibits sudden changes in speed and direction, which are difficult to predict. The complex interaction between the wind’s characteristics (mean speed, turbulence, orientation, vertical gradient) and the building obstacles (number, shape, height) can create at the ground level zones of overspeed and vortices in access ways and open spaces such as plazas and malls. A detailed review of the most common situations that lead to the amplification of the ground level wind speed (e.g. corner effect, wake effect, courtyard effect, channel effect) can be found in Gandemer (1975). This severe and unexpectable amplification of the wind speed near the ground can become annoying and dangerous to pedestrians. The evaluation of the acceptability of a ground level wind environment is not an easy task as it involves not only questions of safety and impediment of movement, but also the highly subjective question of human comfort.
In fact, the comfort or discomfort depends on factors generally difficult to quantify, such as the type of activity and dress, weather conditions and even physical and psychological state of the pedestrian. The first attempt to categorize wind speed acceptability, based on its physical and mechanical effects, is the classical Beaufort Scale. Later, Davenport (1972) attempted to go one-step further and found limits of comfort depending on the type of activity (and associated locality) and wind speed frequency. In the present paper, the wind environment around a complex of three 36-story square-shaped buildings in the vicinity of a 56-story star-shaped building will be investigated through the commercial CFD code FLUENT. The criteria used to assess of the local discomfort risk takes into account not only the mean wind speed but also the effects of turbulence as suggested in Gandemer (1975).
PROBLEM DESCRIPTION As it is illustrated in the Figure 1 and Photo 1, the urban area consists of a 180-meter tall building located in the vicinity of three other 110-meter buildings. All these buildings are sourounded by other smaller ones which are neglected in the CFD numerical simulation.
The CFD modelling was performed with the commercial CFD code FLUENT, which is a general purpose code of the type likely to be used by wind engineering practitioners, based on the control volume method and the PISO algorithm, the adopted solution method for the velocity-pressure fields. The GAMBIT software was used as a mesh generator program to mesh the main body of the domain with four node elements. The standard k-ε turbulence model was used to simulate the turbulence effects.
For the inlet mean velocity, a power law profile Uz/U0=(z/z0)α, was adopted to represent better the real conditions of the wind airflow. An exponent of α=0.28, for this law and a turbulence intensity of I=16%, were chosen to express roughness conditions typical of an urban area. Outlet conditions in terms of zero normal gradients for all quantities were adopted. Also the no slip condition were considered for the ground boundary and all solid obstacle surfaces.
As it is depicted in the Figure 2, the numerical domain extended approximately 20 times the length of the maximum dimension of obstacle surfaces upstream and downstream, which is about 1000 meters. FIGURE 1 – ARRANGMENT OF BUILDINGS IN THE SELECTED URBAN AREA
PHOTO 1 – ARRANGMENT OF BUILDINGS IN THE SELECTED URBAN AREA
Urban Plan Urban 3D View
1st Prominent Wind Direction
3rd Prominent Wind Direction
W
SW
NW
SW
W
NW
2nd Prominent Wind Direction
FIGURE 2 – THE NUMERICAL MODEL AND GRID RESOLUTION WITH APPLIED WIND DIRECTIONS As it is depicted in the Figure 2, the wind flow direction is applied in three prominent ones:
1- West-East direction 2- NorthWest-SouthEast direction 3- SouthWest-NorthEast direction In order to evaluate the impact of the flow characteristics on the buildings environment
which are mainly pedestrains, the following parameter was considered as a pedestrain comfort parameter: Ψ = (U + √2/3k)/Ur (1+Ir) (1)
where U is the local mean velocity, k is the turbulent kinetic energy, I is the turbulence intensity, and the subscript r, means reference conditions. This comfort parameter compares local characteristics of the flow around buildings at a prescribed level with the one that would occur at the same level but with no buildings. Besides the mean velocity U, this parameter takes into account the effect of turbulence in the assumption that it follows an isotropic development, in accordance with the k-ε turbulence model used. The higher is this parameter the higher is the risk of local discomfort induced by the presence of buildings. In the present simulations the parameter ψ was performed at a reference level of 2m above the ground, with Ur and Ir taken from the upstream flow. The mean wind speed at height of 10 meters in the meteorological stations is about 7.5 km/hr.
In each CFD model, the value of Ur, is chosen about 1.5 m/sec. based on the power law profile Uz/2.1=(z/10)0.28.
1000 meters
1st Prominent Wind Direction
3rd Prominent Wind Direction
2nd Prominent Wind Direction
RESULTS AND DISCUSSION As mentioned before, presence of one or few tall buildings near a group of low-rise buildings result in high speed wind in the passages and streets around tall buildings. This leads to discomfort to the pedestrian walking and also to the cyclists and two wheeler drivers.
Comfort or discomfort is an intangible as well as relative phenomenon. It is not possible to exactly define comfort or discomfort level of various physical quantities like temperature, humidity and wind speed. Whereas an individual may feel quite comfortable at a temperature and humidity condition, another individual may complain for his/her discomfort. Similarly wind speed, which may cause discomfort to one person, may be accepted by another person without any complain.
It is not only a particular value of wind speed, which may cause discomfort, but there are other parameters which combinely result in discomfort. They include gustiness, duration, mean wind speed, frequency of occurrence and also the fact that the mean wind speed is achieved in a short duration, i.e. relatively sudden or over a long duration.
Generally a wind speed (U) above 5 m/sec. is considered as uncomfortable wind speed. However wind speed above 10 m/sec. definitely causes unpleasantness and a speed above 20 m/sec. is dangerous (Table 1).
TABLE 1 – SUMMARY OF WIND EFFECTS
Horizontally accelerated flows between three neighboring towers may cause an unpleasant windy ground level pedestrian environment (Figures 3,5,7), which could also be aggravated by ground topography. By inspection of the available wind data, we can find a dominant wind direction that can be used to align the building on the site so as minimize these accelerated flows in highly populated pedestrian areas. Use of porous screens and proper plantation can also improve the studied wind environment. The variation range of calculated velocity from CFD modelings are shown by a box in the Table 1. It can be distinguished from Figures 3,5 and 7, that the case in which wind flow is NorthWest-SouthEast is more effective than the West-East direction and both of which are more effective and frequent than the SouthWest-NorthEast direction. The main portion of the influenced regions are express ways which are frequently known as a main productive source of air pollution in the selected urben area by having a long period of heavy traffics.
FIGURE 3 – THE MAP OF VELOCITY WITH WEST-EAST APPLIED WIND DIRECTION (1.5 M/SEC)
FIGURE 4 – THE MAP OF Ψ WITH WEST-EAST APPLIED WIND DIRECTION
In Figures 4, 6 and 8 contour maps of ψ are presented respectively for multiple wind directions. These representations put in evidence the importance of the channel effect in increasing the discomfort risk in an urban area where buildings are arranged in consecutive sets. In passages between buildings oriented approximately in the wind direction, the maximum value of ψ occurs at the flow channel created between them and is about 30% or 100% higher than the upstream condition, depending on the width of inter-building space.
FIGURE 5 – THE MAP OF VELOCITY WITH NORTHWEST-SOUTHEAST APPLIED WIND DIRECTION (1.5 M/SEC)
FIGURE 6 – THE MAP OF Ψ WITH NORTHWEST-SOUTHEAST APPLIED WIND DIRECTION
It is interesting to notice that as result of the higher channel effect the area affected by values of ψ greater than 1 is more extended in the first and second situations, where the effect is felt even at the very extensive positions. In second and third cases it is also visible the corner effect and its importance to the amplification of ψ near the buildings: values up to 1.95 and 1.8 are reached for cases 1 and 2, respectively.
Concerning the gaps between buildings, a sheltering effect occurs in all situations, giving rise to low values of ψ, which denotes safe areas in terms of risk of discomfort.
FIGURE 7 – THE MAP OF VELOCITY WITH SOUTHWEST-NORTHEAST APPLIED WIND DIRECTION (1.5 M/SEC)
FIGURE 8 – THE MAP OF Ψ WITH SOUTHWEST-NORTHEAST APPLIED WIND DIRECTION CONCLUSIONS AND RECOMMENDATIONS When two or more buildings are constructed in proximity, the fluid flow surrounding the buildings may be significantly deformed and of a much complex nature than usually acknowledged. Knowing the strong dependence of comfort on velocity and turbulence, it is of practical interest to study these flow features associated with these building arrangements and hence to assess the comfort conditions on the neighbor pedestrian circulations. In this paper a real arrangement of tall buildings in a urban area was investigated in terms of the discomfort
risk induced at the pedestrian level. Factors of wind speed amplification were investigated in the selected area, through two simulated cases; namely, corner effect and channel effect.
Reducing the height of the new buildings or increasing their spacement perpendicularly to the flow direction, in order to have wider channels, are adequate solutions to decrease the magnitude of the wind speed amplification. In the selected area, the more affected zones are the ones which are express ways with considerable productivity of air pollutants. Through CFD modeling of tall buildings possible arrangements like the one in the selected area, that would be possible for urban designers to plan the most comfortable and harmless building arrangements in front of some natural phenomenon like windy condition. ACKNOWLEDGEMENT
The International Construction Development Co.( I.C.D. CO ), Iran, has completely supported and funded the project “Evaluation of wind flow effects on a 56-Story Tall Building and its Surrounding Environment” besides the project of evaluation and rehabilitation of Tehran Tower. REFERENCES [1] Davenport, A.G., "An approach to human criteria for environmental wind conditions", Colloquium on building
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[3] Rodriguez, M., Gomes, M.G., Piedade, A.C., "Wind environment around building complexes".
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[6] Miles, S., Westbury, P., "Practical tools for wind engineering in the built environment", QNET-CFD Network newsletter, Vol. 2, No. 2, 2004, 11-14.
