The prevailing wind in CBD?
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| 5/9/2005 13:57:29 Beijing CBD |
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The wind in CBD is really out of the ordinary: "If the wind is Grade 3 in normal environment, the wind underneath Jingguang Center with the height of 200 meters will be Grade 6" said by Mr. Zhang Boyin, the professor of Peking University State Key Laboratory of Turbulence Flow Research. The dense skyscrapers form up a microclimate in CBD, which will lead to the unpredictable changes on wind direction and velocity, called as skyscraper funnelling wind. How to establish up a mild microclimate? More and more developers begin to concern about their constructions.
Trouble of the skyscrapers
When a working staff of Jingguang Center was interviewed by the CCTV program of Approaches to Science, he said, even if there was no wind in other places, the wind here is still blowing, and when the wind in other places was strong, the wind here was ferocious. And then, when he pointed at a narrow tunnel, in normal circumstance, the wind here in the tunnel was pretty strong, hard to walk forward, like a standstill. ?If the wind is Grade 3 in normal environment, the wind underneath Jingguang Center with the height of 200 meters will be Grade 6?said by Mr. Zhang Boyin, the professor of Peking University State Key Laboratory of Turbulence Flow Research.
The reason for such circumstance is simple: the wind in front of the building is lifted up, whereas the wind at the back of the building is dropped down. The dropped wind joins in the wind passing through the two sides of the building to form up the vehement wind. If the distance between two buildings is short, under the effect of the narrow tunnel between buildings, the wind passing through the tunnel produces the Funnelling Effect, leading to ferocious gale. For instance, the Gemini plazas ruined in 911 Event, there was only one narrow tunnel between these binary stars at the height of 411.5m. When the sea breeze goes through them, the man-made tornado at the velocity of 120 km/hr is produced.
In the future, there are many skyscrapers will be constructed in the CBD. When people walk on the street of CBD, it seems like he is passing through the valley. When the wind blows to the high-rise constructions within the 3.99 km2 region, the wind direction and velocity will produce an unpredictable change, so as to cause the troubles for the people.
When suffering the gale to attack, the region with tall buildings standing is also puzzled by poor ventilating. In these areas, the low-speed wind will produce the environmental pollution. the pollutants are easily to the piled to affect the health of the human because of the non-circulating airflow for the wind is hard to reach some corners. According to the relative research, the main pollution sources in CBD are from the dense constructions and the crowded traffic vehicles.
CBD wind tunnel test
The city gale and the inferior air circulation in CBD are hard to be solved. When the architects design the buildings, probably they can easily consider many aspects, such as the appearance and the interior decorations, etc., but it is uncertain for them to consider the wind environment issue when the wind collides with the buildings.
To study the wind environment, it is crucial to understand the local weather materials to ensure the wind direction and velocity. Based on the statistics on wind direction and velocity in Beijing from 1991 to October 2000, the wind frequency in Beijing for the whole year changed with the seasons, the wind by north prevailing in the autumn and winter, the wind by south prevailing in the summer, and both the wind by north and by south prevailing in the spring alternately. The wind direction was controlled by the circulating flow in the northwest highlands and valleys, the wind by south prevailing at daytime and the wind by north prevailing at nighttime. The frequency sum of the northwest wind and southeast wind accounted for 21.09% of that in the whole year, almost including all the gale status in CBD. But it is not to say that there are only two wind directions in CBD, in the high-rise skyscrapers standing CBD, the wind environment is a complicated issue. The wind direction and velocity are the important basis for the simulation test.
In order to further understand the wind environment in CBD, and put forward to the appropriate development proposal, the experts of Peking University Wind Tunnel Project Team put the CBD into the maximal environmental wind tunnel to conduct the test after reduce the scale of CBD as a 1:500 model in 2002. This wind tunnel simulated the wind blowing through the ground roughness to form up the atmospheric boundary layer blowing to CBD constructions. Since there are no proposals for many high buildings when the test was conducted, the simulation for a few buildings is only in fact, introduced by Professor Zhang Boyin, the constructions in CBD impacted to each other on wind environment, when a new building is completed, the neighboring constructions will be affected on the wind environment. However, this CBD wind tunnel simulation test almost reflected the integral wind environment situation after the CBD is completely constructed.
The simulation approach of wind tunnel test is to evenly lay a layer of bran coats on the ground of the model. Under the effect of the prevailing wind provided by Beijing meteorological statistics material, when the wind blows, through the movement of the bran coats, the wind action at the height of the foot passengers could be observed. The testers simulated the situations at different wind velocities and took the pictures, so as to show the wind environment in CBD on intuition. In these shot pictures, the bran coats were blown away completely in the tunnels among the constructions, which indicated that the wind in these areas had a good circulating flow, easily to form the strong wind. But in some other places, the bran coats were piled a lot, which meant that the pollutants in these areas were easily to be piled, not easily to be discharged.
Through this test, which areas are easy to form up the gale, and which areas are easily to form the dead corners hard to be blown by the wind, could be recorded by the photographs, so as to help people to reasonably plan the constructions in CBD.
Construction style changes with wind
There was also wind enlivenment problem in overseas CBD, since their buildings were constructed so early that many problems were caused due to lacking of experience. There was once a report from American magazine: one day in January 1982, in NY Manhattan District standing a lot of skyscrapers, when Miss Rose, the financial analyst walked out of the plaza passing through a glass curtain wall mansion, she was blown to a cement parterre by a ferocious gale suddenly came from her back, two arms severely broken. The criminal of this event was the street hurricane, which was led by the negligence of the developers, architects and municipal planning authorities. In the metropolises, such as New York, Boston and Toronto, and so on, such kind of injury events were easily caused when the foot passengers go through the skyscrapers, thus many civil actions were led. At present, in many overseas cities, such as Boston, and San Francisco, etc., the High-rise Buildings Wind Environment Appraisal Regulations have been established already.
Many large-scale proposed projects in CBD are considering of the wind environment issue. Ms. Li Ying, in charge of the design management of Goldfield International Park, said that, the original conceive for the two office buildings are along south to north direction, but after the architects put forward to the suggestion on funneling effect caused by the narrow distance between two buildings, we have to give up the best orientation for one building, and change as the direction of east to west.
Today, many large-scale projects will initiatively ask for the wind tunnel test by the professionals.
The highlighted new site of CCTV was conducted the wind tunnel test by Canada RWDI Laboratory under the suggestion of the structure design consultant of OAP Engineering Consultancy Limited. In view of the new site facing at Fortune center at the western side, as well as other high-rise buildings around the new site, the wind tunnel test for the new site of CCTV considered the most inferior situation. The test result shows that even in the most inferior situation, this building will not cause any troubles on the wind velocity and the people's comfort.
The capacity of Beijing TV Center was quite big, in order to guarantee the good ventilation in the future, Pecking University was invited to perform the wind tunnel test, which showed that, the wind environment of Beijing TV Center has not big problem, only the main body of the building was renovated a little bit, to change the ground floor as the enclosure type from the original opening type, to avoid the strong wind entering and make the air condition more comfortable inside the building.
Nowadays, more buildings have been conducted for the wind tunnel test in the design phase, so as to maintain the good effect, which indicates that the wind environment is getting more and more concern. If each project can make the consideration to the wind environment issue, Beijing CBD would prevent from the detour that the overseas CBD walked before.
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