Building Aerodynamics
Páginas: 14 (3255 palabras)
Publicado: 4 de marzo de 2013
BU/LD/NG AERODYWAM/CS
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Torn Low$on
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- ".-M 32.40
Description of Wind Calm Light Light Light Moderate Fresh Strong Strong Gale Gale Strong Gale Storm Hurricane
Noticable Effect of Wind S m o k e rises vertically Direction shown by Smoke drift but not by vanes Wind felt on faces: leaves rustle: wind vane moves Leaves and twigs in motion: wind extends a Hag Raises dust and loosepaper: small branches move Small trees in leaf sway Large branches begin to move: telephone wires whistle Whole trees in motion Twigs break off: Personal progress impeded Slight structural damage: chimney pots removed Trees uprooted: considerable structural damage D a m a g e is widespread: unusual in the U K Countryside is devasted: only occurs in tropicai countries
0 1 2 3 4 5 6 7 8 9 10 11 12FIGURE 4.01. THE BEAUFORT SCALE
where 4%B6 means that there i a probability of 0.04 that Beaufort Force 6 s winds will be exceeded. This can be interpreted loosely to mean that Beaufort Force 6 wind speeds will be exceeded for more than 4 % of the time. The use of Beaufort Force has hidden the variable mentioned above as a choice of a value for K. Different phenomena used by the Admiral todefine his different ranges were affected by gusts of different sizes. This caused him no problem because he only needed to know the wind speed for a single environment i e a ship in the open sea, and an hourly-average value suited his ..
Building Aerodynamics C h 4
127
purposes. W h e n applied to comfort purposes, different effects on people occur in gusts of different sizes. Tablesdefining the ranges of the Beaufort Scale (see Figure 4.01) define them in terms of the hourly average wind speed. It is well k n o w n that a value of the hourly average wind speed could define the energy expended in walking across a large site, but it is the gust of a second or so duration which turns an umbrella inside-out or makes your coat tails flap. If W i n d Speed (hourly average) and GustSpeed (say 3 second average) are considered rather than W i n d Speed and Turbulence, then a ratio can be defined between the value of Gust Speed, with that averaging time and a low probability of occurrence in an hour of wind, to the hourly-average value of W i n d Speed, for a stated value of the Intensity of Turbulence. In the Bristol method the value ascribed to this ratio is 1.85, whichrelates to a Turbulence Intensity of 3 0 % for a 3-second average gust speed with a 1 % probability of exceedence. The value of 3 0 % was chosen as a typical value in a shopping precinct, an area where most different uses will be found, and to which m a n y wind tunnel investigations apply. The reason w h y consideration of turbulence is necessary is because different building complexes bring wind andgusts d o w n to ground level differently, and even in the same complex, some areas have high turbulence and others low. In a wind tunnel investigation both should be measured. In the Bristol method, the measured Gust W i n d Speed is converted into an "Equivalent Hourly-Average Gust Speed" by dividing it by 1.85, and then both Equivalent Hourly-Average Gust Speed and W i n d Speed are compared tothe same criteria, which then depends only on use. A ratio of 2.0 is used for some areas overlooking water, where people's response to wind is more tolerant.
USE SYMBOL A B C D E F UNACCEPTABLE 6%>B5 2%>B5 4%>B4 6%>B3 6%>B3 1%>B3 TOLERABLE 2%>B5 2%>B4 6%>B3 6%>B2 4%>B2 4%>B2 CRITERIA 10 9 8 6 6 5 9 7 6 4 3 3
Roads and Car Parks People Around Buildings Pedestrian Walk-through PedestrianStanding Entrance Doors Sitting
FIG. 4.02 USER DEFINITIONS
128
LOCAVTM 1 1 2 3 4 5 6 7 5 6 7 8 9 3 15 3 15 3 15 3 1 5 3 15 3 15 3 3 15 3 1 5 3 1 5 3 15 3 1 5 3 15 92.99 98.41 33.30 40.55 31.27 43.47 32.39 41.47 62.97 65.75 96.81 99.56 37.37 48.89 64.81 23.83 24.39 30.98 42.26 29.84 32.52 75.87 79.54 26.50 31.95 2 6.99 1.59 40.39 32.25 39.38 31.00 39.30 31.74 27.92 23.29 3.19 .4 4 45.54 38.97...
^
Torn Low$on
^
- ".-M 32.40
Description of Wind Calm Light Light Light Moderate Fresh Strong Strong Gale Gale Strong Gale Storm Hurricane
Noticable Effect of Wind S m o k e rises vertically Direction shown by Smoke drift but not by vanes Wind felt on faces: leaves rustle: wind vane moves Leaves and twigs in motion: wind extends a Hag Raises dust and loosepaper: small branches move Small trees in leaf sway Large branches begin to move: telephone wires whistle Whole trees in motion Twigs break off: Personal progress impeded Slight structural damage: chimney pots removed Trees uprooted: considerable structural damage D a m a g e is widespread: unusual in the U K Countryside is devasted: only occurs in tropicai countries
0 1 2 3 4 5 6 7 8 9 10 11 12FIGURE 4.01. THE BEAUFORT SCALE
where 4%B6 means that there i a probability of 0.04 that Beaufort Force 6 s winds will be exceeded. This can be interpreted loosely to mean that Beaufort Force 6 wind speeds will be exceeded for more than 4 % of the time. The use of Beaufort Force has hidden the variable mentioned above as a choice of a value for K. Different phenomena used by the Admiral todefine his different ranges were affected by gusts of different sizes. This caused him no problem because he only needed to know the wind speed for a single environment i e a ship in the open sea, and an hourly-average value suited his ..
Building Aerodynamics C h 4
127
purposes. W h e n applied to comfort purposes, different effects on people occur in gusts of different sizes. Tablesdefining the ranges of the Beaufort Scale (see Figure 4.01) define them in terms of the hourly average wind speed. It is well k n o w n that a value of the hourly average wind speed could define the energy expended in walking across a large site, but it is the gust of a second or so duration which turns an umbrella inside-out or makes your coat tails flap. If W i n d Speed (hourly average) and GustSpeed (say 3 second average) are considered rather than W i n d Speed and Turbulence, then a ratio can be defined between the value of Gust Speed, with that averaging time and a low probability of occurrence in an hour of wind, to the hourly-average value of W i n d Speed, for a stated value of the Intensity of Turbulence. In the Bristol method the value ascribed to this ratio is 1.85, whichrelates to a Turbulence Intensity of 3 0 % for a 3-second average gust speed with a 1 % probability of exceedence. The value of 3 0 % was chosen as a typical value in a shopping precinct, an area where most different uses will be found, and to which m a n y wind tunnel investigations apply. The reason w h y consideration of turbulence is necessary is because different building complexes bring wind andgusts d o w n to ground level differently, and even in the same complex, some areas have high turbulence and others low. In a wind tunnel investigation both should be measured. In the Bristol method, the measured Gust W i n d Speed is converted into an "Equivalent Hourly-Average Gust Speed" by dividing it by 1.85, and then both Equivalent Hourly-Average Gust Speed and W i n d Speed are compared tothe same criteria, which then depends only on use. A ratio of 2.0 is used for some areas overlooking water, where people's response to wind is more tolerant.
USE SYMBOL A B C D E F UNACCEPTABLE 6%>B5 2%>B5 4%>B4 6%>B3 6%>B3 1%>B3 TOLERABLE 2%>B5 2%>B4 6%>B3 6%>B2 4%>B2 4%>B2 CRITERIA 10 9 8 6 6 5 9 7 6 4 3 3
Roads and Car Parks People Around Buildings Pedestrian Walk-through PedestrianStanding Entrance Doors Sitting
FIG. 4.02 USER DEFINITIONS
128
LOCAVTM 1 1 2 3 4 5 6 7 5 6 7 8 9 3 15 3 15 3 15 3 1 5 3 15 3 15 3 3 15 3 1 5 3 1 5 3 15 3 1 5 3 15 92.99 98.41 33.30 40.55 31.27 43.47 32.39 41.47 62.97 65.75 96.81 99.56 37.37 48.89 64.81 23.83 24.39 30.98 42.26 29.84 32.52 75.87 79.54 26.50 31.95 2 6.99 1.59 40.39 32.25 39.38 31.00 39.30 31.74 27.92 23.29 3.19 .4 4 45.54 38.97...
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