非定常性氣流剖面對高層建築風載重之初步研究

Preliminary Investigation of Non-stationary Effects on High-rise Buildings due to Profile Transition

李昜(Yang, Li)

指導教授 ： 羅元隆

淡江大學/工學院/土木工程學系碩士班/碩士(2020年)

https://doi.org/10.6846/TKU.2020.00486

摘要

由於台灣位於熱帶與亞熱帶地區，每年夏秋兩季都會有許多颱風侵襲，其所帶來的強風與大雨經常造成重大經濟損失與人員傷亡，因此建築物之設計風載重一直是我國非常重要的一項問題。在近幾年來隨著建築物空間需求的增加、工程施工技術的進步以及梁柱樓板等材料強度提高，使得建築物主要的跨徑、高度逐漸增加、結構體也變得更比以往結構物複雜、建築物表面的披覆物也更加輕薄細長，因此更加提高了結構物受到風載重影響的敏感性。在颱風、暴風雨發生的期間，其剖面變化即非常見的冪次法則所描述之剖面分布形狀，而是具備了隨著時間變化且甚至在接近地表面處出現了類似噴射流的風速特性，在風速變化上以及在風向的變化上十分劇烈。然而目前的設計風載重理論則是以冪次法則做為最根本的假設。冪次法則假設必須同時遵守在風速與風向的定常性下，才能夠進行頻譜分析。也正是因為如此，全世界的實驗室多半發展成模擬定常性大氣紊流邊界層的風洞機具。理論的發展自然也走向定常性的假設。然而，基於真實的大自然現象所發展出的風載重設計理論不應該只有定常性現象的設計，應當是以最為經濟合理且最能保證結構安全性的設計方法，因此需要考慮風力的非定常性現象。
由本研究的實驗分析結果得知，在模擬流場的部分，可使用位於淡江大學風工程研究中心的主動控制型、開放式複數風扇風洞機來模擬不同氣流剖面。此風洞機具由72顆主動控制型伺服馬達排成12 × 6矩陣式風道，可藉由控制每層高度的風扇轉速來調整氣流剖面的形狀，既可模擬出與傳統風洞機具相同的大氣紊流邊界層流場，亦可模擬颱風、暴風雨等鼻型剖面之特殊形狀流場。此外，尚且能夠模擬出流場在極短時間內變化的風速剖面。本研究模擬四種不同垂直分布形狀且低擾動的平均風速剖面氣流，探討此四種不同垂直分布的平均風力下高層建築物風力分布的特性。在定常性風壓試驗中，可以看出高層建築模型受風剖面影響造成的壓力變化，會隨著不同風速剖面其風速最大之位置所產生較大的壓力現象，有便於找出局部構材在非定常性流場所需要加強考慮之位置。而探討風力的部分，可看出不同垂直分布平均風速剖面的風力是不同的，所以首先在風力係數的定義上，應要與傳統上以建築物高度處的平均風速做為參考風速壓位置的作法不同。接著比較不同剖面所造成的基底剪力、彎矩，探討出大氣邊界層類似的剖面並非最為嚴重的風力分布型態。接著利用風洞機具可於短時間內轉換剖面的特性，探討由大氣紊流邊界層垂直剖面於兩秒內變化至暴風雨之鼻型剖面期間，瞬間風力變化所造成的非定常性效應與一般定常性氣流的差異。

關鍵字

非定常性流場 ； 風壓實驗 ； 高層建物 ； 剖面變換

並列摘要

Taiwan is located in a tropical and subtropical region. There are many typhoons in the summer and autumn periods. The strong winds and heavy rains brought by typhoons have caused severe economic loss and sometimes casualties. The issues related to design wind loads for buildings have been increasingly important due to rapid climate changes. During the occurrence of typhoons and thunderstorms, the mean wind profile is rapidly changed and not necessarily consistent with the shape described by the power law. In a real scenario, the mean wind profile changes with time in different periods when a storm passes by. At specific moments, a sudden acceleration in wind flow at lower altitudes near the ground could occur to form a jet-like flow, which may reach an extremely high wind speed. So is the situation of rapid wind direction changes. Such rapid changes in wind speed or wind direction are frequently observed in nature and often called non-stationary winds in wind engineering society. This research tends to examine the non-stationary effect caused by mean wind profile changes via the multi-fan wind tunnel facility. The multi-fan wind tunnel facility is used as the primary tool for this research. This wind tunnel is consisting of 72 actively-controllable servo motors, arranged in a 12 × 6 matrix format. The shape of the inlet flow profile can be adjusted by changing the fan rotation at each height. The 1.3 × 1.3 m cross-section allows possibilities of simulating typhoons, storms, and any other special-shaped flow profiles. Besides, the servo motors are meant to vary the wind speed in a very short time. This particular function allows for generating accelerating and decelerating flow at different heights.
This study simulates four different vertical shapes of mean wind speed profiles for exploring the characteristics of wind distribution of a high-rise building under these four different vertical wind load distributions. From the results based on stationary tests, different mean wind profiles generate different characteristics of wind load distributions. As the wind incidental angle changes, the local differences can be easily indicated to show how the design wind load differs from case to case. By changing any two of the profiles to each other within a specific short time, a non-stationary flow is simulated. In this study, the low-turbulent atmospherical boundary layer flow is transmitted to the low-turbulent thunderstorm flow within two seconds. The comparison results show how the non-stationary effect significantly alters the wind loading in the along-wind and across-wind directions. It is clearly indicated that in the future, the consideration of the non-stationary effects may make a significant contribution to reshape the estimation of design wind loads.

並列關鍵字

Unsteady flow ； wind pressure ； High-rise building ； Profile transformation

參考文獻

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