透過您的圖書館登入
IP:34.201.69.22
  • 學位論文

加速氣流對於二維方柱模型之氣動力行為探討

Investigation on the Aerodynamic Behavior of Two-dimensional Square Cylinder under Accelerating Flows

指導教授 : 羅元隆
共同指導教授 : 張正興(Cheng-Hsin Chang)

摘要


隨著現今人口密度提高造成建築物高度逐漸上升,對於高樓建築物之安全性以及舒適性的要求提升,結構物抗風之設計成為相當重要的一環。然而世界各地對於風載重之設計皆有一定程度之發展,有著相當多成熟之理論以及規範,不過皆是基於較穩定之風場狀況來進行探討,忽略許多實際存在於大自然中急劇變化的天氣現象,使得在設計上趨於保守,因此颱風、雷雨等急劇變化的天氣特性對於建築物之影響,將會是未來著重探討的項目之一。 本研究使用淡江大學複數風扇風洞實驗室,探討當氣流在急加速的變化下對於二維方柱之氣動力現象。實驗部分可分為兩階段,分別為流場模擬以及氣動力實驗。在流場實驗中,定常性流場設定13種不同的均勻風速剖面;非定常性流場透過調整初始風速、結束風速及加速時間,來定義四種不同的變換方式;氣動力實驗則採用深寬比1之二維方柱模型,依照上述之流場設定進行表面風壓之量測。數據分析上,透過比對算術平均(Arithmetic average)以及集合平均(Ensemble average)之概念,說明數據在定常性及非定常性下之一致性以及正確性。針對渦散特性之探討則使用短時傅立葉轉換(STFT)以及連續小波轉換(CWT)來進行比對,進而找出在時頻分析上的最佳方法。最後透過Yang and Mason(2019)中所提及的無因次化加速度(ap),針對各項氣動力參數進行數據上之計算與探討。 結果顯示,無因次化加速度(ap)之最大值可以用來表示流場開始進行加速的時間點,在初始風速小的Case中ap數值會越大,其所對應之平均及擾動風壓係數數值下降幅度也越大。當ap所表現出之斜率越大,則係數變化之幅度隨之提升,而帶寬越小其回復至定常狀態所需之時間會越快。在加速時間延長的Case中,由於變化之幅度偏低,在各項參數幾乎沒有變化。在渦散特性的比對中,可以看出使用STFT之方式可以有效地進行各時間段的頻率切分,觀察到史特赫數隨著急加速而產生出不穩定之變化的現象,最後以量化的方式呈現出非定常性與定常性結果差異之倍數。

並列摘要


Nowadays with the density of population, the building's height is gradually increasing. The requirements for the safety and comfort of high-rise buildings are improved, so the wind-resistant design of structures has become a very important part. However, the development of the wind load design in various countries is quite mature. It is based on relatively stable wind conditions, ignoring many other dramatic changes phenomenon that exists in nature, lead the building’s design to tend to be conservative. Therefore, the impact of typhoons, thunderstorms, and other rapidly changing weather characteristics on buildings, will be an important project that should be discussed in the future. This study uses the multiple fan wind tunnel laboratory of Tamkang University to investigate the aerodynamic phenomenon of the two-dimensional square cylinder under the accelerative flow. The experiment can be divided into two stages, flow simulation, and aerodynamic experiment. In the stage of flow simulation, first, set up 13 different uniform wind speed profiles under the stationary flow; then, the four different transformation methods by adjusting the initial wind speed, ending wind speed, and acceleration time be defined In the non-stationary flow The aerodynamic experiment uses the two-dimensional square cylinder model and measures the surface pressure of each flow profile case. In data analysis, compare the concepts of arithmetic average and ensemble average to illustrate the accuracy and correctness of data under stationary and non-stationary. For the discussion of the vortex shedding characteristics, short-time Fourier transform (STFT) and continuous wavelet transform (CWT) is used for comparison, and then found the best method in time-frequency analysis. Finally, through the dimensionless acceleration (ap) mentioned in Yang and Mason (2019), discuss the aerodynamic coefficient. The results show that the maximum value of the dimensionless acceleration (ap) can be used to indicate the point in time that the flow starts to accelerate. In the case of the lower initial velocity, the value of ap will be larger, also the magnitude of the average and the R.M.S. of pressure coefficients decrease is greater. The greater slope ap, the magnitude of the coefficient change will be larger. The shorter of bandwidth will be the return faster to a steady-state. In the case of lengthening acceleration time, cause the low amplitude of the change, there is almost no change in each coefficient. In the vortex shedding characteristics, it can be seen that the use of STFT can effectively divide the frequency of each time, and it has been observed that the Strouhal number changes instability during the velocity ramp-up process. Finally, using the method of quantification presents the difference between non-stationary and stationary results.

參考文獻


【1】A. NISHI, H. MIYAGI, K. HIGUCHI, “A Computer-Controlled Wind Tunnel”, Journal Wind Engineering Industrial Aerodynamics, Vol 46&47, pp. 837~846, 1993.
【2】Biggs, J.M., “Wind Load on Truss Bridges”, ASCE, Vol. 119, pp. 879, 1954.
【3】Cermak, J.E., “Application of fluid mechanics to wind engineering”, A freeman-scholar lecture, Journal of Fluids Engineering, ASME, Vol. 97, pp. 9~38, 1975.
【4】Cermak, J.E. “Wind tunnel design for physical modeling of atmospheric boundary layer”, Journal of Engineering Mechanics, Vol. 107, pp. 623~642, 1981.
【5】G. Solari, P. Gaetano, M. P. Repetto, “Thunderstorm response spectrum: Fundamentals and case study”, Journal Wind Engineering Industrial Aerodynamics, Vol 143, pp62~77, 2015.

延伸閱讀