寬深比5的二維矩柱於穩態氣流及加速氣流下之氣動力行為探討

Investigation on the Aerodynamic Behavior of Two Dimensional Rectangular Cylinder of B/D=5 under Steady Flows and Accelerating Flows

蕭愷漢(Kai-Han Hsiao)

指導教授 ： 林堉溢

共同指導教授 ： 羅元隆(Yuan-Lung Lo)

淡江大學/工學院/土木工程學系碩士班/碩士(2022年)

https://doi.org/10.6846/TKU.2022.00447

摘要

寬深比5（BD5）的二維矩形斷面在穩態流場下的氣動力參數及渦散特性可以從文獻觀察出結果。由於實場的強風是較不穩定的，並且不遵循風工程中採用平穩的假設，因此本論文的目的是研究加速氣流下BD5斷面的氣動力行為，並將結果與平滑流和紊流場下的結果進行比較。實驗分別在淡江大學的橋樑斷面風洞和複數風扇風洞中進行。在橋樑斷面風洞中進行了平滑流試驗，然後將結果與參考文獻進行比較，驗證此斷面模型的適用性和可靠性。結果確認後，在複數風扇風洞中進行了紊流和加速流的試驗。由於加速流是屬於非平穩的情況，因此在數據分析中使用了短時傅里葉轉換（STFT）和連續小波轉換（CWT）。

從紊流場與平滑流場比較結果顯示，紊流導致平均壓力恢復區與重新附著位置向上游移動，並且降低升力頻譜中渦散頻率下的能量以及促使渦散頻率下之相位角斜率增加。當流體加速時，觀察到集合平均風壓係數會降低10~20%，擾動風壓係數增加約88~130%。平均阻力係數的趨勢會與背風面風壓孔的壓力變化相同，擾動升力係數在加速期間會稍微下降與擾動風壓係數的趨勢不同，認為是上、下表面壓力互相抵消的關係。另一方面，由於STFT的窗函數限制導致時間解析度不足，因此較難識別出加速期間渦散的變化。相反的，CWT有更好的時頻分析的效果。結果顯示，在加速過程中史特赫數會呈現先增加後減少的現象。同時，當加速度開始上升至加速度最大值時，相位角會保持於0°的現象，隨著加速度開始減小，相位角變化會與定常性的結果相似，可能的原因是加速度的增加降低了渦散對表面的影響。

關鍵字

加速氣流 ； 寬深比5 ； 渦散頻率 ； 相位角

並列摘要

The aerodynamic parameters and vortex properties of the rectangular cross section with the width-to-depth ratio of 5（BD5）under steady flows can be seen in the literatures. Since real strong winds are not steady and do not follow stationary assumption which is adopted in the wind engineering. Therefore, the objective of this thesis is to investigate the aerodynamic behavior of the BD5 cross section under accelerating flows and compare the results with those in smooth flow and the turbulent flow. The experiments were respectively conducted in the Bridge Wind Tunnel and the Multiple Fans Wind Tunnel of Tamkang University. The tests in smooth flow was performed in the former wind tunnel. Then the results were compared with those from the references to verify the applicability and reliability of the section model. After the results were confirmed, the tests in turbulent flow and accelerating flow were then executed in the latter wind tunnel. Since the accelerating flow is non-stationary, Short-Time Fourier Transform（STFT）and Continuous Wavelet Transform（CWT）are used in the data analysis.

Comparisons between the results obtained from the turbulent and smooth flows indicate that turbulence causes the locations of mean wind pressure recovery and reattachment to move upstream and reduces energy of the lift force spectrum at vortex shedding frequency. The slopes between the phase angles of vortex and distances in the turbulent flow are larger. When the flow is accelerating, the ensemble-averaged pressure coefficients were observed to reduce by nearly 10-20%, and the ensemble-fluctuating pressure coefficients were observed to increase by about 88-130%. The varying trend of ensemble-averaged drag coefficients is determined by pressures at the leeward surface. The ensemble-fluctuating lift coefficients, different from pressure coefficients, are observed to reduce slightly during the accelerating period. It is supposed that the pressures on the top surface are somehow offset from that on the bottom surface. On the other hand, since the selected window function in the STFT technique requires finer resolutions in time, the variations of vortices during the accelerating period are hardly identified. Instead, the CWT technique is applied in the subsequent time-frequency analysis. Results indicate that the Strouhal number is increased first and then reduced during the accelerating period. Meanwhile, the phase angles maintain zero as the acceleration is increased to the apex acceleration is reached. As the acceleration is reduced, the variations of phase angles become similar to those in the stationary case. The possible reason could be due to the increase of acceleration alleviating the effects of vortex on the surface.

並列關鍵字

Accelerating Flow ； BD5 ； Vortex Frequency ； Phase Angle

參考文獻

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