加速氣流對於橋面板表面風壓特性之探討

Characteristics of wind pressures on bridge deck model in accelerating wind flow

林彥良(Yen-Liang Lin)

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(2020年)

https://doi.org/10.6846/TKU.2020.00616

摘要

本研究利用位於淡江大學風工程研究中心的主動控制型複數風扇風洞進行深寬比8的橋面板斷面模型氣動力實驗，模擬橋面板受加速氣流時對於風力特性的影響。實驗以兩階段進行，分別進行穩態氣流與加速氣流下，斷面模型表面風壓的瞬時量測，探討在不同的穩態流(低紊流)及加速氣流(低紊流)下，表面風壓分布的特性以及升力、扭轉力係數的差異。結果顯示，在0°風攻角下，上、下表面在x/D = 0 – 0.2 (x為模型迎風面往下游伸展之距離，D則為模型深度)的平均風壓係數有些微提升，x/D = 0.2 – 0.4的平均風壓係數則有些微下降，分別與穩態流有10%及20%的差距；受正風攻角影響時，分離點後風壓係數的增幅會隨風攻角增加而減少，3°風攻角時只增加約5%；負風攻角時，風壓係數在分離點後之增幅保持在10%。風壓係數的擾動在加速流的低風速下，約為穩態流的2.5倍。升力向及扭轉向風力係數在加速氣流下都會比穩態流更大，±3°風攻角下分別增加15%及30%；然而在加速中期可觀察到升力向的擾動值較穩態流增加50%。加速氣流下再接觸點之相關性會比穩態氣流下更低，且再接觸點後相關係數回復狀態不如穩態氣流來得好，穩態氣流約在再接觸點後7公分即回復至-0.9，有良好的負相關，但加速氣流到最後一點才會接近-0.9。最後本研究亦採用小波時頻分析找出0°風攻角在加速段期間渦散頻率由30 Hz上升至90 Hz的現象，而當風攻角不為0°時，再接觸點會出現低頻的能量。

關鍵字

複數風扇風洞 ； 加速氣流 ； 小波分析

並列摘要

This study examined the aerodynamic behavior of the bridge deck model excited by accelerating wind flows generated by the actively-controllable multiple fan wind tunnel at Tamkang university. The experiment had two phases. The first phase is to make a comparison test under steady flows; the second phase is to collect enough ensemble size of testing results under the accelerating flow. The bridge-deck model was made in an aspect ratio of eight, and the inherent turbulence of the approaching wind is around 2.5%. From the results, the pressure coefficient increased by 10% at the normalized distance x/D = 0 – 0.2 in accelerating case if compared to the steady case. When the position went downstream to x/D = 0.2 – 0.4, the pressure coefficient was reduced by 20%. If the wind attack angle was rotated to positive angles, the amplification due to separation is reduced from 10% to 5% under accelerating flows. On the other hand, if the rotation is negative, the pressure coefficient after the separation point remained consistent. The standard deviation value of the pressure coefficient under the accelerating flow is 2.5 times larger than that under the steady flow. Both the lift and the torsion coefficients in accelerating wind flow are larger than those in the steady flow, where the lift force coefficient increased 15%, and the torsion 30%. The correlation coefficients in accelerating wind flow at the reattachment points are lower than those in the steady flow. Whether the wind attack angle is positive or negative, the correlation coefficients in steady flow returned back to -0.9 at 7 cm behind the reattachment points, and the correlation coefficients in the accelerating wind flow returned to -0.9 at the last point on the top face. This study also used the wavelet transform technique to find out the frequency variation of the vortex shedding increasing from 30 Hz to 90 Hz during the acceleration process. The wavelet analysis results showed how energy transmitted from the low-frequency range to the high-frequency range.

並列關鍵字

Multiple Fan Wind Tunnel ； Accelerating Flow ； Wavelet Analysis

參考文獻

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