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標題: 振盪來流下二維方柱紊流流場之數值探討
Numerical Investigations of Turbulent Flows around a Square Cylinder in Oscillating Approaching Flow
作者: 林得雄
Lin, Te-Hsiung
關鍵字: Numerical simulation;數值模擬;Square cylinder;Oscillating flow;Morison equation;方柱;振盪流;莫里森方程式
出版社: 土木工程學系所
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本研究以數值計算方法,模擬二維方柱在平均流速不為零之週期性振盪來流下之鄰近紊流流場及方柱受力行為。在高雷諾數的情況下,改變週期性振盪來流之速度振幅比(AR)從0.1至0.7、Keulegan-Carpenter number (KC) 從0.5至30,以及風攻角(α = 0° 與 22.5° ),目的在探討相應紊流流場之變化及其對二維方柱受力行為之影響。
為了正確地反應出流場中既有之非恆定性與紊流特性,研究中之流場模擬採用微可壓縮流法與動力次網格模型。數值計算之結果以Morison equation為根據,運用最小平方法迴歸出相應之阻力係數、昇力係數,以及方柱受非線性振動力產生的慣性項阻力係數、慣性項昇力係數 。
研究中發現,本數值模式對於振盪來流下方柱之阻力變化預測頗為良好,數值計算之結果可以Morison equation適當地描述;然而,在昇力變化的預測上顯得較不如預期。此外,當來流之無因次振盪週期與方柱之渦散週期相應時會出現共振反應,此時之阻、昇力係數以及慣性項阻、昇力係數為最大值。

The unsteady forces on a square cylinder in oscillating flows with non-zero mean velocities are investigated numerically in a two-dimensional sense. At a high Reynold's number, the major parameters of the flow around a cylinder with a sinusoidal motion are the the amplitude ratio of the approaching-flow velocity (AR ) and the Keulegan-Carpenter number (KC), which vary respectively from 0.1 to 0.7 and 0.5 to 30. The resulting wind loads on the cylinder for various and values are analyzed systematically at two selected attack angles (α = 0° and 22.5° ) to examine the flow effect of the square cylinder.
To predict the unsteady turbulent flow around the square cylinder, a weakly-compressible-flow method together with a dynamic subgrid-scale turbulence model is adopted. According to the Morison equation, the theoretical expressions regarding the variation of the unsteady drag, one can determine the corresponding values of Cd and Cd~ based on a least-square estimation. On the other hand , the corresponding values of Cl and Cl~ are obtained as well, while the theoretical expressions describe the general tendency of the lift variation poorly.
Results show that the occurrence of peaks is apparently due to the effect of resonance between oscillation of the approaching flow and the motion of vortex shedding of the square cylinder. Finally related forces coefficients are presented in help with the analysis of engineering designs.
其他識別: U0005-0808200612194300
Appears in Collections:土木工程學系所

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