本研究主要應用一個具虛擬力的直接施力沉浸邊界法(Direct Forcing
Immersed Boundary Method, DFIB) 來模擬計算一圓柱體在振盪流中
所產生的渦漩引致振動(Vortex Induced Vibration, VIV) 現象。渦
漩引致振動是一個非常實際的工程問題, 一直以來, 各國工程師們皆苦心
設法防止因VIV 現象造成近海相關土木設備和結構物共振或疲勞的嚴重損
害。由於圓柱周圍的渦漩逸放會導致原本作用於圓柱的水動力隨著時間而
有所變化, 所以當水動力作用的頻率與物體自然頻率接近時, 鎖向放大現
象就會發生, 同時也會導致物體具有劇烈的大振幅圓柱振動進而造成結構
物的破壞。在此研究中也發現到一個比鎖向放大的損害更加嚴重的現象,
稱為振鈴。由於此現象的橫向振動頻率會在短時間內突然上升, 導致結構
物的損害相較於鎖向放大來的嚴重。此研究首先探討振盪流經單根固定圓
柱, 經由比較實驗及數值預測水動力係數和速度分量在三個不同的截面積
下來驗證本研究所建立之數值模擬的正確性, 接著此方法應用於模擬振盪
流與單根圓柱在橫向方向的交互作用。探討入流速度的變化對渦流引致振
動的效應並且找出對應的頻率鎖定區域。因此本數值模擬可用來預測發生
鎖向放大的條件與現象。

Numerical simulations of the vortex-induced vibration (VIV) for
a circular cylinder in an oscillatory flow using direct-forcing immersed boundary (DFIB) method are undertaken. Vortex-induced vibration (VIV) of structures is a practical engineering problem.
In last few decades, numbers of engineers have devoted themselves
to prevention of the VIV phenomenon causing serious damages of offshore-related civil equipment and structures. The fluctuating hydrodynamic force induces vibration of structures due to the vortex
shedding around it. Furthermore, this vibration phenomenon causes failure of structures due to lock-in phenomenon. A phenomenon called “springing” also found in this study which is more serious than lock-in, as it has sudden a very high transverse frequency response which can cause serious damage to the structure. A stationary cylinder in an oscillatory flow is considered in this study. The experimental and numerical results for dynamic and velocity components in three different cross-sectional areas have been compared to verify and validate with the available published results. This DFIB method is then applied to simulate a moving circular cylinder in a transverse direction in an oscillatory flow. The effect of the reduced velocity are discussed and the corresponding lock-in region is found. This established model can be useful for prediction of VIV of structures.

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