自然環境中振盪流與固體間交互作用的問題無所不在,尤其又以海洋工程最常見，例如在海洋工
程所需之海洋工作平台、消波塊、海上鑽油平台等。結構物受到週期性來回的波浪與海流水動力
影響下，必需考慮結構安全性的問題。本研究主要利用有限體積法並針對流體與複雜形狀固體之
交互作用問題，發展一個具虛擬力的沉浸邊界法。此研究首先探討振盪流經單根圓柱，比較實驗
及數值預測動力係數來驗證本研究所建立之數值模式的正確性，接著將此方法應用於模擬振盪
流與四根圓柱交互作用。探討Keulegan-Carpenter(KC)數、斜向流動、與四圓柱間距大小對
此交互作用及流場對圓柱造成的升阻力影響。

本研究利用渦度來觀測流場之變化，並利用升阻力係數時序圖及其方均根、頻譜分析、軌跡圖來探討前述三個效應之影響。首先比較不同KC數之影響，當KC數增加時阻力係數之振幅會下降，而升力係數在頻譜分析上，其基頻會變快並且會誘發越多的次頻率成分波。接著改變流場方向而得一與水平夾45度之斜向流動，結果發現在斜向流動中，其升力係數在低KC數會較水平流小，但在高KC數會較水平流大。再著，將四根圓柱間距加大時，在低KC數時，升力係數會隨之變小，但是在高KC數時，升力係數則會隨之變大。本研究藉由分析振盪流流經四根圓柱模擬海洋中平台，瞭解真實結構物周圍流場與受力情形，可提供設計海洋平台者做為參考。

Cylindrical structures are commonly used in offshore engineering, e.g. a tension-leg platform(TLP). The prediction of hydrodynamic loadings on those cylindrical structures due to oscillatory flows is one of the most important issues in the design of those marine structures. The aim of this study is to establish a direct-forcing immersed boundary method to simulate the oscillatory flow past a circular cylinder array in a square arrangement. The finite volume method was used to solve the Navier-Stokes equations. In this study, the effects of Keulegan-Carpenter(KC) number, oblique flow and the gap among four cylinders were investigated.
Numerical results were visualized using vorticity contours so evolutions of oscillatory flow with the cylinder array were presented. Hydrodynamic loadings including in-line and transverse force coefficients were determined and illustrated in the time and spectral domains. It was found that as KC increases the flow pattern transists from a symmetric mode to an asymmetric mode. Meanwhile, the in-line and transverse force coefficients were affected by the increase of KC. The fundamental frequency of the transverse coefficient
shifts and was different from the oscillatory flow. Moreover, when the flow direction changes, the effect of the oblique flow on the hydrodynamic coefficients was reported. The effect of the gap among cylinders on hydrodynamic loadings were explained in this study as well. Essentially, the proposed direct-forcing immersed boundary approach can be useful for scientists and engineers who would like to understand the interaction of the oscillatory flow with an array of cylinders and to estimate hydrodynamic loadings on the
array of cylinders.

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