本研究利用計算流體力學(Computational Fluid Dynamics, CFD)的數值方法對流體力學的控制方程式進行求解與流場模擬，針對離岸風機單樁式基樁與三頻波之間的非線性波群(Non-linear Wave Group)交互作用影響來探討。利用有限體積法(Finite Volume Method, FVM)做為離散求解的數值模擬，數值波浪水槽(Numerical Wave Tank, NWT)中自由液面部分採用體積分率法(Volume of Fraction, VOF)解決空氣與水之兩相流問題；流體與基樁壁面之黏性流問題則採用雷諾平均納維-斯托克斯方程式(RANS)搭配RNG k-ε紊流模型(RNG k-ε Turbulent Model)求解基樁受水動力作用時的暫態情形。在數值水槽模型的網格處理上，對時間步長與網格大小相對關係的庫朗數(Courant Number)進行侷限以增進模擬計算時的穩定與收斂性，因此針對自由液面交界範圍採用體積加密的方式固定其大小，同時也將無因次壁面函數(Non-Dimensional Wall Function) y^+考慮在內，對基樁壁面邊界層處增加使用邊界層網格設定(Inflation)生成六面體網格；並拘束其第一層厚度以得到更為準確的結果，藉此能夠計算出紊流邊界層內的黏性流域對基樁之影響；最後也在水槽的後方採用數值消波(Numerical Beach)的設定，讓波浪在通過基樁一段距離後進行消散以防止回波對結果造成影響。此外在波浪條件的部分，為了了解實際海況的非線性波群作用與波浪調變(Wave Modulation)所造成的影響，本文針對單樁式基樁在三種典型非線性波列之演變進行觀察探討，分別是單一頻率規則波、兩相等振幅之雙頻波、一主頻兩副頻之三頻波的不同條件下之溯升高度位置情形，並導入無因次化調變因子(δ ̂)分析其與基樁周圍之溯升情形。在受力探討的部分，本研究採用除了以Morison的半經驗公式推估流體作用的水平力外，快速傅立葉轉換(Fast Fourier Transform, FFT)搭配Hilbert-Huang Transform和經驗模態分解法(Empirical Mode Decomposition, EMD)來進行能譜時域分析將受力得以分解出準靜態力、動態力與砰擊力(Slamming Force)，使得在波浪荷載的探討與上有較為精確的結果，也有助於日後的離岸風電海洋工程的開發風險影響評估。

In this thesis, the computational fluid dynamics (CFD) numerical method is used to solve fluid mechanics governing equations and a flow field simulation. The influence of the interaction between the strong nonlinear waves of tri-chromatic waves and the mono-pile offshore wind turbine (OWT) foundation is discussed. We use the finite volume method (FVM) as a numerical solution for the discrete solution, and we also solve the problem of two-phase flow using the volume of fraction (VOF) method. And, the viscous flow problem between the fluid and the wall surface of the mono-pile is solved using the Reynolds-average Navier-Stokes equation (RANS) and the RNG k-ε turbulent model. In addition, regarding wave conditions, we realize the effect of nonlinear wave group interaction, wave modulation, wave evolution, and run-up height of nonlinear wave trains for a mono-pile foundation. The wave condition of the tri-chromatic wave consists of one main frequency and two sub-frequencies, and it concerns about eight kinds of different non-dimensional modulation factor (δ ̂) situations. In this study, except for the horizontal force of the Morison's semi-empirical formula estimating fluid action, fast Fourier transform (FFT) is used. Hilbert-Huang transform and empirical mode decomposition (EMD) are also applied for analyzing the energy spectrum in the time domain in order to decompose the total force into quasi-static force, dynamic force, and slamming force, which makes the wave load discussion have more accurate results regarding ocean engineering areas, and it also can help the development of offshore wind turbine risk assessment in the future.

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