非線性波作用下離岸風機基椿之溯升高度和波浪荷載模擬

THE SIMULATION OF RUN-UP HEIGHTS AND WAVE LOADS OF OFFSHORE WIND TURBINES INDUCED BY NONLINEAR WAVES

本研究主要以計算流體力學（computational fluid dynamics, CFD）軟體，並以RANS方程式搭配k-ε紊流模式作為求解器，模擬風機基樁在不同波浪條件下立溯升與波浪荷載情形。在自由液面的設定上，則是使用體積分率（volume of fraction, VOF）方法處理自由液面問題。爾後，再以此方法模擬學樁式、三腳管式以及重力式三種不同單風機基樁型式，在不同波浪條牛影響下的波浪荷戰情形與溯升高度變化經模擬分析後發現，在波浪尖銳度較高、盟期較短的波浪環境下，三種不同種類的基樁皆有較高的溯升高度，但基樁所受到的波力卻較小反立，波浪尖銳度較小週期較長的波浪條件下，雖然溯升高度較小，但波力卻會對基樁造成較大的波浪荷載。最後，本文以史托克斯二階理論為基礎，並以速度停滯頭理論作為波浪溯升半經驗公式之參考依蟬，探討溯升校正因子在不同基樁形式及波浪條件下立變化情形。結果指出，溯升校正因子會隨著波浪尖銳度及最大溯升高度的增加而有減少的趨勢。 A hydrodynamic simulation of wave run-up heights and wave loads on three types of wind turbine foundations, i.e. monopile, gravity-based and tripod support structures, was conducted using a RANS solver for incompressible fluid flows, employing k-ε turbulent closure. The present CFD model based on the commercial software, FLUENT, is provided to calculate the maximum wave heights and wave loads that the foundations may experience during the wave propagation in Stokes second order wave theory. The wave run-up on different designs of wind turbine foundations is discussed by the breakwater effect, which would be the dominant factor deciding the minimum air-gap requirement of working platform. Thus, a series of numerical experiments with different cases of wave steepness ka as well as scattering parameter kA were conducted by comparing the effects of wave characteristics on these support structures. Due to the contribution of the present CFD model, a semi-empirical formula is calibrated based on velocity stagnation head theory for crest kinematics. It is obvious that the increased velocities close to the cylinder is the predominant factor of run-up heights, in which the velocity head is included. Eventually, the results indicate that the difference among the maximum normalized run-up heights of these support structures is smaller for lower wave steepness than those for high wave steepness. In contrast, it is shown that the difference among the wave loads of these foundations is larger for lower wave steepness than those for higher wave steepness. A modified run-up parameter is also obtained by means of numerical simulation and found that the decreased value of modified run-up parameter is accompanied with increased values of wave steepness and the maximum normalized run-up height.