波浪作用下港口附近船舶之受力及運動

Abstract

本文利用邊界元素法建立一數值計算模式,以解析船舶在一可為任意形狀 、水深及具有任意反射率碼頭及防波堤的港池港口附近受波浪作用所引起 的運動與其所波外力。理論解析中應用微小振幅波理論,將波浪因港口附 近海底地形變化及防波堤等結構物存在所引起的反射、繞射波及因船體運 動所引起的散射波融合一併討論,可同時解析港口附近波浪的繞射與散射 問題;而在船體表面條件的處理上,本文假設船體的運動為與波浪同週期的 微小振幅運動,因此可推導出船體的運動學邊界條件,本文則利用其第一階 的運動及動力邊界條件而得船體表面邊界條件,同時並利用其動力邊界條 件與線性的Bernoulli方程式推導而得船體各方向的運動振幅。進行數值 計算時,本文分別以正方形港及實際的八斗子漁港作為計算例 ,並在平面 水槽中進行模型試驗,以驗證數值計算的結果,經由比較數值計算與實驗結 果發現兩者相當吻合,顯示本數值計算模式在解析規則波作用下港口附近 船體運動與受力的妥適性。另外,本文利用所建立的數值計算模式配合永 井(1972)的等能量分割法分解入射波波譜,並將分解所得的各成份波視為 單一頻率的規則波而加以解析,以計算船體受單方向不規則波作用所引起 的運動及受力反應能譜。數值計算時以方形港為例,以Bretschneider- Mitsuyasu波譜作為入射波波譜,並於水槽中進行實驗,以驗證數值計算的 結果,由比較結果顯示實驗與數值計算所得反應能譜,兩者相當吻合,顯示 本數值計算模式配合波譜分解法能有效的處理港口附近船體受不規則波作 用時所引起的運動與受力反應。為瞭解實際波浪對港口附近船體的作用, 本文以前述波譜作為入射波頻率波譜,並引用光易(1975)觀測所得的方向 分佈函數,數值計算方向不規則波對港口附近船體作用所引起的運動反應, 討論入射波波浪之方向性能量集中度對船體運動的影響。 A numerical method for predicting both shipmotions and wave forces exerting on a ship near a harbor entrance of abritary geometry is developed throught usage of three-dimensional integral equation method. Theorical analysis is based on the potential theory with linearized free surface condition, and it is applied to the arbitary body geometry and arbitary water depth. Both scattering wave due to the existing of breakwaters of harbor and variance of sea bottom and radiation wave, which is induced by ship motions,is discussed together in the analytic process. Condition on a ship surface can be obtained by combining first order kinmatic and dynamic boundary condition on the basis of assumption of small amplitude motion of ship. Ship motions were gotten together linearized Bernoulli equation with dynamic boundary condition on ship surface. A fishing boat around a square basin entrance is first computed using the method and the validity of this computation is verified experimentally by a model test. Numerical model was applied to calculate responses of a ship, when it is affected by uni-directional irregular waves, for understanding of the behavior of ship near harbor entrance. When the analysis of ship motions induced by irregular wave proceeding, Bretschneider-Mitsuyasu spectrum was used, and wave spectrum was dealt with the same manner as Nagai's. The spectrum is divided into several spectral components, which process equal wave energy in each frequency interval.The response spectra of ship could be gotten after the calculation of ship response in accordance with the representative frequency of each component wave one by one. The validity of the method is adequately verified by experimental results. Finally, numerical results would be presented when ship is affected by dircetional irregular waves, and directional spreading function of the Mitsuyasu-type is applied.