浮式結構物吸收波能之研究

Abstract

依據二維線性波的理論，應用邊界元素法分析探討浮式結構物於有限水深處吸收入射波波能的效率。本文以每一週期所具有的功率來代表入射波波能，浮式結構物每一週期所吸收的波能與入射波波能的比值定義為效率，當結構物所吸收的波能與入射波波能相同，此時效率定義為最佳效率。本文模擬的結構物共三種：包括直角楔形、對稱楔形及直立平板；其中前兩者的運動方式為垂直振動，而最後者的運動方式則為水平擺動。 首先將數值結果與實驗值比較後發現兩者趨勢一致，證明本文數值模式的妥適性。研究中發現，若結構物後方為開放式的邊界，則結構物難以達到完全吸收波能的情況；當結構物後方為不透水牆時，由於增加反射波的作用，結構物在某些相對水深可以達到最佳效率；透過改變結構物的形狀及結構物與不透水牆的距離，最佳效率所對應的相對水深會隨之改變。換言之，藉由改變結構物的形狀及位置將可得到已知波浪條件的最佳效率。由本研究得知，直立平板其最佳效率所對應的相對水深範圍較廣，其次為對稱楔形，最差者為直角楔形；若考慮效率在0.9以上時，直立平板所對應的相對水深範圍最廣，直角楔形次之，最差者為對稱楔形；因此以本文探討的三種吸能結構物而言，使用直立平板水平振動方式來吸收波能為較佳選擇。

Based on a two dimensional linear water wave theory, the boundary element method is developed and applied to study the effectiveness of a wave energy absorber in water to finite depth. The present study is concerned with wave energy which is defined as the incident wave power per unit crest length, efficiency of an absorber defined as the ratio of the wave energy absorbed by the floating structures per wave period to the available energy of the incident wave, and the best efficiency defined as wave energy absorbed by structures and the available energy of the incident wave are the same. In this thesis, the structures have three forms: a right wedge, a symmetric wedge, and a vertical flat plat; we assume that front two structures are constrained to oscillate in heave, and the third structures is constrained to oscillate in sway. The accuracy of the present numerical model is proved by comparing results of present numerical model, and laboratory experiment. When the boundary in back of the structures is radiation condition, the absorption of wave energy complete hardly by absorber, but exchange radiation boundary for vertical sidewall, the best efficiency was made in some relative depths. By changing the shape of the structures or the distance from structures to sidewall, the best efficiency relative to relative depths are different; in other words, the best efficiency of known wave conditions was got by changing the shape or location of the structures. Using the vertical flat plat to absorb wave energy, the arrange of the best efficiency relative to relative depths is wider than other two; considering above 90 percent of efficiency, using the vertical flat plat to absorb wave energy, the arrange of relative depths is also wider than other two. In summary, about three absorbers of the present study, the available of energy absorbed by vertical flat plat is better than others.