以水槽實驗探討潛沒式穿孔消波裝置之消波效率

Abstract

潛沒式穿孔消波裝置為在水平面下安裝可控且均勻分佈的穿孔結構物。該裝置允許水中生物自由穿梭與棲息，以維持自然環境原始樣貌為目標，實現生態友善的理念，同時保持水面航行的便利性。 本研究在二維斷面實驗平推式造波水槽中，採用二階週期波造波理論生成史托克斯波（Stokes wave）和三階橢圓餘弦波（Cnoidal waves）作為入射波，探討潛沒式穿孔消波裝置的消波效率、消波機制及流場變化。研究中，在消波裝置前後各安裝兩支超音波感測器，使用Goda 兩點法測量入射振幅（aI）、反射振幅（aR）和透射振幅（aT），計算出反射係數（Cr）和透射係數（Ct），再基於能量守恆原理得出消散係數（Cd），以尋找最佳的消波裝置組合。研究中調整了20 組入射波週期（T）、4 組入射波波高（H）、7 組孔隙率（ε）、5 組不同的潛沒深度（d/h），其中3 組位於水平面下、1 組與水平面齊平、1 組突出水面；此外，還有5 組兩道垂直穿孔牆間距（B）及3 組穿孔牆孔洞形狀等六項變因。結果顯示，在不同入射波高下，各項係數的趨勢相似，且較大的入射波高呈現出更高的Cd。研究還發現，垂直牆壁的穿孔形狀對最終消波效率無顯著影響。最終，我們定義的高消波效率為最高的Cd 與最低的Cr 和Ct 的組合。經過分析，最高消波效率的裝置組合為：ε = 0.1，d/h = 0.2，B/L = 0.2 - 0.3。

The submerged perforated wave absorber is a device that consists of controlled, uniformly distributed perforated structures installed below the water surface. This device aims to maintain the natural environment by allowing aquatic organisms to freely move and inhabit the area, embodying the concept of ecological friendliness, while simultaneously preserving the convenience of surface navigation. In this study, a two-dimensional wave flume was used to investigate the wave absorption efficiency, mechanisms, and flow field variations of the submerged perforated wave absorber. Second-order Stokes waves and third-order cnoidal waves are generated as incident waves based on second-order wave theory. To assess the performance of the wave absorber, two ultrasonic sensors were installed both upstream and downstream of the absorber.The Goda two-point method was employed to measure the incident amplitude (aI ), reflected amplitude (aR), and transmitted amplitude (aT), from which the reflection coefficient (Cr) and transmission coefficient (Ct) were calculated. Subsequently, the energy conservation principle was applied to determine the dissipation coefficient (Cd), aiming to identify the optimal configuration of the wave absorber. The study adjusted 20 sets of incident wave periods (T), 4 sets of incident wave heights (H), 7 sets of porosities (ε), and 5 sets of different submersion depths (d/h), where 3 sets were below the water surface, 1 set was flush with the water surface, and 1 set protruded above the water surface. Additionally, 5 sets of distances (B) between two vertical perforated walls and 3 different hole shapes in the perforated walls were tested as variables. The results indicated that, across different incident wave heights, the trends of each coefficient were similar, and higher incident wave heights resulted in higher Cd. It was also found that the shape of the perforations in the vertical walls did not significantly impact the final wave attenuation efficiency. Ultimately, high wave attenuation efficiency was defined as the combination of the highest Cd and the lowest Cr and Ct. Through analysis, the optimal configuration for maximum wave attenuation efficiency was found to be: ε = 0.1, d/h = 0.2, B/L = 0.2 - 0.3.