非線性波群變形之實驗探討

Observations on Wave Groups Transformation in a Long Wave Tank

江文山(Wen-Son Chiang) ； 許文陽(Wen-Yang Hsu) ； 楊瑞源(Ray-Yeng Yang) ； 黃煌煇(Hwung-Hweng Hwung)

《海洋工程學刊》 14卷2期 (2014/06) Pp. 111-120

https://doi.org/10.6266/JCOE.2014.1402.03

摘要

本文於大型波浪斷面水槽中，針對非線性波群在深水區的演變，進行實驗探討。對於未碎波條件，波列的發展約略呈現左右對稱的型態，直到初始波列BFI指數（Benjamin-Feir Index; Janssen, 2003）達一定門檻值以上，則波列的演變發展到前傾，進而碎波的型態。波列發展過程中達最強調變時，也是波群分裂時，較高波群在前傳播，較低波群尾隨在後。對未碎波條件，波列演變過程中能量在各成分波之間的轉換，基本上呈現能量往高頻帶傳輸較往低頻帶者略高；至於碎波的條件，則波列演變過程中能量往低頻帶傳輸較往高頻帶者略高。波列碎波同時尖銳度降低、頻譜變寬，另外發現波譜的主頻呈現往低頻降移的情形。碎波結束後，波譜的寬度又迅速回復到接近初始型態且波譜主頻亦同時轉移回到原主頻，此現象稱為暫時性主頻降移。然此主頻回復的現象，隨著初始波群BFI指數的增加逐漸地不明顯，最終甚至無法回復而形成永久性主頻降移。從不同波群的演變過程觀察，實驗結果顯示最大波高比呈現隨BFI增加而增大，直到BFI指數約1.8達最大值，之後最大波高比則呈現降低的情形。最大波高比出現的位置與實驗中觀察到開始發生碎波的條件一致。

關鍵字

波群 ； 調變 ； BFI指數

並列摘要

The evolution of nonlinear wave groups in deep water is investigated in a super wave tank. BFI (Benjamin-Feir Index; Janssen, 2003) is a good indication to describe the evolution of wave groups in the experiments. For BFI approaching a critical value, wave groups evolves to strong modulation and lead forward. Then, followed by wave breaking. Accompany with the strong modulation, disintegration of wave groups was found. Larger wave propagates in front of the smaller trailing wave. Energy transfer in between the components both the higher and lower frequencies as wave groups propagate downstream. Breaking process plays an important role for energy transfer. For breaking wave groups, the lower frequencies gained much energy than higher ones. However, the energy pumed to higher and lower frequencies are comparable for nonbreaking wave groups. Meanwhile, the spectrum becomes wider and peak frequency downshifts during breaking process. After the breaking process, both of the spectrum width and peak frequency are partially recurred. The peak frequency downshift could be temporatory for lower BFI and permanent for higher BFI. The amplification of max. wave height in wave groups increases with BFI until a maximum at BFI~1.8 and decends. The BFI value which corresponds to the peak amplification of max. wave height matches the condition which wave breaking occurres during the evolution process of wave groups.

並列關鍵字

wave groups ； modulation ； Benjamin-Feir Index (BFI)