| 研究生: |
江文山 Chiang, Wen-Son |
|---|---|
| 論文名稱: |
深水區非線性波列調變之研究 A Study on Modulation of Nonlinear Wave Trains in Deep Water |
| 指導教授: |
黃煌煇
Hwung, Hwung-Hweng |
| 學位類別: |
博士 Doctor |
| 系所名稱: |
工學院 - 水利及海洋工程學系 Department of Hydraulic & Ocean Engineering |
| 論文出版年: | 2005 |
| 畢業學年度: | 93 |
| 語文別: | 英文 |
| 論文頁數: | 144 |
| 中文關鍵詞: | 主頻下移 、不穩定 、調變 、副頻 、再現性 |
| 外文關鍵詞: | sideband, modulation, recurrence, instability, frequency downshift |
| 相關次數: | 點閱:90 下載:5 |
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本文於成功大學水工試驗所之大型波浪水槽(300m*5m*5.2m)進行一系列深水波實驗研究。針對三種典型非線性波列之演變進行觀察研究,分別是初始均勻波列、初始給定主頻及一組微弱副頻組成之弱調變波列與初始給定兩相等振幅而頻率存在微小差異成份波組成之雙頻波列。
實驗結果顯示初始均勻波列之調變乃源自於造波機由靜止啟動至穩態運轉過程中所造出之非穩態波列,同時伴隨衍生出副頻能量。之後,波形調變及副頻能量隨此非穩態波列傳遞而增加,最終達到近似穩定調變,其中有一組(高、低)自然衍生之副頻因成長最快而特別顯著。前人對於此類初始均勻波列衍生之調變的研究侷限於大尖銳度之波列( ),而本文的研究範圍則涵蓋較大區域( )。初始階段一組副頻振幅呈現對稱指數成長,直到碎波後低副頻振幅超越原主頻形成所謂主頻下移現象。
觀察分析一系列相同主頻而主頻與副頻之頻差不同的初始弱調變波列的實験結果,發現波列的演變終將會使得最不穩定副頻突顯出來。其中特別值得提出的是對頻差微小的波列,其最不穩定副頻是透過多次的主頻下移而得以顯露,這是首次被觀察到的現象。其隱含透過副頻不穩定機制,伴隨波列的傳遞主頻會逐漸往更低頻遷移。
針對不同尖銳度之初始弱調變波列所作的實験,顯示在後碎波階段,波列呈現週期性調變與反調變,其中波列大部份能量在原主頻與給定之一組副頻間傳遞。因初始尖銳度的不同,後碎波階段之實驗結果分別呈現出永久性與暫時性主頻下移。亦即前人觀察到之碎波引起永久性主頻下移其實未必是永久性。
雙頻波列演變的實驗分析結果,在非碎波情況,波列呈現初始狀態的近似再現性,對波列演變過程中出現碎波的情況,則發現波列碎波過程中低副頻呈現相對增強,這與初始弱調變波列的實驗結果一致。
因為副頻不穩定機制,非線性波列演變過程中會演化出瞬間極大波乃至發生碎波。相較於Stokes波的碎波極限,觀察到的碎波其對應之尖銳度與波峰水粒子速度對相位速度的比值皆偏小。顯示基於均勻波列之假設推導得到之Stokes波的碎波極限與調變波列中碎波發生之條件並不一致。
To investigate the wave modulation induced by sideband instability in deep water. A series of experiments on the long time evolution of nonlinear wave trains in deep water were carried out in a super wave flume (300m x 5.0m x 5.2m) at Tainan Hydraulics Laboratory. In order to examine the wave modulations which evolve naturally and initially seeded, three typical wave trains, namely uniform wave, imposed sidebands wave and bichromatic wave were generated initially by a piston-type wave maker.
The modulation of wave trains increases with propagation and the corresponding wave spectra show the exponential growth of sideband amplitudes, which further increase due to wave reflection from the flume boundary. Eventually, a quasi steady state of wave train evolution is achieved and a pair (upper and lower) of fastest growth sidebands evolved. Initially, the amplitudes of fastest growth sidebands exhibit a symmetric exponential growth in spatial domain until the onset of wave breaking. The amplitude of lower sideband becomes larger than those of the carrier wave and upper sideband after wave breaking, which is so called frequency downshift. Present results on the evolution of initial uniform wave trains cover a wide range of initial wave steepness (kcac=0.12~0.30) and greatly extended previous researcher’s studies which only confine to the initial larger wave steepness region (kcac>0.20).
The investigations on the evolution of initial imposed sidebands wave trains with constant initial wave steepness but different normalized frequency difference between the carrier wave and the imposed sideband (sideband space) illustrate that the most unstable mode of initial wave train will manifest itself in the evolution of wave train. Especially, the most unstable mode of initial wave train develops through a multiple-downshift of wave spectrum for the wave train with smaller sideband space, which is observed the first time in literature. The evolution of wave train is a periodic modulation and demodulation at post breaking stages, in which most of the energy of wave train transfers cyclically between the carrier wave and two imposed sidebands. Meanwhile, the wave spectrum shows temporal and permanent frequency downshift respectively for different initial wave steepness, which suggests that the permanent frequency downshift induced by wave breaking observed by previous researchers may not be permanent.
Near recurrence of initial status of bichromatic wave train is demonstrated for non-breaking case. However, for breaking case, the amplitude of lower sideband is selectively amplified through the breaking process. The results confirm the findings on the evolution of initial uniform and imposed sidebands wave trains.
The local wave steepness and the ratio of horizontal particle velocity to linear phase velocity at wave breaking in modulated wave group indicate that the breaking criterion of Stokes wave, which is derived based on the uniform wave, is not consistent with the results of modulated wave trains.
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