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Please use this identifier to cite or link to this item: http://ntour.ntou.edu.tw:8080/ir/handle/987654321/34386

Title: 海洋異常波浪發生機制與機率預測研究
A Study on the Mechanism and Prediction of Oceanic Freak Waves
Authors: 董東璟
Contributors: NTOU:Department of Marine Environmental Informatics
國立臺灣海洋大學:海洋環境資訊系
Keywords: 異常波浪;機率預測;現場觀測
Freak wave;Probability forecast;in‐situ measurement
Date: 2012-08
Issue Date: 2013-10-07T02:28:37Z
Publisher: 行政院國家科學委員會
Abstract: 摘要:異常波浪(freak wave)是指海上突然發生的波浪,是一種非線性、不對稱的 波浪,出現和消失的很突然,具有瞬態性,對海上航行船隻和海洋工程設施造成 很大危害,若它發生在近岸,就是我們俗稱的瘋狗浪,對於海邊活動民眾的安全 造成威脅。 異常波浪發生的原因是近年來相當熱門的課題,研究結果顯示,異常波浪 發生原因很多,近年來,波浪的調制不穩定(Modulational instability)所引起能量 的非線性自聚焦(self-focusing)漸被認可是主要生成原因之一,很多學者以B-F 不 穩定(Benjamin-Feir instability)理論來討論異常波浪的發生。簡單地說,異常波浪 的發生與波浪的非線性程度有關,Janssen (2003)定義出BFI 指數(Benjamin-Feir Index)來描述波浪資料的非線性,這個係數與波浪的尖銳度(steepness)和譜型有 關,他又推導出BFI 指數與水位變動峰度(kurtosis)之間的解析解。本研究目的為 採用極值統計理論,推導異常波浪發生機率,並考慮波浪方向性的影響,推導結 果將與實測資料比較驗證,最後結合波浪模式進行異常波浪機率預測。 欲推導異常波浪發生機率需先知最大波高超越機率密度函數,而最大波高 機率密度函數可由波高機率密度函數推導,前人已導至最大波高分布,只要漿液 常波浪條件代入即可導出其超越機率理論式。經由文獻蒐集結果可以研判,異常 波浪發生機率的估算式應為水位變動峰度和波浪數目的函數,也就是說只要計算 出峰度,就可求得異常波浪發生機率,而峰度與BFI 有關,BFI 指數又與波譜有 關,因此,只要波浪模式能夠提供各網格點上的方向波譜,異常波浪的發生機率 即可以預測。 本計畫預計以三年完成,接續目前執行中的計畫成果,在第一年度的工作 主要內容是推導異常波浪超越機率理論式,並與實測異常波浪比較驗證與討論。 同時,第一年度也將著手進行異常波浪的現場與遙測作業,收集更多資料提供後 續計畫應用。計畫第二年度將加入波浪方向性的影響,推導BFI 與方向分佈參數 之關係,將波浪方向特性導入異常波浪發生機率的估算上,推導結果仍將使用現 場實測結果與其比較與檢討。同時,第二年度也將進行異常波浪模擬分析,試圖 掌握異常波浪生成前後至衰滅之演變。本計畫第三年度,則將前兩年的異常波浪 發生機率推導結果與波浪模式結合,藉由波浪模式的方向波譜輸出進行異常波浪 發生機率預測。預測結果以外海浮標測站進行驗證。完成本計畫將對於異常波浪 特性與其發生機率有很深入之瞭解,並可以實現機率預測之目標。
Abstract:Freak waves are relatively large and spontaneous ocean surface waves. They are not necessarily the biggest waves found at sea; they are, rather, surprisingly large waves for a given sea state. They have steep wave form, asymmetry and are highly non‐linear. Freak wave has high impacts to navigation ships in the ocean, ocean engineering structure and also to people at the coast when it propagates to the coastal area. The phenomenon of rogue waves is still a matter of active research, so it is too early to say clearly what the most common causes are or whether they vary from place to place. Presented mechanisms for freak waves include diffractive focusing, focusing by currents, wave‐wave nonlinear effects and forcing by wind. Recently, energy self‐focusing by wave modulational instability is assumed to be the most possible cause. Many researches use Benjamin‐Feir instability theory to simulate the freak waves. Since the cause of freak waves is related to wave‐wave nonlinear effect. Janssen (2003) presented the Benjamin‐Feir Index (BFI) to assess the nonlinear. This index is correlated to wave steepness and spectral shape. The analytical solution of correlation between BFI and kurtosis is solved by Janssen. Mori and Janssen (2006) derived the analytical solution of the probability function of occurrence of freak waves. It is not possible to forecast the height of the freak wave. However, the prediction of the occurrence probability of freak waves now looks possible. Theoretical derivation is achieved under many assumptions, such as the surface elevation follows Gaussian distribution when solving the freak wave occurrence probability. The purpose of this project is to apply the in‐situ freak wave cases to the presented analytical formula to verify the correctness or to study the reasons of the disagreement. This is a three‐year project. In the first year, looking for freak wave cases from the in‐situ wave database will be done. We thought there are difficult to find the right freak waves by conventional definition of freak wave. We will present two new indices to looking for freak waves. In the second year of the project, the in‐situ freak wave cases will be applied to Mori and Janssen (2006) formula. In the last year, directional dispersion will be considered to estimate the probability of freak wave occurrence. BFI index will be derived to correct the parameter describing the wave directional dispersion. The result will be verified by in‐situ freak wave cases. We hope this modification will improve the correctness of prediction of freak wave occurrence.
Relation: NSC101-2628-E019-002-MY3
URI: http://ntour.ntou.edu.tw/handle/987654321/34386
Appears in Collections:[海洋環境資訊系] 研究計畫

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