方向性不規則波之波速

Abstract

本文旨在討論海洋上具方向性不規則波的波速變化，以現場海面資料分析探討實測波 速值與理論波速值的差異，藉以觀察方向分散性以及非線性效應對於波浪波速之物理特性 的影響情形。 波浪行進的方向自四面八方而來，一般的海洋風浪在主頻處波能的方向集中於主方向 的現象比較明顯，但是每個頻率成份波在各個方向成份上均有能量的分佈，對於波速值的 計算均有影響，故主方向以外其他方向成份波仍不可忽視，因此必須作方向分散性的修正 。 以經過長期統計所得到之通用的cos 函數形式來描述方向頻譜，雖然方便於工程上的 應用，但是對於描述每一筆個別的波浪資料而言，則與真實海面上的物理現象會有很大的 差別，故直接運用實際海面方向頻譜的資料，以實計的方向分佈對由線性分散關係式而來 的理論波速值進行修正，可得到一修正過的線性理論值，再與使用波高計陣列經由相位差 法計算而來的實測波速值，並藉由討論二階頻譜、方向頻譜的型態進行對照與比較。 在複雜的海洋波浪情況之下，線性理論波速值只要考慮了方向分散性這個因素，對某 些波浪情況能夠適用，完全能夠適用的比例約為1/8 ，其餘不適用的情形有3/4 經過非線 性效應及波譜型態的討論，大致上也能夠對現象提出解釋，至於可能受到海面上潮流、風 速風向等不定因素影響以致於完全不適用的情形也佔了約1/8 的比例。 故經由本文的研究得知，實際海面上受到非線性效應及水流的影響，於近岸地區，大 部分波浪的波速分散關係之線性理論並不適用。 The phase velocity of directional random sea waves is investigated in this thesis. In order to observe the influence of angular dispersion and nonlinear effect, we discuss the difference between theoritical and actual velocities. The way come from all directions,wind waves in the ocean are rather commonly occured near peak frequency,and more frequently concerntrated toward mean direction. However,waves of individual component to which in many direc- tions are yet not neglectable,it is because that energy distributes over every direction component under each frequency which influences the calculation about phase velocity. Consequently,correction on anguler dispersion is nece- ssary. In engineering application, it is convenient to describe directional wave spectra in the form of cosin function which obtained experiencely after compiling statisics for a long time.However, for a physical datum from actual sea surface, there is great variation when describing an individual batch of wave datum. To calculate theoritical phase velocity directly, apply datum of directional spectrum under actual sea surface to modify theoritical phase velocity acquired from dispersion relation and a batch of theoritical linear velocity value is obtained. After aquiring actual phase velocities from wave guages by phase difference method discussing and comperison would be made through investigating spectral datum in bi-spectra and the shape of direc- tional wave spectra. Under the complicated random sea waves through observing and comparing each phase velocity spectrum,there is of 1/8 the propability that a linear theoritical wave appears and 3/4 nonlinear the result is explaincible through discussions about nonlinear effects and spectral patterns. As for waves influenced by tides,currents,wind and other uncertain factors with the result that the linear theory loses applicability,there is of 1/8 the propability. However, according to this study, linear theory of wave dispersion relation is not applicability to most waves in coastal area under the influences of drift current and nonlinear effects.