近岸波浪頻譜形狀與其非線性研究

Abstract

以台灣近岸海域6個測站的長期波浪觀測資料, 求出個別的波譜形態, 果 發現對同一測站而言, 其波譜形態具一致性。因此基於通用型式NSWAP及 Pierson-Moskowitz波譜建立各測站的波譜模式以供海岸工程應用上的參 考。為了考察這些基於線性理論的波譜模式的適用性 , 本文以風洞水槽 在不同實驗條件下產生風浪資料, 利用波形非對稱 , 二階頻譜以及波速 頻譜等分析方法來探討波浪的非線性。由此發現一階波浪能譜形狀隨著相 對水深變小, 而趨於平緩的現, 並非單純全由波浪非線性交互作用效應所 產生的非線性能量增加所致, 而是在高頻區域線性成份波的能量也會同時 增加所造成的。另外利用台灣近岸海域鼻頭角、成功、興達等三個測站的 波浪實測資料, 以類似的非線性理論之波譜分析法的適用性, 結果發現近 岸之波浪能譜, 其非線性能量相較於線性能量甚小, 因此線性理論的波譜 仍可適用於當相對水深不小於0.1的台灣近岸海域。此外也發現不同的波 浪非線性衡量方法, 皆有其意義與功能, 因此宜相輔使用。 In this study, the local wave spectrum are obtained by six long- term observational wave stations off the nearshore of Taiwan, respectively. It was shown that, at a specific site, the similarity of wave spectral form exist. Therfore, theneralized forms of Pierson-Moskowitz spectrum and JONSWAP spectrum were used to describe the local wave spectrum by selecting siutable spectral form parameters. In order to examine the feasibility of the linear theory of power spectrum, the experimental wind wave were generated in a wind-wave flume, with many different wind velocities and water depths. Based on the experimental data, asymmetry of wave forms, wave bispectra, and phase velocity spectrum were obtained. From these results, it was found that the reason the slope of the power spectral from becomes smaller in shallow water can be attributed th not only the increase of nonlinear wave component energy but also the increase of linear wave component energy on higher frequencynge. On the other hand, wave data were obtained from threervation stations off the nearshore of Taiwan. The fours were applied to investigate the nonlinearity of power spectrum. The result of this study could be utilized as aference toward the application of the linear power spectral model to the ocean waves when the relative water depth was lessan 0.1 in the nearshore region of Taiwan. Furthermore, thelarities, discrepancies and characteristics of the four methods were discussed and compared with one another.