應用波壓推算水位變化之研究

Abstract

本研究利用 Kuo and Chiu(1994)所提出波高與波壓之經驗轉換函數， 配合快速傅立 葉轉換法(FFT)，由水下壓力計所測得之波壓訊號，來推 算水位變化。將 Nielsen(1986)所提出之局部頻率法(LCM)及本文所使用 之快速傅立葉變換法， 同時對不同的波高與波壓間的轉換關係，做比較 分析。研究結果發現，在波壓計放 置深度較淺時，利用 FFT法與 LCM法 經由經驗轉換函數皆可準確的做變換，但隨著 壓力計放置的位置愈深時 ，使用經驗轉換函數與 FFT法，將可得到較佳的準確性。 本文同時也針 對 FFT法中，截斷頻率無法明確判定的問題，提出簡易而明確的判斷 方 式。 此外本文也藉由物理模型試驗，探討淺水域中波壓與波高的轉換關 係。經由 實驗數據分析後發現，淺水域中波浪分裂所造成高階成份的波 壓能量與波高能量間 的轉換函數，與以往的研究一樣，主要的是與一個 由角頻率及波壓計 設置深度組合而成的無因次參數有關，且各階成份波 的轉換函數似乎一樣。此由規則 波實驗數據所迴歸的經驗轉換公式，經 由單一方向不規則波的驗證，證明此經驗轉換 公式可利用來做淺水域中 波壓與波高的變換。 In the present study an empirical transfer function between the wave pressure and wave height,which was established by Kuo and Chiu(1994) ,was applied to predict the history signals of the free water surface elevation from the wave pressure beneath the still water level.The Fast Fourier Transform(FFT) method is used to calculate the power spectral and history signals of wave height.The FFT method is compared with LCM method developed by Nielsen(1986) in this paper.It is found that both of FFT method and LCM method are accurately calculated when the pressure gages are locatted near the still water level.However, when the distance between the pressure gage and the still water level is longer,the FFT method with the empirical transfer function is more efficient and accurate than the LCM method. Furtherwore,the present study has decided on the cutoff frequency range,the weak point of the power spectral analisis which was resolved in this paper. Furthermore,the present study used the physical model test to investigate the transfer function between the wave pressure and the wave height in shallow water. By the analysis from the experimental data, we can find the transfer function is concern with the wave frequency and pressure gage locations. In addition ,the transfer functions of each older nonlinear components seem to be the same.