博士 / 國立成功大學 / 水利及海洋工程學系碩博士班 / 96 / This work presents the simulation of irregular waves interacting with submerged offshore structures. By generating the incident irregular waves in a numerical wave flume and solving the unsteady two-dimensional Navier-Stokes equations with the fully nonlinear free surface boundary conditions for the fluid flows in the flume, the viscous flows are determined and the bed shear stress as well as vortex circulation can be obtained. Four topics were focus: irregular waves with the associated bed shear stress were studied linearly and nonlinearly, vortex formed beneath irregular waves around a submerged breakwater and ripples.
Irregular waves were generated by the wavemaker adopting deterministic spectral amplitude method implemented using the fast Fourier transform algorithm. The accuracy of the generated irregular waves and the viscous flows was confirmed by comparing the predicted wave spectrum with the target spectrum and by comparing the numerical transfer function between the shear stress and the surface elevation with the theoretical transfer function, respectively. Additionally, characteristics of the wave spectra and the associated shear stress spectra were discussed in terms of the spectral frequency, the zeroth spectral moment and the spectral bandwidth parameter. The maximum bottom shear stress caused by irregular waves, computed by this wave model, was compared with that obtained using Myrhaug’s model (1995). The transfer function method was also employed to determine the maximum shear stress, and was demonstrated to be accurate.
Nonlinear properties of irregular waves are preliminary presented using the formula of the second-order bound waves, subharmonics and superharmonics, which deduced by Dean and Sharma (1981). As the associated phases changed, bound waves of the synthesized results show irregularity in spectrum and form randomness in spatial-varied skewness. Similar phenomena were found in the numerical results, although the magnitude of spectral higher harmonic components and the skewness are larger. The spatial-varied skewness of the surface elevation and the bed shear stress shows bound components indeed present, and indicate that the phase difference of surface elevation and bed shear stress has to be considered for building the associated statistical model.
Of the repeatable properties of the present numerical scheme, the incident waves and reflected waves from the submerged breakwater were separated without any assumption. A reliable time duration for analysis was confirmed by the comparison of reflected coefficients in order to ensure that the wave reflected from wavemaker would not affect the interesting duration. A vorticity-based method with a vorticity threshold were developed and made the automatic recognition of vortex become possible, in which the threshold was confirmed for well describing the vortex region. Both the circulations and the selected vortex region were shown in flow fields and vorticity contours. The interaction of the vortices was evidenced and showed the succeed-formed vortices above breakwater increased the breakdown rate of the vortices next to the breakwater three times. Of the irregular-wave train, the vortex of large wave followed by small one lasted longer.
Applying body-fitted boundary, different hydraulic conditions were studied about on and off equilibrium ripple pattern beneath regular and irregular waves. While concerning only the region beneath twice the ripple wave height, the present numerical scheme was verified with the experiment and has a good agreement within the transitional regime. The vortices formed beneath regular waves reached a limited circulation if the bed-orbital-displacement amplitude is larger than the ripple wavelength. The limitation can furthermore be evidenced by the trajectories of the vortex center. The vortices formed beneath irregular waves were recognized automatically as well, although the number of the zero-upcrossing waves and the vortices was not matched.
Identifer | oai:union.ndltd.org:TW/096NCKU5083030 |
Creators | Mao-Lin Shen, 沈茂霖 |
Contributors | Ching-Jer Huang, 黃清哲 |
Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
Language | en_US |
Detected Language | English |
Type | 學位論文 ; thesis |
Format | 220 |
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