| Summary: | 博士 === 國立臺灣海洋大學 === 河海工程學系 === 99 === This study has applied the Hilbert-Huang transform (HHT) scheme to analyze previous experimental data of bed soil fluidization induced by monochromatic waves, nonlinear waves, regular wave groups and irregular wave groups. The HHT scheme is preferable for dealing with nonlinear and non-stationary signals for deriving the time-frequency-energy distributions. To overcome the mode mixing found in its ensemble empirical mode decomposition (EEMD), a post-processing method using masking signal-based EMD (MSEMD) was adopted to modify the mode-mixed intrinsic mode function (IMF).
The results have shown that IMF components with primary frequency had displayed amplitude amplification during the transient stage with building-up excess pore pressure, while those with higher frequencies had started to appear. The nonlinear components of the loading waves were seen to be absent from the pore pressures indicating their very low transmission into the underlying bed soil. It was clearly seen that the wave groups are quite influential to pore pressure responses while components with lower frequencies could be more effective in transmitting wave energy into the bed soil.
The wave dissipation occurred also mainly in the IMF of the primary frequency during fluidized response and much less in nonlinear or higher-frequency components. The dissipation mechanism was found to be exponentially increased with larger Ursell number. Moreover, the velocity fields near above the bed were rather relevant to the bed soil fluidization. During the fluidization response, the amplitudes of horizontal velocity components similarly dissipated as waves mainly in primary frequency ones but much less in higher-frequency ones. On the contrary, the vertical velocity components of both primary frequency and higher-order frequencies were significantly amplified. Meanwhile, the IMF components of bed surface oscillation had demonstrated the same trend of amplitude variations of vertical velocity implying close relationships with the amplification of vertical velocity.
Finally, the empirical model for calculating excess pore pressure given by Jeng et al. (2007) was modified by present laboratory data. Two empirical parameters of the source term regarding soils’ relative density were derived by fitting experimental data of monochromatic wave tests. The modified model was shown to be able to effectively simulate the maximum excess pre pressures as well as reasonably predict the experimental results due to irregular wave group.
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