• Smart Structures and Systems
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• v.23 no.5
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• pp.405-420
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• 2019

In this study, vibration characteristics of offshore wind turbine tower (WTT) with gravity-based foundation (GBF) are identified from dynamic responses under wave-induced excitations. The following approaches are implemented to achieve the objective. Firstly, the operational modal analysis methods such as frequency domain decomposition (FDD) and stochastic subspace identification (SSI) are selected to estimate modal parameters from output-only dynamic responses. Secondly, a GBF WTT model composed of superstructure, substructure and foundation is simulated as a case study by using a structural analysis program, MIDAS FEA. Thirdly, wave pressures acting on the WTT structure are established by nonlinear regular waves which are simulated from a computational fluid software, Flow 3D. Wave-induced acceleration responses of the target structure are analyzed by applying the simulated wave pressures to the GBF WTT model. Finally, modal parameters such as natural frequencies and mode shapes are estimated from the output-only acceleration responses and compared with the results from free vibration analysis. The effect of wave height and period on modal parameter extraction is also investigated for the mode identification of the GBF WTT.

• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• 2003.05a
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• pp.157-164
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• 2003

Introduction of wave model, considered the effect of tide, wind and wave induced currents at the coastal waters of complex bathymetry, is a very important factor for most coastal engineering design and disaster protection problems. As the steady state spectral wave model could simulate depth induced wave shoaling and refraction, current induced refraction effect, steepness induced wave breaking, diffraction, wind wave growth, and wave-wave interaction that redistribute energy, this would support and compensate the gap in the real field of design where other wave models could not deal and cause wrong estimation. In this study, for that sense, we applied the spectral wave model t the large coastal waters near Gaduck Island where the Busan new port construction project is going on, for better understanding and analysis of wave transformation process. We also compared the simulation results with the calculated from the existing model. From such a trial of this study, we hope that broader and sager use of the spectral model in the area of port design and disaster prevention system come through in near future.

• Journal of Ocean Engineering and Technology
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• v.19 no.2 s.63
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• pp.12-18
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• 2005

Typhoon Maemi landed on the southeast coast of Korea and caused a severe storm surge in Jinhae Bay and Masan Bay. The tide gage in Masan Port recorded the storm surge of a maximum of more than 2m and the area of more than 700m from the Seo Hang Wharf was flooded by the storm surge. They had not met such an extremely severe storm surge since the opening of the port. Then storm surge was hindcasted with a numerical model. The typhoon pressure was approximated by Myers' empirical model and super gradient wind around the typhoon eye wall was considered in the wind estimation. The land topography surrounding Jinhae Bay and Masan Bay is so complex that the computed wind field was modified with the 3D-MASCON model. The motion of seawater due to the atmospheric forces was simulated using a one-layer model based on non-linear long wave approximation. The Janssen's wave age dependent drag coefficient on the sea surface was calculated in the wave prediction model WAM cycle 4 and the coefficient was inputted to the storm surge model. The result shows that the storm surge hindcasted by the numerical model was in good agreement with the observed one.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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• pp.44.2-44.2
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• 2016

The solar wind electrons are made of three or four distinct components, which are core Maxwellian background, isotropic halo, and super-halo (and sometimes, highly field-aligned strahl component which can be considered as a fourth element). We put forth a steady-state model for the solar wind electrons by considering both the steady-state particle and wave kinetic equations. Since the steady-state solar wind electron VDFs and the steady-state wave fluctuation spectrum are related to each other, we also investigate the complete fluctuation spectra in the whistler and Langmuir frequency ranges by considering halo- and superhalo-like model electron VDFs. It is found that the energetic electrons make important contributions to the total emission spectrum. Based on this, we complete the steady-state model by considering both the whistler and Langmuir fluctuations. In particular, the Langmuir fluctuation plays an important role in the formation and maintenance of nonthermal electrons.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.27 no.5
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• pp.281-294
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• 2015

In order to quantitatively estimate the nonlinear destructive interaction of wave load with wind load, which is very vital for the optimal design of offshore wind energy converter, we carried out a hydraulic model test and wind tunnel test. As a substructure of offshore wind energy converter, we would deploy the monopile, which is popular due to its easiness in construction. Based on the simulation using Monte Carlo simulation using Kaimal spectrum and cross spectrum, the instantaneous maximum wind velocity is adjusted to 10 m/s. And, considering the wave conditions of the Western Sea where a pilot wind farm is planned to be constructed, $H_s=0.1m$, 0.15 m, 0.2 m is carefully chosen. It turns out that the nonlinear destructive interaction between the wind and wave loads acting on the offshore wind energy converter is more clearly visible at rough seas rather than at mild seas, which strongly support our deduction that a Large eddy, a swirling vortex developed near the bumpy water surface in the opposite direction of the wind, is the driving mechanism underlying nonlinear destructive interaction between the wind and wave loads.

• Journal of Ocean Engineering and Technology
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• v.32 no.1
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• pp.9-20
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• 2018

The present study deals with the conceptual design of a motion reduction device for a floating wave-offshore wind hybrid power generation platform. A damping plate attached to the bottom of a column of a large semi-submersible is introduced to reduce the motion of the platform. Performance analyses on various shapes and configurations of damping plates were performed using the potential flow solver, and the appropriate configuration and size of the damping plate were selected based on the numerical results. In order to see the effect of viscous damping, a small scale model test was performed in a 2D wave flume. The performances of five different damping plates were measured and discussed based on the results of free decay tests and regular wave tests.

• International Journal of Ocean System Engineering
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• v.3 no.2
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• pp.68-76
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• 2013

In this study, a 3D numerical model was used to predict nonlinear wave forces on a cylindrical pile installed in a shallow water region. The model was based on solving the viscous and incompressible Navier-Stokes equations for a two-phase flow (water and air) model and the volume of fluid method for treating the free surface of water. A new application was developed based on the cut-cell method to allow easy installation of complicated obstacles (e.g., bottom geometry and cylindrical pile) in a computational domain. Free-surface elevation, water particle velocities, and inline wave forces were calculated, and the results show good agreement with experimental data obtained by the Danish Hydraulic Institute. The simulation results revealed that the proposed model can, without the use of empirical formulas (i.e., Morison equation) and additional wave analysis models, reliably predict non-linear wave forces on an offshore wind turbine foundation installed in a shallow water region.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.25 no.6
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• pp.422-432
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• 2013

A System Identification method to develop parametric models linking free surface elevation and wave pressure is presented and two models are built allowing for either wave pressure or free surface elevation simulation. Linear, time invariant model structures with static nonlinearities are assumed and solutions are sought in a form of autoregressive model with extra input (ARX). An arbitrary chosen free-surface elevation and wave pressure dataset is used for estimation of the models, which are subsequently verified against datasets with similar pressure gauge depth but different free-surface elevation spectra due to different meteorological conditions. It is shown that free-surface simulation using System Identification methods can perform better than traditional linear transfer function derived from linear wave theory (LTF), while wave pressure simulation quality using presented methods is generally similar to that obtained with corrected LTF.

• Wind and Structures
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• v.10 no.5
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• pp.463-479
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• 2007

A series of wind tunnel sectional model dynamic tests of a twin-deck bridge were conducted at the CLP Power Wind/Wave Tunnel Facility (WWTF) of The Hong Kong University of Science and Technology (HKUST) to investigate the effects of gap-width on the self-excited vibrations and the dynamic and aerodynamic characteristics of the bridge. Five 2.9 m long models with different gap-widths were fabricated and suspended in the wind tunnel to simulate a two-degrees-of-freedom (2DOF) bridge dynamic system, free to vibrate in both vertical and torsional directions. The mass, vertical frequency, and the torsional-to-vertical frequency ratio of the 2DOF systems were fixed to emphasize the effects of gap-width. A free-vibration test methodology was employed and the Eigensystem Realization Algorithm (ERA) was utilized to extract the eight flutter derivatives and the modal parameters from the coupled free-decay responses. The results of the zero gap-width configuration were in reasonable agreement with the theoretical values for an ideal thin flat plate in smooth flow and the published results of models with similar cross-sections, thus validating the experimental and analytical techniques utilized in this study. The methodology was further verified by the comparison between the measured and predicted free-decay responses. A comparison of results for different gap-widths revealed that variations of the gap-width mainly affect the torsional damping property, and that the configurations with greater gap-widths show a higher torsional damping ratio and hence stronger aerodynamic stability of the bridge.

• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.367-375
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• 2020

This paper presents an integrated analysis about dynamic performance of a Floating Offshore Wind Turbine (FOWT) OC4 DeepCwind with semi-submersible platform under real sea environment. The emphasis of this paper is to investigate how the wave mean drift force and slow-drift wave excitation load (Quadratic transfer function, namely QTF) influence the platform motions, mooring line tension and tower base bending moments. Second order potential theory is being used for computing linear and nonlinear wave effects, including first order wave force, mean drift force and slow-drift excitation loads. Morison model is utilized to account the viscous effect from fluid. This approach considers floating wind turbine as an integrated coupled system. Two time-domain solvers, SIMA (SIMO/RIFLEX/AERODYN) and FAST are being chosen to analyze the global response of the integrated coupled system under small, moderate and severe sea condition. Results show that second order mean drift force and slow-drift force will drift the floater away along wave propagation direction. At the same time, slow-drift force has larger effect than mean drift force. Also tension of the mooring line at fairlead and tower base loads are increased accordingly in all sea conditions under investigation.