• Proceedings of the Korea Water Resources Association Conference
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• 2006.05a
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• pp.567-573
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• 2006

An efficient typhoon wave-generating model is applied to northeast Asia sea zone presented that can be used by civil defense agencies for real-time prediction and fast warnings on typhoon-generated wind wave and storm surge. Instead of using commercialized wave models such as WAM, SWAN, the wind waves are simulated by using a new concept of wavelength modulation to enhance broader application of the hyperbolic wave model of the mild-slope equation type. The results simulated along the Korean coasts during Typhoon Nabi (2005) showed reasonable agreement with the recorded wind waves.

• Proceedings of the Korean Environmental Sciences Society Conference
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• 2003.11a
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• pp.37-42
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• 2003

We have analyzed the characteristics of the sea surface winds and wind waves near the Korean marginal seas on the basis of prediction results of the sea surface winds from MM5/KMA model, which is being used for the operation system at the Korea Meteorological observation buoy data to verify the model results during Typhoon events. The correlation coefficients between the models and observation data reach up to about 95％, supporting that these models satisfactorily simulate the sea surface winds and wave heights even at the coastal regions. Based on these verification results, we have carried out numerical experiments about the wave modulation. When there exist an opposite strong current for the propagation direction of the waves or wind direction, wave height and length gets higher and shorter, and vice versa. It is proved that these modulations of wave parameters are well generated when wind speed is relatively week.

• Wind and Structures
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• v.12 no.2
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• pp.103-120
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• 2009

Articulated tower platforms due to its compliant nature are more susceptible to the dynamic effects of wind than conventional fixed platforms. Dynamic response analysis of a double hinged articulated tower excited by low frequency wind forces with random waves is presented in this paper. The exposed super structure of the platform, housing the drilling and production facilities is subjected to mean and fluctuating wind loads, while the submerged portion is acted upon by wind driven waves. The fluctuating component of the wind velocity is modeled by Emil Simiu's spectrum, while the sea state is characterized by Pierson-Moskowitz spectrum. Nonlinearities in the system due to drag force, added mass, variable submergence and instantaneous tower orientation are considered in the analysis. To account for these nonlinearities, an implicit time integration scheme (Newmark's-${\beta}$) has been employed which solves the equation of motion in an iterative fashion and response time histories are obtained. The power spectra obtained from random response time histories show the significance of low frequency responses.

• Journal of Astronomy and Space Sciences
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• v.32 no.1
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• pp.1-11
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• 2015

Earth's magnetopause separating the fast and often turbulent magnetosheath and the relatively stagnant magnetosphere provides various forms of free energy that generate low-frequency surface waves. The source mechanism of this energy includes current-driven kinetic physical processes such as magnetic reconnection on the dayside magnetopause and flux transfer events drifting along the magnetopause, and velocity shear-driven (Kelvin-Helmholtz instability) or density/pressure gradient-driven (Rayleigh-Taylor instability) magnetohydro-dynamics (MHD) instabilities. The solar wind external perturbations (impulsive transient pressure pulses or quasi-periodic dynamic pressure variations) act as seed fluctuations for the magnetopause waves and trigger ULF pulsations inside the magnetosphere via global modes or mode conversion at the magnetopause. The magnetopause waves thus play an important role in the solar wind-magnetosphere coupling, which is the key to space weather. This paper presents recent findings regarding the generation of surface waves (e.g., Kelvin-Helmholtz waves) at the Earth's magnetopause and analytic and observational studies accountable for the linking of the magnetopause waves and inner magnetospheric ULF pulsations, and the impacts of magnetopause waves on the dynamics of the magnetopause and on the inner magnetosphere.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.19 no.1
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• pp.16-28
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• 2007

The results of experimental investigation of coherent structures beneath wind-generated waves in deep water are presented. Vorticity fields of deepwater wind waves were visualized by analyzing the velocity fields obtained by PIV measurements under different wind and fetch conditions. In addition, spatio-temporal evolution of the coherent structures and subsequent changes in vertical profiles of the instantaneous vorticity were qualitatively examined. It was found that a coherent structure is formed right underneath the wave crest and traveled in phase with the surface wave. The direction of rotation of the coherent structure was contrary to the wave orbital motion when wind speed is less than 10 m/s, while was same as the wave orbital motion when wind speed is approximately 13 m/s and wave breaking occurs at the wave crest. In the near-surface region, complex vortex-vortex interactions were observed according to the traveling of the coherent structure. In contrast, coherent structures far below the water surface changed little due to weak influence of orbital motion by the surface waves.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.12 no.1
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• pp.39-52
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• 2000

The triad interactions have been known to be important only for shoaling waves or finite depth wind waves. In deep water, they are insignificant compared with the quadruplet interactions in respect to the evolution of wind waves due to energy transfer among the wave components. However, the triad interactions may be important even for deep water waves because they may closely be related to the wave steepness, which definitely affects wave breaking, drag of air flow over t.'Ie sea, or navigation of ships, especially during the early stage of the development of wind waves. This study reports a series of laboratory experiments, whose data are subjected to bispectral analyses to investigate the triad interactions of deep-water wind waves. It is found that the bicoherence at the spectral peak frequency and the wave steepness are almost directly proportional, indicating that the steep waves with peaked crests and flat troughs are resulted from the triad interactions. Both bicoherence and wave steepness increase with the wave age during the early stage of wave generation and then drop off as the waves grow old. It seems that the energy of the secondary spectral peak developed by the triad interactions during the early stage of wave generation is redistributed to the neighboring frequencies by the quadruplet interactions during the later stage.

• 한국신재생에너지학회:학술대회논문집
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• 2009.11a
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• pp.466-472
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• 2009

Offshore wind energy is gaining more and more attention during this decade. For the countries with coast sites, the water depth is significantly large. This causes attention to the floating wind turbine. Offshore wind turbines are designed and analyzed using comprehensive simulation codes that account for the coupled dynamics of the wind inflow, aerodynamics, elasticity and controls of the wind turbine, along with the incident waves, sea current, hydrodynamics, and foundation dynamics of the support structures. In this work, a three-bladed 5MW upwind wind turbine installed on a floating spar buoy in 320m of water is studied by using of fully coupled aero-hydro-servo-elastic simulation tool. Specifications of the structures are chosen from the OC3 (Offshore Code Comparison Collaboration) under "IEA Wind Annex XXIII-subtask2". The primary external conditions due to wind and waves are simulated. Certain design load case is investigated.

• Journal of Advanced Research in Ocean Engineering
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• v.4 no.2
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• pp.53-65
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• 2018

The load condition significantly influences ship maneuverability in calm water. In this research, the effect of the load condition on turning performance of a very large crude oil carrier (VLCC) sailing in adverse weather conditions is investigated by an MMG-based maneuvering simulation method. The relative drift direction of the ship in turning to the wave direction is $20^{\circ}-30^{\circ}$ in ballast load condition (NB) and full load condition (DF) with a rudder angle $35^{\circ}$ and almost constant for any wind (wave) directions. The drifting displacement in turning under NB becomes larger than that under DF at the same environmental condition. Advance $A_d$ and tactical diameter $D_t$ become significantly small with an increasing Beaufort scale in head wind and waves when approaching, although $A_d$ and $D_t$ are almost constant in following wind and waves. In beam wind and waves, the tendency depends on the plus and minus of the rudder angle.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.30 no.5
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• pp.223-233
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• 2018

The predictability of winter storm waves using KMA's operational wind forecasts has been studied to predict wind waves and swells in the East coast of Korea using SWAN. The nested model were employed along the East coast of Korea to simulate the wave transformation in the coastal area and wave dissipation term of whitecapping is optimized to improve swell prediction accuracy. In this study, KMA's operational meteorological models (RDAPS and LDAPS) are used as input wind fields. In order to evaluate model accuracy, we also simulate wind waves and swells using ECMWF reanalysis and KIOST WRF wind and they are compared with the KMA's operational wave model and the wave measurement data on the offshore and onshore stations. As a result, it has the lowest RMSE and the highest correlation coefficient in the onshore when the input wind fields are KMA's operational meteorological forecasts. In the offshore, all of the simulate results shows good agreements with similar error statistics. It means that it is very feasible to use SWAN model with the modified whitecapping factor and KMA's operational meteorological forecasts for predicting the wind waves and swells in the East coast of Korea.

• Wind and Structures
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• v.11 no.2
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• pp.97-120
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• 2008

Wind and wave loadings have a predominant role in the design of offshore structures in general, and articulated tower in particular for a successful service and survival during normal and extreme environmental conditions. Such towers are very sensitive to the dynamic effects of wind and wind generated waves. The exposed superstructure is subjected to aerodynamic loads while the submerged substructure is subjected to hydrodynamic loads. Articulated towers are designed such that their fundamental frequency is well below the wave frequency to avoid dynamic amplification. Dynamic interaction of these towers with environmental loads (wind, waves and currents) acts to impart a lesser overall shear and overturning moment due to compliance to such forces. This compliancy introduces geometric nonlinearity due to large displacements, which becomes an important consideration in the analysis of articulated towers. Prediction of the nonlinear behaviour of these towers in the harsh ocean environment is difficult. However, simplified realistic mathematical models are employed to gain an important insight into the problem and to explore the dynamic behaviour. In this paper, various modeling approaches and solution methods for articulated towers adopted by past researchers are reviewed. Besides, reliability of articulation system, the paper also discussed the design, installation and performance of articulated towers around the world oceans.