• Journal of Korean Society of Coastal and Ocean Engineers
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• v.31 no.1
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• pp.17-27
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• 2019

We carry out the numerical simulation to test a hypothesis that freak waves can be triggered by the infragravity waves of bound mode underlying the ever-present swells and its constructive interaction with swells using the Tool Box called the ihFoam that has its roots on the OpenFoam, and Bi-spectrum. Numerical simulation is implemented for the SamChcuk LNG Plant where freak waves have been reported in front of the private wharf during its construction phase due to the uncompleted northern breakwater. Infra-gravity waves of bound mode is generated using the difference wave-wave interaction between the local wind waves of 7 s and a swell of 11.4 s based on the Bi-spectrum. For the sake of comparison, numerical simulation for infra-gravity waves of free mode is also carried out. Numerical results show that stem waves along the private wharf for SamChcuk LNG Plant can be triggered by the infra-gravity waves of bound mode coming from the north, which eventually leads to freak waves when encounters the reflected waves from the south jetty.

• Journal of Ocean Engineering and Technology
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• v.29 no.5
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• pp.359-365
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• 2015

A reliability analysis of the gravity-based foundation of anoffshore wind turbine was performed by considering the uncertainties of the design variables, including environmental loads. The limit state functions of the gravity-based foundation were defined using the response limits of the support structures suggested in the DNV standard. The wind load couldbe obtained using the GH_bladed software, and the wave load was calculated using the Morison equation. Then, the extreme distributions of the wind and wave loads were estimated by applying the peak over threshold (POT) method to the wind and wave load data. The probability distribution characteristics of the soil properties were defined with reference to a southwest coast geotechnical survey report. The reliability index was evaluated for each failure mode using a first-order reliability method.

• Ocean and Polar Research
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• v.24 no.3
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• pp.263-266
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• 2002

Gravity waves have been searched for with a new all-sky camera system over Korean Peninsular. The all-sky camera consists of a 37mm/F4.5 Mamiya fisheye lens with a 180 dog field of view, interference filters and a 1024 by 1024 CCD camera. The all-sky camera has been tested near Daejeon city, and moved to Mt. Bohyun where the largest astronomical telescope is operated in Korea. A clear wave pattern was successfully detected in OH filter images over Mt. Bohyun on July 18, 2001, indicating that small scale coherent gravity waves perturbed OH airglow near the mesopause. Other wave features are since then observed with Na 589.8nm and OI 630.0nm filters. Since a Japanese all-sky camera network has already detected traveling ionospheric disturbances (TID) over the northeast-southwest range of Japanese islands, we hope our all-sky camera extends the coverage of the TID's observations to the west direction. We plan to operate our all-sky camera all year around to study seasonal variation of wave activities over the mid-latitude upper atmosphere.

• Journal of Ocean Engineering and Technology
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• v.10 no.3
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• pp.62-73
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• 1996

Numerical simulation is made of the three-dimensional wave breaking motion about a part of a floating offshore structure containing a circular cylinder mounted vertically onto a lower hull in regular periodic gravity wave generated by a numerical wave maker. TUMMAC-VIII finite-difference method is newly developed for such a problem. By use of density-function technique the three-dimensional wave breaking motion is approximately implenented in the framework of rectangular grid system. A porosity technique is devised for the implementation of the no-slip bydy boundary conditions. The generation of breaking waves by the interaction of incident waves with the structure is well simulated and interesting features of breaking waves are revealed with containing degree of quantitative and qualitative accuracy.

• Atmosphere
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• v.16 no.4
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• pp.303-318
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• 2006

A parameterization of gravity wave drag induced by cumulus convection (GWDC) proposed by Chun and Baik is implemented in the KMA operational global NWP model (GDAPS), and effects of the GWDC on the forecast for July 2005 by GDAPS are investigated. The forecast result is compared with NCEP final analyses data (FNL) and model's own analysis data. Cloud-top gravity wave stresses are concentrated in the tropical region, and the resultant forcing by the GWDC is strong in the tropical upper troposphere and lower stratosphere. Nevertheless, the effect of the GWDC is strong in the mid- to high latitudes of Southern Hemisphere and high latitudes of Northern Hemisphere. By examining the effect of the GWDC on the amplitude of the geopotential height perturbation with zonal wavenumbers 1-3, it is found that impact of the GWDC is extended to the high latitudes through the change of planetary wave activity, which is maximum in the winter hemisphere. The GWDC reduces the amplitude of zonal wavenumber 1 but increases wavenumber 2 in the winter hemisphere. This change alleviates model biases in the zonal wind not only in the lower stratosphere where the GWDC is imposed, but also in the whole troposphere, especially in the mid- to high latitudes of Southern Hemisphere. By examining root mean square error, it is found that the GWDC parameterization improves GDAPS forecast skill in the Southern Hemisphere before 7 days and partially in the Northern Hemisphere after about 5 days.

• Korean Journal of Materials Research
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• v.14 no.11
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• pp.775-779
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• 2004

For improvement of heat dissipation performance, heat analysis is conducted on the newly designed heat sinks under two convection conditions by using computational fluid dynamics(CFD). Three types of heat sink, plate, wave and top vented wave, are used, and convection conditions are the variations of gravity direction at natural convection and of fan location at forced convection. The results of analysis showed that the heat resistances of top vented wave heat sink were $0.17^{\circ}C$/W(forced convection) and $0.48^{\circ}C$/W(natural convection). In the case of natural convection, gravity direction affected heat flow change, and protection against heat performance was superior in case of z-axis gravity direction. Under the forced convection, all the heat sinks revealed superior thermal characteristics in the fan position of z-axis rather than y-axis. In this study, it was observed that the top vented wave type heat sink showed the best ability of heat radiation comparing with the others.

• Journal of The Korean Astronomical Society
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• v.24 no.2
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• pp.129-149
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• 1991

An attempt has been made to examine the characteristics of acoustic and MHD waves generated in stellar convection zones($4000\;K\;{\leq}\;T_{eff}\;{\leq}\;7000\;K$, $3\;{\leq}\;\log\;g\;{\leq}\;4.5$). With the use of wave generation theories formulated for acoustic waves by Stein (1967), for MHD body waves by Musielak and Rosner (1987, 1988) and for MHD tube waves by Musielak et al.(l989a, 1989b), the energy fluxes are calculated and their dependence on effective temperature, surface gravity and megnetic field strength are analyzed by optimization techniques. In computing magneto-convection models, the effect of magnetic fields on the efficiency of convection has been taking into account by extrapolating it from Yun's sunspot models(1968; 1970). Our study shows that acoustic wave fluxes are dominant in F and G stars, while the MHD waves dominant in K and M stars, and that the MHD wave fluxes vary as $T_{eff}^4{\sim}T_{eff}^7$ in contrast to the acoustic fluxes, as $T_{eff}^{10}$. The gravity dependence, on the other hand, is found to be relatively weak; the acoustic wave fluxes ${\varpropto}\;g^{-0.5}$, the longitudinal tube wave fluxes ${\varpropto}\;g^{0.3}$ and the transverse tube wave fluxes ${\varpropto}\;g^{0.3}$. In the case of the MHD body waves their gravity dependence is found to be nearly negligible. Finally we assesed the computed energy fluxes by comparing them with the observed fluxes $F_{ob}$ of CIV(${\lambda}1549$) lines and soft X-rays for selected main sequence stars. When we scaled the corrected wave fluxes down to $F_{ob}$, it is found that these slopes are almost in line with each other.

• Journal of Ship and Ocean Technology
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• v.4 no.4
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• pp.45-55
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• 2000

A formal solution method using the complex analysis is given for the problems derived by Longuet-Higgins(1963). The same method is applied to a new perturbation problem of higher approximation. Interpretation of its solution made it possible to confirm that the rough agree-ment of Longuet-Higgins\`s prediction with experimental data of Cox(1958) was mainly due to the fact that the gravity effect in the perturbation problem was neglected for the case when the basic gravity wave not sufficiently steep.

• Journal of Astronomy and Space Sciences
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• v.24 no.4
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• pp.327-338
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• 2007

We estimate the momentum fluxes of short-period gravity waves which are observed in the OI 557.7 nm nightglow emission with all-sky camera at Mt. Bohyun ($36.2^{\circ}\;N,\;128.9^{\circ}\;E$) in Korea. The intrinsic phase speed ($C_{int}$), the intrinsic period (${\tau}_{int}$), and vertical wavelength (${\lambda}_z$) are also deduced from the horizontal wavelength (${\lambda}_h$), observed period (${\tau}_{ob}$), propagation direction (${\phi}_{ob}$), observe phase speed (${\upsilon}_{ob}$) of the gravity wave on the all-sky images. The neutral winds to deduce intrinsic wave parameters are measured with Fabry-Perot interferometer on Shigaraki ($34.8^{\circ}\;N,\;13.1^{\circ}\;E$) in Japan. We selected 5-nights of observations during the period between July 2002 and December 2006 considering of the weather and instrument conditions in two observation sites. The mean values of intrinsic parameter of gravity waves are $({\tau}_{int})\;=\;12.9\;{\pm}\;6.1\;m/s,\;({\lambda}_z)\;=\;12.9\;{\pm}\;6.5,\;and\;(C_{int})\;=\;40.6\;{\pm}\;11.6\;min$. The mean value of calculated momentum fluxes for four nights besides of ${\lambda}_z\;<\;6\;km$ is $12.0\;{\pm}\;15.2\;m^2/s^2$. It is needed the long-term coherent observation to obtain typical values of momentum fluxes of the mesospheric gravity waves using all-sky camera and the neutral wind measurements.

• 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.