• Journal of the Korean Society for Marine Environment & Energy
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• v.15 no.4
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• pp.357-362
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• 2012

Despite the Wave observations data is very important information to human life at sea, the technology development and research for wave equipments are lacking. In this study, the wave observation system using GPS was evaluated the quality of wave observation data by comparing of long-term observations. The result of the comparison of the acceleration sensor (Hippy-40) and GPS sensor (Mose-1000), the correlation coefficient of the significant wave height and significant wave periods is 0.997 and 0.990 respectively. Also in case of BIAS, the significant wave height is 0.014 m, the significant wave period is -0.212 sec. It makes no significant differences whether the acceleration sensor (Hippy-40) and GPS sensor (Mose-1000). These results of the wave observation data using GPS quality will be evaluated as very good.

• The Journal of the Acoustical Society of Korea
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• v.18 no.3E
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• pp.9-20
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• 1999

To investigate the characteristics of internal waves (IWs) and their effects on acoustic wave propagation, a series of sea experiment were performed in the east coast of Donghae city, Korea in 1997 and 1998 where the water depth varies between 130 and 140 m. Thermistor strings were deployed to measure water temperatures simultaneously at 9 depths. CW source signals with the frequencies of 250,670 and 1000 Hz were received by an array of 15 hydrophones. Through the Wavelet transform analysis, the IWs are characterized as having typical periods of 2-17 min and duration of 1-2 hours. The IWs exist in a group of periods rather than in one period. Underwater acoustic signals also show obvious energy peaks in the periods of less than 12 min. Consistency in the periods of the two physical processes implies that acoustic waves react to the IWs through some mechanisms like mode interference and travel time fluctuation. Based on the thermistor string data, mode arriving structures are analyzed. As thermocline depth varies with time, it may cause travel time difference as much as 4-10 ms between mode 1 and 2 over 10 km range. This travel time difference causes interference among modes and thus fluctuation from range-independent stratified ocean structure. In real situations, however, there exist additional spatial variation of IWs. Model simulations with all modes and simple IWs show clear responses of acoustic signals to the IWs, i.e., fluctuations of amplitude and phase.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.66.1-66.1
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• 2015

We present the first observational detection of radial and azimuthal oscillations in full halo coronal mass ejections (HCMEs). We analyze nine HCMEs well-observed by LASCO from Feb 2011 to Jun 2011. Using the LASCO C3 running difference images, we estimated the instantaneous apparent speeds of the HCMEs in different radial directions from the solar disk center. We find that the development of all these HCMEs is accompanied with quasi-periodic variations of the instantaneous radial velocity with the periods ranging from 24 to 48 mins. The amplitudes of the instant speed variations reach about a half of the projected speeds. The amplitudes are found to anti-correlate with the periods and correlate with the HCME speed, indicating the nonlinear nature of the process. The oscillations have a clear azimuthal structure in the heliocentric polar coordinate system. The oscillations in seven events are found to be associated with distinct azimuthal wave modes with the azimuthal wave number m=1 for six events and m=2 for one event. The polarization of the oscillations in these seven HCMEs is broadly consistent with those of their position angles with the mean difference of $42.5^{\circ}$. The oscillations may be connected with natural oscillations of the plasmoids around a dynamical equilibrium, or self-oscillatory processes, e.g. the periodic shedding of Alfvenic vortices. Our results indicate the need for advanced theory of oscillatory processes in CMEs.

• Journal of the Earthquake Engineering Society of Korea
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• v.11 no.4
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• pp.65-77
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• 2007

Site response analyses were performed based on equivalent linear technique using shear wave velocity profiles of 162 sites collected around the Korean peninsula. The site characteristics, particularly the shear wave velocities and the depth to the bedrock, are compared to those in the western United States. The results show that the site-response coefficients based on the mean shear velocity of the top 30m ($V_{S30}$) suggested in the current code underestimates the motion in short-period ranges and overestimates the motion in mid-period ranges. The current Korean code based on UBC is required to be modified considering site characteristics in Korea for the reliable estimation of site amplification. From the results of numerical estimations, new regression curves were derived between site coefficients ($F_{a}\;and\;F_{v}$) and the fundamental site periods, and site coefficients were grouped based on site periods with reasonable standard deviations compared to site classification based on $V_{S30}$. Finally, new site classification system and modification of design response spectra are recommended considering geotechnical characteristics in Korea.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2011.04a
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• pp.389-394
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• 2011

Existing FPS(Friction Pendulum System) is isolation system which is possible to isolate structures by pendulum characteristic from ground vibration. Structural natural frequency could be decided by designing the radius of curvature of FPS. Thus, response vibration could be reduced by changing natural frequency of structures from FPS. But effective periods of recorded seismic wave were various and estimation of earthquake characteristic could be difficult. If effective periods of seismic wave correspond to natural frequency of structures with FPS, resonance can be occurred. Therefore, CFPBS(Cone-type Friction Pendulum Bearing System) was developed for controlling the response acceleration and displacement by the slope of friction surfaces. Structural natural frequency with CFPBS can be changed according to position of ball on the friction surface which was designed cone-type. Therefore, Divergence of response could be controlled by CFPBS which had constantly changing natural frequency with low modal participation factor in wide-range. In this study, Seismic performance of CFPBS was evaluated by numerical analysis and shaking table test.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.21 no.7
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• pp.599-608
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• 2011

FPS(friction pendulum system) is an isolation system which is possible to isolate structures from earthquake by pendulum characteristic. Natural frequencies of the structures could be determined by designing the radius of curvature of FPS. Thus, response vibration could be reduced by changing natural frequency of structures from FPS. But effective periods of recorded seismic wave were various and estimation of earthquake characteristic could be difficult. If effective periods of seismic wave correspond to natural frequency of structures with FPS, resonance can be occurred. Therefore, CFPBS(cone-type friction pendulum bearing system) was developed for controlling the acceleration and displacement of structure by the slope of friction surfaces. Structural natural frequency with CFPBS can be changed according to position of ball on the friction surface which was designed cone-type. Therefore, superstructures on CFPBS could be isolated from earthquake. In this study, seismic performance of CFPBS was evaluated by numerical analysis and shaking table test.

• Journal of the Korean Society of Marine Environment & Safety
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• v.22 no.6
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• pp.750-757
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• 2016

This study presents the wave run-up height around single and multiple surface-piercing cylinders according to wave period and steepness. In order to simulate 3D incompressible viscous two-phase turbulent flow, the present study employed a volume of fluid (VOF) method with realizable $k-{\varepsilon}$ turbulence model based on commercial Computational Fluid Dynamics (CFD) software, "STAR-CCM". The wave periods at model scale were 1.269s and 1.692s for a single cylinder and 1.716s for multiple cylinders. In each case, wave steepness of has 1/30 and 1/16 were used, respectively. Consequently, the results for wave run-up height with regard to wave steepness and period were compared with those of relevant previous experimental studies. The numerical simulation results showed a good qualitative agreement with experiments.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.31 no.2
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• pp.88-99
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• 2019

This study presents the results of analysis for the wave data that were consecutively collected from February 2013 to November 2018 at the location of 1.6 km offshore from Namhangjin beach. The water depth at the location is 30.5 m and waves were measured by AWAC (Acoustic Wave And Current meter). By using wave-by-wave analysis and spectral analysis, wave heights and periods were evaluated and then the relationships between the quantities obtained by the two methods were proposed based on linear regression analysis. In addition, monthly and yearly variations of the significant wave height and period, and the peak wave direction were analyzed. Moreover, the relationship between the significant wave height and period was newly suggested. Variability and probability distribution of the significant wave period with respect to the significant wave height were also examined.

• Journal of the Korean Society of Marine Environment & Safety
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• v.28 no.5
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• pp.700-711
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• 2022

In this study, deepwater design waves were estimated for 16 wave directions and various return periods based on statistical analysis of extreme waves observed for more than 20 years at three stations (Chilbal-do, Geomun-do, Donghae). These values were compared with design waves estimated based on the omni-directional wave data. The Weibull distribution was used as the probability distribution function whose parameters were determined by the least square method. The Kolmogorov-Smirnov test was applied for the goodness of fit test. Notably, the directional design waves were smaller than the omni-directional design wave for every wave direction. The maximum 50-year wave heights for directional sectors were 7.46 m (NNE), 12.05 m (S), and 9,59 m (SSW) at Chilbal-do, Geomun-do and Donghae whereas those for uni-directional wave data were 7.91 m, 13.82 m and 10.38 m, respectively. This implied possible under-estimation of the deepwater design waves for 16 wave directions being currently used in the design of offshore and coastal structures.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2004.05a
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• pp.30-35
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• 2004

Several roll motion reduction devices are reviewed and suggested for the application in FPSO. The firstly suggested solution is the shape of the bilge. The next is a bilge keel. The last suggestion is the ART (anti-rolling tank). Typical U-tube type ART is designed for a FPSO and examined extensively by model experiment. The model section was made of transparent acryl. Free decay test, forced oscillation test and wave test were carried out at a two-dimensional wave flume. U-tube type ART is effective only when the natural periods of ART and ship are same. Therefore, the divided U-tube type ART with split plate is suggested for the reduction of the roll motion of a FPSO over the wide range of the roll period.