• Journal of the Institute of Convergence Signal Processing
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• v.23 no.1
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• pp.15-20
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• 2022

An improvement of computational resources with a large scale cluster service is available to the individual person, which has been limited to the original industry and research institute. Therefore, the application of powerful computational resources to the engineering design has been increased fast. In naval and marine industry, the application of Computational Fluid Dynamics, which requires a huge computational effort, to a design of ship and offshore structure has been increased. Floating bodies such as the ship or offshore structure is exposed to ocean waves, current and wind in the ocean, therefore the precise modelling of those environmental disturbances is important in Computational Fluid Dynamics. Especially, ocean waves has to be nonlinear rather than the linear model based on the superposition due to a nonlinear characteristics of Computational Fluid Dynamics. In the present study, a fast reconstruction technique is suggested and it is validated from a series of simulations by using the Computational Fluid Dynamics.

• Atmosphere
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• v.34 no.2
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• pp.153-176
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• 2024

At 0843 UTC 30 May 2021, a commercial aircraft encountered severe turbulence at z = 11.5 km associated with the rapid development of Mesoscale Convective System (MCS) in the Gyeonggi Bay of Korea. To investigate the generation mechanisms of Near-Cloud Turbulence (NCT) near the MCS, Weather Research and Forecasting model was used to reproduce key features at multiple-scales with four nested domains (the finest ∆x = 0.2 km) and 112 hybrid vertical layers. Simulated subgrid-scale turbulent kinetic energy (SGS TKE) was located in three different regions of the MCS. First, the simulated NCT with non-zero SGS TKE at z = 11.5 km at 0835 UTC was collocated with the reported NCT. Cloud-induced flow deformation and entrainment process on the downstream of the overshooting top triggered convective instability and subsequent SGS TKE. Second, at z = 16.5 km at 0820 UTC, the localized SGS TKE was found 4 km above the overshooting cloud top. It was attributed to breaking down of vertically propagating convectively-induced gravity wave at background critical level. Lastly, SGS TKE was simulated at z = 11.5 km at 0930 UTC during the dissipating stage of MCS. Upper-level anticyclonic outflow of MCS intensified the environmental westerlies, developing strong vertical wind shear on the northeastern quadrant of the dissipating MCS. Three different generation mechanisms suggest the avoidance guidance for the possible NCT events near the entire period of the MCS in the heavy air traffic area around Incheon International Airport in Korea.

• Journal of the Korean Geotechnical Society
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• v.30 no.1
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• pp.17-25
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• 2014

Offshore wind turbine foundations are subjected to wind, current and wave loadings. Hence, both static and cyclic behaviors of foundation's soil are important for the design of offshore wind turbine foundation. Undrained cyclic behaviors of soils depend upon the number of loading cycles, vertical effective stress, cyclic shear strain, relative density, and the combination of cyclic and average shear stresses. In order to evaluate the effect of average and cyclic shear stresses on the undrained cyclic behavior of marine silty sand, cyclic direct simple shear (CDSS) tests are performed with relative density of 85%, vertical effective stress of 200 and 300 kPa, and failure criteria of either 15% double amplitude cyclic shear strain (${\gamma}_{cyc}$) or permanent shear strain (${\gamma}_{p}$). The results are presented in the form of design graphs or contour diagrams. The undrained cyclic behavior of marine silty sand is found to be dependent on cyclic and average shear stresses and/or the combination of both shear stresses. It is found that when significant average shear stress exists the permanent or progressive shear strain is the govering failure criteria instead of cyclic shear strain.

• Journal of the Korea Concrete Institute
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• v.26 no.6
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• pp.751-759
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• 2014

Currently, due to global warming, occurrences of extreme climate phenomena such as heat wave, heavy snow, heavy rain, super typhoon are continuously increasing all over the world. Due to these extreme climate phenomena, concrete structures and infrastructures are exposed to serious deterioration and damage. However, researches on construction technologies and standards to confront the climate change generated problems are needed presently. In order to better handle these problems, the validity of the present concrete mixture proportions are evaluated considering wind speed and sunlight exposure time based on climate change record in Seoul, Korea. The specimens cured at various wind speed and sunlight exposure time conditions were tested to obtain their compressive and split tensile strengths at various curing ages. Moreover, performance based evaluation (PBE) method was used to analyze the target strength satisfaction percentage of the concrete cured for the curing conditions. From the probabilistic method of performance evaluation of concrete performance, feasibility and usability of current concrete mix design practice for climate change conditions can be evaluated.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.23 no.3
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• pp.226-235
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• 2011

Field measurements of the winds and waves were carried out for one year at multiple locations inside and outside of the Pohang New Harbor in order to clarify the reason of downtimes frequently occurring at most of the harbor quays and to establish an efficient countermeasure. In addition, the downtime records of the quays and precipitation data provided by Korea Meteorological Agency were acquired for mutual comparison and comprehensive analysis of the cause of downtimes. Except the influence of precipitation, it was found that the downtimes occurred when the height of waves entering into the harbor incurred by either one of swell, wind seas, or mixture of both, exceeded a threshold. The seiche whose period ranges from 5 to 80 minutes, which was suspected as a possible cause of the downtimes, is shown to have no direct relation with the downtimes. Meanwhile, the height of far-infra-gravity waves whose period ranges between 0.5 and 3 minutes, propagating to the harbor mouth forced by short period waves, showed almost proportional relationship with the height of short period waves. Based on the result of this study, it is concluded that the downtime problems of Pohang New Harbor can be greatly improved by effectively preventing the entrance of short period waves such as swell or wind seas.

• Journal of the Korea institute for structural maintenance and inspection
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• v.19 no.5
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• pp.45-55
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• 2015

Currently, extreme weather events such as super typhoon, extreme snowfall, and heat wave are frequently occurring all over the world by natural and human caused factors. After industrial growth in the 1970s, earth's temperature has risen sharply. due to greenhouse effect. Global warming can be attributed to gases emitted from using fossil fuel such as average carbon dioxide, perfluorocarbons, nitrous oxide, and methane. Especially, carbon dioxide has the highest composition of about 90%. in the fossile fuel usage emitted gas. Concrete has excellent durability as a building material climate change. However, due to various of physical and chemical environmental effect such as conditions during its curing process, the performance degradation may occur. Carbon dioxide in the atmosphere causes steel corrosion and durability decreases by lowering the alkalinity of concrete. Therefore, in this study, concrete durability performance with respect to carbonation from curing conditions change due to wind speed and sunshine exposure time. Concrete carbonation experiment are performed. using wind speed (0, 2, 4, 6) m/s and sunlight exposure time (2, 4, 6, 8) hrs. Also, performance based evaluation through the satisfaction curve based on the carbonation depth and carbonation rate test results are performed.

• Journal of the Korean Society of Marine Environment & Safety
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• v.28 no.7
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• pp.1201-1208
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• 2022

In general, to maximize the supply and efficiency of floating offshore wind power generation energy, the motion caused by wave attenuation of the substructure must be reduced. According to previous studies, the motion response was reduced due to the vortex viscosity generated by the damping plate installed in the lower structure among the waves. In this study, a 5 MW semi-submersible OC5 platform and two platforms with attenuation plates were designed, and free decay experiments and numerical calculations were performed to confirm the effect of reducing motion due to vortex viscosity. As a result of the model test, when the heave free decay tests were conducted at drop heights of 30 mm, 40 mm, and 50 mm, compared with the OC5 platform, the platform with two types of damping plates attached had relatively improved motion damping performance. In the model test and numerical calculation results, the damping plate models, KSNU Plate 1 and KSNU Plate 2, were 1.1 times and 1.3 times lower than OC5, respectively, and the KSNU Plate 2 platform showed about two times better damping performance than OC5. This study shows that the area of the damping plate and the vortex viscosity are closely related to the damping rate of the heave motion.

• Journal of Navigation and Port Research
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• v.46 no.6
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• pp.483-490
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• 2022

Floating LiDAR is a system that provides a new paradigm for wind condition observation, which is essential when creating an offshore wind farm. As it can save time and money, minimize environmental impact, and even reduce backlash from local communities, it is emerging as the industry standard. However, the design and verification of a stable platform is very important, as disturbance factors caused by fluctuations of the buoy affect the reliability of observation data. In Korea, due to the nation's late entry into the technology, a number of foreign equipment manufacturers are dominating the domestic market. The west coast of Korea is a shallow sea environment with a very large tidal difference, so strong currents repeatedly appear depending on the region, and waves of strong energy that differ by season are formed. This paper conducted a study examining buoys suitable for LiDAR operation in the waters of Korea, which have such complex environmental characteristics. In this paper, we will introduce examples of optimized design and verification of ship-type buoys, which were applied first, and derive important concepts that will serve as the basis for the development of various platforms in the future.

• The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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• v.23 no.1
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• pp.1-19
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• 2018

In order to compare significant wave height (SWH) data from multi-satellites (GFO, Jason-1, Envisat, Jason-2, Cryosat-2, SARAL) and SWH measurements from Ieodo Ocean Research Station (IORS), we constructed a 12 year matchup database between satellite and IORS measurements from December 2004 to May 2016. The satellite SWH showed a root mean square error (RMSE) of about 0.34 m and a positive bias of 0.17 m with respect to the IORS wave height. The satellite data and IORS wave height data did not show any specific seasonal variations or interannual variability, which confirmed the consistency of satellite data. The effect of the wind field on the difference of the SWH data between satellite and IORS was investigated. As a result, a similar result was observed in which a positive biases of about 0.17 m occurred on all satellites. In order to understand the effects of topography and the influence of the construction structures of IORS on the SWH differences, we investigated the directional dependency of differences of wave height, however, no statistically significant characteristics of the differences were revealed. As a result of analyzing the characteristics of the error as a function of the distance between the satellite and the IORS, the biases are almost constant about 0.14 m regardless of the distance. By contrast, the amplitude of the SWH differences, the maximum value minus the minimum value at a given distance range, was found to increase linearly as the distance was increased. On the other hand, as a result of the accuracy evaluation of the satellite SWH from the Donghae marine meteorological buoy of Korea Meteorological Administration, the satellite SWH presented a relatively small RMSE of about 0.27 m and no specific characteristics of bias such as the validation results at IORS. In this paper, we propose a conversion formula to correct the significant wave data of IORS with the satellite SWH data. In addition, this study emphasizes that the reliability of data should be prioritized to be extensively utilized and presents specific methods and strategies in order to upgrade the IORS as an international world-wide marine observation site.

• Smart Structures and Systems
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• v.15 no.2
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• pp.283-297
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• 2015

Jacket-type offshore structures are always exposed to severe environmental conditions such as salt, high speed of current, wave, and wind compared with other onshore structures. In spite of the importance of maintaining the structural integrity for an offshore structure, there are few cases to apply a structural health monitoring (SHM) system in practice. The impedance-based SHM is a kind of local SHM techniques and to date, numerous techniques and algorithms have been proposed for local SHM of real-scale structures. However, it still requires a significant challenge for practical applications to compensate unknown environmental effects and to extract only damage features from impedance signals. In this study, the impedance-based SHM was carried out on a 1/20-scaled model of an Uldolmok current power plant structure in Korea under changes in temperature and transverse loadings. Principal component analysis (PCA)-based approach was applied with a conventional damage index to eliminate environmental changes by removing principal components sensitive to them. Experimental results showed that the proposed approach is an effective tool for long-term SHM under significant environmental changes.