• The Transactions of the Korean Institute of Power Electronics
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• v.27 no.4
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• pp.356-366
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• 2022

This paper presents a mode transition method that applies frequency compensation technique of an LLC resonant converter for stable mode transition. LLC resonant converters used in various applications require high efficiency and high power density. However, because of circuit property, a wider voltage gain range equates to a greater circuit loss, so maintaining high efficiency at all voltage gain ranges is difficult. In this case, full bridge-half bridge mode transition method can be used, which maintains high efficiency even in a wide voltage gain range. However, this method causes damage to the circuit through overcurrent by the mode transition. This study analyzes the cause of the problem and proposes a mode transition method that applies frequency compensation technique to solve the problem. The proposed method verifies the stable transition through simulation analysis and experimental results.

• Geomechanics and Engineering
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• v.18 no.6
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• pp.651-659
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• 2019

The IB-SEM numerical method combines the spectral/hp element method and the rigid immersed boundary method. This method avoids the problems of low computational efficiency and errors that are caused by the re-division of the grid when the solids move. Based on the Fourier transformation and the 3D immersed boundary method, the 3D IB-SEM system was established. Then, using the open MPI and the Hamilton HPC service, the computational efficiency was increased substantially. The flows around a cylinder and a sphere were simulated by the system. The surface of the cylinder generates vortices with alternating shedding, and these vortices result in a periodic force acting on the surface of the cylinder. When the shedding vortices enter the flow field behind the cylinder, a recirculation zone is formed. Finally, the three-dimensional pore flow was successfully investigated.

• Journal of Korean Society of Transportation
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• v.28 no.4
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• pp.41-59
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• 2010

In this study, the indicators for evaluating the level of green growth in transportation are proposed. The authors reviewed the existing philosophies such as sustainable development and eco-efficiency and explained the differences between the philosophies and green growth. The planning hierarchy and indicator framework are proposed and the idea of indicators is also introduced. In order to decide the weighting value for objectives and targets, a questionnaire survey for transportation experts was conducted. The survey results show that low-carbon/environmentally-friendly development is the most important objective. In addition, the results reveal that transportation experts do not fully understand the concept of green growth and cannot differentiate the concept of green growth from that of sustainable development and eco-efficiency.

• Journal of Korean Society of Water and Wastewater
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• v.33 no.3
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• pp.215-224
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• 2019

Recent focus on wastewater treatment includes energy-saving and renewable energy generation for energy-independence of water infrastructures. Aeration and pumping in biological wastewater removal processes account for nearly 30-60% of the total electricity cost in real wastewater treatment plants. In this study, the performance and microbial characteristics were investigated in sequencing batch reactor under typical oxygen and oxygen limited condition. Under typical DO ($7.55{\pm}0.99mg/L$) and low DO ($0.23{\pm}0.08mg/L$) conditions, COD removal was stable over 91 % during SBR operation. Ammonia removal efficiency was reduced from 95.6 % to 89.2 % when DO concentration was dropped sharply. Phosphorus removal efficiency also reached 77% at oxygen-limited condition. The results indicated that removal efficiency both ammonia and phosphorus was influenced by DO condition. Microbial analysis revealed that Proteobacteria and Bacteroidetes at phylum level was dominant in typical DO and low DO conditions and DO concentration did not much affect phylum distribution. Population decrease of genera of nitrifying bacteria(Dokdonella) and Dechloromonas spp. affect removal efficiency of nitrogen and phosphorus at low DO condition.

• Transportation Technology and Policy
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• v.8 no.3
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• pp.41-50
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• 2011

• Journal of Korean Society of Transportation
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• v.35 no.5
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• pp.458-467
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• 2017

The expressway of the Korea has an important role in freight movement because 76 percent of the commodity is transported by trucks. However, there has been few indices on the role of expressways regarding freight transportation and truck traffic. The objective of this study is to propose four freight transportation related indices using ITS-related system such as TCS and HS-Wim: total truck's travel miles ($veh{\cdot}km/year$), total freight transport miles ($ton{\cdot}km/year$). efficiency of truck's travel ($veh{\cdot}km/km$), and efficiency of freight movement ($ton{\cdot}km/km$). These truck and freight related indices were estimated and compared by two different data sources: traffic volume data using VDS and OD data using TCS. These indices were designed to estimated on real time and updated every day and month.

• Journal of the Korea Safety Management & Science
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• v.8 no.4
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• pp.129-149
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• 2006

Six sigma, contrary to TQM, offers the concrete process for accomplishing the management innovation. Nowadays it has become as a standard means. Many companies claim to stand for durable management and would like to get customers' satisfaction and more profit with total six sigma activities. Under the present conditions we propose how to maximize efficiency with six sigma application on transportation management that is an issue among many companies.

• Journal of Korean Society of Transportation
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• v.32 no.4
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• pp.336-346
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• 2014

Recently interest and concern about the effectiveness of the transportation investment policy have been increasing because lots of transportation projects reflected to the National Transportation Plan are suspended or delayed during pre-feasibility study. For instance, the criticism such as overlap and deficiency in consistency in appraisal system were aroused because of the waste of budget, time and man-power etc. This is because the Korea government adopted pre-feasibility study for the purpose of efficient use of national budget, but the methodology and the appraisal contents are similar to the existing feasibility study. Also, the feasibility appraisal study was adopted in 1999 for the purpose of establishing efficient transport networks but the feasibility study has been substituting the function which should be conducted by feasibility-appraisal study. In this study, by reviewing the current situation and the purport of introducing these policies, the understanding of these policies could be increased. Then, the plans for improving the efficiency of transportation appraisal system are suggested in the context of strengthening the connectivity between these policies. This study is expected to play an important role in increasing the rationality, consistency and efficiency of transportation investment policies.

• The Journal of the Korea Contents Association
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• v.21 no.4
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• pp.88-95
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• 2021

In Korea, which achieved economic growth due to export industries, the economic importance of the foreign port transportation industry is very great. The government took the initiative in fostering the foreign port transportation industry, and as a result, it developed into one of the top 10 foreign port transport countries in the world. However, due to the poor management of the small and medium-sized foreign port transport business and the trade dispute between countries, the foreign port transportation industry has faced a long-term invasion. Therefore, in order for these companies to recover again, they will, among other things, have to reorganize their operational inefficiency to carry out efficient production activities. In this study, we want to analyze the management efficiency of the top 10% companies in the unlisted domestic outbound shipping industry, present the project value of inefficient enterprises, and confirm the reference collection of companies that may be subject to benchmarking. For this purpose, a total of 10 foreign port transportation industry were analysed for efficiency and scale profitability (RTS) of the CCR and BCC models. The analysis showed that there were a total of six entities with BCC efficiency and five entities with CCR efficiency. In addition, there were a total of five entities with both BCC and CCR efficiency.

• Journal of Navigation and Port Research
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• v.42 no.2
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• pp.107-116
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• 2018

Planning strategies to achieve higher competitiveness of ports are becoming increasingly important in business environment. Therefore, strategic competitive position and efficiency analysis needs to be performed to increase ports' effectiveness and competitiveness. This matches with one of targets of new concept e-Navigation to increase the agility and efficiency of ports. The purpose of this study was to apply Boston Consulting Group matrix to analyze competitive positioning of major ports in Korea and China in term of several main cargo types and then use a combination of Data Envelopment Analysis and Principal Component Analysis model to calculate efficiencies. Results show that, at the moment, Chinese ports are still on the top with high position and efficiency score for the representative-Shanghai port. However, result also points out that except container type, Korean ports have chance to compete in other cargo types. Moreover, Gwangyang port is regarded as efficient. It has better position time. It is believed that Gwangyang port together with Busan port can compete with Chinese port in the near future.