• Ocean Systems Engineering
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• v.14 no.2
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• pp.141-156
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• 2024

The turbine system converts the kinetic energy of water flow to electricity by rotating the rotor in a restricted waterway between the seabed and free surface. A turbine system's immersion depth and rotation direction are significantly critical in the turbine's performance along with the shape of the rotor. This study has investigated the hydrodynamic performance of the Savonius hydrokinetic turbine (SHT) according to the immersion depth and rotation direction using computational fluid dynamics (CFD) simulations. The instantaneous torque, torque coefficient, and power coefficients are calculated for the immersion ratios Z/D ranging [0.25, 3.0] and both clockwise (CW) and counterclockwise (CCW) rotations. A flow visualization around the rotor is shown to clarify the correlation between the turbine's performance and the flow field. The CFD simulations show that the CCW rotation produces a higher power at shallow immersion, while the CW rotation performs better at deeper immersion. The immersion ratio should be greater than the minimum of Z/D=1.0 to obtain the maximum power production regardless of the rotation direction.

• Journal of Power Electronics
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• v.8 no.3
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• pp.259-267
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• 2008

Power quality improvement in a three-phase four-wire system is achieved using a new topology of DSTATCOM (distribution static compensator) consisting of a star/delta transformer with a tertiary winding and a three-leg VSC (voltage source converter). This new topology of DSTATCOM is proposed for power factor correction or voltage regulation along with harmonic elimination, load balancing and neutral current compensation. A tertiary winding is introduced in each phase for a delta connected secondary in addition to the star-star windings and this delta connected winding is responsible for neutral current compensation. The dynamic performance of the proposed DSTATCOM system is demonstrated using MATLAB with its Simulink and Power System Blockset (PSB) toolboxes under varying loads. The capacitor supported DC bus of the DSTATCOM is regulated to the reference voltage under varying loads.

• The Journal of the Acoustical Society of Korea
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• v.17 no.3E
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• pp.58-66
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• 1998

In this paper, we analyze the effects of interference canceller on the performance of multitone DS/CDMA system proposed by Vandendorpe[5]. There are various kinds of interference canceller suggested by different researchers including parallel and successive cancellers and we adopt a canceller used by Yoon et al.[9] which is a kind of parallel canceller. We consider three kinds of interferences, that is, multipath interference(MPI), interchannel interference(ICI) and multiple access interference(MAI). The ICI is the interference between multitones. The equations for variances. are derived for the inteferences and thermal noise used for signal to noise ratio calculation. We also consider RAKE reception over multipath channel which is modeled as lowpass equivalent linear filter and three stage interference canceller used for performance improvement. We show the performance results for number of canceller stage, diversity order and number of users and draw some conclusions that interference canceller is effective in multitone DS/CDMA system and the performance is further improved with the higher order of diversity and larger number of PN chips.

• International Journal of Fluid Machinery and Systems
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• v.3 no.1
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• pp.39-49
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• 2010

In this paper, a numerical study has been carried out to investigate the influence of jet fan design variables on the performance of a jet fan. In order to achieve an optimum jet fan design and to explain the interactions between the different geometric configurations in the jet fan, three-dimensional computational fluid dynamics and the DOE method have been applied. Several geometric variables, i.e., hub-tip ratio, meridional shape, rotor stagger angle, number of rotor-stator blades and stator geometry, were employed to improve the performance of the jet fan. The objective functions are defined as the exit velocity and total efficiency at the operating condition. Based on the results of computational analyses, the performance of the jet fan was significantly improved. The performance degradations when the jet fan is operated in the reverse direction are also discussed.

• Journal of the Korea Safety Management & Science
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• v.14 no.1
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• pp.189-200
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• 2012

Many companies have endeavored to build a quality cost management system in order to be more productive business organization. This study shows the detail procedures of constructing a quality cost management system which is believed to be appropriate for their business system. That is, the method to calculate the quality cost and the linking logic between the quality improvement and its financial impact are explained based on a particular industry case. In this sense, the changes of business performance measures such as market share, customer satisfaction, etc. were analyzed in the longitudinal perspective for the consecutive 4 years (2003~2006). As the quantitative results of this study, the improvement activities based on the quality cost management system resulted in the 32% reduction of quality cost and the 121% increase of business profit, compared 2005 with 2006. In the qualitative perspective, the successive practice of quality cost reduction and the job information sharing in business unit were obtained by providing the best practices and bench-marking cases. Finally, the customer satisfaction has increased so that the customer-friendly management system has been accomplished. With these efforts, the 3.4% increase of the market share and the 3% increase of the customer satisfaction were obtained in 2005. As the future study, the current study can be extended to the concept of COPQ (cost of poor quality) which focuses on the hidden quality cost of the whole business activities. Such extension of analysis will help us understand the wider role of a quality cost management system in the business.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.39A no.7
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• pp.365-371
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• 2014

Chaos communication system is possible to improve the system security by using chaos signal. Further, it is possible to reduce the possibility of eavesdropping, and have strong characteristics from interference signal and jamming signal. However, BER(Bit Error Rate) performance of chaos system is worse than digital communication system. By this reason, researches in order to improve the BER performance of chaos communication system are being actively studied. In previous studies, we proposed a novel chaos map for BER performance improvement, and called it 'Boss map'. Also, we proposed a novel chaos transceiver for BER performance improvement. However, BER performance is evaluated differently according to delay time in transceiver. Therefore, in order to use Boss map effectively, we should find the optimal delay time in proposed chaos transceiver. In this paper, when Boss map is used, we evaluate BER performance of CDSK(Correlation Delay Shift Keying) system and novel chaos transceiver according to delay time. After evaluation of BER performance according to delay time, we find a delay time that is possible to have best BER performance in CDSK system and novel chaos transceiver.

• International Journal of Control, Automation, and Systems
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• v.6 no.4
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• pp.534-543
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• 2008

To use the Global Positioning System (GPS) carrier phase measurement for precise positioning, the integer ambiguities at the early stage of most algorithms must be determined. Furthermore, if a precise positioning is to be applied to real time navigation, fast determination and validation methods for integer ambiguity are essential. In this paper, the Wald test that simultaneously determines and validates integer ambiguities is used with assistance of the Inertial Navigation System (INS) to improve its performance. As the Wald test proceeds, it assigns a higher probability to the candidate that is considered to be true at each time step. The INS information is added during the Wald test process. Large performance improvements were achieved in convergence time as well as in requiring fewer observable GPS satellites. To test the performance improvement of the Wald test with the INS information, experimental tests were conducted using a ground vehicle. The vehicle moved in a prescribed trajectory and observed seven GPS satellites. To verify the effect of the INS information on the Wald test, the convergence times were compared with cases that considered the INS information and cases that did not consider the INS information. The results show that the benefits of using the INS were emphasized as fewer GPS satellites were observable. The performance improvement obtained by the proposed algorithm was shown through the fast convergence to the true hypothesis when using the INS measurements.

• 한국신재생에너지학회:학술대회논문집
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• 2011.05a
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• pp.87.1-87.1
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• 2011

The residential Fuel Cell system has high efficiency of 85% with transferring natural gas to electrical power and heat, directly and it is a friendly environmental new technology in that $CO_2$ emission can reduce 40% compared with conventional power generator and boiler. The residential fuel cell system consists of two main parts which have electrical and hot storage units. The electrical unit contains a fuel processor, a stack, an inverter, a control unit and balance of plant(BOP), and the cogeneration unit has heat exchanger, hot water tank, and auxiliaries. 5kW class fuel process was developed and tested from 2009, it was evaluated for long-term durability and reliability test including with improvement in optimal operation logic. Stack development was crried out through improvement of design and evaluation protocol. Development of system controller was successfully accomplished through strenuous efforts and original control logic was optimized in 5kW class PEMFC system. In addition, we have been focused on development of system process and assembly technology, which bring about excellent improvement of reliability of system. The 5kW class PEMFC system was operated under dynamic conditions for 1,000 hours and it showed a good performance of total efficiency and durability.

• Journal of Korea Multimedia Society
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• v.20 no.12
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• pp.1970-1979
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• 2017

Computerization of educational administration following educational informatization of government has been steadily improved for the purpose of teachers' offload and job efficiency, finally resulting that NEIS(National Education Information System) has been completed. The NEIS consists of Nationwide service of NEIS, Business portal system of NEIS, Authentication management system and so on. Students, parents and civil petitioners handle civil affairs through Nationwide service of NEIS and teachers and persons of task conduct theirs business by accessing the Business portal system of NEIS. At this time, users have to obtain their certification from Authentication management system. Previous Studies were mainly focused on the evaluation about its performance according to the introduction of NEIS. But from now on there is a growing interest in security assessment and an efficient method for security improvement to check if NEIS works properly. Therefore, in this thesis, we'll propose an analytic framework in which security assessment is carried out after comprehending the fault structures through performing Fault Fishbone Analysis based on the Fault Tree Analysis. As a result of the system applied, the system had the highest rate of improvement to 47.7 percent.

• The Journal of Information Systems
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• v.21 no.2
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• pp.1-25
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• 2012

In the field of education in Korea, University Information Disclosure System was conducted forcibly in December 2008 by Ministry of Education, Science and Technology. As a result, information quality provided to the user has been improved dramatically. Based on the preceding studies of information success model, the purpose of this study was to prove empirically three facts on the organization's performance, user's attitude, and gaps between two factors by improvement in the quality of information. To accomplish the purpose, literature review and statistical analysis were done using the university evaluation data published by JoongAng Daily from 2007 to 2011. The results of this study showed that the information quality had positive effect on organization's performance, user's attitude like a reputation, and the gaps between two factors. The results of this study had a great expectation for the role of offering useful insight and information in the field of information system and education.