• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.17 no.7 s.124
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• pp.596-604
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• 2007

This article investigates the effects of geometric configuration on the vibro-acoustic characteristics of in-plane vibration of a thick annular disc. Disc thickness and outer radius for a given inner radius are selected as independent variables having reasonable ranges. Variations in structural eigensolutions for radial modes are investigated using pre-developed analytical method. Based on these data, far-field sound pressure distributions due to the modal vibrations for a given geometry are also calculated using an analytical solution. Modal sound powers and radiation efficiencies are calculated from the far-field sound pressure distributions and vibratory velocity distributions on the radial surfaces. Based on the results explained above, the geometric configuration that minimizes modal sound radiations in a given frequency range is determined. Finally sound power and radiation efficiency spectra for a unit harmonic force from the selected geometric configuration are obtained from structural and acoustic modal data using the modal expansion technique. Multi-modal sound radiations of the optimized disc that are obtained using proposed analytical methods are confirmed with numerical results. Using the procedure introduced in this article, sound radiation due to in-plane modes within a specific frequency range can be minimized by the disc geometry modifications in a comprehensive and convenient manner.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.38 no.2
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• pp.169-179
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• 2010

Satisfying the same probability of loss of control and essentially two fail operative performance with a triplex computer architecture requires a lot of modification of the conventional redundancy management design techniques, previously employed in quadruplex digital flight control computer. T-50 FCS for triplex redundancy management design applied an advanced digital flight control architecture with an I/O controller which is functionally independent of the digital computer to achieve the same reliability and special failure analysis and isolation schemes for fail operational goals with a triplex configuration. The analysis results indicated that the triplex flight control system is to satisfy the safety requirement utilizing the advanced flight control techniques and the system performance of the implemented flight control system was verified by failure mode effect test.

• Applied Chemistry for Engineering
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• v.33 no.1
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• pp.96-102
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• 2022

Reactants of PEMFC are hydrogen and oxygen in gas phases and fuel cell overpotential could be reduced when reactants are smoothly transported. Numerous studies to modify cathode flow field design have been conducted because oxygen mass transfer in high current density region is dominant voltage loss factor. Among those cathode flow field designs, a block in flow field is used to forced supply reactant gas to porous gas diffusion layer. In this study, the block was installed on a simple fuel cell model. Using computational fluid dynamics (CFD), effects of forced convection due to blocks on a polarization curve and local current density contour were studied when different air flow rates were supplied. The high current density could be achieved even with low air supply rate due to forced convection to a gas diffusion layer and also with multiple blocks in series compared to a single block due to an increase of forced convection effect.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.18 no.4
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• pp.527-535
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• 2017

Heat management is one of the most critical issues in Polymer Electrolyte Membrane Fuel Cells (PEMFCs) installed inside the fuel cell power pack of a fuel cell battery hybrid UPS. If the heat generated by the chemical reaction in the fuel cell is not rapidly removed, the durability and performance of the fuel cell may be affected, which may shorten its lifetime. Therefore, the objective of this study is to select and propose a proper cooling method for the fuel cells used in the fuel cell power pack of a UPS. In order to find the most appropriate cooling method, the various design factors affecting the cooling performance were studied. The numerical analysis was performed by a commercial program, i.e., COMSOL Multiphysics. Firstly, the surface temperature of the 1 kW class fuel cell stack with the cooling fans placed at the top was compared with the one with the cooling fans placed at the bottom. Various rotation speeds of the cooling fan, viz. 2,500, 3,000, 3,500, and 4,000 RPM, were tested to determine the proper cooling fan speed. In addition, the influence of the inhaled air flow rate was investigated by changing the porous area of the grille, which is the entrance of the air flowing from the outside to the inside of the power pack. As a result, it was found that for the operating conditions of the 1 kW class PEMFC to be acceptable, the cooling fan was required to have a minimum rotating speed of 3500 RPM to maintain the fuel cell surface temperature within an acceptable range. The results of this study can be effectively applied to the development of thermal management technology for the fuel cells inside the fuel cell power pack of a UPS.

• Journal of the Korea institute for structural maintenance and inspection
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• v.18 no.5
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• pp.96-106
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• 2014

The increased vibration level of the railway bridge could make significant noise and, also, cause structural damages such as fatigue cracks. Related to these subjects, a spherical elastomeric bridge bearing, which is layered by hemispherical rubber and steel plates, was investigated in terms of its vibration performance. Several different shape factors could be considered by changing the curvature of hemispherical surface and size in rubber and steel plate thicknesses in the manufacturing stage. The performance of the spherical elastomeric bearing for the reduction in vibration was compared with that of the conventional bearing by performing vibration experiments on a scale-downed model. The rubber material characteristics and spherical shape are found to be important parameters in reducing the bridge vibration.

• Journal of Advanced Marine Engineering and Technology
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• v.38 no.4
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• pp.367-375
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• 2014

In numbers of kinds of heat exchanger, the shell-tube heat exchanger is the most commonly used type of heat exchanger in the industry field. In order to improve the thermal performance of the heat exchanger, this study was analyzed heat transfer characteristics according to arrangement of baffle and direction of baffle and bump phase of baffle about shell-tube heat exchanger using appropriate SST (Shear Stress Transport) turbulence model for flow separation and boundary layer analysis. As the boundary condition for CFD (Computational Fluid Dynamics) analysis, the inlet temperature of shell side was constantly 344 K and the variation of the water flow rate was 6, 12, 18 and 24 l/min. As the result of analysis, zigzag baffle arrangement enhances heat transfer rate and pressure drop. Furthermore, in the direction of the baffle, heat transfer rate is more improved with vertical type and angle $45^{\circ}$ type than existing type, and pressure drop was little difference. Also, the bump shape of baffle surface contributes to heat transfer rate and pressure drop improvement due to the increased heat transfer area. Through analysis results, we knew that the increase of the heat transfer was influenced by flow separation, fluid residual time, contact area with the tube, flow rate, swirl and so on.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.8
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• pp.855-860
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• 2011

We numerically simulated a dissolved air flotation (DAF) tank to predict the performance of the pilot facility. The flow was assumed to be two-dimensional and two-phase. The velocity distributions in the separation zones of differently shaped DAFs were compared to find the effect of the shape on the performance. The results showed that the typical flow pattern that appeared in a well-designed DAF-tank was generated in the separation zone of the base model. This flow pattern could be maintained while the baffle height was sufficiently tall regardless of the other geometric parameters. However, the baffle height and angle, the contact zone width, and the perforated plate affected the uniformity of the downward flow in the separation zone. Except for the baffle height, the base model used in this study showed a better uniformity of downward flow than did other models with different geometric parameters.

• The Journal of the Korea Contents Association
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• v.20 no.9
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• pp.672-680
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• 2020

This study aimed to elucidate the differences in nursing service experience according to the participants' characteristics and experience of hospitalization and to identify the relation between nursing service experience and perceptions of the health system. It analyzed the data from a "Survey of health care service experience" conducted among 574 participants who had been hospitalized. The results showed that the mean nursing service experience of participants who had never used other hospitals before hospitalization with over ten days of hospital stay was higher compared with the other participants. Moreover, those who used other hospitals before hospitalization reported lower nursing service experience scores. In the classification based on the change of the inpatient room, the difference in the nurse service experience according to the number of hospitalization days was significant only in patients who had no change in the inpatient room in the multi-person room. The nursing service experience scores were positively related to general satisfaction with the hospitalization experience, health system satisfaction, and trust in the health system. This finding implies that the nursing service experience should be interpreted by separating the inpatient experience, and the strategies to improve the nursing service are needed. In addition, the survey should include items related to the characteristics of medical institutions such as nurse staffing, to identify factors affecting the experience of nursing services.

• Journal of the Korea Concrete Institute
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• v.26 no.3
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• pp.287-297
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• 2014

Heat of hydration of mass concrete is one of the most important factors that significantly affect structural quality and construction period. Therefore, appropriate methods to control heat of hydration are essential technologies for mass concrete construction. In this study, probability of thermal cracking was checked by thermal analysis prior to the construction of a turbine foundation in a domestic power plant. Subsequently, changes of concrete mix proportion and an effective curing method were proposed to control heat of hydration of mass concrete structures. Concrete manufactured by slag cement was proposed instead of concrete produced by ordinary Portland cement, and an automated curing method was proposed to improve the curing method using typical moist curing with blanket. The automated curing method maintains the temperature difference between center and surface of concrete below a setting value by temperature monitoring. Concrete with slag cement was used for actual construction. One of two identical turbine foundations was cured by an insulated curing method, and the other was cured by the automated curing method to compare the curing methods. And then, the effects of control of heat of hydration were evaluated based on temperature/strain monitoring and crack investigations.

• The Proceeding of the Korean Institute of Electromagnetic Engineering and Science
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• v.17 no.4 s.60
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• pp.48-52
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• 2006

2006 CISPR 총회가 스웨덴 스톡홀름 KISTA에서 개최되었다. CISPR I 회의는 9월 18일부터 20일까지 3일간 WG2 및 WG4를 중심으로 진행되었으며, WG1과 WG3 회의는 9월 20일 오후에 있었다. 우리나라에서는 WG1에 두 가지 제안을 발표하였다. 하나는 2004년 중국 상해 회의에서 처음 제안하여 2005년 남아공 케이프타운 회의에서 Task Force가 구성된 대형 시험품에 대한 CISPR 20의 외부 방사내성 시험레벨에 관한 것이다. 2006년 1월 미국 산타페에서 열린 TF 회의에서 시험결과에 대한 기술적 토론이 있었으며, Round Robin Test(RRT)를 실시하기로 결정되었다. RRT시험은 7월 중순에 실시되었으며, 참여한 시험소는 영국의 SONY, 네덜란드의 Philips, 독일의 SHARP 이었으며, 국내의 여러 EMC 시험소도 추후에 참여하였다. RRT 시험의 결과 및 분석이 이번 스톡홀름 회의에서 발표되었다. 또한, CISPR 13의 방사성 방출(RE)의 측정거리 기준점의 변경에 관한 것이다. 본 고에서는 TV 수신기에 대한 RRT 시험결과 및 분석에 대한 발표 내용을 소개하고자 한다. CISPR 20의 외부 전자기장 내성 시험은 0.8 m인 개방형 스트립라인(Open Strip-line: OSL)내에 시험품을 설치하고 $150kHz{\sim}150MHz$에서 시험하도록 규정하고 있다. OSL에 설치할 수 없는 대형기기는 IEC 61000-4-3에 따라서 주파수 범위 $80MHz{\sim}150MHz$까지 동일한 한계치로 전자파무반사실에서 시험하도록 규정되어 있다. 이에 대해 국내 제조업체 및 EMC 전문가들로 구성된 CISPR I 국내 위원회에서는 IEC 61000-4-3에 의한 전자파무반사실(챔버)에서의 내성시험이 CISPR 20에 의한 OSL에서의 내성시험보다 가혹하다는 문제 제기가 있었다. 이것이 본 안건에 대한 배경이다 지금까지 진행되어 왔던 내용을 요약해 보면 2004년 상해 회의에서, 수치해석 및 측정비교를 통하여 CISPR 20에서 규정하고 있는 OSL 전기장 교정방법의 개선의 필요성과 OSL에서 시험할 수 없는 대형 시험품에 대하여 시험레벨을 조정할 것을 지적하였다. 2005년 케이프타운 회의에서는 실제적으로 TV 방송 수신기 및 관련기기가 두가지 방법으로 시험되었을 때 시험결과에 미치는 영향을 시험품의 전기장 내성 레벨로 비교하였으며, 전자파무반사실에서 시험할 경우 시험레벨이 12 dB 낮추도록 보정되어야 할 것을 제안하였다. 2006년 이번 회의에서는 세계 각국의 시험소와 비교시험을 통하여 나타난 OSL과 전자파무반사실 간의 시험결과의 차이 및 이에 대한 원인을 분석하여 보고하였다. OSL과 전자파무반사실 비교시험에서 나타난 차이는 평균적으로 9 dB의 차이로 나타났으며, 주요 원인은 OSL에서 적용하고 있는 시험품을 OSL내부에 설치한 후 시험품에 의해 변경된 전기장의 세기를 보정해 주는 인자(k2)로 인한 차이(6 dB)와 OSL 내부의 전기장의 세기와 전자파무반사실 균일장 영역의 전기장의 세기의 차이(3 dB)이다.