• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2009년도 제33회 추계학술대회논문집
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• pp.382-386
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• 2009

극심한 작동환경에 노출되는 부품은 일반적으로 수명이 줄어들게 된다. 액체로켓의 터보펌프는 높은 회전속도로 인한 높은 원심력과 높은 온도와 같은 환경에서 작동한다. 이와 같은 환경에서 작동하는 터보펌프는 극심한 피로와 크리프의 영향으로 재료의 내구성이 떨어지게 된다. 75톤급 터보펌프 터빈의 내구성을 확보하기 위하여 피로와 크리프의 영향을 고려한 손상률을 계산하였다. 터보펌프 피로해석에는 ABAQUS/CAE와 MSC.Fatigue를 사용하여 해석을 수행하였으며, 크리프 해석을 수행하기 위해서 Larson-Miller parameter 곡선과 로빈슨 법칙을 사용하였다. 본 연구에서는 터보펌프의 터빈에 대한 내구성을 확보하기 위하여 피로의 영향과 크리프의 영향을 비교, 분석하였다.

• Wind and Structures
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• 제30권6호
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• pp.617-631
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• 2020

Wake-induced aerodynamics of yawed circular cylinders with smooth and grooved surfaces in a tandem arrangement was studied. This pair of cylinders represent sections of stay-cables with smooth surfaces and high-voltage power conductors with grooved surfaces that are vulnerable to flow-induced structural failure. The study provides some insight for a better understanding of wake-induced loads and galloping problem of bundled cables. All experiments in this study were conducted using a pair of stationary section models of circular cylinders in a wind tunnel subjected to uniform and smooth flow. The aerodynamic force coefficients and vortex-shedding frequency of the downstream model were extracted from the surface pressure distribution. For measurement, polished aluminum tubes were used as smooth cables; and hollow tubes with a helically grooved surface were used as power conductors. The aerodynamic properties of the downstream model were captured at wind speeds of about 6-23 m/s (Reynolds number of 5×10⁴ to 2.67×10⁵ for smooth cable and 2×10⁴ to 1.01×10⁵ for grooved cable) and yaw angles ranging from 0° to 45° while the upstream model was fixed at the various spacing between the two model cylinders. The results showed that the Strouhal number of yawed cable is less than the non-yawed case at a given Reynolds number, and its value is smaller than the Strouhal number of a single cable. Additionally, compared to the single smooth cable, it was observed that there was a reduction of drag coefficient of the downstream model, but no change in a drag coefficient of the downstream grooved case in the range of Reynolds number in this study.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2008년도 추계 학술발표회 제20권2호
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• pp.517-520
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• 2008

중력식 콘크리트 댐은 댐의 자중에 의해 수압 등의 외력에 저항하여 역학적인 안정을 유지하는 댐으로 크게 내부콘크리트와 외부콘크리트로 분류되어 시공된다. 성덕 다목적댐은 경북내륙지역의 안정적 용수공급 및 보현천, 길안천유역의 홍수피해 경감을 위해 경상북도 청송군에 건설 중인 댐으로, 댐길이 274m 댐높이 58.5m 총 저수용량 27.9${\times}$10$^6$ m$^3$ 규모의 중력식 다목적 댐이다. 본 댐의 분할 타설고 설정 및 수화열에 의한 온도 균열 유무 조사 등의 목적으로 상류 가물막이댐에 대한 시험시공을 실시하였다.본 연구에서는 성덕 다목적댐의 내부 및 외부콘크리트의 배합, 강도 특성 및 단열온도 등의 역학적 특성과 발열 특성을 실내실험을 통하여 확인하고, 상류 가물막이댐의 시험시공시 온도 및 응력 계측을통한 현장에서의 수화열과 온도응력 특성을 검토하였다.

• 한국해양공학회지
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• 제24권1호
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• pp.60-67
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• 2010

Recently, there has been an increase in the construction of various types of coastal structures for efficient wave dissipation, seawater exchange, and so on. Among these, a perforated slit caisson has been widely used to reduce the reflected wave energy and the wave pressure on the structure. Therefore, many studies on the wave force on a caisson, as well as the wave reflection from it, have been carried out with laboratory experiments and numerical analyses, considering it as a 2-D problem. However, because a structure like a perforated slit caisson has a variable 3-D shape, waves forces should be considered as a 3-D problem. Therefore, in this paper, a fully-nonlinear 3-D numerical model (LES-WASS-3D) is proposed to examine the reflection characteristics of a perforated slit caisson with two chambers. The numerical model, LES-WASS-3D, was verified in a 3-D wave field by a comparison with existing experimental data for wave reflection coefficients. Then, using the numerical results, the reflection from a perforated slit caisson with two chambers was examined in relation to wave steepness, chamber width, and the shape/porosity of perforated slit.

• 한국건축시공학회:학술대회논문집
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• 한국건축시공학회 2008년도 추계 학술논문 발표대회
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• pp.199-203
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• 2008

With recent trend in domestic and global market requiring architectures' conversion into skyscrapers seasoned with the features of landmarks, structural problems in relation with explosive spatting during fire emergencies are arising as controversial issues. Accordingly, many productive researches have been made in relation to the reinforcement techniques for improving fire resistance and the number of applications in the field is gradually increasing. In this study, a ductile outline form using ECC (Engineered Cementations Composites) was made with improvements on the structure and fire resistance to examine its applicability. Also, currently in Japan, the number of studies and applications is increasing focusing on reduction of construction time and improvement of workability with application of Half-PCa method. However, using such method of construction, large structural members decrease the utilization of space and architecture-wise, there is a disadvantage of the weight increase. Therefore, in such context, it would be worth reducing the weight of the structural members by reducing the size using ECC. In addition, its excellent pseudo strain-hardening due to fiber may have great effects on seismic designs. In the mean time, this study planned 3 equal conditions for mix water, PVA fiber and additives excluding binder and refractory to evaluate the mechanical properties of resistance against pressure and internal force. Finally, an evaluation was executed on the fire resistance of the newly made ductile outline form. As a result, from ECC-I to ECC-III, all showed excellent mechanical properties due to pseudo strain-hardening and in the fire resistance test conducted with ISO 834 heating curve, most of them tended to be in the range of the reference temperature (538℃-180min), so there was no occurrence of any explosive spatting.

• 한국결정성장학회:학술대회논문집
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• 한국결정성장학회 1996년도 The 9th KACG Technical Annual Meeting and the 3rd Korea-Japan EMGS (Electronic Materials Growth Symposium)
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• pp.422-448
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• 1996

Low-temperature epitaxial growth of Si and SiGe layers of Si is one of the important processes for the fabrication of the high-speed Si-based heterostructure devices such as heterojunction bipolar transistors. Low-temperature growth ensures the abrupt compositional and doping concentration profiles for future novel devices. Especially in SiGe epitaxy, low-temperature growth is a prerequisite for two-dimensional growth mode for the growth of thin, uniform layers. UHV-ECRCVD is a new growth technique for Si and SiGe epilayers and it is possible to grow epilayers at even lower temperatures than conventional CVD's. SiH and GeH and dopant gases are dissociated by an ECR plasma in an ultrahigh vacuum growth chamber. In situ hydrogen plasma cleaning of the Si native oxide before the epitaxial growth is successfully developed in UHV-ECRCVD. Structural quality of the epilayers are examined by reflection high energy electron diffraction, transmission electron microscopy, Nomarski microscope and atomic force microscope. Device-quality Si and SiGe epilayers are successfully grown at temperatures lower than 600℃ after proper optimization of process parameters such as temperature, total pressure, partial pressures of input gases, plasma power, and substrate dc bias. Dopant incorporation and activation for B in Si and SiGe are studied by secondary ion mass spectrometry and spreading resistance profilometry. Silicon p-n homojunction diodes are fabricated from in situ doped Si layers. I-V characteristics of the diodes shows that the ideality factor is 1.2, implying that the low-temperature silicon epilayers grown by UHV-ECRCVD is truly of device-quality.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2003년도 하계학술대회 논문집 Vol.4 No.2
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• pp.787-790
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• 2003

A rf triode magnetron sputtering system is designed and installed its construction in vacuum chamber. In order to calibrate the rf triode magnetron sputtering for thin films deposition processes, the effects of different glow discharge conditions were investigated in terms of the deposition rate measurements. The basic parameters for calibrating experiment in this sputtering system are rf power input, gas pressure, plasma current, and target-to-substrate distance. Because a knowledge of the deposition rate is necessary to control film thickness and to evaluate optimal conditions which are an important consideration in preparing better thin films, the deposition rates of copper as a testing material under the various sputtering conditions are investigated. Furthermore, a triode sputtering system designed in our team is simulated by the SIMION program. As a result, it is sure that the simulation of electron trajectories in the sputtering system is confined directly above the target surface by the force of $E{\times}B$ field. Finally, some teats with the above 4 different sputtering conditions demonstrate that the deposition rate of rf triode magnetron sputtering is relatively higher than that of the conventional sputtering system. This means that the higher deposition rate is probably caused by a high ion density in the triode and magnetron system. The erosion area of target surface bombarded by Ar ion is sputtered widely on the whole target except on both magnet sides. Therefore, the designed rf triode magnetron sputtering is a powerful deposition system.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2008년도 추계학술대회 논문집 Vol.21
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• pp.395-395
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• 2008

Water-assisted synthesis of carbon nanotubes (CNTs) has been intensively studied in recent years, reporting that water vapor enhances the activity and lifetime of metal catalyst for the CNT growth. While most of these studies has been focused on the supergrowth of CNTs at high temperature, rarely has the similar approach been made for the CNT synthesis at low temperature. Since the metal catalyst are much less active at lower temperature, we expect that the addition of water vapor may increase the activity of catalyst more largely at lower temperature. We synthesized multi-walled CNTs at temperature as low as $360^{\circ}C$ by introducing water vapor during growth. The water addition caused CNTs to grow ~3 times faster. Moreover, the water-assisted growth prolonged the termination of CNT growth, implying the enhancement of catalyst lifetime. In general, a thinner catalyst layer is likely to produce smaller-diameter, longer CNTs. In a similar manner, the water vapor had a greater effect on the growth of CNTs for a smaller thickness of catalyst in this study. To figure out the role of process gases, CNTs were grown in the first stage and then exposed to each of process gases in the second stage. It was shown that water vapor and hydrogen did not etch CNTs while acetylene led to the additional growth of CNTs even faster in the second stage. As-grown CNTs were characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), and Raman spectroscopy.

• 한국전기전자재료학회논문지
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• 제30권10호
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• pp.656-664
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• 2017

This paper presents the design and fabrication of a high power piezoelectric ultrasonic surgery unit for multi-purpose dental implantation. A conventional piezoelectric ultrasonic surgery units consists of a transducer and a tip. However, the drawback of this simple structure is that the output performance of the transducer considerably changes with the change of the tips. An ultrasonic surgery unit that has an additional booster between the transducer and the tip can solve this problem to some extent; for this, an optimal structural design for the transducer is required. We used the Bolted Langevin Transducer (BLT) as the basic transducer; it consists of piezoelectric ceramics and a metal body. It's structure was optimized using mathematical methods to determine the length and radius of the tail and head masses. Additionally, the booster was also subjected to the same methods. Using these mathematical methods, optimal results in terms of the resonance frequency (24.96 kHz), displacement ($14.27{\mu}m$), and pressure (2.8 MPa), could be obtained. The validity of this proposed surgery unit was confirmed experimentally, exhibiting a cutting force of around 7% higher than that of a conventional surgery unit.

• 한국항공우주학회지
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• 제45권3호
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• pp.241-251
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• 2017

EFD-CFD 워크샵 중 Case3인 RAE Wing A 형상에 대해 전산유체역학 해석을 수행하였다. 해석자로 3개의 상용 CFD 해석 코드(Star-CCM+, Fluent, CFX)와 오픈 소스 CFD 코드(SU2)를 사용하였다. 격자는 상용 격자 생성 코드인 Pointwise를 이용하여 격자의 수에 따라 4가지를 생성하였다. 마하수 0.4, 0.8에서 는 받음각 2도, 마하수 0.9에서 받음각 1도에서 해석하였다. coarse격자부터 fine 격자까지는 압력계수 곡선과 수직력 계수는 비슷한 결과를 보였으나 항력 계수에서는 차이가 있었다. Star-CCM+과 Fluent의 해석에서 차분법의 차수가 높을수록 충격파의 위치를 앞으로 예측하였다. 수렴에 다다르는 계산시간은 Fluent, Star-CCM+, CFX의 순으로 나타났고 SU2는 이에 비해 수렴하는데 많은 시간이 들었다.