• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집E
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• pp.754-759
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• 2001

This paper presents unsteady computational investigations and wind tunnel tests on the flow field around a square cylinder with a gap between the body and the ground plane. Two-dimensional unsteady, incompressible Navier-Stokes codes are developed for the computation of the viscous turbulent flows. By computing the flow around a square cylinder without ground effect, three two-equation turbulence models are evaluated and the developed code is validated. The results show a good agreement with experimental values and other computational results. Critical gap height at which the formation of Karman vortex streets is interrupted, is demonstrated and another transition regime is pointed out

• 한국초전도저온공학회:학술대회논문집
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• 한국초전도저온공학회 2002년도 학술대회 논문집
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• pp.123-126
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• 2002

Samples of Nb/Al-Al$O_{x}$/Nb tunnel junction with the size of 50$\mu$m $\times$ 50$\mu$m were fabricated by employing self-aligning and reactive ion etching technique. In the samples with high-quality, $V_{m}$ value (the product of the critical current and subgap resistance measured at 2 mV) was 34 mV at the critical current density $J_{c}$ = 500 A/$cm^{2}$ and $V_{g}$ value (the gap voltage) was 2.8 mV. In the higher $J_{c}$, voltage fluctuation in the current rising at the gap voltage was observed. The $V_{m}$ and $J_{c}$ value were 8 mV and 900 A/$cm^{2}$, respectively.

• 한국정보디스플레이학회:학술대회논문집
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• 한국정보디스플레이학회 2008년도 International Meeting on Information Display
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• pp.660-663
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• 2008

In this study, it was mainly focused on the mechanism and reliability performances of PI/PET composites after many times of curving. We developed a new process of spacer for flexible display to improve the maintenance of cell gap. This new process used the laser carving technology, which is widely applied on printing press, to produce the pattern of spacers and shaped both of the alignment film and spacers simultaneously by press of pattern. Assembling the spacer-shaped film and plastic substrates together well and it shows an excellent performance on the maintenance of cell gap and reliability of curving.

• 한국결정성장학회지
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• 제14권1호
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• pp.17-20
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• 2004

본 논문의 목적은 초전도체에 자기장 H가 인가되고 있는 경우. 임계온도에서의 비열 불연속폭을 열역학적 방법을 사용하여 인가자기장 H의 해석적 함수로서 정확히 구하는 데에 있다. 그리고 이러한 결과를 이용하여 몇몇 초전도체의 비열 불연속폭에 대한 수치를 계산하고 그 의미를 분석할 것이다 .특히 초전도체의 일반적 현상인 상태변이 시의 비열불연속 현상이 특정 세기의 자기장하에서는 일어나지 않는다는 주목할 만한 결과에 대해서도 언급할 것이다.

• International Journal of Aeronautical and Space Sciences
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• 제17권2호
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• pp.139-148
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• 2016

The passive control methods such as horizontal and vertical fences on the lower surface of the bluff body were applied to suppress the vortex shedding and enhance the aerodynamic stability of flow. For investigating the effects of the passive control methods, wind tunnel experiments on the unsteady flow field around a bluff body near a moving ground were performed. The boundary layer and velocity profiles were measured by the Hot Wire Anemometer (HWA) system and the vortex shedding patterns and flow structures in a wake region were visualized via the Particle Image Velocimetry (PIV) system. Also, it is a measuring on moving ground condition that the experimental values of the critical gap distances, Strouhal numbers and aerodynamic force FFT analyses. Through the experiments, we found that the momentum supply due to moving ground caused the vortex shedding at the lower critical gap distance rather than that of fixed ground. The horizontal and vertical fences increase the critical gap distance and it can suppress the vortex shedding. Consequently, the stability characteristics of the bluff body near a moving ground could be effectively enhanced by the simple passive control such as the vertical fences.

• Journal of Electrical Engineering and Technology
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• 제13권3호
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• pp.1326-1336
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• 2018

This paper proposes a numerical approach to predict the critical flashover voltages of air gaps under lightning impulses. For an air gap, the impulse voltage waveform features and electric field features are defined to characterize its energy storage status before the initiation of breakdown. These features are taken as the input parameters of the predictive model established by support vector machine (SVM). Given an applied voltage range, the golden section search method is used to compute the prediction results efficiently. This method was applied to predict the critical flashover voltages of rod-rod, rod-plane and sphere-plane gaps over a wide range of gap lengths and impulse voltage waveshapes. The predicted results coincide well with the experimental data, with the same trends and acceptable errors. The mean absolute percentage errors of 6 groups of test samples are within 4.6%, which demonstrates the validity and accuracy of the predictive model. This method provides an effectual way to obtain the critical flashover voltage and might be helpful to estimate the safe clearances of air gaps for insulation design.

• Journal of Advanced Marine Engineering and Technology
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• 제12권3호
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• pp.57-67
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• 1988

Although handicapped by the inability to bridge the size gap between small laboratory sample and large engineering component, the V charpy test sample method does possess certain advantages, such as each of preparation, simplicity of test method, speed, low cost in test machinery, and low cost per test. On the other hand, the COD test method does posses advantages, which reduce the size gap between the laboratory sample and actual engineering component. Consequently, the correlation between V charpy absorbed energy and the critical COD value is required for estimating critical COD value from the simple V charpy test results. In this paper, the high tensile strength steel AH36 plate specimens having a single edge cracked notch were investigated to find out the correlation between V charpy absorbed energy and critical COD value in the welded parts under such various welding methods as shielded metal arc welding, the submerged arc welding and the electro gas welding by means of V charpy impact test and static 3-point bending test. Main results obtained are as follow ; 1. The relationships between V charpy absorbed energy Wc' and critical COD value ($\delta_c$)show; $\delta_c$=0.0065 Wc'+0.1906. 2. Ductile- brittle transition behaviours can be estimated by means of fracture appearance and general yielding behaviours. 3. The V charpy absorbed energy of SMAW is higher than that of SAW, EGW and similar relationships are obtained in the COD tests.

• 대한기계학회논문집B
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• 제20권9호
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• pp.2981-2990
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• 1996

Dual solutions for steady natural convection of air between two horizontal concentric cylinders are numerically investigated in the range of $D_i$/TEX>/L(=diameter of inner cylinder/gap width)$\leq$10. It is found that, when the Rayleigh number based on the gap width exceeds a certain critical value, a new flow pattern forming two counter-rotating eddies in the half of the annulus can be realized, which is different from the crescent-shaped flow commonly observed. In the new flow pattern, the fluid near the top of the hot inner cylinder moves downward. This solution is found for D$_{i}$/L.geq.0.3, but not for$D_i$/TEX>/L$\leq$0.2. As $D_i$/TEX>/L increase, the critical Rayleigh number is decreased, and tends to a finite limit.t.

• 품질경영학회지
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• 제43권3호
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• pp.397-408
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• 2015

Purpose: Express mail service(EMS) supplied by 'Korea POST'is an international postal service which brings letters, documents and parcels to a destination abroad in the fastest and safest way. This service is in a tough competition with global companies such as TNT, FedEx, DHL or UPS. To build an effective strategy for being competitive, an intensive study on EMS service quality should be accomplished from a customer's perspective. Methods: Kano model and improvement gap analysis are used to categorize and prioritize EMS service quality attributes. A 5-point Lickert scale is employed to apply the Kano model and improvement gap analysis. Each quality attribute can be classified as weak in comparison with competitors, critical for improvement, or potentially competitive. Results: Price, on-time delivery and delivery quality are to be kept as is; fast delivery, parking convenience and staff expertise can be potentially competitive; and the other attributes are considered to be critical for improvement. Conclusions: Fast reimbursement and proper compensation when shipping items are lost or damaged are to be improved with high priority. Efforts to secure fast delivery, parking convenience and staff expertise are needed to be competitive in the future.

• 한국초전도ㆍ저온공학회논문지
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• 제11권4호
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• pp.33-36
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• 2009

An air gap between the pancake windings was provided in this paper to increase the central magnetic field of a high temperature superconducting (HTS) magnet consisting of pancake windings. Unlike the LTS magnet, providing an air gap between the pancake windings increases the central magnetic field of a HTS magnet. Furthermore, the uniformity of the magnetic field near the center of the magnet increased because the pancake windings spread out in wider area. Effects of the air gap on the central magnetic field of an HTS magnet was described in this paper, Calculation of the critical current was carried out by using E-J relation of the HTS wire and the optimization technique was adopted to obtain the appropriate critical current which could maximize the central magnetic field. Pancake windings with BSCCO-2223 HTS wire were wound on glass epoxy bobbin. 6 double pancake windings with 200 turns were used to construct a HTS magnet. Characteristics of the HTS magnet including the central magnetic field and the uniformity of the magnetic field were measured and compared with the results of calculation.