• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집E
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• pp.45-50
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• 2001

This experimental study concerns the characteristics of a transitional flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one rotating. The pressure losses and skin-friction coefficients have been measured for the fully developed flow of a 0.2 % aqueous solution of sodium carbomethyl cellulose (CMC) at a inner cylinder rotational speed of $0{\sim}600$ rpm. The transitional flow has been examined by the measurement of pressure losses, to reveal the relation of the Reynolds numbers with the skin-friction coefficients, in the laminar and transitional flow regimes. The occurrence of transition has been checked by the gradient change of pressure losses and skin-friction coefficient with respect to the Reynolds numbers. The increasing rate of skin-friction coefficient due to the rotation is uniform for laminar flow regime, whereas it is suddenly reduced for transitional flow regime and, then, it is gradually declined for turbulent flow regime. Consequently, the critical(axial-flow) Reynolds number decrease as the rotational speed increases. Thus, the rotation of inner cylinder promotes the early occurrence of transition due to the onset of taylor vortices.

• 산업공학
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• 제21권4호
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• pp.365-377
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• 2008

In this paper, for constructing the Korean Hazard Information System (KHIS), we conceptually design a hazard evaluation process. We first deal with a hazard evaluation process focused on flood hazard to give the most immense damage and loss. The hazard evaluation process is consist of a damage evaluation process and a loss evaluation process, and is used for transforming hazards from natural disasters into economic measures. The proposed process is developed based on the famous FEMA (Federal Emergency Management Agency)'s $HAZAS^{@MH}$methodology. We modify the FEMA's process to be mutually exclusive and collectively exhaustive, that is all losses from the hazards are included into the estimation process but the losses are not duplicated in the process. In addition to this, we define the loss process specifically by considering the characteristics from the hazard environments of Korea. We can expect that KHIS for evaluating economic losses from natural hazards can be developed based on the conceptual design for the economic loss evaluation process, and KHIS can be used as a useful tool for analyzing the feasibilities of mitigation plans in central/local governments.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1997년도 하계학술대회 논문집 F
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• pp.2047-2049
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• 1997

Computers and automatic equipment are very sensitive to the disturbances such as voltage transients, voltage sag, and harmonics. These disturbances cause them to have a malfunction or fault which brings about damages and losses. UPS (uninterruptible power supply) and SPS(stand-by power supply) have been used to provide the required voltage in a critical load without disturbances. However, UPS has appreciable losses due to the operation of the inverter in full rated power at all times. SPS, although whose inverter losses are smaller than those of UPS, transfers disturbances with a short duration to the load, due to the limited reaction time. In this research, a dynamic voltage compensator, which can make up for the weakness of UPS and SPS, is proposed. The operation of the proposed system was verified by a computer simulation. A hardware scaled-model was fabricated and tested to conform the feasibility of the actual system development.

• 대한전자공학회논문지TC
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• 제37권10호
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• pp.18-25
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• 2000

본 논문에서는 ATM망에서의 UBR 서비스를 이용한 네트워크 구성에서의 TCP 혼잡 제어 알고리즘에 따른 성능 분석을 하였다. TCP 혼잡 제어 알고리즘에는 Slow Start, Congestion Avoidance, Fast Recovery, Fast Retransmit등으로 이루어져 있다. TCP Reno, TCP Vanilla을 사용하여 ATM-UBR 서비스 망을 구축하여 성능을 비교 분석하였다. 그러나 Fast Retransmit, Recovery 알고리즘의 특성에 의하여 다중 패킷 손실이 발생할 때의 ATM-UBR 네트워크에서도 성능 저하 현상이 현저하게 발생하였다. 이러한 현상에 대한 해결책으로 본 논문은 새로운 Fast Retransmit, Recovery 알고리즘을 제시하여 성능을 개선하였다.

• 천문학회지
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• 제44권2호
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• pp.49-58
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• 2011

We calculate the energy spectra of cosmic ray (CR) protons and electrons at a plane shock with quasi-parallel magnetic fields, using time-dependent, diffusive shock acceleration (DSA) simulations, including energy losses via synchrotron emission and Inverse Compton (IC) scattering. A thermal leakage injection model and a Bohm type diffusion coefficient are adopted. The electron spectrum at the shock becomes steady after the DSA energy gains balance the synchrotron/IC losses, and it cuts off at the equilibrium momentum $p_{eq}$. In the postshock region the cutoff momentum of the electron spectrum decreases with the distance from the shock due to the energy losses and the thickness of the spatial distribution of electrons scales as $p^{-1}$. Thus the slope of the downstream integrated spectrum steepens by one power of p for $p_{br}$ < p < $p_{eq}$, where the break momentum decreases with the shock age as $p_{br}\;{\infty}\;t^{-1}$. In a CR modified shock, both the proton and electron spectrum exhibit a concave curvature and deviate from the canonical test-particle power-law, and the upstream integrated electron spectrum could dominate over the downstream integrated spectrum near the cutoff momentum. Thus the spectral shape near the cutoff of X-ray synchrotron emission could reveal a signature of nonlinear DSA.

• KDI Journal of Economic Policy
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• 제35권3호
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• pp.1-38
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• 2013

본 연구는 공공부문과 민간투자사업자를 포함한 일반균형모형을 사용하여 재정사업과 BTL 민간투자사업의 후생 차이를 비교 분석한다. 다른 모든 조건이 동일할 때 정부가 세입 세출을 동일하게 유지해야 한다고 가정할 경우, 민간투자사업하에서의 국민경제 후생수준이 재정사업하에서의 후생수준보다 높은 것으로 나타난다. 정부가 균형재정의 제약을 회피할 목적으로 민간투자사업을 무리하게 추진하는 경우 미래의 임대료가 모두 부채로 간주되기 때문에 상당한 사회적 비용이 초래될 수 있음을 명시적으로 보인다.

• 에너지공학
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• 제11권4호
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• pp.299-305
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• 2002

본 연구에서 실험과 수치해석 검증은 지름비 0.52인 동심환형관내에서 안쪽축이 회전하고 바깥쪽축이 고정된 유동장의 유동특성을 수행하였다. 압력손실과 마찰계수는 안쪽축이 0~600 rpm 회전시 물과 0.2% CMC 수용액을 완전히 발달된 유동장에서 측정하였다. 천이유동은 표면마찰계수(C$_{f}$ )에 대하여 로스비수(Ro)와 레이놀즈수(Re)관계를 나타내기 위하여 압력손실 측정에 의해 확인하였다. 천이발생은 레이놀즈수에 대하여 압력손실과 마찰계수의 구배변화에 의해 조사하였다. 회전으로 인한 마찰계수의 증가율은 천이영역에서 대하여 갑자기 감소함에 반하여 층류영역에서 균일하며, 난류영역에서는 점차적으로 감소함을 알 수 있었다.

• 한국항공우주학회지
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• 제33권5호
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• pp.99-105
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• 2005

장기체공 소형 무인기 실용화를 위해서는 연료소비율이 우수한 엔진 개발이 매우 중요하다. 본 연구에서는 4행정 글로우 플러그 엔진을 가솔린 엔진으로 개조하였다. 고공에서의 엔진 성능예측에 필요한 시뮬레이션 프로그램 개발을 위하여 지상에서 엔진 성능을 측정하였다. 측정결과 고속에서 윤활부족으로 인하여 엔진 마찰력이 급격히 증가함을 알 수 있었다. 지상 시험결과를 토대로 개발된 엔진성능 예측 프로그램에 의하면 고도가 상승할수록 연료소비율이 악화되는데 이는 윤활부족에 의한 마찰력 손실은 고도에 관계없이 거의 일정하기 때문이다.

• 신재생에너지
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• 제2권2호
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• pp.4-8
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• 2006

Reduction of optical losses in crystalline silicon solar cells by surface modification is one of the most important issues of silicon photovoltaics. Porous Si layers on the front surface of textured Si substrates have been investigated with the aim of improving the optical losses of the solar cells, because an anti-reflection coating and a surface passivation can be obtained simultaneously in one process. We have demonstrated the feasibility of a very efficient porous Si AR layer, prepared by a simple, cost effective, electrochemical etching method. Silicon p-type CZ (100) oriented wafers were textured by anisotropic etching in sodium carbonate solution. Then, the porous Si layer were formed by electrochemical etching in HF solutions. After that, the properties of porous Si in terms of morphology, structure and reflectance are summarized. The surface morphology of porous Si layers were investigated using SEM. The formation of a porous Si layer about $0.1{\mu}m$ thick on the textured silicon wafer result in an effective reflectance coefficient Reff lower than 5% in the wavelength region from 400 to 1000nm. Such a surface modification allows improving the Si solar cell characteristics.

• Journal of Power Electronics
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• 제9권6호
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• pp.823-834
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• 2009

This paper proposes a high-efficiency single-stage ac/dc converter. The proposed converter features low voltage stresses and reduced switching losses. It operates at the boundary of discontinuous- and continuous-conduction modes by employing variable switching frequency control. The turn-on switching loss of the switch can be reduced by turning it on when the voltage across it is at a minimum. The voltage across the bulk capacitor is independent of the output loads and maintained within the practical range for the universal line input, so the problem of high voltage stress across the bulk capacitor is alleviated. Moreover, the voltage stress of the output diodes is clamped to the output voltage, and the output diodes are turned off at zero-current. Thus, the reverse-recovery related losses of the output diodes are eliminated. The operational principles and circuit analysis are presented. A prototype circuit was built and tested for a 150 W (50V/3A) output power. The experimental results verify the performance of the proposed converter.