• 한국추진공학회지
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• 제14권6호
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• pp.25-30
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• 2010

최근 이중목 노즐(Dual Throat Nozzle, DTN)을 사용하여 추진체의 추력을 제어하는 방법이 많은 주목을 받고 있다. 이중목 노즐은 공동을 사이에 두고 두 개의 노즐 목을 가지도록 설계된다. 본 연구에서는 DTN의 유동특성을 조사하기 위하여, 수치해석적인 방법을 적용하였으며, 2차 유동의 질량유량과 노즐 압력비를 변화시켰다. 수치해석에서는 2차원, 압축성 Navier-Stokes 방정식을 풀기 위하여, 유한체적법을 적용하였다. 그 결과 본 수치해석은 실험결과를 잘 예측하였으며, DTN을 이용한 추력벡터 제어는 추력계수와 유출계수의 항으로 상세하게 설명하였다.

• KIEE International Transactions on Power Engineering
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• 제2A권3호
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• pp.121-128
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• 2002

This study proposes a new method for transmission system expansion planning using fuzzy integer programming. It presents stepwise cost characteristics analysis which is a practical condition of an actual system. A branch and bound method which includes the network flow method and the maximum flow - minimum cut set theorem has been used in order to carry out the stepwise cost characteristics analysis. Uncertainties of the permissibility of the construction cost and the lenient reserve rate and load forecasting of expansion planning have been included and also processed using the fuzzy set theory in this study. In order to carry out the latter analysis, the solving procedure is illustrated in detail by the branch and bound method which includes the network flow method and maximum flow-minimum cut set theorem. Finally, case studies on the 21- bus test system show that the algorithm proposed is efficiently applicable to the practical expansion planning of transmission systems in the future.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2001년도 추계학술대회 논문집 전력기술부문
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• pp.350-353
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• 2001

This study proposes a new method for the transmission system expansion planning using the fuzzy integer programming. It presents stepwise cost characteristics analysis which is a practical condition of an actual systems. A branch and bound method which includes the network flow method and the maximum flow-minimum cut set theorem has been used in order to proceed the stepwise cost characteristics analysis. Uncertainties of the permission of the construction cost and not strict reserve rate and load forecasting of expansion planning have been included and also processed using fuzzy set theory in this study. In order to proceed the latter analysis, the solving procedure is illustrated in detail by branch and bound method which includes the network flow method and maximum flow-minimum cut set theorem. Finally, case studies on 21-bus test system show that the algorithm proposed is efficiently applicable to the practical expansion planning of transmission systems in future.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2010년도 제34회 춘계학술대회논문집
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• pp.339-342
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• 2010

최근 이중목 노즐(Dual Throat Nozzle, DTN)을 사용하여 추진체의 추력을 제어하는 방법이 많은 주목을 받고 있다. 이중목 노즐은 공동을 사이에 두고 두 개의 노즐 목을 가지도록 설계된다. 본 연구에서는 DTN의 유동특성을 조사하기 위하여, 수치해석적인 방법을 적용하였으며, 2차유동의 질량 유량을 변화시켰다. 수치해석에서는 2차원, 압축성 Navier-Stokes방정식을 풀기 위하여, 유한체적법을 적용하였다. 그 결과 본 수치해석은 실험결과를 잘 예측하였으며, DTN을 이용한 추력벡터 제어는 추력계수와 유출계수의 항으로 상세하게 설명하였다.

• Journal of Communications and Networks
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• 제12권1호
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• pp.86-94
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• 2010

In this paper, we present a decentralized algorithm to find minimum cost quality of service (QoS) flow subgraphs in network coded multicast schemes. The main objective is to find minimum cost subgraphs that also satisfy user-specified QoS constraints, specifically with respect to rate and delay demands. We consider networks with multiple multicast sessions. Although earlier network coding algorithms in this area have demonstrated performance improvements in terms of QoS parameters, the proposed QoS network coding approach provides a framework that guarantees QoS constraints are actually met over the network.

• Tribology and Lubricants
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• 제17권3호
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• pp.198-208
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• 2001

A Parametric study based on a computer analysis of the lubrication system of a four-cylinder gasoline engine is illustrated. Through the parametric study, the effects of various aeration levels on the change of oil flow rate and pressure are investigated. Also, at high oil temperature and low engine speed, the effect of oil aeration level on oil flow characteristics in lubrication system is investigated. The illustrated results may give to designers the guide lines of oil aeration level for the safe design of engine lubrication systems in terms of minimum pressure at crank oil bore.

• Tribology and Lubricants
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• 제28권6호
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• pp.303-314
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• 2012

In this research, spool valves with spiral grooves are suggested and their lubrication characteristics are investigated by numerical analysis. The three-dimensional flow field is obtained by solving the Navier-Stokes equations in dimensionless form, so that the performance variables such as lateral force, friction force and volume flow rate are determined. Also, the lubrication characteristics of spool valves with spiral grooves are compared with those with typical grooves under variable working conditions. It is shown that spool valves with spiral grooves can get better performance in aspect of mitigation of uneven pressure distribution surrounding spool. Moreover, it is found that the minimum distance between spool edges and grooves, the type of spiral groove, and the groove angle have noticeable effect on the lubrication characteristics.

• 산업경영시스템학회지
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• 제25권5호
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• pp.9-14
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• 2002

A capital investment problem is essentially one of determining whether the anticipated cash inflows from a proposed project are sufficiently attractive to invest funds in the project. The net present value(NPV) criterion and internal rate of return(IRR) criterion are widely used as means of making investment decisions. A positive NPV means the equivalent worth of the inflows is greater than the equivalent worth of outflows, so, the project makes profit. Business people are familiar with rates of return because they all borrow money to finance ventures, even if the money they borrow is their own. Thus they are apt to use the IRR in preference to the NPV. The IRR can be defined as the discount rate that causes the net present value of a cash flow to equal zero. Why the project are accepted if the project's IRR is greater than the investor's minimum attractive rate of return\ulcorner Against the NPV, the definition cannot distinctly explain the concept of the IRR as decision criterion. We present a new definition of the IRR as the ratio of profit on the invested capital.

• 한국태양에너지학회 논문집
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• 제27권3호
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• pp.45-54
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• 2007

For a conventional natural-circulation type solar water heater, the pressure head is limited by the height between the storage tank and hot water tap. Therefore, it is difficult to provide sufficient hot water flow rate for general usage. This study deals with a design modification of the storage tank to utilize the tap-water pressure to increase hot-water supply Based on fluid dynamic and heat transfer theories, a series of modeling and simulation is conducted to achieve practical design requirements. An experimental setup is built and tested and the results are compared with theoretical simulation model. The storage tank capacity is 240 l and the outer diameter of piping was 15 mm. Number of tube turns tested are 5, 10, and 15. Starting with initial storage tank temperature of $80^{\circ}C$, the temperature variation of the supply hot water is investigated against time, while maintaining minimum flow rate of 10 1/min. Typical results show that the hot water supply of minimum $30^{\circ}C$ can be maintained for 34 min with tap-water supply pressure of 2.5 atm, The relative errors between modeling and experiments coincide well within 10% in most cases.

• 한국전기전자재료학회논문지
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• 제15권7호
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• pp.564-569
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• 2002

Heteroepitaxial GaN films were grown on sapphire substrates in order to study the effects of the buffer layer's surface roughness and thermal treatment on the epitaxial layer's quality. For this, GaN buffer layers were grown at $550^{\circ}C$ with various TMGa flow rates and durations of growth, and annealed at $1010^{\circ}C$ for 3 min after the temperature was raised by 23 ~ $92^{\circ}C/min$, and then GaN epitaxial layers were grown at $1000^{\circ}C$. It has been found that the buffer layer's surface roughness and the thermal treatment condition are critical factors on the quality of the epitaxial layer. When a buffer layer was frown with a TMGa flow rate of $24\mu mole/min$ for 30 sec, the surface roughness of the buffer lather was minimum and when the thermal ramping rate was $30.6^{\circ}C/min$ on this layer, the successively grown epitaxial layer's crystalline and optical qualities were optimized with a specular morphology. The minimum full width at half maximum(FWHM) of GaN(0002) x-ray diffraction peak and that of near-band-edge(NBE) peak from a room temperature photoluminescence (PL) were 5 arcmin and 9 nm, respectively.