• 천문학회보
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• 제38권2호
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• pp.45.1-45.1
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• 2013

Type Ia supernovae (SNe Ia) are a powerful tool to investigate the expansion history of the universe, because their peak luminosity is as bright as a galaxy and is known as an excellent standard candle. Since the discovery of the acceleration of the universe based on the observations of SNe Ia, higher than ever accuracy of their peak luminosity is needed to investigate various problems in cosmology. We started a project to improve the accuracy of the calibration of the peak luminosity of SNe Ia by measuring accurate distances to nearby resolved galaxies that host SNe Ia. We derive accurate distances to the SN Ia host galaxies using the method to measure the luminosity of the tip of the red giant branch (TRGB). In this study we present the results for M66 and M96 in the Leo I Group which are nearby spiral galaxies hosting SN 1989B and SN 1998bu, respectively. We obtain VI photometry of resolved stars in these galaxies from F555W and F814W images in the Hubble Space Telescope archive. We derive the distances to these galaxies from the luminosity of the TRGB. With these results we derive absolute maximum magnitudes of two SNe (SN 1989B in M66 and SN 1998bu in M96). We derive a value of the Hubble constant from the optical magnitudes of these SNe Ia and SN 2011fe in M101 based on our TRGB analysis. This value is similar to the values derived from recent estimates from WMAP9 and Planck results, but smaller than other recent determinations based on Cepheid calibration for SNe Ia luminosity.

• 한국대기환경학회지
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• 제34권2호
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• pp.191-206
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• 2018

In this study, the HAPs(Harzard Air Pollutants) emission factor level of Starex and Grand Carnival was tested using PEMS (Portable Emissions Measurement System) on real road driving. As a result of RDE (Real Driving Emission test), the overall vehicle speed pattern showed similar characteristics. The real-road driving test at constant speed revealed emission factor is inversely proportional relationship to constant speed. Results of accelerating with speed limit on the real-road were shown as followings; Uran (less than 45 km/h)>Rural (<45 km/h, less than 80 km/h)>Motorway (>80 km/h). Moreover, the sudden acceleration and deceleration in driving at high speed was the increasing factor to the HAPs emission factor. This tendency is considered to be influenced by the operating environment on real roads.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2009년도 한국우주과학회보 제18권2호
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• pp.34.1-34.1
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• 2009

After SNIa and WMAP observations during the last decade, the discovery of the accelerated expansion of the universe is a major challenge to particle physics and cosmology. There are currently three candidates for the dark energy which results in this accelerated expansion: $\cdot$ a non-zero cosmological constant, $\cdot$ a dynamical cosmological constant (quintessence scalar field), $\cdot$ modifications of Einstein's theory of gravity. The scalar field model like quintessence is a simple model with time-dependent w, which is generally larger than -w1. Because the different w lead to a different expansion history of the universe, the geometrical measurements of cosmic expansion through observations of SNIa, CMB and baryon acoustic oscillations (BAO) can give us tight constraints on w. One of the interesting ways to study the scalar field dark-energy models is to investigate the coupling between the dark energy and the other matter fields. In fact, a number of models which realize the interaction between dark energy and dark matter, or even visible matter, have been proposed so far. Observations of the effects of these interactions will offer an unique opportunity to detect a cosmological scalar field. In this talk, after briefly reviewing the main idea of the three possible candidates for dark energy and their cosmological phenomena, we discuss the interactinng dark-energy model, paying particular attention to the interacting mechanism between dark energy with a hot dark matter (neutrinos). In this so-called mass-varying neutrino (MVN) model, we calculate explicitly the cosmic microwave background (CMB) radiation and large-scale structure (LSS) within cosmological perturbation theory. The evolution of the mass of neutrinos is determined by the quintessence scalar field, which is responsible for the cosmic acceleration today.

• 한국추진공학회지
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• 제2권1호
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• pp.13-20
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• 1998

다목적으로 활용할 수 있는 터보축엔진의 개발을 위한 정상상태 및 동적모사 프로그램을 개발하였다. 개발비, 개발시간, 개발위험도의 절감을 위해 가스발생기 부분은 성능이 잘 알려진 기존의 터어보제트 엔진을 활용하였으며 수명연장을 위해 터빈재질을 교체하고, Larson-Miller 곡선을 이용하여 약 3000hr 이상의 수명을 확보하기 위한 최대회전속도와 최대 터빈입구온도를 결정하였다. 추가되는 동력터어빈의 구성품 성능선도는 압축기 터어빈의 성능선도를 축척하여 사용하였다. 정상상태 성능해석에는 유량 및 일평형 방정식을 이용하였으며, 가스발생기와 동력터빈의 공회전 상태에서부터 최대 회전속도까지 동력터빈은 10% 간격, 가스발생기는 5%RPM 간격으로 해석하였다. 동적모사시에는 일정유량평형방법(Constant Flow Method : CMF)을 이용하였으며, 급 가속의 상황을 가정하고 연료유량이 Step 증가하도록 Scheduling 하였다. 이 때 터빈 입구온도에 오버슈트가 발생하여 제한온도를 초과하는 것을 확인할 수 있었다.

• 전기학회논문지
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• 제62권11호
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• pp.1571-1577
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• 2013

Electrical tree structure is one of the most important influencing factors for electrical treeing characteristics in polymers. In this paper, we focused on the structure characteristics of electrical treeing in epoxy resins (original) insulation under different high-frequency voltages (60, 500, 1000Hz). Effects of voltage frequency on the ac electrical treeing phenomena in an epoxy resins were carried out in needle-plate electrode arrangement. To measure the treeing initiation and propagation, and the breakdown rate, constant AC of 10 kV with three different voltage frequencies (60, 500 and 1,000 Hz) was applied to the specimen in needle-plate electrode specimen at $30^{\circ}C$ of insulating oil bath. At 60 Hz, the treeing initiation time was 360 min and the propagation rate was $6.85{\times}10^{-4}mm/min$, and the morphology was dense branch type. As the voltage frequency increased, the treeing initiation time decreased and the propagation rate increased. At 1,000 Hz, the treeing initiation time was 0 min and the propagation rate was $7.81{\times}10^{-2}mm/min$, and the morphology was dense bush type.

• 상하수도학회지
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• 제12권1호
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• pp.81-87
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• 1998

The possibility of collision of two particles slowly settling one after another in water can be described using the collision efficiency factor in differential sedimentation (${\alpha}_{DS}$). ${\alpha}_{DS}$ was found to be a function of several parameters particle size, particle size ratio, Hamaker constant, density of liquid and particle, gravity acceleration. Previous researches were limited to the case when there is no electric repulsion assuming that the suspension is destabilized. In this paper, ${\alpha}_{DS}$ is calculated for the stabilized condition. The relative trajectory of two particles are calculated including hydrodynamics, attraction and repulsion forces. Ionic strength and surface potential significantly affect the collision possibility of two settling particles. Depending on the surface potential and ionic strength, ${\alpha}_{DS}$ value is divided into three regions; stable, unstable and transition zone. ${\alpha}_{DS}$ increases as the ionic strength increases, and as the surface charge decreases. This result can be used to model both destabilized and stabilized suspension incorporating the collision efficiency factors of the other coagulant mechanisms such as fluid shear and Browian motion.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2016년도 전력전자학술대회 논문집
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• pp.333-334
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• 2016

This paper proposes a new cascoded configuration for hybrid energy storage system (HESS) which consists of batteries and supercapacitor (SC) for Electric Vehicle applications. In this configuration,a resonant LLC converter is interfacedin series with a battery module and it converts a part of the energy from the batteries and transfer it to the dc-link bus. The LLC converter is controlled by a phase-shift angle between the primary and secondary switches to maintain a constant dc-link voltage and obtain soft-switching conditions for all the primary switches. By placing the SC moduleina cascoded concept, the rated voltage of SC can be reduced significantly compared with the conventional topologies. It helps save the cost and reduce the number of SC cells. The proposed configuration can operate with four different modes: feeding load, acceleration, regenerative braking andSC charging. A scaled-down prototype converter (2 kW, 600V output) is designed and tested to verify the advantages of the proposed topology. The maximum efficiency obtained with the proposed topology is 99%.

• 대한기계학회논문집B
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• 제32권2호
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• pp.158-165
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• 2008

The carbon dioxide properties change sharply near the critical or pseudo-critical point in the heat transfer processes. The reduction in turbulent, convective heat transfer parameters observed in some supercritical data and in experiments with common gases can be due to property variation, acceleration, buoyancy or combinations of these phenomena, depending on the conditions of the applications. In this study, the measurement for the secondary flow driven by buoyancy was carried out on the supercritical carbon dioxide turbulent flows in the different boundary condition with the constant mass flow rate. The available measuring techniques were used to clarify the behaviour of any supercritical fluid. Laser Doppler Velocimeter (LDV) and a special device was used to measure the secondary velocity and turbulent characteristics of the supercritical flows.

• 대한기계학회논문집
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• 제17권9호
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• pp.2294-2303
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• 1993

This paper investigated the characteristics of the turbulent incompressible flow past the orifice ring in an axi-symmetric pipe. The flow field was the turbulent pulsatile flow for Reynolds number of $2{\times}10^{5}$ which was defined based on the maximum velocity and the pipe diameter at the inlet, with oscillating frequence $(f_{os})=1/4{\pi}$ which was considered as quasi-steady state frequence. In the present investigation, finite analytic method was used to solve the governing equations in Navier Stokes and turbulent transport formulations. Particularly at high Reynolds number and low oscillation frequency, the effects of orifice ring on the flow were numerically investigated. The separation zone behind the orifice ring during the acceleration phase was found to be decreased. However, during the deceleration phase, the separation behind the orifice ring for pulsatile flow continuously grow to a size even larger than that in steady flow. The pressure drop in steady flow was found to be constant and always positive while for pulsatile flow the pressure drop change with time. And large turbulent kinetic energy, dissipation rate were found to be located in the region where the flow passes through the orifics ring. The maximum turbulent kinetic energy, generally occurs along the shear layer where the velocity gradient is large.

• 한국생산제조학회지
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• 제8권2호
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• pp.94-94
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• 1999

Natural gas is considered to be on e of the most promising candidates for a clean substitute fuel and a great amount of research on the compressed natural gas(CNG) fueled vehicle has been performed. In this s tudy, we try to understand the property of CNG fuel with using CNG engine experiment. In order to present the direction and application of CNG, we experiment with various operating conditions that is, spark timing, A/F ratio, air quantity and fuel quantity, etc. 11,967 cc engine was used in the experiment and the engine fuel ratio was determined in the way that the performance of dedicated CNG engine is corresponded to that of existing diesel engine. The performance and dedicated CNG engine were measured by changing the fuel injection timing. The dedicated CNG engine was proved to be good in describing the experimental results and according to the actual road test, acceleration and constant speed driving for dedicated CNG engine was better than existing diesel engine.