• 한국자동차공학회논문집
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• 제7권8호
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• pp.66-75
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• 1999

In two-stroke engines, methods of evaluating scavenging effciency have been studied in parallel with trials for improvement of scavenging process. But simulating methods have a wide difference with difference with scavenging process in real engines and methods of evaluating scavening efficiency in real engines are very difficult and very expensive. This study shows that there is a possbility of evaluating scavenging efficiency in real engines very easily by analysis of in-cylinder pressure data. And as a characteristics two-stroke engines, the poly tropic indices in the process of compression are varies with degreeof scavenging and good representatives of scavenging efficiency.

• International Journal of Air-Conditioning and Refrigeration
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• 제9권2호
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• pp.51-61
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• 2001

A numerical study has been conducted to characterize the transient pressure in a simple water supply pipe system with an air chamber by utilizing a commercial code that employs the method of characteristics. Some results produced for validation in the study agree quite well with the previously reported. Several parameters are than varied. Among them are the valve closure time, the wave speed, the static pressure, the polytropic exponent, the air chamber volume, the diameter and the shape of orifice in the air chamber, etc, while the water temperature and velocity are kept constant at $20^\circ{C}$ and 0.8m/s, respectively. Results reported in this parametric study may be useful to understand the unsteady behavior of the system.

• 대한기계학회논문집
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• 제14권5호
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• pp.1200-1210
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• 1990

본 연구에서는 먼저, 단열 압축기 또는 단열압축 과정에 대한 기존의 효율들, 즉 등 엔트로피 효율과 폴리트로픽 효율 및 엑서지 효율에서의 출력에 해당하는 양의 의미를 검토하고, 이들을 엑서지 출력을 일반화 시킨 형태의 일종으로서 설명하며, 기 타의 가능한 효율을 정의하고, 나아가 같은 논리를 압축과정 중 외부와 열교환이 있는 비단열 압축과정에 대하여 확장 적용시켜 비단열 압축과정에서의 열역학 제2법칙적 출 력 및 효율들을 정의하고 그들의 특성을 살펴보려 한다.

• 설비공학논문집
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• 제12권8호
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• pp.782-790
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• 2000

A numerical study has been conducted to characterize the transient pressure in a building water supply system with an air chamber by utilizing a commercial code that employs the method of characteristics. Some results produced for the purpose of verification in the study agree quite well with the previously reported. Several parameters are then varied. Among them are the valve closure time, the wave speed, the static pressure, the polytropic exponent, the air chamber volume, the inner diameter and the shape of orifice in the air chamber, etc, while the water temperature and velocity are kept constant at $20^P{circ}C $,/TEX> and 0.8 m/s, respectively, Results reported in this parametric study may be useful to understand the unsteady behavior of the system.

• Korean Journal of Hydrosciences
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• 제7권
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• pp.77-86
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• 1996

An air chamber is design to keep the pressure from exceeding a predetermined value, or to prevent low pressures and colum separation. Therefore, it can be used to protect against rapid transients in a pipe system following abrupt pump stoppage. In this research, an air chmber was applied to a hypthetical pipe system to analyze attenuation effect of pressure head for different air volumes, locations, chamber areas, coefficients of orifice loss and pollytropic exponents. With an increase of air volume, the maximum pressure head at pump site is decreased and the minimum pressure head is imcreased. For different locations and areas of the chamber, the attenuation effects do not show much difference. Also, as the orifice loss coefficient increases, the maximum pressure head is decreased. For different polytropic exponents, isothermal process shows lower maximum pressure head than that of the adiabatic process.

• 한국유체기계학회 논문집
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• 제11권5호
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• pp.22-29
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• 2008

This study is performed to understand the effect of the variation in the passage area of a two-dimensional impeller on its performance characteristics. We observe the results with changing the area ratio of inlet to outlet about $1{\sim}2.8$. A comparison between the experimental and numerical results was performed for the same configuration in order to verify the reliability of the CFD code. Overall characteristics in the passages of impeller were analyzed in detail including streamline, Mach number, pressure and polytropic efficiency distribution. When the passage area ratio exceeds 2, the pressure ratio is high. An area ratio of 2.3 showed the highest efficiency. The results will be used as useful reference data to establish the design concept of two-dimensional impeller and to improve its performance.

• Journal of Astronomy and Space Sciences
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• 제10권2호
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• pp.163-188
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• 1993

적색편이 공간에서 보이는 국부 요동지역 주변의 초면은 프리이드만 배경 우주에 포함된 툴만 시공간 모형 관점에서 보면 국부 중력장에 유도된 국부 팽창율이 원인이 됨을 알고 있다. 초면은 국부적인 것으로서 요동지역에 포함된 질량에 크게 지배되고 물질들의 동력학적 상태와 밀접한 관계가 았다. 은하단의 밀도 분포 모형으로 광학 관측과 X-선 관측을 잘 맞추는 아벨 윤곽과 폴리트로프적 밀도 분포 윤곽을 택하여 틀만 시공간 모형으로 계산한 적색편이 공간에 나타나는 초면과 코마 은하단과 페르세우스 은하단 성분은하들에서 관측되는 적색편이-중심거리 형태와 비교하였을 때, 초면은 은하단의 크기와 질량에 한계값을 설정할 수 있었고, 뜨거운 기체의 물질 상태를 짐작할 수 있는 단서를 제공한다.

• 천문학회보
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• 제43권1호
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• pp.56.1-56.1
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• 2018

Tidal disruption events (TDEs) provide evidence for quiescent supermassive black holes (SMBHs) in the centers of inactive galaxies. TDEs occur when a star on a parabolic orbit approaches close enough to a SMBH to be disrupted by the tidal force of the SMBH. The subsequent super-Eddington accretion of stellar debris falling back to the SMBH produces a characteristic flare lasting several months. The theoretically expected bolometric light curve decays with time as proportional to $t^{-5/3}$. However, the light curves observed in most of the optical-UV TDEs deviate from the $t^{-5/3}$ decay rate especially at early time, while the light curves of some soft-X-ray TDEs are overall in good agreement with the $t^{-5/3}$ law. Therefore, it is required to construct the theoretical model for explaining these light curve variations consistently. In this paper, we revisit the mass fallback rates analytically and semi-analytically by taking account of the structure of the star, which is simply modeled by the polytrope. We find the relation between a polytropic index and the power law index of the mass fallback rate. We also discuss whether and how the decay curves, which we derived, fit the observed ones.

• Journal of Advanced Marine Engineering and Technology
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• 제26권1호
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• pp.116-124
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• 2002

The bubble model by Keller and Prosperetti is adapted to solve the nonlinear oscillation of a gas bubble. This formulation leads to accurate results since it introduces the energy equation instead of the polytropic assumption for the bubble interior. The numerical method used in this study is stable enough to handle large amplitude of bubble oscillation. The numerical results show some interesting nonlinear phenomena fur the bubble oscillator. The excitation changes the natural frequency of the bubble and makes some harmonic resonances at $f/f_0=1/2, 1/3$ and so on. The natural frequency of a bubble oscillator decreases compared with the linear case result, which means that the nonlinear bubble oscillation system is a "softening"system. In addition, the frequency response curve jumps up or down at a certain frequency. It is also found that there exist multi-valued regions in the frequency response curve depending on the initial conditions of bubble. The dependency of the bubble motion on the initial condition can generate extremely large pressure and temperature which might be the cause of the acoustic cavitation and the sonoluminescence.inescence.

• 대한기계학회논문집B
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• 제20권12호
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• pp.4053-4061
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• 1996

Internal flow characteristics within Y-jet atomizers and the local drop size distribution and cross-sectional averaged drop size at the outside were investigated with the liquid and air injection pressures, mixing port length of atomizers, and the liquid properties taken as parameters. To examine the effect of the liquid viscosity, glycerin-water mixtures were used in this study. The liquid viscosity plays only a minor role in determining the internal flow pattern and the spatial distribution shape of drops, but the drop sizes themselves generally increase with increasing of the liquid viscosity. An empirical correlation for the liquid discharge coefficient at the liquid port was deduced from the experimental results; the liquid discharge coefficient strongly depends on the liquid flow area at the mixing point which is proportional to the local volumetric quality(.betha.$_{Y}$), and the volumetric quality was included in the correlation. Regardless of the value of the liquid viscosity, the compressible flow through the gas port was well represented by the polytropic expansion process(k=1.2), and the mixing point pressure could be simply correlated to the aspect ratio( $l_{m}$/ $d_{m}$) of the mixing port and the air/liquid mass flow rate ratio( $W_{g}$/ $W_{f}$) as reported in the previous study.udy.udy.y.