• 천문학회지
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• 제28권1호
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• pp.97-107
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• 1995

The stability of the geometrically thin, two-temperature hot accretion disk is studied. The general criterion for thermal instability is derived from the linear local analyses, allowing for advective cooling and dynamics in the vertical direction. Specifically, classic unsaturated Comptonization disk is analysed in detail. We find five eigen-modes: (1) Heating mode grows in thermal time scale, $(5/3)({\alpha}{\omega})^{-1}$, where alpha is the viscosity parameter and w the Keplerian frequency. (2) Cooling mode decays in time scale, $(2/5)(T_e/T_i)({\alpha}{\omega})^{-1}$, where $T_e\;and\;T_i$ are the electron and ion temperatures, respectively. (3) Lightman-Eardley viscous mode decays in time scale, $(4/3)(\Lambda/H)^2({\alpha}{\omega})^{-1}$, where $\Lambda$ is the wavelength of the perturbation and H the unperturbed disk height. (4) Two vertically oscillating modes oscillate in Keplerian time scale, $(3/8)^{1/2}\omega^{-1}$ with growth rate $\propto\;(H/\Lambda)^2$. The inclusion of dynamics in the vertical direction does not affect the thermal instability, adding only the oscillatory modes which gradually grow for short wavelength modes. Also, the advective cooling is not strong enough to suppress the growth of heating modes, at least for geometrically thin disk. Non-linear development of the perturbation is followed for simple unsaturated Compton disk: depending on the initial proton temperature perturbation, the disk can evolve to decoupled state with hot protons and cool electrons, or to one-temperature state with very cool protons and electrons.

• Environmental Engineering Research
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• 제16권1호
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• pp.47-51
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• 2011

The oxygen transfer rate is a parameter that characterizes the gas-liquid mass transfer in surface aerators. Gas-liquid transfer mechanisms in surface aeration tanks depend on two different extreme lengths of time; namely, macromixing and micromixing. Small scale mixing close to the molecular level is referred to as micromixing; whereas, macromixing refers to mixing on a large scale. Using experimental data and numerical simulations, macro- and micro-scale parameters describing the two extreme time scales were investigated. A scale up equation to simulate the oxygen transfer rate with micromixing times was developed in geometrically similar baffled surface aerators.

• 천문학회보
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• 제37권1호
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• pp.75.2-75.2
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• 2012

Turbulence is ubiquitous in astrophysical fluids such as the interstellar medium and intracluster medium. To maintain turbulent motion, energy must be injected into the fluids. In turbulence studies, it is customary to assume that the fluid is driven on a scale, but there can be many different driving mechanisms that act on different scales in astrophysical fluids. We expect different statistical properties of turbulence between turbulence with single driving scale and turbulence with double driving scales. In this work, we perform 3-dimensional incompressible MHD turbulence simulations with energy injection in two ranges, 2${\surd}$12 (large scale) and 15

• Journal of the Korean Data and Information Science Society
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• 제21권3호
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• pp.555-560
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• 2010

One-dimensional approach to two-dimensional warranty data involves modeling us- age as a function of time. Iskandar (1993) suggests a simple linear model for usage. However, simple linear form of intensity function is of limited value to model the situa-tion where the intensity varies over time. In this study Weibull intensity is considered where the scale parameter is expressed in terms of different models. We will nd out how each parameter in the model a ects the warranty cost and which model gives a bigger number of failures within the two-dimensional warranty region.

• 한국전자통신학회논문지
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• 제2권1호
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• pp.10-18
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• 2007

본 논문에서는 MTS(Multiple Time Scale) 트래픽 제어 프레임워크를 TCP(Transfer Control Protocol) 기반의 신뢰할 수 있는 전송 및 윈도우 기반 혼잡제어로 확대 적용한다. 이 작업은 TCP의 대역폭 소비 반응의 적극성을 LTS 네트워크 상태의 함수 형태, 즉 RTT(Round-Trip Delay Time)가 결정한 피드백 루프의 한계를 넘어서는 정보의 형태를 조정하는 LTS(Large Time Scale) 모듈과 TCP를 연계시키는 방법으로 수행된다. 혼잡 제어 성능 평가 방식은 자기 유사성 네트워크 트래픽의 물리적 모델링으로부터 얻은 시뮬레이션 기반 하에서 결과를 나타낸다. 자기 유사 버스트 환경 하에서 RTT가 450ms일 때 소스 트래픽이 초과되지 않는 경우에 TCP-SSC(Selective Slope Control)의 성능 이득은 각각 ${\alpha}$가 1.05일 45%정도 높아지는 반면에 ${\alpha}$가 1.95일 때는 20%정도의 성능 이득을 얻을 수 있다. 그러므로 비율 기반 피드백 혼잡 제어에 TCP-MTS를 적용함으로서 TCP-SCC 처리 이득의 성능이 약 2배정도의 개선이 이루어짐을 시뮬레이션 결과로부터 알 수 있다.

• Structural Engineering and Mechanics
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• 제41권4호
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• pp.559-573
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• 2012

An adaptive modeling and simulation technique is introduced for the effective and reliable fluid-structure interaction analysis using MSC/Dytran for large-scale complex pressurized liquid containment. The proposed method is composed of a series of the global rigid sloshing analysis and the locally detailed fluid-structure analysis. The critical time at which the system exhibits the severe liquid sloshing response is sought through the former analysis, while the fluid-structure interaction in the local region of interest at the critical time is analyzed by the latter analysis. Differing from the global coarse model, the local fine model considers not only the complex geometry and flexibility of structure but the effect of internal pressure. The locally detailed FSI problem is solved in terms of multi-material volume fractions and the flow and pressure fields obtained by the global analysis at the critical time are specified as the initial conditions. An in-house program for mapping the global analysis results onto the fine-scale local FSI model is developed. The validity and effectiveness of the proposed method are verified through an illustrative numerical experiment.

• 한국항공우주학회지
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• 제32권5호
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• pp.58-64
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• 2004

본 논문에서는 양시등급 항공기 동력학에서 빠른 종속시스템의 극점들을 그대로 둔 채 느린 종속시스템의 극점만을 재배치함으로써 페루프의 근사치 해를 획득하는 방법이 제안되었다. 행렬대각화를 통하여 얻어지는 이러한 근사치 해는 수정된 것과 수정되지 않은 것 두 종류로 구분된다. 이들의 차이는 전자의 경우 오차가 $O({\varepsilon})$이며 후자의 경우는 오차가 $O({\varepsilon}^2)$이다. 두 가지 해는 모두 감소해이지만 충분한 견실성을 보여준다. 제안된 기법의 우수성은 항공기 종방향 운동 모델의 시뮬레이션을 통하여 확인되었다.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 2003년도 ICCAS
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• pp.508-513
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• 2003

This paper is concerned with a mixed control with aerodynamic fin and side thrusters applied to an agile missile using two-time scale dynamic inversion and linear time-varying control technique. The nonlinear dynamic inversion method with the weighting function allocates the desired control inputs (aerodynamic fin and side thrusters) to track a reference trajectory, and the time-varying control technique guarantees the robustness for the uncertainties. Closed-loop stability is achieved by the assignment of the extended-mean of these linear time-varying eigenvalues to the left half complex plane. The proposed schemes are validated by nonlinear simulations.

• 한국연소학회:학술대회논문집
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• 한국연소학회 1997년도 제15회 KOSCO SYMPOSIUM 논문집
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• pp.163-173
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• 1997

This article presents an application of a large-scale structural mixing model (Broadwell et al. 1984) to the blowout of turbulent reacting jets discharging perpendicularly into an unconfined cross air-flow. In an analysis of a common stability curve, a plausible explanation can be made that the phenomenon of blowout is related only to the mixing time scale of the two flows. The most notable observation is that the blowout distance is traced at fixed positions at all times according to the velocity ratio R. Measurements of the lower blowout limits in the liftable flame agree qualitatively with the blowout parameter ${\varepsilon}$, proposed by Broadwell et al. Good agreement between the results calculated by a modified blowout parameter ${\varepsilon}^'$ and experimental results confirms the important effect of a large-scale structure in specifying the stabilization feature of blowouts.

• 한국연소학회지
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• 제14권3호
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• pp.29-36
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• 2009

Simulation is performed to analyze the characteristics of turbulent spray combustion in a diesel engine condition. An extended Conditional Moment Closure (CMC) model is employed to resolve coupling between chemistry and turbulence. Relevant time and length scales and dimensionless numbers are estimated at the tip and the mid spray region during spray development and combustion. The liquid volume fractions are small enough to support validity of droplets assumed as point sources in two-phase flow. The mean scalar dissipation rates (SDR) are lower than the extinction limit to show flame stability throughout the combustion period. The Kolmogorov scales remain relatively constant, while the integral scales increase with decay of turbulence. The chemical time scale decreases abruptly to a small value as ignition occurs with subsequent heat release. The Da and Ka show opposite trends due to variation in the chemical time scale. More work is in progress to identify the spray combustion regimes.