• Journal of The Korean Astronomical Society
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• v.40 no.4
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• pp.91-97
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• 2007

The Fokker-Planck (FP) model is one of the commonly used methods for studies of the dynamical evolution of dense spherical stellar systems such as globular clusters and galactic nuclei. The FP model is numerically stable in most cases, but we find that it encounters numerical difficulties rather often when the effects of tidal shocks are included in two-dimensional (energy and angular momentum space) version of the FP model or when the initial condition is extreme (e.g., a very large cluster mass and a small cluster radius). To avoid such a problem, we have developed a new integration scheme for a two-dimensional FP equation by adopting an Alternating Direction Implicit (ADI) method given in the Douglas-Rachford split form. We find that our ADI method reduces the computing time by a factor of ${\sim}2$ compared to the fully implicit method, and resolves problems of numerical instability.

• Transactions of the Korean Society of Automotive Engineers
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• v.10 no.6
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• pp.44-50
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• 2002

The spray characteristics of GDI(Gasoline Direct Injection) injector affects on engine efficiency and emission of a GDI engine. Thus, many researchers have investigated the spray characteristics and the mixture formation of GDI injector. In this study, it was tried to provide the fundamental data for GDl injector design which effects on the spray macroscopic characteristics such as penetration and spray angle. In addition, the mixture formation analyzed by using entropy analysis. The entropy analysis is based on the concept of statistical entropy, and it identifies the degree of homogeneity in the fuel concentration. The results show that as injection pressure increases but as ambient pressure increases, spray penetration decreases and spray angle doesn't affected by increasing injection pressure and ambient temperature. From the entropy analysis results, we could find that the direct diffusion phenomena is a dominant factor in the formation of a homogeneous mixture at downstream of GDI spray especially in vaporizing conditions.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.1 no.1
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• pp.87-96
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• 1989

Flows through impellers of centrifugal compressors are calculated by a streamline curvature method. A method for the exit boundary condition is suggested in the present paper. Flow angles are assumed to be deviated from the blade angle parabolically. The maximum deviation is adjusted for the whole angular momentum to balance with the empirically estimated value by using Stanitz' slip-factor. The present method is verified to reasonably simulate flows through the impeller, when the 3-dimensionality of the flow is not strong. It is also shown that the method can be applied for the design of the splitter in the impeller.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.1
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• pp.81.2-81.2
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• 2014

To figure out the effect of rotation on the final mass of Pop III stars, 1D stellar evolution simulations of the evolution of mass-accreting protostars are performed, with zero metalicity and high constant mass accretion rates. The protostar reaches the Keplerian rotation very soon after the onset of mass accretion, but it may continue mass accretion via angular momentum transport induced by viscous stress or magnetic field. However, as the accreting star evolves, the envelope expands rapidly when the total mass reaches $5{\sim}6M_{\odot}$ and the corresponding Eddington factor sharply increases. Strong radiative pressure with rotation imposes different criteria for breakup at the stellar surface, and the so-called 'critical rotation (${\Omega}{\Gamma}$-limit)' is reached. As a result mass accretion rate has to be significantly lowered. This implies that characteristic masses of Pop III stars would be significantly lowered than the previous expectation.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.12 no.8
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• pp.717-724
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• 2000

It is important to investigate characteristics of flow of refrigerant/oil mixture circulating in a refrigeration system. Therefore the oil concentration in refrigerant/oil mixture should be measured exactly by the adequate measuring instrument. In this paper, the oil concentration was measured by density monitoring system(DMS) in the liquid-line of a inverter-driven heat pump. Experimental result follows ; the main factor that have an effect on oil concentration refrigerant/oil mixture circulating in a refrigeration system is the momentum and kinematic viscosity of refrigerant/oil mixture compressed by scroll compressor.

• Kyungpook Mathematical Journal
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• v.60 no.4
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• pp.853-867
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• 2020

The aim of this study was to analyze the momentum and heat transfer of a rotating nanofluid with conducting spherical dust particles. The fluid flows over a stretching surface under the influence of an external magnetic field. By applying similarity transformations, the governing partial differential equations were trans-formed into nonlinear coupled ordinary differential equations. These equations were solved with the built-in function bvp4c in MATLAB. Moreover, the effects of the rotation parameter ω, magnetic field parameter M, mass concentration of the dust particles α, and volume fraction of the nano particles 𝜙, on the velocity and temperature profiles of the fluid and dust particles were considered. The results agree well with those in published papers. According to the result the hikes in the rotation parameter ω decrease the local Nusselt number, and the increasing volume fraction of the nano particles 𝜙 increases the local Nusselt number. Moreover the friction factor along the x and y axes increases with increasing volume fraction of the nano particles 𝜙.

• Journal of Korea Water Resources Association
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• v.48 no.8
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• pp.625-634
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• 2015

The diverse patterns of separation zone according to the marked bed discordance by dredging at confluence in addition to the confluence angle of tributary and discharge ratio between tributary and main channels have been analyzed by CCHE2D model simulation. The separation zone is defined by inside of zero velocity boundary at down-stream of confluence. The separation zone dose not formed at the $30^{\circ}$ of confluence angle of tributary. The size of separation zone increases as the discharge ratio and confluence angle increase in general. The separation zone decreases as the dredging depth increases which shows the relative momentum reduction compared by the flow volume increasing by dredging at confluence. The contraction factor with the variation of confluence angle and discharge ratio has been investigated and confirmed the corresponding conveyance decreasing results in backwater effect. The regression equation of shape factor with confluence angle and discharge and dredging depth ratios has been suggested.

• IEIE Transactions on Smart Processing and Computing
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• v.2 no.4
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• pp.240-247
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• 2013

The main aim of this study was to suppress the angular velocity against strong winds during storms and analyze the stability and performance of the phase plane method. The utilization of small-scale wind turbine system has become common in agriculture, houses, etc. Therefore, it is considered to be a scheme for preserving the natural energy or avoiding the use of fossil fuels. Moreover, settling small-scaled wind turbines is simpler and more acceptable compared to ordinary huge wind turbines. In addition, after converting the energy there is no requirement for distribution. Therefore, a much lower cost can be expected for small-scaled wind turbines. On the other hand, this system cannot be operated continuously because the small-scaled wind turbine consists of a small blade that has low inertia momentum. Therefore, it may exceed the boundary of angular velocity, which may cause a fault in the system due to the centrifugal force. The aim of this study was to reduce the angular velocity by controlling the stall factor. Stall factor control consists of two control methods. One is a shock absorber that is loaded in the junction of the axis of the blade of the wind turbine gear wheel and the other is pitch angle control. Basically, the stall factor itself exhibits nonlinear behavior. Therefore, this paper confirmed the feasibility of stall factor control in producing desirable performance whilst maintaining stability.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.47 no.6
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• pp.438-445
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• 2019

In this paper, we present a new approach to extract the g-sensitivity scale-factor error for a MEMS gyroscope. MEMS gyroscopes, based on the use of both angular momentum and the Coriolis effect, have a g-sensitivity error due to mass unbalance. Generally, the g-sensitivity error is not considered in general use of gyroscopes, but it deserves our attention if we are to develop for tactical class performance and reliability. The g-sensitivity error during vehicle flight increases navigation error; so it must be analyzed and compensated for the use of MEMS IMU for high dynamics vehicle systems. Therefore, we analyzed how to extract the g-sensitivity scale-factor error from the inertial sensor error model. Furthermore we propose a new method to extract the g-sensitivity error using flight motion simulator. We verified our proposed method with experimental results.

• Journal of the Korean Society of Marine Environment & Safety
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• v.28 no.7
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• pp.1222-1230
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• 2022

The rotor blade is an important component of a tidal stream turbine and is affected by a large thrust force and load due to the high density of seawater. Therefore, the performance must be secured through the geometrical and structural design of the blade and the blade structural safety to which the composite material is applied. In this study, a 1 MW class large turbine blade was designed using the blade element momentum (BEM) theory. GFRP is a fiber-reinforced plastic used for turbine blade materials. A sandwich structure was applied with CFRP to lay-up the blade cross-section. In addition, to evaluate structural safety according to flow variations, static load analysis within the linear elasticity range was performed using the fluid-structure interactive (FSI) method. Structural safety was evaluated by analyzing tip deflection, strain, and failure index of the blade due to bending moment. As a result, Model-B was able to reduce blade tip deflection and weight. In addition, safety could be secured by indicating that the failure index, inverse reserve factor (IRF), was 1 or less in all load ranges excluding 3.0*Vr of Model-A. In the future, structural safety will be evaluated by applying various failure theories and redesigning the laminated pattern as well as the change of blade material.