• Journal of The Korean Astronomical Society
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• v.29 no.spc1
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• pp.201-204
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• 1996

Magnetic fields are thought to playa role in a wide variety of important astrophysical processes, from angular momentum transport and jet formation in accretion disks to corona formation in stars. Unfortunately, the dynamics of magnetic fields in astrophysical plasmas are extremely complicated, and the success of current theoretical models and computer simulations seems to be inversely correlated with the amount of observational detail available to us. Here I will discuss some of the more striking conflicts between numerical simulations and observations, and present an explanation for them based on an important dynamical process which is not adequately modeled in current numerical simulations. These processes will lead to the formation of flux tubes in stars and accretion disks, in accordance with observations. I will discuss some of the implications of flux tube formation for stellar and accretion disk dynamos.

• 한국전산유체공학회:학술대회논문집
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• 2003.10a
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• pp.213-216
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• 2003

This paper put forward the controlled spin intensity method for the tangentially-fired furnaces to solve the problems existed in the counter-tangential operation. The numerical simulation was used in this paper to discuss some basic principles.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.329-332
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• 2002

A theoretical study is made of the steady flow of a compressible fluid in a rapidly rotating finite cylinder. Flow is generated by imposing mechanical and/or thermal disturbances at the rotating endwall disks. Both the Ekman and Rossby numbers are small. A detailed consideration is given to the energy budget for a control volume in the Ekman boundary layer. A combination of physical variables, which is termed the energy contents, consisting of temperature and modified angular momentum, emerges to be relevant. The distinguishing features of a compressible fluid, in contrast to those of an incompressible fluid, are noted. For the Taylor-Proudman column to be sustained, in the interior, it is shown that the net energy transport between the solid disk wall and the interior fluid should vanish. Physical rationalizations are facilitated by resorting to the concept of the afore-stated energy content.

• 제어로봇시스템학회:학술대회논문집
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• 1995.10a
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• pp.472-475
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• 1995

In this paper, we investigate so-called the falling cat problem. It is well known that a cat, when released from an upside down configuration starting from rest, is able to land on her feet without violating angular momentum conservation. This has being an interesting problem for engineers for a long time. We consider a model of a falling cat as connected two rigid columns, which is a nonholonomic system. We design the controller for it, using time- state control form of the model and exact linearization technique. Finally, we test the controller thorough simulation on the model of a falling cat.

• 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.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.1
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• pp.66.1-66.1
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• 2010

Using two-dimensional numerical hydrodynamic simulations, we investigate the star formation rate (SFR) in turbulent, multiphase, galactic gaseous disks. Our simulation domain is axisymmetric, and local in the radial direction and global in the vertical direction. Our models include galactic rotation, vertical density stratification, self-gravity, radiative heating and cooling, and thermal conduction, but do not include spiral-arm features. Turbulence in our models is driven by momentum feedback from supernova explosion events occurring in localized dense regions formed by thermal and gravitational instabilities. Self-consistent radiative heating, representing enhanced/reduced FUV photons from the star formation, is also taken into account. By controlling three parameters (the gas surface density, the stellar disk density, and the angular rotation rate) that characterize local galactic disks, we explore how the SFR depends on the background environmental state. We also discuss the relation between the SFR and the gas surface density found in our numerical models in comparison with observations.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.27.1-27.1
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• 2017

There is a rich diversity of galaxy properties and we are starting to understand some of the drivers for differences between galaxies. Much progress has been has been made in the last decade, thanks in large part to massively multiplexed surveys using single fibres, but we still lack a complete picture of how galaxies are built. I will discuss how large-scale integral field surveys can address a number of the outstanding questions in the field, starting with the current SAMI Galaxy Survey, and then looking towards the Hector instrument that will carry out integral field surveys of order 50,000-100,000 galaxies. With SAMI we can start to address how mass and environment influence galaxy structure and history, and I will discuss examples such as the environmental quenching of star formation and the distribution of angular momentum. With larger samples afforded by Hector we can go beyond simply mass and environment, to separate galaxies based on their merger or accretion history, as well as their larger-scale environment.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.4
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• pp.503-508
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• 1997

The change of the structure of homogeneous turbulence subject to irrotational strains has been studied in an anti-Morel type diffuser (center matched cubic contour) using the hot wire anemometry. It was observed that the profiles of mean velocities and turbulence velocities along the center line were stable at the entrance region but rapidly changed near the matching point. The wall induced turbulence at the entrance region grows fast and was diffused toward the center at downstream. It was also observed that the axial turbulence grows faster than the radial one in the middle region of the diffusing flow and that the diffusing process has the vortex compression mechanism due to the conservation of angular momentum. These phenomena are frequently observed at the initial flow region of the free jet.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.2
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• pp.616-623
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• 1996

This paper investigates the linear stability of reacting mixing layers with special emphasis on the existence of multiple unstable modes. The governing equations for laminar flows are from two-dimensional compressible boundary-layer equations. The chemistry is a finite rate single step irreversible reaction with Arrhenius kinetics. For the incompressible reacintg mixing layer with variable density. A necessary condition for instability has been derived. The condition requires that the angular momentum, not the vorticity, to have a maximum in the flow domain. New inflectional modes of instability are found to exist in the outer part of the mixing layer. For the compressible reacting mixing layer, supersonic unstable modes may exist in the abscence of a generalized inflection point. The outer modes at high Mach numbers in the reacting mixing layer are continuations of the inflectional modes of low Mach number flows. However, the generalized inflection point is less important at supersonic flows.

• Journal of the Korean Society of Combustion
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• v.5 no.1
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• pp.55-66
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• 2000

A general purpose program for the analysis of flows in a gas turbine combustor is developed. The program uses non-staggered grids based on finite volume method and the cartesian velocities as primitive variables. A flow inside the C-type diffuser is simulated to check the boundary fitted coordinate. The velocity profiles at cross section agree well with experimental results. A turbulent diffusion flame behind a bluff body is simulated for the combustion simulation. Simulated results show good agreement with experimental data. Finally, a turbulent flow with swirl in a gas turbine combustor was simulated. The results show two recirculating region and simulated velocity fields agree well with experimental data. The distance between two recirculating regions becomes shorter as swirl angle increases. Swirl angle changes angular momentum and streamlines in flow fields.