• 한국정밀공학회지
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• 제18권7호
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• pp.127-133
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• 2001

The effects of triangular and rectangular discharge current waveforms on anode erosion in wire electric discharge machining is investigated based on heat transfer analysis of half-space subject to time and space-dependent heat flux. The thermal load on the anode is simulated by a hear flux that is calculated from time-dependent discharge channel radius and energy. Evolution of the melting front during discharge, the molten volume at the experimental result for qualitative verification of the analysis. It is demonstrated that the triangular current waveform is more efficient in eroding the anode than the rectangular one.

• International Journal of Aeronautical and Space Sciences
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• 제7권2호
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• pp.26-32
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• 2006

This paper targets a direct and quantitative prediction of characteristics of unstable waves in a combustion chamber, which employs the governing equations derived in terms of amplification factors of flow variables. A freshly formulated nonlinear acoustic equation is obtained and the analysis of unsteady waves in a rocket engine is attempted. In the present formalism, perturbation method decomposes the variables into time-averaged part that can be obtained easily and accurately and time-varying part which is assumed to be harmonic. Excluding the use of conventional spatially sinusoidal eigenfunctions, a direct numerical solution of wave equation replaces the initial spatial distribution of standing waves and forms the nonlinear space-averaged terms. Amplification factor is also calculated independently by the time rate of changes of fluctuating variables, and is no longer an explicit function for compulsory representation. Employing only the numerical computation, major assumptions inevitably inherent, and in erroneous manner, in up to date analytical methods could be avoided. With two definitions of amplification factor, 1-D stable wave and 3-D unstable wave are examined, and clearly demonstrated the potentiality of a suggested theoretical-numerical method of combustion instability.

• 한국경제지리학회지
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• 제17권2호
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• pp.249-263
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• 2014

1950년대에 조성된 RTP는 계획적으로 조성된 첨단 산-학 복합단지임과 동시에 외생적 발전 전략으로 클러스터 기반을 구축한 후, 성장 과정에서 내생적 발전 역량을 갖추게 된 흥미로운 사례이다. 본 논문에서는 외생적 발전 전략에 의해 계획적으로 조성된 RTP가 어떻게 성공적인 진화 경로를 밟게 되었는지, 그 과정과 요인을 트리플 힐릭스 공간의 구축 과정을 통해 분석하였다. RTP의 클러스터 형성 초기에는 지식 공간의 존재와 역할이 중요했으나, RTP의 고도성장기에 지식 공간이 미친 영향은 상대적으로 미흡했다. 반면에, NC 과학기술위원회는 트리플 힐릭스 합의 공간으로서, RTP의 조성단계에서부터 지금까지 클러스터의 경로파괴적 진화를 견인한 핵심 주체라고 할 수 있다. 그리고 RTP의 지속적 진화 과정에는 노스캐롤라이나 생명공학연구센터(NCBC)와 노스캐롤라이나 마이크로일렉트릭센터(MCNC) 등과 같은 혁신 공간은 클러스터의 트리플 힐릭스 상호작용을 유발하는 핵심 중개기관의 역할을 수행했다. 오늘날, RTP에는 합의 공간을 중심으로 지식 공간과 혁신 공간이 연계되는 구조를 통해 트리플 힐릭스 공간이 구축되어 있으며, 이를 통해 기존 산업 생태계의 역동성을 유지하면서도 새로운 산업 생태계가 성장하는 이른바 역동적 진화 구조를 나타내고 있다.

• 천문학논총
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• 제32권1호
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• pp.101-103
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• 2017

We have carried out the near-infrared spectroscopic observations of recent classical novae (e.g., V2468Cyg, V1280Sco) within a few years from the outburst with AKARI as a part of AKARI Open Time Observing Program for Phase 3-II "Spectral Evolution of Novae in the Near-Infrared based on AKARI Observations (Proposal ID: SENNA)". The homogeneous datasets of near-infrared spectra from $2.5{\mu}m$ to $5{\mu}m$ with AKARI/IRC collected in this program are useful to infer the physical conditions of the shell formed by the ejected materials, to examine the chemical properties of the ejecta gas, and to examine the properties of dust formed in the nova ejecta.

• Journal of Astronomy and Space Sciences
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• 제38권1호
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• pp.65-82
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• 2021

For the vast majority of geostationary satellites currently in orbit, station keeping activities including orbit determination and maneuver planning and execution are ground-directed and dependent on the availability of ground-based satellite control personnel and facilities. However, a requirement linked to satellite autonomy and survivability in cases of interrupted ground support is often one of the stipulated provisions on the satellite platform design. It is especially important for a geostationary military-purposed satellite to remain within its designated orbital window, in order to provide reliable uninterrupted telecommunications services, in the absence of ground-based resources due to warfare or other disasters. In this paper we investigate factors affecting the robustness of a geostationary satellite's orbit in terms of the maximum duration the satellite's station keeping window can be maintained without ground intervention. By comparing simulations of orbit evolution, given different initial conditions and operations strategies, a variation of parameters study has been performed and we have analyzed which factors the duration is most sensitive to. This also provides valuable insights into which factors may be worth controlling by a military or civilian geostationary satellite operator. Our simulations show that the most beneficial factor for maximizing the time a satellite will remain in the station keeping window is the operational practice of pre-emptively loading East-West station keeping maneuvers for automatic execution on board the satellite should ground control capability be lost. The second most beneficial factor is using short station keeping maneuver cycle durations.

• Journal of Advanced Marine Engineering and Technology
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• 제29권1호
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• pp.60-67
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• 2005

Generally. in the molecular dynamics simulations. the Verlet neighboring list algorithm is used for the reduction of a simulation time On the other hand. the application of the Verlet neighboring list forces the time evolution of a simulation system to follow an unrealistic path in a phase space. In equilibrium state, it does not matter with the simulation results because the individual molecule's motion is originally random and any effect due to a small deviation from a real time evolution can be completely ignored. However, if an unsteady state is involved. such a deviation may significantly affect to the results. That is, there is a Possibility that the simulation results Provide ones with any misleading data In this study we evaluated the effect due to the Verlet neighboring list in performing the simulation of a non-equilibrium state and suggested the method to avoid it.

• 천문학회지
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• 제40권4호
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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.

• 천문학회보
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• 제35권2호
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• pp.74.2-74.2
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• 2010

We investigate driven magnetohydrodynamic (MHD) turbulence by including the effects of expansion and collapse of background medium. The main goal is to quantify the evolution and saturation of strength and characteristic lengths of magnetic fields in expanding and collapsing media. Our findings are as follows. First, with expansion and collapse of background medium, the magnetic energy density per comoving volume does not saturate; either it keeps decreasing or increasing with time. The magnetic energy density relative to the kinetic energy density strongly depends on the expanding or collapsing rate. Second, at scales close to the energy injection (or driving) scale, the slope of magnetic field power spectrum shallows with expansion but steepens with collapse. Third, various characteristic lengths, relative to the energy injection scale, decrease with expansion but increase with collapse. We discuss the astrophysical implications of our findings.

• 대한수학회지
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• 제50권6호
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• pp.1311-1332
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• 2013

This paper concerns the evolution of a closed hypersurface of the hyperbolic space, convex by horospheres, in direction of its inner unit normal vector, where the speed equals a positive power ${\beta}$ of the positive mean curvature. It is shown that the flow exists on a finite maximal interval, convexity by horospheres is preserved and the hypersurfaces shrink down to a single point as the final time is approached.

• 천문학회보
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• 제36권2호
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• pp.108.1-108.1
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• 2011

Turbulence is ubiquitous in astrophysical fluids such as the interstellar medium(ISM) and the intracluster medium(ICM). There are many driving mechanisms which can inject energy into the fluid in variety driving scales, But the plausible driving scale of ISM/ICM turbulence are yet unknown. Therefore, understanding different statistical properties between turbulence with single driving scale and turbulence with double driving scale is required. In this work, we performed 3-dimensional isothermal compressible, magnetohydrodynamic(MHD) turbulence simulations. We drive turbulence in the Fourier space in two ranges, 2