• Communications of the Korean Mathematical Society
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• v.38 no.1
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• pp.223-234
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• 2023

The main purpose of this paper is to study the lightlike hypersurface (M, $\overline{g}$) of cosymplectic space form $\overline{M}$(c). In this paper, we computed the Gauss and Codazzi formulae of (M, $\overline{g}$) of cosymplectic manifold ($\overline{M}$, g). We showed that we can't obtain screen semi-invariant lightlike hypersurface (SCI-LH) of $\overline{M}$(c) with parallel second fundamental form h, parallel screen distribution and c ≠ 0. We showed that if second fundamental form h and local second fundamental form B are parallel, then (M, $\overline{g}$) is totally geodesic. Finally we showed that if (M, $\overline{g}$) is umbilical, then cosymplectic manifold ($\overline{M}$, g) is flat.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.5
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• pp.722-730
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• 2001

The local heat transfer of an axisymmetric submerged air jet impinging on a heated flat plate is investigated experimentally with the variation of mesh-screen solidity. The screen installed in front of the nozzle exit modifies the flow structure and local heat transfer characteristics. The mean velocity and turbulence intensity profiles of streamwise velocity component are measured using a hot-wire anemometry. The temperature distribution on the heated flat surface is measured with thermocouples. The smoke-wire flow visualization technique was employed to understand the near-field flow structure qualitatively for different mesh screens. Large-scale toroidal vortices and high turbulence intensity enhance the heat transfer rate in the stagnation region. For a higher solidity, turbulence intensity become higher which increases the local heat transfer at small nozzle-to-plate spacings such as L/D<6. The local and average Nusselt numbers of impinging jet from the $\sigma$(sub)s=0.83 screen at L/D=2 are about 5.6∼7.5% and 7.1% larger than those for the case of no screen, respectively. For the nozzle-to-plate spacings larger than 6, however, the turbulence intensities for all tested screens approach to an asymptotic curve and the mean velocity along the jet centerline decreases monotonically. As the nozzle-to-plat spacing increases for high solidity screens, the heat transfer rate decreases due to the reduction in turbulence intensity and jet momentum.

• Journal of Korea Foundry Society
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• v.11 no.1
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• pp.79-84
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• 1991

The process of solidification of metal is accompanied by liquid-solid change and known as Stefan's heat conduction problem on the moving boundary. In this study the temperature distribution in ingot and metallic mould during casting was analyzed by the two dimensional heat conduction theory. The transient temperature distribution was numerically calculated using a finite element method on the nodal point of mesh screen representing ingot and mould cross section. The theory was applied on the casting of aluminum(purity ; 99%) in flat ingot mould of GC25. The analysis will make it possible to calculate an optimum mould shape of which temperature gradient becomes minimum.

• Proceedings of the KIEE Conference
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• 2005.07a
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• pp.629-631
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• 2005

The DASPRO (Protection Coordination Program for DAS) is one of the application programs for Distribution Automation System (DAS) of the KEPCO. Application programs developed by KEPCO provide optimal operation in distribution network. The DASPRO includes diverse functions to operate network as construction of database using existing information. creation of diagram for protection coordination. calculation of fault current. evaluation network. power quality monitoring, etc. The DASPRO can examine validity of setting values for protective devices by comparing to T-C curves. Unless the protection coordination is correct, the most suitable setting value for protection devices are displayed on the screen. In this paper, I will introduce the contents of the DASPRO.

• Laser Solutions
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• v.10 no.4
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• pp.18-22
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• 2007

In this paper, we suggested the femtosecond laser processing using the mask which makes Gaussian spatial beam distribution to a normalized distribution by Fresenel diffraction. Holography pattern of the size of $320{\times}320{\mu}m^2$ on the Cr thin film on glass substrate with a pixel size of $5{\times}5{\mu}m^2$ was fabricated according to the pattern generated by the iterative Fourier transform algorithm(IFTA) algorithm. We analysed the damage threshold with an assumption the power distribution as Gaussian profile as 45 $mJ/cm^2$. The regenerated image of letters through the diffractive pattern was well recognized at the screen.

• Proceedings of the KIEE Conference
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• 2000.07d
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• pp.2889-2891
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• 2000

We developed the training simulator for KEPCO operators to maintain small-scale distribution automation system effectively. The training simulator has the same screen form and operating environments in comparison with using system in field. It is to be wished that the operating techniques of KEPCO operators will progress to solve the fear of system operation and to training simulations of distribution conditions again and again.

• Communications of the Korean Mathematical Society
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• v.27 no.4
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• pp.763-770
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• 2012

In this paper, we study the geometry lightlike hypersurfaces (M, $g$, S(TM)) of a semi-Riemannian manifold ($\tilde{M}$, $\tilde{g}$) of quasi-constant curvature subject to the conditions: (1) The curvature vector field of $\tilde{M}$ is tangent to M, and (2) the screen distribution S(TM) is either totally geodesic in M or totally umbilical in $\tilde{M}$.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2001.10a
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• pp.158-161
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• 2001

Light guide can be manufactured with a several method dot-screen method, injection method and DSF method. The advantage of DSF method is three-dimensional optical distribution.

• Communications of the Korean Mathematical Society
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• v.29 no.2
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• pp.311-317
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• 2014

We study lightlike hypersurfaces of a semi-Riemannian space form $\tilde{M}(c)$ admitting a semi-symmetric non-metric connection. First, we construct a type of lightlike hypersurfaces according to the form of the structure vector field of $\tilde{M}(c)$, which is called a ascreen lightlike hypersurface. Next, we prove a characterization theorem for such an ascreen lightlike hypersurface endow with a totally geodesic screen distribution.

• Communications of the Korean Mathematical Society
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• v.30 no.2
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• pp.109-121
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• 2015

In this paper, we investigate horizontal lightlike hypersurfaces of Robertson-Walker spacetimes. Some results involving the unique existence of the screen distribution and the symmetry of the induced Ricci curvature tensor of horizontal lightlike hypersurfaces are presented. We also obtain some properties concerning the symmetry and the parallelism of the second fundamental forms of such lightlike hypersurfaces.