• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.3
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• pp.309-319
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• 2011

We experimentally investigated lifted propane jet flames diluted with nitrogen to obtain flame-stability maps based on heat-loss-induced self-excitation. We found that heat-loss-induced self-excitations are caused by conductive heat loss from premixed flame branches to trailing diffusion flames as well as soot radiation. The conductive-heat-loss-induced self-excitation at frequencies less than 0.1 Hz is explained well by a suggested mechanism, whereas the oscillation of the soot region induces a self-excitation of lift-off height of the order of 0.1 Hz. The suggested mechanism is also verified from additive experiments in a room at constant temperature and humidity. The heat-loss-induced self-excitation is explained by the Strouhal numbers as a function of the relevant parameters.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.18 no.5 s.120
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• pp.503-510
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• 2007

This paper proposes the design scheme of internal triple band antenna intended for using in GSM900, DCS1800, and PCS1900 bands. The suggested folding metal plates of the two branches are mounted on a dielectric coated ground plane for size miniaturization and durability. Return losses are overlapped when length of metal branches are controlled. This is important technique for wide band operation. For the suggested antenna geometry its return loss was calculated by HFSS 9 simulator, and was shown to be -10 [dB] less within the required band. Also, gain and radiation pattern of antenna were measured using far field measurement system in an anechoic chamber. The measured peak gain is more than 3.0 [dBi], and the average gain is over -1.0 [dBi] for the triple band, which is regarded as satisfactory for the internal antenna application. Also, the radiation pattern for two frequencies shows a similar shape each other within the required band.

• Fire Science and Engineering
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• v.20 no.4 s.64
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• pp.1-12
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• 2006

In this study, CFD computer simulations by FDS are carried out in order to confirm the performance of the combined operation of both sprinkler system and smoke curtain of 0.54 m depth installed for cooling and blocking the smoke which propagates beneath the sloped ceiling of a stairway corridor of which dimensions are 17.92 m long, 4.00 m wide, and 6.12 m high. It is shown that the response time of sprinklers decreases with fire size and it increases more about 1.1 second in case without smoke curtain than in case with smoke curtain, that the time of smoke transport from the fire source to the stairway outlet decreases considerably with fire size, and that the delay effect of smoke transport is not related to the sprinkler system, whether it is operated or not. This study shows that the combined operation of both sprinkler system and smoke curtain is very effective in smoke cooling, but it is a little for effect on smoke blockage. Although the hazard of skin burn due to radiative heat flux from hot smoke layer is decreased by spray cooling effect, the hazard of smoke suffocation and the weakening of visibility is increased by smoke downdrag and the turbulence of smoke-air mixing due to water spray. These conditions may result in preventing occupants from going out of the stairway during evacuation.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.23 no.2
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• pp.87-94
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• 2023

As the latest weapon systems and electronic equipments are increasingly demanding stealth technology to improve the survivability of allies, it is necessary to implement low-observability technology that reduces the radar cross section(RCS). In order to implement this stealth technology, a method for low RCS characteristics by applying a shape design or a electromagnetic wave absorber is widely used. However, active phased array antennas have structural limitations in shape design, also when a absorber is applied to it, the performance of the antenna is degraded. Therefore, in this paper, in order to realize the low RCS characteristics of the active phased array antenna operating in the X-band, individual radiating elements suitable for applying the radio wave absorber were selected, and a 13x13 array antenna was designed and manufactured. Next, by comparing the measured results of the relative RCS and electrical performance for the manufactured antenna according to the presence and type of the absorber, it is shown that the electrical performance is maintained at an equal or higher level while obtaining the low RCS characteristics. Thereby the method proposed in this paper for implementing the low RCS characteristics was validated. Finally, it was confirmed that when the wave absorber is applied to the array antenna, the limitation of its performance deterioration can be overcome.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.7
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• pp.729-738
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• 2010

The characteristics of lifted butane flames diluted with nitrogen have been investigated experimentally in order to elucidate the mechanism of individual flame oscillation modes. Flame oscillations in laminar free-jet lift-off flames are classified into the following five regimes: a stabilized lift-off regime (I), a heat-loss-induced oscillation (II), a buoyancy-induced oscillation along with a heat-loss-induced oscillation (III), a combined form of an oscillation prior to blow-out and a heat-loss-induced oscillation (IV), and a combination of an oscillation prior to blow-out and a buoyancy-induced oscillation along with a heat-loss-induced oscillation (V). The characterization of the individual flame oscillations modes are presented and discussed using Strouhal numbers and their relevant parameters by the analysis of the power spectrum for temporal variation of the lift-off height.

• Journal of KIISE:Computing Practices and Letters
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• v.14 no.5
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• pp.517-521
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• 2008

A worm is a malware that propagates quickly from host to host without any human intervention. Need of early worm detection has changed research paradigm from signature based worm detection to the behavioral based detection. To increase effectiveness of proposed solution, in this paper we present mechanism of detection and prevention of worm in distributed fashion. Furthermore, to minimize the worm destruction; upon worm detection we propagate the possible attack aleγt to neighboring nodes in secure and organized manner. Considering worm behavior, our proposed mechanism detects worm cycles and infection chains to detect the sudden change in network performance. And our model neither needs to maintain a huge database of signatures nor needs to have too much computing power, that is why it is very light and simple. So, our proposed scheme is suitable for the ubiquitous environment. Simulation results illustrate better detection and prevention which leads to the reduction of infection rate.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.39C no.11
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• pp.1209-1215
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• 2014

The Uniform Geometrical Theory of Diffraction(UTD) is an effective technique to analyze the radio wave by ray-based calculation of the diffraction coefficient and the reflection coefficient. In this paper, user body shadowing based on the UTD is investigated when the user is close to the mobile device. Two cases are introduced. One is that the user is calling with the mobile attached to the ear(Head Model). The other is that the mobile is located about 15cm away from the user's chest(Chest Model) Radiation Model and Scattering Model are applied to these cases presenting which model is applicable.

• Korean Journal of Optics and Photonics
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• v.11 no.6
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• pp.385-389
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• 2000

A phase-retardation function which was derived from Wigner distribution function (WDF) is used to increase a focal depth of a radially symmetric optical system. The WDF for one-dimensional signal is represented as a two-dimensional function of phasespace ($\chi,\zeta$), and a normalized irradiance is described as a form of the Strehl ratio (SR). The increase in the focal depth is accomplished by delivering a shearing tilt a that represents a characteristic of free space propagation with simple manipulation in the WDF space. In this paper we propose a method for evaluating the focal depth quantitatively by representing the phaseretardation function in terms of the focal depth term. In order to verify the validity of the proposed method, we compared the numerically analyzed result with that of J. Sochki's study. study.

• Publications of The Korean Astronomical Society
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• v.10 no.1
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• pp.15-30
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• 1995

It has been recognized that the morphologies of the SNRs from the radio observation are "barrel shaped". To interpret the mechanism of the radiation and the physical state of the environments, we have analytically calculated the dynamical structure of the interacting region in the case where the ejectum has a steep power-law density profile($\rho{\sim}r^{-n}$) and the ambient medium has a shallow power-law density profile($\rho{\sim}r^{-s}$), assuming that the cosmic rays are isotropically accelerated in the shock wave and the magnetic fields are very weak. The calculated synchrotron radio maps show that the emission from the equator is intense and the emissions from the central and polar regions are less intense. Also the thicknesses of the shell are strongly dependent on s and weakly on n. The azimuthal intensity ratio $\alpha$ increases as the efficiency of the cosmic ray acceleration increases and s decreases. We compared the results with the morphology of the SNR A. D. 1006(type I SNR). It does agree with the case of s = 0, w = 0.3 - 0.5. This value for w is consistent with the results by Eichler(1979). It provides us the evidence of the cosmic ray acceleration in the shock wave.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.10 no.8
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• pp.1352-1358
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• 2006

In this paper, we have fabricated and tested a broad band bow-tie slot antenna with dual reflector. If we put 1/2 length thin and long slot on a wide metal plate, the slot antenna radiates efficient and strong radio wave as same as 1/2 dipole antenna does. we made with bow-tie form slot which has wider broad band than normal rectangular slot. At first, we made and test a single reflector slot antenna. To enlarge the broad band, we inserted another reflection plate between the slot antenna and reflection plate. After the test, we could have known that the low band(VSWR<2.0) has about $22%(793MHz\sim992MHz)$, high band(VSWR<2.0) has about 61% (1626MHz\sim3064MHz).