• The Transactions of The Korean Institute of Electrical Engineers
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• v.57 no.11
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• pp.2011-2014
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• 2008

We measured the permittivity and electromagnetic wave propagation velocity of used insulation oil with wide frequency range including ultra-high frequency by time domain reflectometry. The permittivity or propagation velocity is essential for locating discharge faults of oil filled power transformer. We derived 2.21 as a permittivity and $2.03{\times}10^8 m/s$ as a velocity from the measurement of pulse travelling time along a coaxial line filled with used insulation oil or air. The permittivity measurement system we designed shows high measurement accuracy and the convenience for field use.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.34 no.1
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• pp.59-69
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• 2021

Recently, large-scale offshore and coastal structures have been constructed owing to the increasing interest in eco-friendly energy development. To achieve this, precise simulations of waves are necessary to ensure the safe operations of marine structures. Several experiments are required in the field to understand the offshore wave; however, in terms of scale, it is difficult to control variables, and the cost is significant. In this study, numerical waves under various wave conditions are produced using a piston-type wavemaker, and the produced wave profiles are verified by comparing with the results from a numerical wave tank (NWT) modeled using the smoothed particle hydrodynamics (SPH) method and theoretical equations. To minimize the effect by the reflected wave, a mass-weighted damping zone is set at the right end of the NWT, and therefore, stable and uniform waves are simulated. The waves are generated using the linear and Stokes wave theories, and it is observed that the numerical wave profiles calculated by the Stokes wave theory yield high accuracy. When the relative depth is smaller than two, the results show good agreement irrespective of the wave steepness. However, when the relative depth and wave steepness are larger than 2 and 0.04, respectively, the errors are negligible if the measurement position is close to the excitation plate. However, the error is 10% or larger if the measurement position is away from the excitation location. Applicable target wave ranges are confirmed through various case studies.

• Journal of Mechanical Science and Technology
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• v.19 no.8
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• pp.1611-1620
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• 2005

This paper shows the use of wavelet transformation combined with inverse acoustics to reconstruct the surface velocity of a noise source. This approach uses the boundary element analysis based on the measured sound pressure at a set of field points, the Helmholtz integral equations and wavelet transformation for reconstructing the normal surface velocity field. The reconstructed field can be diverged due to the small measurement errors in the case of nearfield acoustic holography (NAH) using an inverse boundary element method. In order to avoid this instability in the inverse problem, the reconstruction process should include some form of regularization for enhancing the resolution of source images. The usual method of regularization has been the truncation of wave vectors associated with small singular values, although the order of an optimal truncation is difficult to determine. In this paper, a wavelet transformation is applied to reduce the computation time for inverse acoustics and to enhance the reconstructed vibration field. The computational speed-up is achieved, with solution time being reduced to $14.5\%$.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.27 no.5
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• pp.471-481
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• 2016

We introduce the design technique and test results of a millimeter-wave compact antenna test range. Physical optics is employed for the analysis of the plane wave collimated by an electrical large parabolic reflector of the compact range in the test zone. The performance of the manufactured compact range is verified with the field probing test on the quiet zone and the measurement of high gain antenna. A millimeter-wave compact range designed in the frequency range of 75~110 GHz with a diameter of the test zone of 50 cm shows the magnitude variation of less than 0.75 dB.

• Journal of the Korean Society for Nondestructive Testing
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• v.20 no.4
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• pp.322-328
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• 2000

In this paper, we were investigated the detectability on various specimen in self-designed test field using the GPR system with three antenna elements. The GPR system was constantly radiated 730MHz frequency. To examine the detectability on various condition, the test were experimented using different materials, size and buried depth. As an adjusted wave-propagation velocity, the location of hyperbolic curve pattern were displayed B-scan CRT. And the pattern was exactly positioned when it was compared to the real buried-depth. Therefore, we can confirm similarity between the wave-propagation velocity and previous results.

• The Journal of the Korea Contents Association
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• v.7 no.9
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• pp.212-219
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• 2007

This study were developed with the preventive structures of shore erosion using the seawater circulation system in wave dissipation block of the natural affinity. The shore protection structures were established to excellent by the hydraulic model experiment on reflectivity and stability and wave overtopping in comparison with existing other structures. These structures, in order to analyzes also as the shore protection and the erosion preventive, were examined with the field applications of performance and the capacity of prevention, respectively, from field construction of the pending positions. As the result, the structures were ensured with the applications and the efficiency as the shore protection structure of erosion preventive by certifying accumulated sediment deposits in the field measurement and monitoring.

• Journal of the Korean Solar Energy Society
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• v.27 no.4
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• pp.137-146
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• 2007

This study is about the passive cooling effects of three outdoor solar shading facilities as trees, pergola with wistaria vine and membrane shading structure, which are expected to provide cool spots in the summer. Field observations of measuring thermal environment of selected facilities is executed. Thermal environment measuring was categorized as short wave radiation, long wave radiation, net radiation, globe temperature, surface temperature measured by infrared camera. Heat transfer mechanism is analyzed with overall data from field measurement. Results from this study are as below; 1) Radiation balance measured on shaded surface under membrane shading structure was 17%($86W/m^2$) of the unshaded surface radiation balance($511W/m^2$). 2) Surface temperature comparison between vegetation and membrane of the shading structure is performed at 3 o'clock in the afternoon. Surface temperature of vegetation was same as air temperature and that of membrane was $5^{\circ}C$ higher than air temperature. Vegetation transpiration is considered as the causing factor which make those differences. 3) Results from this study could be used as fundamental data for reducing heat island phenomena and continuos research on this subject would be needed.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.14 no.1
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• pp.51-64
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• 2002

To investigate long-period wave responses in Sokcho Harbor and Cheongcho lagoon, field measurements were made for long-and short-period waves and current velocities using a Directional Waverider, a ultrasonic-type wave gauge, four pressure-type wave gauges, and a current meter. From the data analysis, it was found that the Helmholtz resonant periods of Sokcho Harbor and Cheongcho lagoon are about 13.6 and 54.5 minutes, respectively, and the dominant period of wave induced current in the passage between Sokcho Harbor and Cheongcho lagoon is about 55.2 minutes which depends on Helmholtz resonant condition of the Cheongcho lagoon. It was also found that the energy level of the far-infra-gravity waves during storm conditions is very high compared with that during calm sea conditions. To investigate relationships between far-infra-gravity waves and short-period waves at offshore station, regression analyses were carried out especially for 1) heights, 2) periods, 3) direction and height, 4) height and period between short-and far-infra-gravity waves, respectively. The results showed that the long-period wave height is highly correlated with the short-period wave height. However, no special trend was found for the other relations. In the future far-infra-gravity wave heights on return period around Sokcho Harbor region can be suggested by using extreme value analyses of long term measured data.

• Journal of Magnetics
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• v.4 no.3
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• pp.73-75
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• 1999

By appliying an slternate pulsed magnetic field -generated by using a sequential ignition circuit and a magnet exciting circuit- with peak value of about 10 T to the rod type Nd based magnet Nd2Fe12.7Cr1.3B with length of 5 mm and diameter of 3.6 mm, the basic magnetic properties such as saturation magnetization, residual magnetization, coercivity, maximum energy products, magnetic anisotropy and anisotropic field are investigated with obtaining the major and minor J-H loops of the magnet. The increase in coercivity due to eddy currents in ac measurement of coercivity is calculated considering eddy current loss by analyzing a wave of generating magnetic field. The average coercivity calculated for the magnet is about 12.2 kOe, anisotropy magnetic field and anisotropic constant are measured as 60 kOe 2.43 Mj/$m^3$, respectively.

• Proceedings of the Acoustical Society of Korea Conference
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• 1998.06e
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• pp.259-262
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• 1998

In this paper, we have studied measurement technique for inhomogeneous residual stress using acoustic microscopy with quadrature detector. In experiment, aluminum tensile specimen with the flaw has been made and loaded by Instrone. A spherical typed acoustic transducer of center frequency 5MHz has been used for sending a longitudinal acoustic wave into a stressed specimen. It has been shown in experimental results that the phase has largely changed around the flaw that residual stress has been largely distributed and acoustic microscopy has been used in the field of residual stress measurement.