• Journal of the Korean Ceramic Society
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• v.42 no.7 s.278
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• pp.449-454
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• 2005

A waveguide method is developed to study the materials with relatively large dielectric constants at microwave range. Basically, the method is similar to the previous waveguide methods represented by short-circuit line and transmission/reflection measurement methods. However, the complex permittivity is not determined by the shift in resonance frequencies, but by numerical analysis of measured scattering parameters. In order to enhance microwave penetration into the specimen with relatively large permittivity, a dielectric plate with lower permittivity is employed for impedance matching. The influences of air gap between the specimen and waveguide wall are evaluated, and the corresponding errors are estimated. The propagation of higher order modes is also considered. Experimental results for several reference ceramics are presented.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.5
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• pp.891-898
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• 2001

This paper develops a master manipulator which can reflect a force from a slave manipulator effectively. It many have a big workspace, can represent a human operators manipulation perfectly, and is composed of a position control part, an orientation control part and an end effector control part. This paper also develops a graphic simulator using the Visual C++ and OpenGL in the Window operating system. It can be used to make a virtual slave manipulator and set a virtual working environment, and provide a visual information from a desired view point. A virtual teleoperation system is developed by connecting the developed master manipulator to the graphic simulator using an interfacing hardware bilaterally. It is used for performing a virtual object exploration experiment. In the experiment, two virtual objects are used. They are virtual wall and virtual hexahedron which have 0.7N/mm and 2.2 N/mm stiffness respectively. The experiments are performed under six different working conditions. The experiment results will show the effectiveness of the reflected force from the slave manipulator for improving the efficiency and stability of the teleoperation task.

• International Journal of Aeronautical and Space Sciences
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• v.17 no.2
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• pp.253-259
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• 2016

The typical method for performing an absolute radiometric calibration of a Synthetic Aperture Radar (SAR) System is to analyze its response, without interference, to a target with a known Radar Cross Section (RCS). To minimize interference, an error-free calibration site for a Corner Reflector (CR) is required on a wide and flat plain or on an area without disturbance sources (such as ground objects). However, in reality, due to expense and lack of availability for long periods, it is difficult to identify such a site. An alternative solution is the use of a Triangular Trihedral Corner Reflector (TTCR) site, with a surrounding protection wall consisting of berms and a hollow. It is possible in this scenario, to create the minimum criteria for an effectively error-free site involving a conventional object-tip reflection applied to all beams. Sidelobe interference by the berm is considered to be the major disturbance factor. Total interference, including an object-tip reflection and a sidelobe interference, is analyzed experimentally with SAR images. The results provide a new guideline for the minimum criteria of TTCR site design that require, at least, the removal of all ground objects within the fifth sidelobe.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2011.04a
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• pp.415-418
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• 2011

Thermal fatigue is one of the life-limiting damage mechanisms in the nuclear power plant conditions. The turbulent mixing of fluids of different temperatures induces rapid temperature changes to the pipe wall. The successive thermal transients cause varying cyclic thermal stresses. These cyclic thermal stresses cause fatigue crack nucleation and growth similar to the cyclic mechanical stresses. The aim of this study was to fulfil the need by developing an real crack manufacturing method, which would produce realistic cracks. The test material was austenitic STS 304, which is used as pipelines in the reactor coolant system of a nuclear power plants. In order to fabricate thermal fatigue crack similar to realistic crack, successive thermal transients were applied to the specimen. Thermal transient cycles were combined with heating (60sec) and cooling cycle (30sec). And, In order to identify ultrasonic characteristic, it was performed the ultrasonic reflection measuring method for the fabricated specimen. From the results of ultrasonic reflection measuring testing, it was conformed that A-scan results(average 83% of real crack depth) for the TFC reference specimen was more enhanced NDT reliability than results(average 38% of real crack depth) for the EDM notch reference specimen.

• Journal of the Korean Institute of Navigation
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• v.22 no.4
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• pp.51-57
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• 1998

According to the rapid development of the electric industry, the demand of the frequency allocation and the usage of electromagnetic wave are increased due to automation of modem society. Electromagnetic wave absorbers for anechoic chamber are needed to broaden the effective frequency bandwidth, reduce the thickness, and decrease the weight. There are various absorbers proposed for the above conditions, but they could not decisively solve the above requirements. The Electromagnetic wave absorber made by a conventional ferrite tile has, for example, broadened the effective frequency bandwidth by the way of forming air layer(practically in urethane foam, etc.) on the ferrite tile. Therefore, an air layer is formed between a reflection plate and a sintered Ni-Zn ferrite tile of 7 mm in thickness, which has reflectivity less than -20 dB from 30 MHz to 400 MHz in bandwidth. In this paper, a broadband electromagnetic wave absorber are designed, which has the reflection characteristics less than -20 dB from 30 MHz to 8,000 MHz in the bandwidth. A super broadband electromagnetic wave absorber is achieved by inserting square Ferrite Cylinders Type with the thickness less than 23.5 mm and with the frequency band from 30 MHz to 8,000 MHz under the above tolerance limits. The purpose of this research is on the development of a universal anechoic chamber for measuring radiated electromagnetic wave or immunity of electronic equipments, GTEM-cell and also a wall material for preventing TV ghost, etc.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.3 no.1
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• pp.43-50
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• 1999

Electromagnetic wave absorbers for anechoic chamber are needed to broaden the useful frequency bandwidth, reduce the thickness, and decrease the weight. There are various absorbers proposed for the above conditions, but they could not decisively solve the above requirements. The Electromagnetic wave absorber made by a conventional ferrite tile has, for example, broadened the useful frequency bandwidth by the way of forming air layer. Therefore, an air layer is formed absorber between a reflection plate and a sintered Ni-Zn ferrite tile of 7 mm in thickness, which has reflectivity less than -20 dB from 30 MHz to 450 MHz in frequency band, far narrower than the aimed bandwidth. The purpose of this paper is on the development of a universal anechoic chamber for measuring radiated electromagnetic wave or immunity of electronic equipments, GTEM-cell, wall material for prevention of TV ghost, etc. Accordingly, in this paper, a broadened electromagnetic wave absorber is designed, which has the reflection characteristics less than -20 dB from 30 MHz to 5,430 or 8,000 MHz in the bandwidth. Then we will design a super broadband electromagnetic wave absorber by inserting square Ferrite Cylinders Type with the thickness less than 23.5 m in three-layed type and with the frequency band from 30 MHz to 5,430-8,000 MHz under the above tolerance limits.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.24 no.12
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• pp.969-976
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• 2014

Nowadays, it is required for naval ships to estimate 3D underwater radiated noise pattern in all direction at peak frequencies of hull vibration for the reduction of being detected and doing the effective operation. For this purpose, the numerical method has to be developed to calculate 3D underwater radiated noise pattern with experimental data. It is very difficult to obtain the experimental data for the real ship. Alternative to get the experimental results is to use NAH(near-field acoustic holography) in acoustic tank with experimental model. Application of NAH in acoustic tank for the experimental model needs some investigation of reflection wave from the wall of the acoustic tank and unmeasured zone of the experimental model due to the supporting structure for it. In this study, the effect of reflection wave in the acoustic tank and unmeasured area of the experimental model when using the NAH was investigated with experiment and numerical model. From these, it is known for the error due to reflection wave can be reduced when the distance between the measurement plane and source is being shorten. Also, unmeasured area of the experimental model gives rise to some error in the estimation of the far-field acoustic pressure.

• The Journal of the Acoustical Society of Korea
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• v.27 no.1
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• pp.1-11
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• 2008

To overcome the mid frequency limitation of geometrical acoustic techniques, the phased geometrical method was suggested by introducing the phase information into the sound propagation from the source. By virtue of phase information, the phased tracing method has a definite benefit in taking the interference phenomenon at mid frequencies into account. Still, this analysis technique has suffered from difficulties in dealing with low frequency phenomena, so called, wave nature of sound. At low frequencies, diffraction at corners, edges, and obstacles can cause errors in simulating the transfer function and the impulse response. Due to the use of real valued absorption coefficient, simulated results have shown a discrepancy with measured data. Thus, incorrect phase of the reflection characteristic of a wall should be corrected. In this work, the uniform theory of diffraction was integrated into the phased beam tracing method (PBTM) and the result was compared to the ordinary PBTM. By changing the phase of the reflection coefficient, effects of phase information were investigated. Incorporating such error compensation methods, the acoustic prediction by PBTM can be further extended to low frequency range with improved accuracy in the room acoustic field.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.29 no.6
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• pp.286-304
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• 2017

In this study, we proposed a new-type of circular perforated caisson breakwater consisting of a bundle of latticed blocks that can be applied to a small port such as a fishing port, and numerically investigated the hydraulic characteristics of the breakwater. The numerical method used in this study is OLAFOAM which newly added wave generation module, porous media analysis module and reflected wave control module based on OpenFOAM that is open source CFD software published under the GPL license. To investigate the applicability of OLAFOAM, the variations of wave pressure acting on the three-dimensional slit caisson were compared to the previous experimental results under the regular wave conditions, and then the performance for irregular waves was examined from the reproducibility of the target irregular waves and frequency spectrum analysis. As a result, a series of numerical simulations for the new-type of circular perforated caisson breakwaters, which is similar to slit caisson breakwater, was carried out under the irregular wave actions. The hydraulic characteristics of the breakwater such as wave overtopping, reflection, and wave pressure distribution were carefully investigated respect to the significant wave height and period, the wave chamber width, and the interconnectivity between them. The numerical results revealed that the wave pressure acting on the new-type of circular perforated caisson breakwaters was considerably smaller than the result of the impermeable vertical wall computed by the Goda equation. Also, the reflection of the new-type caisson breakwater was similar to the variation range of the reflection coefficient of the existing slit caisson breakwater.

• Journal of the Korean Society of Visualization
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• v.10 no.1
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• pp.40-46
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• 2012

Recently micro shock tube is extensively being used in many diverse fields of engineering applications but the detailed flow physics involved in it is hardly known due to high Knudsen number and strong compressibility effects. Unlike the macro shock tube, the surface area to volume ratio for a micro shock tube is very large. This unique effect brings many complexities into the flow physics that makes the micro shock tube different compared with the macro shock tube. In micro shock tube, the inter- molecular forces of working gas can play an important role in specifying the flow characteristics of the unsteady shock wave flow which is essentially generated in all kinds of shock tubes. In the present study, a CFD method was used to predict and visualize the unsteady shock wave flows using the unsteady compressible Navier-Stokes equations, furnished with the no-slip and slip wall boundary conditions. Maxwell's slip equations were used to mathematically model the shock movement at high Knudsen number. The present CFD results show that the propagation speed of the shock wave is directly proportional to the initial pressure and diameter of micro shock tube.