• Journal of the Korean institute of surface engineering
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• v.35 no.5
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• pp.305-311
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• 2002

Optical measurements have been used to study the biaxial tensile strain in heteroeptaxial ZnTe epilayers on the (100) GaAs substrate by hot wall epitaxy (HWE) with Zn reservoir. It is effect on the low-temperature photoluminescence spectrum of the material. Optimum growth condition has been determined by a four-crystal rocking curve (FCRC) and a low temperature photoluminescence measurement (PL). It was found that Zn partial pressure from Zn reservoir has a strong influence on the quality of grown films. Under the determined optimum growth condition, ZnTe epitaxial films with thickness of 0.72~24.8$\mu\textrm{m}$ were grown for studying the effect of the thickness on crystalline quality. The PL and FCRC results indicated that the quality of ZnTe films becomes higher rapidly with increase of thickness up to 6$\mu\textrm{m}$. The best value of the FWHM of the four crystal rocking curve, 66 arcsec, was obtained on the film with 12$\mu\textrm{m}$ in thickness. The PL spectrum shows the splitted strong free exciton emissions and very weak deep band emissions. These results show the high quality of films.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1201-1206
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• 2004

Sonoluminescence (SL) characteristics such as pulse shape, radiance and spectrum radiance from submicron bubbles were investigated. In this study, a set of analytical solutions of the Navier-Stokes equations for the gas inside bubble and equations obtained from mass, momentum and energy equations for the liquid layer adjacent the bubble wall were used to estimate the gas temperature and pressure at the collapse point, which are crucial parameters to determine the SL characteristics. Heat transfer inside the gas bubble as well as at the liquid boundary layer, which was not considered in the most of previous studies on the sonoluminescence was taken it into account in the calculation of the temperature distribution inside the bubble. It was found that bremsstrahlung is a very possible mechanism of the light emission from either micron or submicron bubbles. It was also found that the peak temperature exceeding $10^{6}$ K in the submicron bubble driven at 1 MHz and 4 atm may be due to the rapid change of the bubble wall acceleration near the collapse point rather than shock formation.

• Proceedings of the KSME Conference
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• 2001.11b
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• pp.294-299
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• 2001

An experimental study was made of turbulent separated and reattaching flow over a blunt body, where unsteady wake was generated by a spoke wheel-type wake generator with cylindrical rods. The influence of unsteady wake was scrutinized by altering the rotating direction (CW and CCW) and the normalized passing frequency $(0{\leq}St_H{\leq}0.20)$. The Reynolds number based on the cylindrical rod was $Re_d=375$. A phase-averaging technique was employed to characterize the unsteady wake. The effect of different rotating directions was examined in detail, which gave a significant reduction of $X_R$. The wall pressure fluctuations on the blunt body were analyzed in terms of the spectrum and the coherence.

• Proceedings of the KIEE Conference
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• 1997.07d
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• pp.1333-1335
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• 1997

We have made disperse diazo black D(DBD) thin films using Langmuir-Blodgett(LB) and vacuum-evaporation technique. Physical and optical properties of the films were investigated. Solution was made with a concentration of $10^{-3}mol/{\ell}$ using chloroform. Moving wall apparatus, (NL-LB140S-MWC) was employed to make the LB films. X,Y and Z-type LB films were manufactured and studied UV/visible absorbance spectra and morphology of surface using atomic force microscopy(AFM). Vacuum-evaporated DB D thin films were made at a pressure of $10^{-5}$ torr. The absorption peaks were observed at 200 and 400 nm in the LB films and vacuum-deposited films. We have also studied photoluminescence spectrum of the DBD films.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 1997.11a
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• pp.493-496
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• 1997

We have made disperse diazo black D(DB-D) ultrathin films using Langmur-Blodgett(LB) and vacuum-evaporation technique. Physical and electrical properties of the films were investigated. Solution was made with a concentration of 10$^{-3}$ mol/$\ell$ using chloroform. Moving wall apparatus (NL-LB140 S-MWC) was employed to make the LB films. X,Y and Z-type LB films were manufactured and studied by UV/visible absorbance spectra and morphology of surface using atomic force microscopy. Vacuum-evaporated DB-D think films were made at a pressure of 10$^{-5}$ torrr. The absorption peaks appear at 200 and 40nm in the LB films and vacuum-deposited films. And we have studied photoluminescence spectrum of the DB-D films. Also TGA and DSC properties of the DB-D have been observed and current -voltage characteristics of the DB-D LB films have been measured along the perpendicular direction.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.2
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• pp.353-361
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• 2002

An experimental study was made of turbulent separated and reattaching flow over a blunt body, where unsteady wake was generated by a spoke wheel-type wake generator with cylindrical rods. The influence of unsteady wake was scrutinized by altering the rotating direction (CW and CCW) and the normalized passing frequency (0 St$\_$H/ 0.20). The Reynolds number based on the cylindrical rod was Re$\_$d/=375. A phase-averaging technique was employed to characterize the unsteady wake. The effect of different rotating directions was examined in detail, which gave a significant reduction of x$\_$R/. The wall pressure fluctuations on the blunt body were analysed in terms of the spectrum and the coherence.

• Journal of the Korean institute of surface engineering
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• v.24 no.3
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• pp.119-124
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• 1991

Recently, the trend of research in hard coating is concentrate on developing the process of ionization rate under low operating pressure, to get the thin film with high adhesion and dense microstructures. In this study ionization rate enhancement type PVD process using permanent magnet is developed, which enhances the ionization rate by confining the plasma suppressing the wall loss of electron. By the result to investigate the characteristic of glow discharge, the ionization rate of this process is enhanced about twice as high as that of triod BARE process (about 26%), and more dense TiN microstructures are obtained in this process. Cylindrical ion energy analyzer is made and attached in front of a quadrupole mass filter for the analysis of the energy distribution of reactive gas and activated gas ions from the plasma zone. To analyze the operation mechanism of ion energy analyzer, computer simulation is performed by calculation the electric field environment using finite element method. By these analyses of ion energy distribution of outcoming ions from the plasma zone, it is found that magnetic field enhances ion kinetic energy as well as ionization rate. The other results of this study is that the foundation of feed-back system is constructed, which automatically control the partial pressure of reactive gas. In can be possible by recording the data of mass spectrum and ion energy analysis using A-D converter.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.32 no.6
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• pp.531-552
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• 2020

In the previous study, both the wave characteristics at the tip of composite breakwater and on caisson were investigated by applying olaFlow numerical model of three-dimensional regular waves. In this paper, the same numerical model and layout/shape of composite breakwater as applied the previous study under the action of one directional irregular waves were used to analyze two and three-dimensional spatial change of wave force including the impulsive breaking wave pressure applied to trunk of breakwater, the effect of rear region, and the occurrence of diffracted waves at the tip of caisson located on the high crested rubble mound. In addition, the frequency spectrum, mean significant wave height, mean horizontal velocity, and mean turbulent kinetic energy through the numerical analysis were studied. In conclusion, the larger wave pressure occurs at the front wall of caisson around the still water level than the original design conditions when it generates the shock-crushing wave pressure in three-dimensional analysis condition. Which was not occurred by two-dimensional analysis. Furthermore, it was confirmed that the wave pressure distribution at the caisson changes along the length of breakwater when the same significant incident wave was applied to the caisson. Although there is difference in magnitude, but its variation shows the similar tendency with the case of previous study.

• 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.

• The Journal of the Acoustical Society of Korea
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• v.40 no.1
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• pp.55-63
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• 2021

In this study, a numerical methodology is proposed for evaluating valve flow noise in a pipe conveying high pressure gas, and the effects of perforated plates on reduction of such valve flow noise are quantitatively analyzed. First, high-accurate unsteady compressible Large Eddy Simulation techniques are utilized to predict flow and flow noise by a valve in a high-pressure pipe. The validity of the numerical result is confirmed by comparing the predicted wall pressure spectrum with the measured one. Next, the acoustic power of downstream-propagating acoustic waves due to the valve flow is analyzed using an acoustic power formula for acoustic waves propagating on mean flow in a pipe. Based on the analysis results, perforated plates are designed and installed downstream of the valve to suppress the valve flow noise and the acoustic power of downstream-going acoustic waves is predicted by using the same numerical procedure. The reduction by 9.5 dB is confirmed by comparing the predicted result with that of the existing system. Based on these results, the current numerical methodology is expected to be used to reduce valve flow noise in an existing system as well as in a design stage.