• Magazine of the Korean Society of Agricultural Engineers
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• v.36 no.1
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• pp.128-140
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• 1994

Coastal areas, especially embayments are apt to be polluted easily and many embayments in Korea are already suffering from pollution problems. To manage such pollution, it is strongly needed to develop technique to trace movements of pollution. Such technique cove- ring the embayment affected by the tidal influence, should take account both of the convection and the diffusion motions which cause lots of problems in numerical calculation. In this study, a Eulerian-Lagrangian Analysis(ELA) model was applied to Young Il bay and tested for its applicablity, which was developed by using the Eulerian-Lagrangian Method that reduce the numerical disperison and oscillation by way of solving convection and diffusion terrns separately. Concentration of Chemical Oxygen Demand(COD) and Suspend Solid(SS) of the embay- ment area were estimated by the model and compared with the observed values and the sound results were obtained. At the same time the diffsion coefficient and decay coefficient for Chemical Oxygen Demand in the Young II Bay were found as Dx = Dy = 20m$^2$/sec, kd=2.5 ${\times}$ 10-5/sec respectively, and for Suspend Solid, Dx =Dy = 30m$^2$/sec, kd=5.0${\times}$ 10-5/sec

• The KSFM Journal of Fluid Machinery
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• v.20 no.2
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• pp.63-68
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• 2017

This paper predicts the rotordynamic force coefficients of tilting pad journal bearings (TPJBs) with ball-socket pivot and compares the predictions to the published test data obtained under load-between-pad (LBP) configuration. The present TPJB model considers the pivot stiffness calculated based on the Hertzian contact stress theory. Due to the compliance of the pivot, the predicted journal eccentricity agree well with the measured journal center trajectory for increasing static loads, while the early prediction without pivot model consideration underestimates it largely. The predicted pressure profile shows the significant pressure development even on the unloaded pads along the direction opposite to the loading direction. The predicted stiffness coefficients increase as the static load and the rotor speed increase. They agree excellently with test data from open literature. The predicted damping coefficients increase as the static load increases and the rotor speed decreases. The prediction underestimates the test data slightly. In general, the current predictive model including the pivot stiffness improves the accuracy of the rotordynamic performance predictions when compared to the previously published predictions.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2010.11a
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• pp.519-522
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• 2010

Flow-induced vibration analyses have been conducted for a 3D compressor blade model. Advanced computational analysis system based on computational fluid dynamics(CFD) and computational structural dynamics has been developed in order to investigate detailed dynamic responses of designed compressor blades. A fully implicit time marching scheme based on the Newmark direct integration method is used for computing the coupled aeroelastic governing equations of the 3D compressor blade for fluid-structure interaction problems. Detailed dynamic responses and instantaneous pressure contours on the blade surfaces considering flow-separation effects are presented to show the multi-physical phenomenon of the rotating compressor blade.

• Journal of Korean Port Research
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• v.12 no.2
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• pp.269-280
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• 1998

A Wave-induced current model is developed in our study and this model is composed with wave transform model and current model. Two types of wave model are used in our study one is Copeland(1985) type which is applied in the offshore region and the other is Watanabe and Maruyama(1984) type which is applied in the surf zone. The depth-integrated and time-averaged governing equation of an unsteady nonlinear form is used in the wave induced current model. Lateral mixing radiation stresses surface and bottom stresses are considered in our current model. Copeland’s(1976) is used as a surface friction formula. Numerical solutions are obtained by Leendertse scheme and compared with Noda’s(1974) experimental results for the uniform slope coastal region test and Nishimura & Naruyama’s (1985) experimental results and numerical simulation results for the detached breakwater. The results from our wave model and wave model and wave-induced current model show good agreements with the others and also show nonlinear effects around the detached breakwater. The model in this study can be applied in the surf zone considering the friction stresses.

• Proceedings of the Optical Society of Korea Conference
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• 2000.08a
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• pp.62-63
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• 2000

다양한 분야의 요구에 따라 반사율이 99.995% 이상인 고반사 저손실 반사경의 제작이 가능해 지면서 반사경의 산란, 투과, 흡수 및 손실 등을 측정하는 여러 측정 방법들도 더불어 연구되어 왔다. 적용 분야에 따라 사용될 반사경의 특성이 결정되는데 그 중 중력파 측정 장치$^{(1)}$ , 광자 감쇠 분광기$^{(2)}$ 등의 분야에서는 투과, 산란 및 흡수가 모두 작은 저손실 반사경을 요구한다. 한편 링 레이저 자이로스코프의 경우 반사경의 산란이 곧 lock-in을 결정하여 성능을 제한하는 중요한 변수가 된다. 따라서 이 응용의 경우 반사경의 산란을 정확히 측정하는 일은 중요하며 위치에 따른 산란분포 정보를 알면 링 레이저 자이로스코프의 성능 예측과 개선이 더욱 쉬워진다. 산란을 측정하는 직접적인 방법에는 ARS(angle resolved scattering)과 TIS(total integrated scattering)이 있는데, 본 연구에서는 TIS 측정 장비를 반사경의 위치에 따른 산란분포까지 측정할 수 있도록 mapping 기능을 첨가하고 이온빔 스퍼터링에 의해 제작된 저산란 반사경을 측정하기 위해 높은 분해능을 갖도록 구성하였다. 반사경의 응용분야의 필요에 의해 45$^{\circ}$ 산란을 측정하였다. (중략)

• Korean Journal of Optics and Photonics
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• v.31 no.6
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• pp.290-294
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• 2020

In this paper, we quantitatively analyze the effect of lens aberration on the degradation of beam-coupling efficiency of a tiled coherent-beam combining system. The Zernike polynomial is used to quantify the aberration of the lens, and Fresnel diffraction is applied to numerically simulate the change in the peak light intensity when combined at a distance. The results of this paper will be useful for quantitative prediction of the beam-combining efficiency that is degraded by aberration of the lens, and it is expected to be helpful for the optimal design of a practical tiled coherent beam-combining system.

• Journal of the Korean Society of Propulsion Engineers
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• v.25 no.1
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• pp.42-49
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• 2021

As a measure of susceptibility on the combustion instability, thermo-acoustic instabilities in rocket combustion system was considered for the estimation of the operational stability margin. Growth rate, which governs the asymptotic stability behavior of the system, was determined from the dynamic data measured during combustion tests in order to understand the dynamic characteristics of combustor system. Frequency transform technique was first applied to determine the system parameters such as growth rate and/or damping coefficient for an interested mode from the time series pressure data, and the PDFs of pressure amplitude were extracted from the amplitude envelope of pressure oscillation for the stochastic analysis.

• Journal of the Korean Geotechnical Society
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• v.37 no.12
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• pp.7-23
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• 2021

Underground utility tunnels, which contribute to supply of electricity, communication, water and heat, are critical lifelines of an urban area. In case service is discontinued or functional disruption happens, there will be a huge socio-economic impact. For the improved seismic design and evaluation of underground structures, this study proposes a ground displacement measure when the site is subjected to a scenario earthquake based on hazard-consistent source spectra and site amplification/attenuation. This measure provides a rational estimation of ground displacement and can be an alternative to existing response displacement methods.

• Journal of Radiation Protection and Research
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• v.41 no.1
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• pp.41-47
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• 2016

Background: For the accurate dose assessment in radiation therapy, energy spectrum of the photon beam generated from the linac head is essential. The aim of this study is to develop the technique to accurately unfolding the energy spectrum with the transmission analysis method. Materials and Methods: Clinical linear accelerator and Monet Carlo method was employed to evaluate the transmission signals according to the thickness of the observer material, and then the response function of the ion chamber response was determined with the mono energy beam. Finally the energy spectrum was unfolded with HEPROW program. Elekta Synergy Flatform and Geant4 tool kits was used in this study. Results and Discussion: In the comparison between calculated and measured transmission signals using aluminum alloy as an attenuator, root mean squared error was 0.43%. In the comparison between unfolded spectrum using HEPROW program and calculated spectrum using Geant4, the difference of peak and mean energy were 0.066 and 0.03 MeV, respectively. However, for the accurate prediction of the energy spectrum, additional experiment with various type of material and improvement of the unfolding program is required. Conclusion: In this research, it is demonstrated that unfolding spectra technique could be used in megavoltage photon beam with aluminum alloy and HEPROW program.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.5 no.4
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• pp.653-660
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• 2001

In this paper, we designed 2303.15MHz frequency synthesizer for the purpose of the phase noise prediction. For the modeling of phase noise generated in the designed system through introducing the noise-modeling method suggested by Lascari we analyzed a variation of phase noise as according as that of offset frequency. Especially, for the third-order system of the PLL among some kinds of phase noise generated from VCO we analyzed the aspect of 1/f-noise appearing troubles in the low frequency band. Since it is difficult to analyze mathematically 1/f-noise in the third-order system of the PLL, introducing the concept of pseudo-damping factor has made an ease of the access of the 1/f-noise variance. we showed a numerical formula of 1/f-noise variance in the third-order system of the PLL which is compared with that of 1/f-noise variance in the second-order system of the PLL. As a result, In case of txco we found the reduce rapidly along the offset frequency after passed through that phase-noise was -160dBc/Hz before passed through a loop at 10kHz offset frequency and -162.6705dBc/kHz after passed through the loop, -180dBc/Hz at 100kHz offset frequency and -560dBc/kHz after passed through the loop. We can notice that the variance of third-order system more occurs (or the variance of second-order system in connection with noise bandwidth and variance factor of second-order and third-order system.