• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2006년도 제27회 추계학술대회논문집
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• pp.191-195
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• 2006

우주 발사체의 성공적인 비행을 위해서는 로켓 엔진의 성능 분산 관리가 필수적이다. 양산되는 엔진의 성능편차를 정량적으로 예측하기 위한 해석을 수행하고 성능영향계수를 이용하여 보정에 필요한 차압을 산출하였다. 별도의 추력제어 시스템을 갖추지 않은 엔진의 진공 추력 분산은 +9.1%, -8.7%로 나타났으며 엔진 혼합비 오차는 +9.7%, -9.6%에 달했다. 보정에 필요한 요구차압은 동일한 혼합비 보정에 대해 연소기 배관 산화제 측에서 더 작게 나타났으나 가스발생기 배관의 요구차압은 더 크게 요구된다.

• Journal of the Optical Society of Korea
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• 제10권3호
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• pp.112-117
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• 2006

We experimentally investigated the transmission characteristics of 400 Gbit/s (10 Gbit/s ${\times}$ 40 channels) WDM signals with 100 GHz channel spacing over 323 km of installed NZ_DSF. The installed fiber has optical properties of 0.28 dB/km attenuation, 4.3 ps/nm/km dispersion, $0.083ps/nm^2/km$ dispersion slope and less than $0.05ps/km^{1/2}$ PMD coefficient. In this experiment, two cases of dispersion compensation schemes, the lumped type and the distributed type, were compared. The results implied that the distributed type dispersion compensation in which dispersion compensation devices are inserted at the end of the each span showed better transmission performance than the lumped one in which dispersion compensation devices are located at the transmitter and receiver sites. From the analysis of the experimental results, we verified that different transmission performance comes from the power penalty induced by XPM in the distributed scheme is lower than the lumped scheme case.

• Journal of applied mathematics & informatics
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• 제35권3_4호
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• pp.241-260
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• 2017

An analysis is made to study the solute transport in a Casson fluid flow through an annulus in presence of oscillatory flow field and determine how this flow influence the solute dispersion along the annular region. Axial dispersion coefficient and the mean concentration expressions are calculated using the generalized dispersion model. Dispersion coefficient in oscillatory flow is found to be a function of frequency parameter, Schmidt number, and the pressure fluctuation component besides its dependency on yield stress of the fluid, annular gap and time in the case of steady flow. Due to the oscillatory nature of the flow, the dispersion coefficient changes cyclically and the amplitude and magnitude of the dispersion increases initially with time and reaches a non - transient state after a certain critical time. This critical value varies with frequency parameter and independent of the other parameters. It is found that the presence of inner cylinder and increase in the size of the inner cylinder inhibits the dispersion process. This model may be used in understanding the dispersion phenomenon in cardiovascular flows and in particular in catheterized arteries.

• 상하수도학회지
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• 제21권2호
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• pp.187-193
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• 2007

Reynolds and Froude numbers, along with tracer tests, have been widely used to estimate hydrodynamics in full-scale horizontal sedimentation basin in water treatment plants (WTPs). In spite of this, the former numbers do not reflect actual operating conditions, instead most often being based on the dimensions of the structures. The index and graphic analyses most often used were found to be too subjective to accurately analyze tracer test results. In order to overcome these limitations, dispersion number was applied for analysis of tracer test results. Tracer tests were conducted in eight full-scale sedimentation basins in six WTPs, and then analyzed by index and graphic analyses as well as by dispersion number. The dispersion number was more useful as an indicator of the hydrodynamic behavior in the basin. It was also found that as dispersion number decreased, flow approached the ideal for plug flow in which case, higher sedimentation efficiency could be expected.

• Smart Structures and Systems
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• 제21권2호
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• pp.195-205
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• 2018

Research on Lamb wave-based damage identification in plate-like structures depends on precise knowledge of dispersive wave velocity. However, boundary reflections with the same frequency of interest and greater amplitude contaminate direct waves and thus compromise measurement of Lamb wave dispersion in different materials. In this study, non-reflecting boundaries were proposed in both numerical and experimental cases to facilitate time-frequency characterization of Lamb wave dispersion. First, the Lamb wave equations in isotropic and laminated materials were analytically solved. Second, the non-reflecting boundaries were used as a series of frames with gradually increased damping coefficients in finite element models to absorb waves at boundaries while avoiding wave reflections due to abrupt property changes of each frame. Third, damping clay was sealed at plate edges to reduce the boundary reflection in experimental test. Finally, the direct waves were subjected to the slant-stack and short-time Fourier transformations to calculate the dispersion curves of phase and group velocities, respectively. Both the numerical and experimental results suggest that the boundary reflections are effectively alleviated, and the dispersion curves generated by the time-frequency analysis are consistent with the analytical solutions, demonstrating that the combination of non-reflecting boundary and time-frequency analysis is a feasible and reliable scheme for characterizing Lamb wave dispersion in plate-like structures.

• 한국전자파학회논문지
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• 제18권6호
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• pp.664-673
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• 2007

본 논문에서는 참고문헌 [1], [2]에서 무손실 매질을 가정하고 보고하였던 ID-FDTD 기법을, 손실 매질로 확장시켜 정리하였다. 확장된 내용으로는 기법의 안정도 분석(stability analysis), 손실 매질에 대한 ID-FDTD 기법의 분산 관계(dispersion relation), 무손실 손실 매질 모두에 적용할 수 있는 보정치를 수식 등 크게 세 가지이다. 이러한 분석 결과 ID-FDTD 기법은 참고문헌 [3]의 standard 기법보다 큰 시간 증분에서도 안정성을 유지하면서도 적은 분산 오차(dispersion error)를 가지는 것을 확인할 수 있었다. 이는 두 가지 경우의 전자기파 산란 문제를 계산함으로써 검증하였다.

• 한국안전학회지
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• 제37권2호
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• pp.1-9
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• 2022

Most factories deal with toxic or flammable chemicals in their industrial processes. These hazardous substances pose a risk of leakage due to accidents, such as fire and explosion. In the event of chemical release, massive casualties and property damage can result; hence, quantitative risk prediction and assessment are necessary. Several methods are available for evaluating chemical dispersion in the atmosphere, and most analyses are considered neutral in dispersion models and under far-field wind condition. The foregoing assumption renders a model valid only after a considerable time has elapsed from the moment chemicals are released or dispersed from a source. Hence, an initial dispersion model is required to assess risk quantitatively and predict the extent of damage because the most dangerous locations are those near a leak source. In this study, the dispersion model for initial consequence analysis was developed with three-dimensional unsteady advective diffusion equation. In this expression, instantaneous leakage is assumed as a puff, and wind velocity is considered as a coordinate transform in the solution. To minimize the buoyant force, ethane is used as leaked fuel, and two different diffusion coefficients are introduced. The calculated concentration field with a molecular diffusion coefficient shows a moving circular iso-line in the horizontal plane. The maximum concentration decreases as time progresses and distance increases. In the case of using a coefficient for turbulent diffusion, the dispersion along the wind velocity direction is enhanced, and an elliptic iso-contour line is found. The result yielded by a widely used commercial program, ALOHA, was compared with the end point of the lower explosion limit. In the future, we plan to build a more accurate and general initial risk assessment model by considering the turbulence diffusion and buoyancy effect on dispersion.

• ETRI Journal
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• 제18권2호
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• pp.75-86
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• 1996

A new method is proposed for the analysis of optical properties of stationary transverse electirc (TE) nonlinear waves in the five-layer waveguide which consists of a linear guiding layer with two nonlinear bounding layers sandwiched between a semi-infinite clad and a substrate. By using the relation of the interface electric fields, we obtain the generalized form of nonlinear dispersion equations as an analytic and flexible form. In order to verify the dispersion equation, we apply the dispersion equation to the analysis of the symmetric five-layer waveguide. The nonlinear dispersion curves for several thicknesses of the nonlinear thin film is also presented.

• 한국정밀공학회지
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• 제27권7호
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• pp.49-54
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• 2010

The cryogenic ball milling was performed on carbon nanotubes (CNTs) at an extremely low temperature to increase the dispersion of CNTs. The effects of milling speed and time on the deagglomeration and structural changes of CNTs were studied. FESEM was used to analyze the dispersion and the change of particle size before and after milling process. Transmission electron microscopic (TEM) analysis was also investigated the effect of cryogenic ball milling on the morphological characteristics of CNTs. The structural changes by the cryogenic ball milling process were further confirmed by x-ray diffraction (XRD) and Raman spectroscopic analysis. The results showed that the agglomeration of CNTs was significantly reduced and amorphous structure was observed at high milling speed. However, the milling time has no great effect on the dispersion property and structural change of CNTs compared with milling speed.

• 한국공작기계학회논문집
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• 제10권1호
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• pp.133-140
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• 2001

Recently, to assure the integrity of a structural components such as piping pressure vessels and thinning structure, Lamb wave inspection technique has been used in material evaluation. It is very important to select the optimal Lamb wave mode and to analyze the signal accurately because of its unique dispersion properties grnerating several modes within the speci-men. It this study, the feasibility of material evaluation applications using wavelet analysis of Lamb wave has been veir-fied experimentally. These results show as follows; 1)dispersion characteristic of each mode in dispersion curve is demon-strated that A0 mode propagating material surface is useful mode having the lest energy loss and not sensitive to surface condition. 2) it can be detected even the micro defect ($1\times2mm$) fabricated in ultrasonic probe flaw distance (290mm) to axis direction. 3) the wavelet transform which is called "time-frequency analysis" shows the Lamb wave propagation due to the change of materials characterization can be evaluated at each frequency and experimental group velocity of Lamb wave agrees quite well with that of simulated dispersion curve.ion curve.