• Korean Journal of Optics and Photonics
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• v.10 no.4
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• pp.294-300
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• 1999

The density matrix is solved more rigorously, compared with the third-order pertubation method used in the conventional theory, for a semiconductor laser amplifier. Then the coupled wave equations are derived to explain the wavelength conversion due to the spectral hole burning in the semiconductor optical amplifier. It is shown that our results can explain the effect of saturation of the population density on the electric polarization, which affects the four-wave mixing and wavelength conversion, better than the conventional theroy where the third-order pertubation is used.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.33 no.5
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• pp.311-318
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• 2020

This study introduces a technique for improving the elastomeric-isolator performance using modular devices. The modular devices are shear resistance block, polymer spring, displacement acceptance guide, and anti-falling block. They are installed on the elastomeric isolator as a supplementary device. Each modularized device improves the isolator performance by performing step-by-step actions according to the seismic intensity and displacement. The PRB isolation device works in four stages, depending on the seismic magnitude, to satisfy the target performance. It is designed to accommodate design displacement in the first stage and large magnitude of earthquakes in the second and third stages. This design prevents superstructures from falling in the fourth stage due to large-magnitude earthquakes by increasing the capacity limit of the elastomeric isolator. In this study, the PRB isolation device is analyzed using finite element analysis to verify that the PRB isolation device works as intended and it can withstand loads corresponding to large-magnitude earthquakes. The performance of the PRB isolation device is validated by the analysis, which is further corroborated by actual experiments.

• Journal of the Korean Society of Hazard Mitigation
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• v.10 no.1
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• pp.111-117
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• 2010

A three-dimensional numerical model is employed to investigate wave deformation due to a submerged structure. The three-dimensional numerical model solves the spatially averaged Navier-Stokes equations for two-phase flows. The LES(large-eddy-simulation) approach is adopted to model the turbulence effect by using the Smagorinsky SGS(sub-grid scale) closure model. The two-step projection method is employed in the numerical solutions, aided by the Bi-CGSTAB technique to solve the pressure Poisson equation for the filtered pressure field. The second-order accurate VOF(volume-of-fluid) method is used to track the distorted and broken free surface. A simple linear wave is generated on a constant depth and compared with analytical solutions. The model is then applied to study wave deformation due to a submerged structure and the predicted results are compared with available laboratory measurements.

• Journal of Energy Engineering
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• v.5 no.1
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• pp.50-55
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• 1996

A numerical simulation on a primary calcinator of porcelain was performed with using Fluent to calculate the heat efficiency by studying velocity vector and temperature profile according to variables such as the location of outlet and porcelain. Control-Volume based Finite Difference Method and Up-wind scheme are used for discretization of differential equation. SIMPLEC Algorithm and standard k-$\varepsilon$ turbulent model are selected to resolve the pressure-velocity coupling and the turbulent. The result of simulation showed that the whole velocity vector field in a calcinator was varied greatly according to the location of outlet. But the whole temperature profile at each zone was still high regardless of the location of outlet because of the radiation. But the temperature of a case with a outlet at sidepart of preheating or cooling zone was little high compared to the case with a outlet on the top of preheating zone. The velocity vector field and temperature profile in a calcinator were almost not affected by the location of porcelain, but the temperature inside a porcelain was much affected according to the place where it was located. The heat efficiency in a calcinator was 44.6% and the gas temperature in the outlet was about 1000 K.

• Bulletin of the Society of Naval Architects of Korea
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• v.27 no.4
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• pp.3-14
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• 1990

For a moored body on the sea surface, incident waves with narrow-banded spectra excite the body oscillations of short and long periods. Since the period of slow oscillations can be as long as the natural period of the moored body in horizontal modes, resonance can occur and resulting large motions may cause significant strains in mooring cables. By using the perturbation method of multiple scales, the large slow motion can be analyzed without solving any second-order potentials explicitly. To the leading order, the flows associated with the fast and slow motions interact only parametrically and thus they can be studied separately. It is found that the slow motion strongly depends on the mooring stiffness. In particular, if the moring stiffness is considerably weak compared to the body inertia, the slow motion is highly amplified near resonance. It is also shown that the slow motion is associated with the generation of long waves.

• Journal of Advanced Navigation Technology
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• v.26 no.3
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• pp.144-160
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• 2022

Research on urban air mobility (UAM) is being actively conducted as a method of next-generation transportation. eVTOL, an airplane to be used for urban air mobility, is classified into a complex type, a tilt rotor type, a tilt wing type, a tilt duct fan type, and a multicopter type according to the propulsion method. In this study, conceptual design was performed for the next generation eVTOL of the new tilt rotor type in accordance with the existing design requirements. The aerodynamic analysis programs of OpenVSP and XFLR5 were used to perform aerodynamic analysis. The power required for each flight mission stage was calculated, the battery and motor were selected accordingly, and MTOW (Maximum Take-Off Weight) was predicted by estimating the weight of each component.

• Journal of the Korean Society for Railway
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• v.12 no.1
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• pp.25-30
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• 2009

The present study deals with numerical investigation for smoke behavior in rescue station by using the commercial CFD code (FLUENT Ver 6.3). With the use of the MVHS(Modify Volumetric Heat Source) model modified from the original VHS(Volumetric Heat Source) model, a 10 MW mode was adopted for simulation and the MVHS model can describe the generation of product and the oxygen consumption at the stoichiometric state. In addition, the present simulation includes the species conservation equations for the materialization of heat source and the estimation of smoke movement. From the results, the smoke flows are moving along the ceiling because of thermal buoyancy force and as time goes, the smoke gradually moves downward at the vicinity of the entrance. Moreover, without using ventilation, it is found that the smoke flows no longer spread across the cross-passages because the pressure in the non-accident tunnel is higher than that in the accident tunnel.

• Journal of the Korean Society of Propulsion Engineers
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• v.19 no.1
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• pp.9-17
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• 2015

The basic flow configuration is composed of a plane, double shear layer where relatively thin mid gas layer is sandwiched between air and fuel stream. The present study describes numerical investigations concerning the combustion enhancement according to a variation of mid layer thickness. In this case, the effect of heat release in turbulent mixing layers is important. For the numerical solution, a fully conservative unsteady $2^{nd}$ order time accurate sub-iteration method and $2^{nd}$ order TVD scheme are used with the finite volume method including k-${\omega}$ SST model. The results consists of three categories; single shear layer consists of fuel and air, inert gas sandwiched between fuel and air, cold fuel gas sandwiched between fuel and air. The numerical calculations has been carried out in case of 1, 2, 4 mm of mid layer thickness. The height of total gas stream is 4 cm. The combustion region is broadened in case of inert gas layer of 2, 4 mm thickness and cold fuel layer of 4 mm thickness compared with single shear layer.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1995.10a
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• pp.39-43
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• 1995

대형 상용차용 내연기관에서 방출되는 소음의 음압 레벨은 경우에 따라 140dBA 이상으로 환경소음 배출규제에 부합되는 배출 소음을 위하여는 소음기의 적절한 설계가 필요하다 하겠다. 소음기는 방출 소음을 저감시키는 기본적인 작용 이외에 엔진의 효율을 저하시키는 배압(back-pressure) 상승의 효과를 동반할 뿐만 아니라 차체에의 장착을 위하여 크기와 무게의 제약을 받는다. 따라서 소음기는 설계자의 경험과 과거에 사용되어 온 소음기에 대한 이해, 소음기를 구성하는 단위 요소에 대한 음향학적 해석이론, 제작 과정에서의 경험과 튜닝 등에 바탕을 두고 설계되어져 왔다. 본 연구에서는 대형 상용차에 장착되는 기존 소음기 구성요소의 투과 손실(Transmission Loss: TL)을 전달 행렬법으로 해석하여 음향학적 특성을 규명하고 개별 소음기 요소에 대한 기존의 연구 결과를 바탕으로 공명기와 다공 요소를 이용하여 기존 소음기를 재설계함으로써 배출 소음의 저감을 이루어 나간 과정을 소개하고 이를 적용 사례를 중심으로 살펴보고 있다. 소음기의 설계를 위하여 고려할 수 있는 음향학적 요소는 그 기능과 형태면에서 다양하나 본 연구에서는 대형 상용차용 소음기에 주로 사용 가능한 공명기와 다공관을 주된 설계요소로 생각하였다. 공명기는 공명 주파수 대역의 소음을 저감하는 역할을 하므로 일정한 엔진 회전수 하에서 엔진의 방출 소음중 폭발 성분에 의한 소음을 줄이는데 효과적으로 사용될 수 있지만 가속 주행시에는 회전수(rpm)의 변화에 따라 폭발 주기가 변화하게 되므로 공명기의 설계에 주의를 기울여야 한다. 내연 기관용 소음기에 빈번하게 쓰이는 다공 요소의 해석 방법으로는 Sullivan[1], Kim and Yoon[2] 등의 분할 접근 방법(Segmentation approach)과 Jayaraman and Yam[3], Munjal[4], Peat[5] 등의 연성 제거 방법(Decoupling Approach)등이 제시되었고 평면파 영역에 한하여 해서되어져 왔다. 본 논문에서는 분할 접근 방법(Segmentation Approach)을 이용하여 다공 요소로 이루어진 소음기를 해석하는데 적용하였다.

• Proceedings of the Korea Information Processing Society Conference
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• 2002.11b
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• pp.1531-1534
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• 2002

주로 음성 트래픽을 처리하는 2G CDMA/PCS 시스템과 달리 CDMA2000 및 IxEV-DO 와 같은 차세대 이동통신시스템은 음성 트래픽뿐 만 아니라 패킷형태의 데이터 트래픽도 처리해야 하므로 차세대 이동통신망의 설계 및 디멘져닝을 위해서는 무엇보다도 데이터 트래픽의 주요 특성인 버스트성(Burstiness)과 자기유사성(Self-similarity)이 반영된 트래픽 모델이 요구된다. 이러한 관점에서 본 논문에서는 데이터 트래픽 특성이 서로 다른 다수의 데이터 트래픽의 통합되어 망에 입력되는 경우의 성능을 시뮬레이션 하였고, 그 결과를 해석적 모델인 Norros의 Effective Bandwidth Formula및 Z. FAN의 Bahadur-Rao Theorem을 적용한 Formula와도 비교하였다.