• Wind and Structures
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• 제27권4호
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• pp.255-267
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• 2018

This paper investigated the wind-snow flow around the bogie region of a high-speed train under crosswinds using a coupled numerical method of the unsteady Realizable $k-{\varepsilon}$ turbulence model and discrete phase model (DPM). The flow features around the bogie region were discussed and the influence of bogie fairing height on the snow accumulation on the bogie was also analyzed. Here the high-speed train was running at a speed of 200 km/h in a natural environment with the crosswind speed of 15 m/s. The mesh resolution and methodology for CFD analysis were validated against wind tunnel experiments. The results show that large negative pressure occurs locally on the bottom of wheels, electric motors, gear covers, while the positive pressure occurs locally on those windward surfaces. The airflow travels through the complex bogie and flows towards the rear bogie plate, causing a backflow in the upper space of the bogie region. The snow particles mainly accumulate on the wheels, electric motors, windward sides of gear covers, side fairings and back plate of the bogie. Longer side fairings increase the snow accumulation on the bogie, especially on the back plate, side fairings and brake clamps. However, the fairing height shows little impact on snow accumulation on the upper region of the bogie. Compared to short side fairings, a full length side fairing model contributes to more than two times of snow accumulation on the brake clamps, and more than 20% on the whole bogie.

• 대한조선학회논문집
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• 제42권4호
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• pp.368-378
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• 2005

The effects of rotary oscillation on the unsteady laminar flow past a circular cylinder. are numerically investigated in the present study. The numerical solutions for the 20 Wavier-Stokes equation are obtained using a finite volume method Tn the framework of an overlapping grid system. The vortex formation behind a circular cylinder and the hydrodynamics of wake flows for different rotary oscillation conditions are analyzed from the results of numerical simulation. The lock-on region is defined as the region that the natural shedding frequency due to the Karmann Vortex shedding and the forcing frequency due to the forced oscillating a cylinder are nearly same, and the quasi-periodic states are observed around that region. At the intersection between lock-on and non-lock-on region the shedding frequency is bifurcated. After the bifurcation, one frequency fellows the forcing frequency($S_f$) and the other returns to the natural shedding frequency($St_0$). in the quasi-periodic states, the variation of magnitudes and relevant phase changes of $C_L$ with forcing phase are examined.

• 한국수소및신에너지학회논문집
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• 제31권3호
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• pp.328-336
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• 2020

In the Francis turbine, the leakage flow through the runner gaps which are between the runner and the stator structure influences the internal flow and hydraulic performance. Thus, the investigation for the flow characteristics induced by the runner gaps is important. However, the runner gaps are often disregarded by considering the time and cost of the numerical analysis. Therefore, in this study, the flow characteristics according to runner gaps of the Francis turbine model were investigated including the leakage flow of the runner cone. The three-dimensional unsteady Reynolds-averaged Navier-Stokes analyses were conducted using a scale-adaptive simulation shear stress transport as a turbulence model for observing the influence of the leakage flow on the internal flow and hydraulic performance. The efficiencies were decreased slightly with runner gaps; and the complicated flows were captured in the gaps.

• 한국대기환경학회지
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• 제22권1호
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• pp.35-41
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• 2006

The steel foundries with electric arc furnaces handling metal scraps have recently gained an attention as a potential source of dusts. The present study focuses on the analysis of dust emissions and removals during furnace charging and melting processes by commercial CFD software named FLUENT. A body fitted grid system consisting of 880,000 meshes was first generated by Gambit for the electric arc furnace with the capacity of 60 ton/cycle and then FLUENT was invoked to solve the corresponding NavierStokers equation for the momentum, temperature and dust concentration. The entire processes from metal charging to metal melting were simulated to investigate the unsteady behaviors of fluid flows and dust concentrations. The model simulation results showed that as the top of the electric arc furnace opened for metal charging, hot plumes bursted out from the furnace rose strongly by buoyance and escaped mostly through the main hood. Therefore, the capacity of main hoods determined the vent efficiency in the metal charging process. As the furnace was closed after the metal charging and the metal melting processes was followed, the hot flow stream stretching from the furnace to the main hood was dissipated fast and the flow from the inlet of the bottom of the left hand side to the main and monitoring hoods constituted the main stream. And there was only a slow flow in the right hand side of the furnace. Therefore, the dust concentrations were calculated higher in the left hand side of the furnace, which was consistent with observations.

• 한국항공우주학회지
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• 제48권9호
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• pp.711-716
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• 2020

탄화수소계열 연료를 기반으로 혼합모사추진제를 사용하여 액적을 생성하고 열에너지를 가하면서 초임계 환경으로 분무되는 거동을 가시화하였다. 혼합모사추진제는 임계압력과 임계온도가 상이한 데칸과 메틸사이클로헥산을 선정하였다. 초임계 환경으로 분무되는 유동은 Rayleigh 분열로 액적을 생성하며 Oh 수와 Re 수를 구하여 Rayleigh 분열영역임을 확인하였다. 혼합모사추진제의 온도는 Tr=0.49에서 Tr=1.34까지 변화를 주었다. 유량은 0.7~0.8 g/s로 유지하였다. 액적은 열에너지를 가할수록 분열 길이가 짧아지며 덩어리진 형태로 떨어진다. 액체 상을 가시화하는 장치에서 2차 액적(second droplet)이 형성되는 것을 확인하였고 Tr=1.34일 때 부분적으로 불안정한 상태의 초임계 상태로 액상이 보이지 않는다.

• Journal of Advanced Marine Engineering and Technology
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• 제37권4호
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• pp.351-357
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• 2013

플립칩 패키징에서 언더필 공정은 칩과 기판 사이를 에폭시로 채워서 본딩하는 공정으로 제품의 신뢰성 향상을 위해 수행되어 진다. 이 언더필 공정은 모세관 현상에 의해서 이루어지는데 유체의 계면과 범프의 배열이 계면 운동에 미치는 영향으로 인하여 공정 중 예기치 않은 공기층을 형성하게 된다. 본 연구에서는 모세관 언더필 유동에서 나타나는 비정상 계면 유동을 가시화하여 범프 배열에 따른 레이싱 효과와 계면의 병합 현상에 대하여 고찰하였다. 그 결과, 플립칩 내부의 범프가 고밀도일수록 유체의 흐름방향과 수직방향의 유동이 더욱 활발하게 진행되어 더 많은 공기층이 형성되었으며, 엇갈린 배열일 경우 직각 배열에 비해 이러한 현상이 더 지배적으로 나타난다.

• 한국자동차공학회논문집
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• 제2권1호
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• pp.9-25
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• 1994

In the present study, the Reynolds-averaged Navier-Stokes equations, together with the equations of the $k-{\varepsilon}$ model of turbulence, were solved numerically in a general body-fitted coordinate system for three-dimensional turbulent flows around the six basic shapes of the magnetically levitated train(MAGLEV). The numerical computations were conducted on the MAGLEV model configurations to provide information on shapes of this type very near the elevated track at a constant Reynolds number of $1.48{\times}10^{6}$ based on the body length. The coordinate system was generated by numerically solving a set of Poisson equations. The convective transport equations were discretized using the finite-analytic scheme which employed analytic solutions of the locally-linearized equations. A time marching algorithm was employed to enable future extensions to be made to handle unsteady and fully-elliptic problems. The pressure-velocity coupling was treated with the SIMPLER-algorithm. Of particular interests were wall effect by the elevated track on the aerodynamic forces and flow characteristics of the six models calculated. The results indicated that the half-circle configuration with extended sides and with smooth curvature of sides was desirable because of the low aerodynamic forces and pitching moment. And it was found that the separation bubble was occured at wake region in near the elevated track.

• 한국항공우주학회지
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• 제30권8호
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• pp.46-55
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• 2002

본 연구에서는 저가형 PC-클러스터 환경에서 운영 가능한 고속 병렬처리 기법을 활용하여 전기체 항공기 및 발사체 형상을 고려할 수 있는 천음속/초음속 비선형 플러터 해석시스템을 개발하였다. 이는 이론적으로 진보된 수치해석 기법인 전산구조동역학(CSD), 유한요소법(FEM) 및 전산유체역학(CFD) 기법을 동시에 연계하고 있으며, 각종 비행체의 공탄성안정성 설계 과정에서 공학적으로 매우 정밀한 데이터 제공이 가능하다. 개발된 공탄성 해석시스템의 뛰어난 응용성을 보이기 위해 국내에서 개발 중인 초음속 항공기의 전기체 형상에 대해 천음속/초음속 비선형 공탄성 해석을 수행하였다.

• 한국수자원학회논문집
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• 제38권9호
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• pp.727-736
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• 2005

본 연구에서는 McCormack 기법의 2차 정확도 및 단순성의 장점과 불연속 흐름을 해석할 수 있는 상류이송기법의 장점을 결합하여 상류이송형 McCormack 기법을 새로이 개발하였다. 이 기법은 생성항을 효과적으로 처리할 수 있는 장점도 지니고 있다. 본 연구에서 개발된 기법을 해석해를 가진 가상적인 하도에 적용한 결과 기존 McCormack 기법에서 발생하던 수치진동없이 해석해를 잘 재현할 수 있는 것으로 나타났다. 또한 실제 하천에 대한 적용을 위해 하상과 하폭의 변화가 매우 심한 한강 하류구간에 적용한 결과 기존 McCormack 기법이 해석할 수 없는 정상류나 부정류를 모두 잘 해석할 수 있는 것으로 나타났다. 본 연구에서 개발한 상류이송형 McCormack 기법은 복잡한 자연하도의 흐름해석을 위해 효과적으로 사용될 수 있을 것으로 판단된다.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2000년도 가을 학술발표회 논문집
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• pp.705-712
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• 2000

The fluid generally flows through fractures in crystalline rocks where most of underground storage facilities are constructed because of their low hydraulic conductivities. The fractured rock is better to be conceptualized with a discrete fracture concept, rather continuum approach. In the aspect of fluid flow in underground, the simultaneous flow of groundwater and gas should be considered in the cases of generation and leakage of gas in nuclear waste disposal facilities, air sparging process and soil vapor extraction for eliminating contaminants in soil or rock pore, and pneumatic fracturing for the improvement of permeability of rock mass. For the purpose of appropriate analysis of groundwater-gas flow, this study presents an unsteady-state multi-phase FEM fracture network simulator. Numerical simulation has been also conducted to investigate the hydraulic head distribution and air tightness around Ulsan LPG storage cavern. The recorded hydraulic head at the observation well Y was -5 to -10 m. From the results obtained by the developed model, it shows that the discrete fracture model yielded hydraulic head of -10 m, whereas great discrepancy with the field data was observed in the case of equivalent continuum modeling. The air tightness of individual fractures around cavern was examined according to two different operating pressures and as a result, only several numbers of fractures neighboring the cavern did not satisfy the criteria of air tightness at 882 kPa of cavern pressure. In the meantime, when operating pressure is 710.5 kPa, the most areas did not satisfy air tightness criteria. Finally, in the case of gas leaking from cavern to the surrounding rocks, the resulted hydraulic head and flowing pattern was changed and, therefore, gas was leaked out from the cavern ceiling and groundwater was flowed into the cavern through the walls.