• Journal of the Korean Society of Propulsion Engineers
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• v.22 no.2
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• pp.66-73
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• 2018

A computational fluid dynamics simulation of pyrotechnic material combustion inside a cylindrical closed vessel was carried out using the Eulerian-Lagrangian method. The 5th order upwind WENO scheme and the improved delayed detached eddy turbulence model were implemented to capture shock waves. The flow structure was analyzed inside the cylindrical vessel with a pressure sensor installed at the side wall center. The analysis revealed that the pressure oscillated because of the shock wave vibration. Additionally, the simulation results with four different sensor tab depths implied that, inside the sensor tab, eddies were generated by the excessively large gap between the sensor diaphragm and the side wall. These eddies caused irregularity to the measured time-pressure curve, which is an undesirable characteristic.

• Journal of the Korean Society for Railway
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• v.16 no.3
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• pp.169-174
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• 2013

The effect of modifying the shape of a high-performance EMU train on the aerodynamic drag is studied here using Computational Fluid Dynamics(CFD) based on three dimensional Steady-state Navier-Stokes equation and two equation turbulence modeling. FLUENT 12 and Gambit 2.4.6 are employed for a numerical simulation of the aerodynamic drag of a streamlined-shape train as well as a proto type train. The characteristics of the aerodynamic drag of trains in tunnels are analyzed in a comparison with these characteristics in an open space. The contribution of the aerodynamic drag of each case is also investigated to establish principal pertaining to drag reduction for urban trains in tunnels. The aerodynamic drag of a streamlined train was reduced to 9.8% relative to a proto-type train with a blunt nose and a protruding roof facility and underbody shape: the running resistance is expected to be reduced by as much as 4% at a running speed of 80km/h.

• Proceedings of the SAREK Conference
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• 2008.11a
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• pp.546-550
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• 2008

In this study, 3-dimensional Computational Fluid Dynamics (CFD) analysis was induced to simulate air flow and particle motion in the axial flow cyclone separator. The commercialized CFD code FLUENT was used to visualize pressure drop and particle collection efficiency inside the cyclone. We simulated 4 cyclone models with different shape of vane, such as turning angle or shape of cross section. For the air flow simulation, we calculated the flow field using standard ${\kappa}-{\varepsilon}$ turbulence viscous model. Each model was simulated with different inlet or outlet boundary conditions. Our major concern for the flow filed simulation was pressure drop across the cyclone. For the particle trajectory simulation, we adopted Euler-Lagrangian approach to track particle motion from inlet to outlet of the cyclone. Particle collection efficiencies of various conditions are calculated by number based collection efficiency. The result showed that the rotation angle of the vane plays major roll to the pressure drop. But the smaller rotation angle of vane causes particle collection efficiency difference with different inlet position.

• Nuclear Engineering and Technology
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• v.55 no.5
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• pp.1802-1813
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• 2023

Conjugate heat transfer between liquid metal and solid is a common phenomenon in a liquid-metal-cooled fast reactor's fuel assembly and heat exchanger, dramatically affecting the reactor's safety and economy. Therefore, comprehensively studying the sophisticated conjugate heat transfer in a liquid-metal-cooled fast reactor is profound. However, it has been evidenced that the traditional Simple Gradient Diffusion Hypothesis (SGDH), assuming a constant turbulent Prandtl number (Pr_t,, usually 0.85 - 1.0), is inappropriate in the Computational Fluid Dynamics (CFD) simulations of liquid metal. In recent decades, numerous studies have been performed on the four-equation model, which is expected to improve the precision of liquid metal's CFD simulations but has not been introduced into the conjugate heat transfer calculation between liquid metal and solid. Consequently, a four-equation model, consisting of the Abe k - ε turbulence model and the Manservisi k_𝜃 - ε_𝜃 heat transfer model, is applied to study the conjugate heat transfer concerning liquid metal in the present work. To verify the numerical validity of the four-equation model used in the conjugate heat transfer simulations, we reproduce Johnson's experiments of the liquid lead-bismuth-cooled turbulent pipe flow using the four-equation model and the traditional SGDH model. The simulation results obtained with different models are compared with the available experimental data, revealing that the relative errors of the local Nusselt number and mean heat transfer coefficient obtained with the four-equation model are considerably reduced compared with the SGDH model. Then, the thermal-hydraulic characteristics of liquid metal turbulent pipe flow obtained with the four-equation model are analyzed. Moreover, the impact of the turbulence model used in the four-equation model on overall simulation performance is investigated. At last, the effectiveness of the four-equation model in the CFD simulations of liquid sodium conjugate heat transfer is assessed. This paper mainly proves that it is feasible to use the four-equation model in the study of liquid metal conjugate heat transfer and provides a reference for the research of conjugate heat transfer in a liquid-metal-cooled fast reactor.

• Journal of the Korean Society of Marine Environment & Safety
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• v.24 no.2
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• pp.246-252
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• 2018

In this paper, the friction drag force of 3D submerged body is investigated by considering the surface roughness, the first grid height, and the Reynolds number using open CFD source code, OpenFOAM 4.0. A procedure for estimating drag components by CFD code is set up and suggested in this study. In the 3D submerged body, because of the form factor in the 3D computations, the friction resistance with the small roughness of $12{\mu}m$ obtains different result with the smooth wall. As the Reynolds number increased, the boundary layer becomes thinner and the fiction resistance tends to decrease. In the computations for the effect of y+, the friction resistance and wall shear stress are excessively predicted when the y+ value deviates from the log layer. This is presumably because the boundary layer becomes thicker and the turbulence energy is excessively predicted in the nose due to the increase in y+ value. As the roughness increases, the boundary layer becomes thicker and the turbulence kinetic energy on the surface increases. From this study, the drag estimation method, considering the roughness by numerical analysis for ships or offshore structures, can be provided by using the suggested the y+ value and surface roughness with wall function.

• Fire Science and Engineering
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• v.29 no.5
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• pp.42-50
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• 2015

Three simulation approaches for turbulence were applied for the computation of propane dispersion in a simplified real-scale urban area with one building:, Large Eddy Simulation (LES), Detached Eddy Simulation (DES), and Unsteady Reynolds Averaged Navier-Stokes (RANS). The computations were performed using FLUENT 14, and the grid system was made with ICEM-CFD. The propane distribution depended on the prediction performance of the three simulation approaches for the eddy structure around the building. LES and DES showed relatively similar results for the eddy structure and propane distribution, while the RANS prediction of the propane distribution was unrealistic. RANS was found to be inappropriate for computation of the gas dispersion process due to poor prediction performance for the unsteady turbulence. Considering the computational results and cost, DES is believed to be the optimal choice for computation of the gas dispersion in a real-scale space.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.41 no.7
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• pp.516-523
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• 2013

The tip clearance effect on counter-rotating ducted fan of VTOL UAV in hovering condition, was investigate using computational analysis. The $k-{\omega}$ SST turbulence model is employed in this study. The numerical results of baseline model are validated by wind tunnel test in hovering and forward conditions. It is observed that if tip clearance of one rotor in the counter-rotating ducted fan increase then the thrust coefficient of another rotor increases. In Addition to this, when the tip clearance of the rear rotor increases, the thrust of the ducted fan is improved due to increasing of average total pressure at exit plane.

• Journal of Korean Society of Occupational and Environmental Hygiene
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• v.18 no.2
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• pp.161-170
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• 2008

푸쉬-풀 환기시스템은 도금조와 같이 흡인해야 할 거리가 상대적으로 긴 경우에 많이 사용되고 있다. 그러나, 창문이나 출입문을 통한 방해기류가 푸쉬-풀 환기시스템의 오염물질 제어효율을 심각하게 훼손시키고 있다고 추측하고 있으나 이에 대한 세부적인 연구가 부족한 상태에 있다. 따라서, 본 연구에서는 전산유체역학(Computational fluid dynamics)을 이용하여 푸쉬-풀 환기시스템에서의 방해기류의 방향과 세기가 흡인효율에 어떠한 영향을 미치는지에 대해 평가해 보았다. 선형흡인효율(Linear capture efficiency) 방법을 이용하여 푸쉬-풀 환기시스템에서 가상의 개방조에서 발생한 오염물질이 푸쉬-풀 시스템에 의하여 포집되지 못하고 누출되는 구역이 어딘지를 찾아낼 수 있었다. 전산유체역학 컴퓨터시뮬레이션은 AIRPAK2.1 (FLUENT CODE) 소프트웨어를 사용하였다. 푸쉬-풀 후드시스템에 방해기류가 강하게 작용하면 상대적으로 강한 와류가 발생하는데, 일반적인 난류모델인 ${\kappa}-{\varepsilon}$모델은 와류현상을 충분히 보여주지 못한 반면에 RNG 모델을 사용했을 때 실험결과를 적절히 모사해낼 수 있었다. RNG 모델을 이용하여 세가지 방향, 즉 푸쉬에서 풀 방향으로, 풀에서 푸쉬 방향으로 그리고 그에 수직되는 방향으로 방해기류가 있을 때의 푸쉬-풀 환기시스템의 흡인효율을 분석하였다. 방해기류가 0.25m/s이하일 때에는 흡인효율이 거의 떨어지지 않았으나, 방해기류가 0.6m/s에서 흡인효율이 40-70%로 떨어짐을 알 수 있었다. 따라서, 방해기류를 감소시킬 수 있는 방안에 대해서도 연구를 해야 되겠지만, 방해기류 존재 하에서 충분한 흡인 효율을 유지할 수 있는 푸쉬-풀 후드 설계기준에 대한 연구도 필요할 것으로 판단된다.

• Journal of Korea Water Resources Association
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• v.57 no.5
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• pp.347-357
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• 2024

Numerous prior studies have delineated the size distribution of noncohesive sediment in suspension, focusing on mean size and standard deviation. However, suspensions comprise a heterogeneous mixture of sediment particles of varying sizes. The transport dynamics of suspended sediment in turbulent flow are intimately tied to settling velocities calculated based on size and density. Consequently, understanding the grain size distribution becomes paramount in comprehending sediment transport phenomena for noncohesive sediment. This study aims to introduce a straightforward modeling approach for simulating the grain size distribution of suspended sediment amidst turbulence. Leveraging insights into the contrast between cohesive and noncohesive sediment, we have meticulously revised a stochastic flocculation model originally designed for cohesive sediment to aptly simulate the grain size distribution of noncohesive sediment in suspension. The efficacy of our approach is corroborated through a meticulous comparison between experimental data and the grain size distribution simulated by our newly proposed model. Through numerical simulations, we unveil that the modulation of grain size distribution of suspended sediment is contingent upon the sediment transport capacity of the carrier fluid. Hence, we deduce that our simplified approach to simulating the grain size distribution of suspended sediment, integrated with a sediment transport model, serves as a robust framework for elucidating the pivotal bulk properties of sediment transport.

• Journal of the Korean Society of Propulsion Engineers
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• v.3 no.4
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• pp.58-66
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• 1999

The flow characteristics of supersonic ejectors is often subject to compressibility, unsteadiness and shock wave systems. The numerical works carried out thus far have been of one-dimensional analyses or some Computational Fluid Dynamics(CFD) which has been applied to only a very simplified configuration. For the design of effective ejector-pump systems the effects of secondary mass flow on the supersonic ejector flow should be fully understood. In the present work the supersonic ejector-pump flows with a secondary mass flow were simulated using CFD. A fully implicit finite volume scheme was applied to axisymmetric compressible Navier-Stokes equations. The standard two-equation turbulence model was employed to predict turbulent stresses. The results obtained showed that the flow characteristics of constant area mixing tube types were nearly independent of the secondary flow rate, but the flow fields of ejector system with the second-throat were strongly dependent on the secondary flow rate due to the effect of the back pressure near the primary nozzle exit.