• Wind and Structures
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• 제15권4호
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• pp.313-329
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• 2012

There have existed for a number of years good practice guidelines for the use of Computational Fluid Dynamics (CFD) in the field of wind engineering. As part of those guidelines, details are given for the size of flow domain that should be used around a building of height, H. For low-rise buildings, the domain sizes produced by following the guidelines are reasonable and produce results that are largely free from blockage effects. However, when high-rise or tall buildings are considered, the domain size based solely on the building height produces very large domains. A large domain, in most cases, leads to a large cell count, with many of the cells in the grid being used up in regions far from the building/wake region. This paper challenges this domain size guidance by looking at the effects of changing the domain size around a tall building. The RNG ${\kappa}-{\varepsilon}$ turbulence model is used in a series of steady-state solutions where the only parameter varied is the domain size, with the mesh resolution in the building/wake region left unchanged. Comparisons between the velocity fields in the near-field of the building and pressure coefficients on the building are used to inform the assessment. The findings of the work for this case suggest that a domain of approximately 10% the volume of that suggested by the existing guidelines could be used with a loss in accuracy of less than 10%.

• 대한조선학회논문집
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• 제34권1호
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• pp.1-10
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• 1997

본 연구는 실제 선박 주위의 점성 유동에 대해 RANS방정식을 사용하여 해석함으로써 그 계산 방법의 타당성 및 선형 설계에의 유효성을 입증하고, 모형선 크기에 따른 점성 유동의 영향, 즉 척도효과에 대한 기초 연구를 목적으로 하였다. 높은 레이놀즈수에서의 난류유동을 계산하기 위해 k-${\varepsilon}$ 난류모형을 채용하였으며, 물체 근처에서는 벽법칙을 사용하였다. 선체의 3차원 형상을 효과적으로 처리하기 위해 물체적합좌표계를 이용하여 실제영역에서 유도된 지배방정식을 계산영역으로 변환시켰으며, 유한체적법을 사용하여 이산화시켰다. 압력 계산은 SIMPlE법을 사용하였으며, 이산화된 식들은 TDMA를 이용한 선순법으로 해를 구하였다. 실제 계산대상 선박은 4410 TEU급 콘테이너 운반선과 50,000 DWT급 살물 운반선으로 모형선 크기와 실선 크기에 대해 점성유동을 해석하여 비교하였으며, 모형선에 대해서 저항시험, 프로펠러 면에서의 반류분포 조사 시험, 그리고 한계유선 조사시험을 수행하여 계산결과와 비교 검토하였다. 계산결과는 선미 유동장에서의 평균속도와 압력 분포에 있어서 선미 형상에 따른 효과와 척도효과를 잘 묘사하고 있다. 특히, 계산된 프로펠러 변에서의 반류분포와 선체 표변에서의 한계유선 분포는 실험과 정성적으로 잘 일치하고 있으며, 점성저항 추정에 있어서는 실험 값과 ${\pm}5%$ 이내로 예측하고 있음을 보여 주고 있어 선형 개발의 설계 도구로 활용될 수 있음을 알 수 있다.

• 한국전산유체공학회지
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• 제13권4호
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• pp.1-7
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• 2008

In the process of continuous hot-dip galvanizing, it is well known that the gas wiping through an air knife system is most effective because of its uniformity in coating thickness, possibility of thin coating, workability in high speed, and simplicity of control. However, gas wiping used in the galvanizing process brings about a problem of splashing at the strip edge above a certain high speed of process. It is also known that the problem of edge splashing is more harmful than that at the mid strip surface. For a given liquid(of a certain viscosity and surface tension), the onset of splashing mainly depends upon the strip velocity, the gas-jet pressure, and the nozzle's stand-off distance. In these connections in the present study, we proposed three kinds of air knife system having nozzles of constant expansion rate, and compared the jet structures issuing from newly proposed nozzle systems with the result by a conventional one. In numerical analysis, the governing equations are consisted of two-dimensional time dependent Navier-Stokes equations, and the standard k-${\varepsilon}$ turbulence model is employed to solve turbulence stress and so on. As the result, it is found that we had better use the constant expansion-rate nozzle which can be interpreted from the point view of the energy saving for the same coating thickness. Also, we better reduce the size of separation bubble and enhance the cutting ability at the strip surface, by using an air-knife having constant expansion-rate nozzle.

• 대한기계학회논문집B
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• 제23권10호
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• pp.1327-1339
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• 1999

Characteristics of two-phase flow and heat transfer were numerically investigated in a submerged gas Injection system. Effects of both the gas flow rate and bubble size were investigated. In addition, heat transfer characteristic and effects of heat transfer were investigated when temperature of the injected gas was different from that of the liquid. The Eulerian approach was used for the formulation of both the continuous and the dispersed phases. The turbulence in the liquid phase was modeled by the use of the standard $k-{\varepsilon}$ turbulence model. The interphase friction and heat transfer coefficient were calculated by means of correlations available in the literature. The turbulent dispersion of the phases was modeled by introducing a "dispersion Prandtl number". The plume region and the axial velocities are increased with increases in the gas flow rate and with decreases in the bubble diameter. The turbulent flow field grows stronger with the increases in the gas flow rate and with the decreases in the bubble diameter. In case that the heat transfer between the liquid and the gas is considered, the axial and the radial velocities are decreased in comparison with the case that there is no temperature difference between the liquid and the gas when the temperature of the injected gas is higher than the mean liquid temperature. The results in the present research are of interest in the design and the operation of a wide variety of material and chemical processes.

• Nuclear Engineering and Technology
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• 제54권9호
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• pp.3310-3316
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• 2022

Owing to the rising concerns about the safety of nuclear power plants (NPP), it is essential to study the venturi scrubber in detail, which is a key component of the filtered containment venting system (FCVS). FCVS alleviates the pressurein containment byfiltering and venting out the contaminated air. Themain objective of this research was to perform a CFD investigation of different configurations of a circular, non-submerged, self-priming venturi scrubber to estimate and improve the performance in the removal of elemental iodine from the air. For benchmarking, a mass transfer model which is based on two-film theory was selected and validated by experimental data where an alkaline solution was considered as the scrubbing solution. This mass transfer model was modified and implemented on a unique formation of two self-priming venturi scrubbers in series. Euler-Euler method was used for two-phase modeling and the realizable K-ε model was used for capturing the turbulence. The obtained results showed a remarkable improvement in the removal of radioactive iodine from the air using a series combination of venturi scrubbers. The removal efficiency was improved at every single data point.

• 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.

• International Journal of Naval Architecture and Ocean Engineering
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• 제4권2호
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• pp.151-161
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• 2012

The present study numerically investigates the aerodynamic characteristics of two-dimensional wings in the vicinity of the ground by solving two-dimensional steady incompressible Navier-Stokes equations with the turbulence closure model of the realizable k-${\varepsilon}$ model. Numerical simulations are performed at a wide range of the normalized ground clearance by the chord length ($0.1{\leq}h/C{\leq}1.25$) for the angles of attack ($0^{\circ}{\leq}{\alpha}{\leq}10^{\circ}$) in the prestall regime at a Reynolds number (Re) of $2{\times}10^6$ based on free stream velocity $U_{\infty}$ and the chord length. As the physical model of this study, a cambered airfoil of NACA 4406 has been selected by a performance test for various airfoils. The maximum lift-to-drag ratio is achieved at ${\alpha}=4^{\circ}$ and h / C = 0.1. Under the conditions of ${\alpha}=4^{\circ}$ and h / C = 0.1, the effect of the Reynolds number on the aerodynamic characteristics of NACA 4406 is investigated in the range of $2{\times}10^5{\leq}Re{\leq}2{\times}10^9$. As Re increases, $C_l$ and $C_d$ augments and decreases, respectively, and the lift-to-drag ratio increases linearly.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집B
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• pp.499-504
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• 2001

A validation of a 3D CFD model for predicting local subcooling of moderator in the vicinity of calandria tubes in a CANDU reactor is performed. The small scale moderator experiments performed at Sheridan Park Experimental Laboratory(SPEL) in Ontario, Canada[1] is used for the validation. Also a comparison is made between previous CFD analyses based on 2DMOTH and PHOENICS, and the current model analysis for the same SPEL experiment. For the current model, a set of grid structures for the same geometry as the experimental test section is generated and the momentum, heat and continuity equations are solved by CFX-4.3, a CFD code developed by AEA technology. The matrix of calandria tubes is simplified by the porous media approach. The standard $k-\varepsilon$ turbulence model associated with logarithmic wall treatment and SIMPLEC algorithm on the body fitted grid are used and buoyancy effects are accounted for by the Boussinesq approximation. For the test conditions simulated in this study, the flow pattern identified is a buoyancy-dominated flow, which is generated by the interaction between the dominant buoyancy force by heating and inertial momentum forces by the inlet jets. As a result, the current CFD moderator analysis model predicts the moderator temperature reasonably, and the maximum error against the experimental data is kept at less than $2.0^{\circ}C$ over the whole domain. The simulated velocity field matches with the visualization of SPEL experiments quite well.

• 산업기술연구
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• 제22권A호
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• pp.59-65
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• 2002

The objective of this study is to provide the quantitative and qualitative computational data about the aerodynamic performance of Gurney flap on NACA 00XX airfoils and to show the optimum Gurney flap height for each airfoil. The test was performed on 7 different airfoils from NACA 0006 to NACA0024, which have a 3% chord(=c) thickness interval. For every NACA 00XX airfoil, Gurney flap heights were changed by 0.5% or 0.25% chord interval from 0 to 2.0%c to study their effects. The aerodynamic characteristics of clean and Gurney flap airfoil were compared, and the influences of Gurney flap on each airfoil were compared. As a CFD (Computational Fluid Dynamics) solver, FLUENT, based on Navier-Stokes code, was used to calculate the flow field around the airfoil. The fully-turbulent results were obtained using the standard $k-{\varepsilon}$ two-equation turbulence model. The test results showed that Gurney flap increased the lift coefficient much more than the drag coefficient over a certain range of the lift coefficient, so the lift-to-drag ratio, which is the important index of airfoil performance, was increased. Based on the test results, the relationship between the airfoil thickness and the optimum Gurney flap heights was suggested.

• 한국전산유체공학회지
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• 제15권1호
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• pp.56-63
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• 2010

Since emission regulations for vehicles have become more stringent, SCR technology has drawn a strong attention in order to reduce NOx emissions. Optimal design of a reductant injection nozzle and a multi-hole plate located between the cone and catalyst is critical in that the uniform distribution of reductant is necessary to maximize the NOx conversion efficiency and minimize the slip of reductant in SCR. In this work, an LPG fuel(C3H8 in vapor state) was used as a reductant for LPG vehicles. A Realizable k-$\varepsilon$ model is used for turbulence, and SCR body is defined as porous media with inertia and viscous resistances measured in this work. Effect of the number of nozzle holes on the flow mixing index was analyzed, which revealed that a four hole nozzle shows the best performance in terms of uniformity of flow. An installment of a multi-hole plate at the entrance of catalyst was evaluated with flow mixing index, uniformity of flow, and pressure drop. A multi-hole plate with gradual hole diameter change in three steps showed the best uniformity of flow within the conditions suggested in this work.