• 설비공학논문집
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• 제27권4호
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• pp.220-227
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• 2015

Complex thermal hydraulic characteristics exist inside the reactor because the reactor internals consist of fuel assembly, internal structures and so on. In this study, to examine the effect of Reynolds-Averaged Navier-Stokes (RANS)-based two-equation turbulence models in the analysis of flow distribution inside a 1/5 scaled-down APR+, simulation was performed using the commercial computational fluid dynamics software, ANSYS CFX R.13 and the predicted results were compared with the measured data. It was concluded that reactor internal flow pattern was locally different depending on the turbulence models. In addition, the prediction accuracy of k-${\varepsilon}$ model was superior to that of other two-equation turbulence models and this model predicted the relatively uniform distribution of core inlet flow rate.

• 대한기계학회논문집B
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• 제20권8호
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• pp.2650-2669
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• 1996

A new low Reynolds number nonlinear second moment turbulence closure was introduced to analyze a square sectioned 180.deg. bend flow. Inclusion of nonlinear return to isotropy term and cubic mean pressure strain term has brought out a marked improvement in the level of agreement with measured velocity profiles. Optimization of present closure was performed by comparison of computed velocity profiles with the experimental ones with variation of nonlinear return to isotropy term and quadratic and cubic pressure-strain model. Progressive vortex breakdown due to the interaction of primary and secondary flows was well captured by using the optimized second moment turbulence closure.

• 대한기계학회논문집B
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• 제21권7호
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• pp.873-881
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• 1997

A numerical simulation has been carried out for three-dimensional turbulent flows around an Ahmed body. The Reynolds-averaged Navier-Stokes equation is solved with the SIMPLE method in general curvilinear coordinates system. Several k-.epsilon. turbulence models with two convective difference schemes are evaluated for the performance such as drag coefficient, velocity and pressure fields. The drag coefficient, the velocity and pressure fields are found to be changed considerably with the adopted k-.epsilon. turbulence models as well as the finite difference schemes. The results of simulation prove that the RNG k-.epsilon. model with the QUICK scheme predicts fairly well the tendency of velocity and pressure fields and gives more reliable drag coefficient. It is also demonstrated that the large difference between simulations and experiment in the drag coefficient is due to relatively high predicted values of pressure drag from vertical rear end base.

• 한국추진공학회지
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• 제17권3호
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• pp.21-29
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• 2013

초임계조건의 기체수소/액체산소 화염의 난류유동 및 온도장에 대해 난류모델을 이용한 해석이 수행되었다. 실제유체의 연소유동을 해석하기 위하여 화염편모델에 SRK 상태방정식이 도입되었다. 수정된 압력-속도-밀도 연계알고리듬이 초임계유동에 적용되었다. 수정된 알고리듬을 토대로 6개의 대류항 차분법과 4개의 난류모델의 상대적인 성능비교가 이루어졌다. 선택된 난류모델들은 실제유체 연소유동의 다양한 특징을 고려하기 위해서 수정이 필요함을 나타내었다.

• 한국전산유체공학회지
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• 제13권3호
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• pp.62-68
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• 2008

This paper reports briefly on the computational results of a turbulent Rayleigh-Benard convection with the elliptic-blending second-moment closure (EBM). The primary emphasis of the study is placed on an investigation of accuracy and numerical stability of the elliptic-blending second-moment closure for the turbulent Rayleigh-Benard convection. The turbulent heat fluxes in this study are treated by the algebraic flux model with the temperature variance and molecular dissipation rate of turbulent heat flux. The model is applied to the prediction of the turbulent Rayleigh-Benard convection for Rayleigh numbers ranging from Ra=$2{\times}10^6$ to Ra=$10^9$ and the computed results are compared with the previous experimental correlations, T-RANS and LES results. The predicted cell-averaged Nusselt number follows the correlation by Peng et al.(2006) (Nu=$0.162Ra^{0.286}$) in the 'soft' convective turbulence region ($2{\times}10^6{\leq}Ra{\leq}4{\times}10^7$) and it follows the experimental correlation by Niemela et al. (2000) (N=$0.124Ra^{0.309}$) in the 'hard' convective turbulence region ($10^8{\leq}Ra{\leq}10^9$) within 5% accuracy. This results show that the elliptic-blending second-moment closure with an algebraic flux model predicts very accurately the Rayleigh-Benard convection.

• 신재생에너지
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• 제2권2호
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• pp.28-36
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• 2006

Numerical analysis of wind turbine scale effect was performed by using commercial CFD code, Fluent. For the numerical analysis of wind turbine, the three dimensional Navier-Stokes solver with various turbulence models was tested. As a turbulence mode, the realizable k-e turbulence model was selected for the simulation of wind turbines. To validate the present method, performance of NREL (National Renewable Energy Laboratory) Phase VI wind turbine model was analyzed and compared with its wind tunnel test and blind test data. Using the present method, numerical simulations for various size of wind tunnel models were carried out and characteristics were analyzed in detail. For wind tunnel test model, the size of nacelle may not be scaled down precisely because of available motor. The effect of nacelle size was also computed and analyzed though CFD simulation. The present results showed the good correlations in pre-stall region but much to be improved in post-stall region. In 2006 and 2007, the performance and the scale effect of standard wind turbine model will be tested in KARI(Korea Aerospace Research Institute) LSWT(Low Speed Wind Tunnel) and the present results will be validated with the wind tunnel data.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집B
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• pp.517-522
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• 2001

The exit edges of a diffuser are smoothly rounded, and a wall is located perpendicularly to a diffuser exit. The fluid is discharged towards the radial direction of a diffuser after impinging against a wall from a diffuser. In this flow path, pressure loss coefficients have been calculated by the variables of Reynolds number at a diffuser inlet, distance between a diffuser exit and a wall, and turbulence models. As a result, it was calculated that $h/D_0$ ratio between $0.35\sim0.4$ has the minimum pressure loss coefficient regardless of Reynolds number and turbulence models. It was also found that in case of the flow with relatively high Reynolds number at a diffuser inlet, the pressure loss coefficients by RNG $k-\varepsilon$ model have a tendency to be near to those by standard $k-\varepsilon$ model at small ratio of $h/D_0$, but to those by RSM at large ratio.

• 한국수소및신에너지학회논문집
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• 제32권6호
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• pp.483-488
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• 2021

With continuous emission of environmental pollutants and an increase in greenhouse gases such as carbon dioxide, demand to seek other types of energy sources, alternative energy, was needed. Hydrogen, an eco-friendly energy, is attracting attention as the ultimate alternative energy medium. Hydrogen storage technology has been studied diversely to utilize hydrogen energy. In this study, the gas behavior of hydrogen in the storage tank was numerically examined under charge conditions for the Tpe III hydrogen tank. Numerical results were compared with the experimental results to verify the numerical implementation. In the results of pressure and temperature values under charge condition, the Realizable k-ε model and Reynold stress model were quantitatively matched with the smallest error between numerical and experimental results.

• 대한조선학회논문집
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• 제50권6호
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• pp.436-449
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• 2013

In this paper, a numerical analysis is carried out to study the characteristics of supercavitating flows and the drag of relatively simple two-dimensional and axisymmetric bodies which can be used for supercavity generation device, cavitator, of a high-speed underwater vehicle. In order to investigate the suitability of numerical models, cavity flows around the hemispherical head form and two-dimensional wedge are calculated with combinations of three turbulence models(standard $k-{\epsilon}$, realizable $k-{\epsilon}$, Reynolds stress) and two cavitation models(Schnerr-Sauer, Zwart-Gerber-Belamri). From the results, it is confirmed that the calculated cavity flow is more affected by the turbulence model than the cavitation model. For the calculation of steady state cavity flows, the convergence in case of the realizable $k-{\epsilon}$ model is better than the other turbulence models. The numerical result of the Schnerr-Sauer cavitation model is changed less by turbulence model and more robust than the Zwart-Gerber-Belamri model. Thus the realizable $k-{\epsilon}$ turbulence model and the Schnerr-Sauer cavitation model are applied to calculate supercavitating flows around disks, two dimensional $10^{\circ}$ and $30^{\circ}$ wedges. In case of the disk, the cavitation number dependences of the cavity size and the drag coefficient predicted are similar to either experimental data or Reichardt's semi-empirical equations, but the drag coefficient is overestimated about 3% higher than the Reichardt's equation. In case of the wedges, the cavitation number dependences of the cavity size are similar to experimental data and Newman's linear theory, and the agreement of the cavity length predicted and Newman's linear theory becomes better as decreasing cavitation number. However, the drag coefficients of wedges agree more with experimental data than those of Newman's analytic solution. The cavitation number dependences of the drag coefficients of both the disk and the wedge appear linear and simple formula for estimating the drag of supercavitating disks and wedges are suggested. Consequently, the CFD scheme of this study can be applied for numerical analysis of supercavitating flows of the cavitator and the cavitator design.

• 한국수자원학회논문집
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• 제56권spc1호
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• pp.1059-1069
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• 2023

댐붕괴흐름은 댐이 갑자기 붕괴하여 제어가 어려운 상태의 고속흐름이 방출되는 현상이다. 이 연구에서는 3차원의 댐붕괴흐름을 모의하기 위해 OpenFOAM을 사용하여 층류 및 난류 모델을 적용하고 그 결과를 비교하였다. 난류 모의를 위해 레이놀즈 평균 나비에-스토크스 (Reynolds-Averaged Navier-Stokes) 모델, 구체적으로 k-ε 모델을 사용하였다. 수리모형실험과 함께 수정된 다층 블록 장애물 시나리오를 대상으로 두 가지 모델을 평가하였다. 두 모델 모두 댐붕괴흐름을 효과적으로 재현하였으며, 난류 모델은 흐름의 변동성을 감소시키는 역할을 보여줬다. 그러나 난류 모델에서의 과도한 에너지소산은 수위를 과소 평가하게 하는 것으로 나타났다. 수치기법 및 격자 해상도를 개선하여 적용한 결과 흐름재현성이 향상되었는데 이는 특히 구조물 근처의 난류흐름에서 두드러졌다. 모델 안정성의 경우 난류모델의 사용여부보다는 수치기법 및 격자 해상도의 개선에 더 크게 영향을 받았다. k-ε 모델에 내재된 시간평균처리의 특성은 불연속성과 불안정성이 두드러진 댐붕괴흐름을 재현하는 데 한계가 있음을 나타냈다. RANS 모델을 포함한 난류모의는 방대한 계산자원이 필요하지만, 층류 모델과 비교하여 성능 향상이 제한적이었다. 댐붕괴흐름을 정확히 재현하기 위해 LES (Large Eddy Simulation) 및 DNS (Direct Numerical Simulation)과 같은 고급 난류 모델의 사용이 권장되며, 이를 위해서는 미세한 공간 및 시간 스케일의 구성이 필수적이다. 이 연구를 통해 댐붕괴흐름을 모의할 때 기본적으로 사용할 수 있는 주요 접근법과 적용가능성을 측정하였으며, 구조물 근처에서 난류흐름에 대한 정확한 표현의 중요성을 강조할 수 있었다.