• Journal of Advanced Marine Engineering and Technology
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• v.23 no.5
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• pp.711-718
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• 1999

Current turbulence models including modified $K-{\varepsilon}-{\tau}$ turbulence model do not predict compression effect on turbulence accurately in an internal combustion engine. The $K-{\varepsilon}-{\tau}$ turbulence model was suggested to improve the predictability of compression effect by We et al. In this paper a numeri-cal study was performed to clarify the applicability of the $K-{\varepsilon}-{\tau}$ turbulenc model to the calculation of the in-cylinder flow of an axisymmetric engine. THe results using $K-{\varepsilon}-{\tau}$ turbulence model are compared to those from the modified $K-{\varepsilon}-{\tau}$ turbulence model and experimental data. The mean veloc-ity and rms velocity profiles using $K-{\varepsilon}-{\tau}$ turbulence model showed a better agreement with an experimental data than those of modifid $K-{\varepsilon}-e$ turbulence model.

• Transactions of the Korean Society of Automotive Engineers
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• v.4 no.5
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• pp.119-129
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• 1996

This study is concerned with the critical evaluation of predicative capability of a k-$\varepsilon$ turbulence model using the Renormalization Group(RNG) theory. The present numerical model for solution of the Navier-Stokes System is based on the modified PISO algorithms. Computations have been performed with the RNG-based K-$\varepsilon$ model for the two-dimensional flow over a backward-facing step, a confined coaxial jet, and a swirling flow in a swirl combustor. Numerical results are compared with experimental data in terms of mean flow velocities, turbulent kinetic energy, and turbulent stresses. Numerical results clearly indicate that the RNG-based K-$\varepsilon$ turbulence model shows a significant improvement over a standard K-$\varepsilon$ model in predicting the turbulent flows with flow separation and swirl.

• Journal of the Korean Society of Propulsion Engineers
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• v.17 no.1
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• pp.61-69
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• 2013

Numerical simulations have been performed for assessment of compressibility correction of two-equation turbulent models suitable for large scale separation flows perturbed by a pintle strokes. Two-equation turbulence models, the low Reynolds k-${\varepsilon}$ and the k-${\omega}$ SST models with or without compressibility correction proposed by Wilcox and Sarkar are evaluated. The detail flow structures are observed and static pressures along nozzle wall are compared with experimental results. Mach disk location and pressure recovery profiles in flow separation region are noticeably distinct between turbulent models of k-${\varepsilon}$ and k-${\omega}$ SST. The compressible effect corrections to those models improve resolving of separation flow behaviors. The compressibility corrections to k-${\varepsilon}$ model have provided very comparable results with test data.

• Transactions of the Korean Society of Automotive Engineers
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• v.4 no.3
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• pp.139-146
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• 1996

An extended $k-\varepsilon$ tuebulence model modified by considering the streamline curvature effect and standard $k-\varepsilon$ turbulence model have been applied for three dimensional analysis of turbulece flow in a $90^{\circ}$ curved duct. By comparision of the results with the experimental data, the modified extended $k-\varepsilon$ model gave closer agreement with experimental data than the results from standard $k-\varepsilon$ model owing to an extra time scale of the production rate and parameter describing effects of streamline curvature included in the dissipation rate equation.

• Journal of the Korean Society of Propulsion Engineers
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• v.17 no.1
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• pp.18-25
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• 2013

Assessment and validation of RANS turbulence models are conducted for the optimal analysis of supersonic converging-diverging nozzle through the comparison between computational results and experimental data. One/two equation turbulence closures such as Spalart-Allmaras, RNG k-${\varepsilon}$, and k-${\omega}$ SST are employed to simulate the two-dimensional nozzle flow. Computational results with the turbulence models mentioned fairly well predict shock structure of the nozzle-inside and pressure distribution along the wall. Especially, SST model among the employed ones shows the best agreement to experimental results.

• Journal of Advanced Marine Engineering and Technology
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• v.20 no.5
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• pp.39-44
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• 1996

To overcome weak poinks of the standard k-${\varepsilon}$ turbulence model when applied to complex turbulent flows, various modified models were proposed. But their effects are confined to special flow fields. They have still some problems. Recently, an anisotropic k-${\varepsilon}$ turbulence model was also proposed to solve the drawback of the standard k-${\varepsilon}$ turbulence model. This study is concentrated on the evaluation of the anisotropic k-${\varepsilon}$ turbulence model by the analysis of axisymmetric swirling turbulent flow. Results show that the anisotropic k-${\varepsilon}$ turbulence model has scarecely the fundamentally physical mechanism of predicting the swirling structure of flow.

• Journal of computational fluids engineering
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• v.15 no.4
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• pp.1-8
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• 2010

This article describes the numerical investigation of turbulent blood flow in the stenosed artery bifurcation under periodic acceleration of the human body. Numerical analyses for turbulent blood flow were performed with different magnitude of periodic accelerations using a modified turbulence model which was considering drag reduction of non-Newtonian fluid. The blood was considered to be a non-Newtonian fluid which was based on the power-law viscosity. In order to validate the modified $k-{\varepsilon}$ model, numerical simulations were compared with the standard $k-{\varepsilon}$ model and the Malin's low Reynolds number turbulence model for power-law fluid. As results, the modified $k-{\varepsilon}$ model represents intermediate characteristics between laminar and standard $k-{\varepsilon}$ model, and the modified $k-{\varepsilon}$ model showed good agreements with Malin's verified power law model. Moreover, the computing time and computer resource of the modified $k-{\varepsilon}$ model were reduced about one third than low Reynolds number model including Malin's model.

• 한국전산유체공학회:학술대회논문집
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• 1995.10a
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• pp.31-36
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• 1995

비유선형의 물체 주위의 유동은 정체유동, 경계층 박리 및 주기적 와열 생성 등의 복잡한 유동현상이 공존한다. 본 연구에서는 비교적 단순한 형상인 정사각주 주위의 비정상 난류 유동을 2-방정식 와점성 난류모델인 표준 $k-{\varepsilon}$ 모델과 RNG $k-{\varepsilon}$ 모델을 이용하여 예측할 수 있는지를 검증하였다. 정교하게 수행된 최근의 실험과 대와류모사(LES)의 결과를 검증을 위한 비교의 자료로 삼았다. 적절한 난류모델의 선정과 더불어 시간 정확도, 공간 정확도 및 대류항 처리법 등이 해석결과에 미치는 영향도 살펴보았다. 기존의 표준 $k-{\varepsilon}$모델은 정체점 부근에서 난류 운동에너지를 과도하게 생성하는 근본적인 문제점 때문에 실험 및 LES의 결과를 제대로 예측할 수 없었다. 난류운동에너지의 초과 예측에 따른 운동량의 과도한 혼합으로 인해, 항력계수 및 양력계수의 비정상성 뿐 아니라 평균 항력계수도 부정확하게 예측하였다. RNG $k-{\varepsilon}$ 모델을 사용한 경우에는 정체점 주위 유동현상의 예측이 상당히 향상되어 항력계수 및 양력계수의 평균치, 진폭 및 비정상성의 주기 등을 정확하게 예측하는 것이 가능하였다. 그러나 이 경우에도 예측의 정확도가 시간 증분과 격자의 크기 및 대류항 처리법등에 영향을 받으며, 특별히 대류항 처리법에 상당히 민감하게 변하는 것을 알 수 있었다. 향상된 유동예측은 RNG $k-{\varepsilon}$ 모델의 난류에너지 소산율 방정식의 개선된 항이 과도하게 생성된 난류에너지를 정체점 부근에서 제거하기 때문에 가능하다는 것을 알 수 있었다.의 20세 이하 골절 및 탈구가$30.3\%까지 감소하게되어 년도가 증가함에 따라 청장년 층에 비하여 소아골절 및 탈구가 전체적으로 감소하는 경향을 보였다. 스키골절의 부위별 발생빈도는 1990년 이전까지 하지골절 및 탈구가 많았으나 이후 점차 상지의 골절 탈구가 증가하였다 하지에서 가장 많은 골절은 경골 골절이었으며, 경골골절은 회전력에 의한 나선형골절이 $76.5\%로 가장 많았고 년도에 따른 변화는 보이지 않았다. 스키손상의 발생빈도는 초기에 비하여 점차 감소하는 경향을 보였으며, 손상의 특성도 부위별, 연령별로 다양한 변화를 나타내었다.해가능성을 가진 균이 상당수 검출되므로 원료의 수송, 김치의 제조 및 유통과정에서 병원균에 대한 오염방지에 유의하여야 할 것이다. 확인할 수 있었다. 이상의 결과에 의하면 고농도의 유기물이 함유된 음식물쓰레기는 Hybrid Anaerobic Reactor (HAR)를 이용하여 HRT 30일 정도에서 충분히 직접 혐기성처리가 가능하며, 이때 발생된 $CH_{4}$를 회수하여 이용하면 대체에너지원으로 활용 가치가 높은 것으로 판단된다./207), $99.2\%$(238/240), $98.5\%$(133/135) 및 $100\%$ (313)였다. 각각 두 개의 요골동맥과 우내흉동맥에서 부분협착이나 경쟁혈류가 관찰되었다. 결론: 동맥 도관만을 이용한 Off pump CABG를 시행하여 감염의 위험성을 증가시키지 않으면서 영구적인 신경학적 합병증을 일으키지 않았고 좋은 혈관 개존율을 보여주었다. 따라서 동맥 도관을 이용한 Off pump CABG는 관상동맥의 협착의 정도에 따라 효율적으로 시행

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.8
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• pp.1150-1157
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• 2003

Nonlinear relationship between Reynolds stresses and the rate of strain of nonlinear k-$\varepsilon$models is evaluated theoretically by using the boundary layer assumptions against the turbulence-driven secondary flows in noncircular ducts and then their prediction performance is validated numerically through the application to the fully developed turbulent flow in a square duct. Typical predicted quantities such as mean axial and secondary velocities, turbulent kinetic energy and Reynolds stresses are compared with available experimental data. The nonlinear k-$\varepsilon$ model adopted in a commercial code is found to be unable to predict accurately duct flows with the prediction level of secondary flows one order less than that of the experiment.

• Journal of Korea Water Resources Association
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• v.30 no.3
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• pp.201-210
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• 1997

To analyze the flow through open-channel constrictions using $\kappa$-$\varepsilon$ turbulence mode, a numerical model is developed. The simulated results agree well with existing experimental data which attributes to the adequate input of turbulent eddy-viscosity by turbulence model. A stream function and velocity distributions enable the analysis of flow characteristics at the downstream of constriction. Turbulent eddy viscosities over channel are spatially varied with stream pattern. For the evaluation of rapidly varied flow, the eddy-viscosity input by turbulence model is required instead of the empirical effective viscosity to solve a shallow water equation.