• Title/Summary/Keyword: Realizable ${\kappa}-{\varepsilon}$ turbulence model

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Numerical Analysis of High-Reynolds-Number Flow around Axisymmetric Body (축대칭체 주위 고 레이놀즈수 유동에 대한 수치해석)

  • Choi, Jung-Kyu;Kim, Hyoung-Tae
    • Journal of the Society of Naval Architects of Korea
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    • v.45 no.6
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    • pp.631-636
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    • 2008
  • In this paper, numerical analysis based on the RANS equation and the Realizable ${\kappa}-{\varepsilon}$ turbulence model is carried out for flows around an axisymmetric body at three Reynolds numbers($1.22{\times}10^7$, $1.0{\times}10^8$, $1.5{\times}10^8$) and the numerical results are compared with experiments data. Computed velocity distributions agree well with experiments as the Reynolds number increases. Pressure distributions agree well with the results of the potential flow except the tail region but differ from experiments for the parallel middle body as well as tail region. Pressure gradients show a good agreement with those of potential flow and experiment except the tail region. Friction coefficients show that the numerical results generally are lower than the experimental results estimated from the measured velocity. The difference of friction coefficients between the calculation and the experiment increases with growing of a boundary layer.

A STUDY ON THE IMPROVEMENT OF κ-εTURBULENCE MODEL FOR PREDICTION OF THE RECIRCULATION FLOW (재순환유동 예측을 위한 κ-ε 난류모델 개선에 대한 연구)

  • Lee, Y.M.;Kim, C.W.
    • Journal of computational fluids engineering
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    • v.21 no.2
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    • pp.12-24
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    • 2016
  • The standard ${\kappa}-{\varepsilon}$ and realizable ${\kappa}-{\varepsilon}$ models are adopted to improve the prediction performance on the recirculating flow. In this paper, the backward facing step flows are used to assess the prediction performance of the recirculation zone. The model constants of turbulence model are obtained by the experimental results and they have a different value according to the flow. In the case of an isotropic flow situation, decaying of turbulent kinetic energy should follow a power law behavior. In accordance with the power law, the coefficients for the dissipation rate of turbulent kinetic energy are not universal. Also, the other coefficients as well as the dissipation coefficient are not constant. As a result, a suitable coefficients can be varied according to each of the flow. The changes of flow over the backward facing step in accordance with model constants of the ${\kappa}-{\varepsilon}$ models show that the reattachment length is dependent on the growth rate(${\lambda}$) and the ${\kappa}-{\varepsilon}$ models can be improved the prediction performance by changing the model constants about the recirculating flow. In addition, it was investigated for the curvature correction effect of the ${\kappa}-{\varepsilon}$ models in the recirculating flow. Overall, the curvature corrected ${\kappa}-{\varepsilon}$ models showed an excellent prediction performance.

Case study comparisons of computational fluid dynamics modeling versus tracer test to evaluate the hydraulic efficiency of clearwell (정수지 내 추적자 실험과 CFD(전산유체역학)의 상관관계 분석)

  • Kim, Tae-Kyun;Choi, Young-June;Jo, Young-Mahn
    • Journal of Korean Society of Water and Wastewater
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    • v.25 no.5
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    • pp.635-642
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    • 2011
  • Hydraulic efficiency was a vital component in evaluating the disinfection capability of clearwell. Current practice evaluates these system based on the tracer test only. In this paper, CFD(Computational Fluid Dynamics) was applied on the clearwell for alternating or supplementing the tracer test. The baffle factor derived from the CFD modeling closely matched the values obtained from full scale tracer testing. And, for suggesting proper numerical model in clearwell; the turbulence model, discretization scheme, convergence criteria were investigated through separate simulation runs. The model validation was conducted by comparing the simulated data with experimental data. In the turbulence model, the realizable ${\kappa}-{\varepsilon}$ model and the standard ${\kappa}-{\varepsilon}$ model were found to be more appropriate than RNG ${\kappa}-{\varepsilon}$ model. The residuals of convergence criteria should be used as not $10^{-3}$ but $10^{-4}$ or $10^{-5}$. In discretization scheme, the difference of simulated values in 1st, 2nd, 3rd upwind scheme was found to be insignificant. Moreover, the result of this study suggest that CFD modeling can be a reliable alternative to tracer testing for evaluating the hydraulic efficiency.

Investigation of the Prediction Performance of Turbulence and Combustion Models for the Turbulent Partially-premixed Jet Flame (난류 부분예혼합 제트화염에 대한 난류 및 연소모델의 예측성능 검토)

  • Kim, Yu Jeong;Oh, Chang Bo
    • Fire Science and Engineering
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    • v.28 no.4
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    • pp.35-43
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    • 2014
  • The prediction performance of 9 model sets, which combine 3 turbulent models and 3 combustion models, was investigated numerically for turbulent partially-premixed jet flame. The standard ${\kappa}-{\varepsilon}$ (SKE), Realizable ${\kappa}-{\varepsilon}$ (RKE) and Reynolds stress model (RSM) were used as a turbulence model, and the eddy dissipation concept (EDC), steady laminar flamelet (SLF) and unsteady laminar flamelet model (ULF) were also adopted as a combustion model. The prediction performance of those 9 model sets was evaluated quantitatively and qualitatively for Sandia D flame of which flame structure was measured precisely. The flame length was predicted as, from longest to shortest, RSM > SKE > RKE, and the RKE predicted the flame length of the jet flame much shorter than experiment. The flame temperature was over predicted by the combination of RSM + SLF or RSM + ULF while the flame length obtained by RSM + SLF and RSM + ULF was well agreed with the experiment. The combination of SKE + SLF and SKE + ULF predicts well the flame length as well as the temperature distribution. The SKE turbulence model was most superior to the other turbulent models, and SKE + ULF showed the best prediction performance for the structure of turbulent partially-premixed jet flame.

Numerical Investigation for Drag Prediction of an Axisymmetric Underwater Vehicle with Bluff Afterbody (기저부를 갖는 축대칭 수중운동체의 저항예측에 관한 수치적 연구)

  • Kim, Min-Jae
    • Journal of the Korea Institute of Military Science and Technology
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    • v.13 no.3
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    • pp.372-377
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    • 2010
  • The objective of this study is to predict the drag of an axisymmetric underwater vehicle with bluff afterbody using CFD. FLUENT, commercial CFD code, is used to simulate high Reynolds number turbulent flows around the vehicle. The computed drag coefficients are compared to available experimental data at various Reynolds numbers. Four widely used two-equation turbulence models are investigated to evaluate their performance of predicting the anisotropic turbulence in a recirculating flow region, which is caused by flow separation arising from the base of the vehicle. The simulations with Realizable ${\kappa}-{\varepsilon}$ and ${\kappa}-{\omega}$ SST turbulence models predict the anisotropic turbulent flows comparatively well and the drag prediction results with those models show good agreements with the experimental data.

Numerical Analysis on the Internal Flow Field Characteristics of Wind Tunnel Contractions with Morel's Equation (모렐 식을 갖는 풍동수축부의 내부유동장 특성에 대한 수치해석)

  • Kim, Jang-Kweon;Oh, Seok-Hyung
    • Journal of Power System Engineering
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    • v.22 no.1
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    • pp.11-17
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    • 2018
  • The steady-state, incompressible and three-dimensional numerical analysis was carried out to evaluate the internal flow fields characteristics of wind tunnel contractions made by Morel's curve equations. The turbulence model used in this study is a realizable ${\kappa}-{\varepsilon}$ well known to be excellent for predicting the performance of the flow separation and recirculation flow as well as the boundary layer with rotation and strong back pressure gradient. As a results, when the flow passes through the interior space of the analytical models, the flow resistance at the inlet of the plenum chamber is the largest at $Z_m=300$, 400 mm, but the smallest at $Z_m=700mm$. The maximum turbulence intensity in the test section is about 2.5% when calculated by the homogeneous flow, so it is improved by about 75% compared to the 10% turbulence intensity at the inlet of the plenum chamber due to the contraction.

A VOLUME OF FLUID METHOD FOR FREE SURFACE FLOWS AROUND SHIP HULLS (선체주위 자유수면 유동 해석을 위한 VOF법 연구)

  • Park, I.R.
    • Journal of computational fluids engineering
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    • v.20 no.1
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    • pp.57-64
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    • 2015
  • This paper describes a volume of fluid(VOF) method, mRHRIC for the simulation of free surface flows around ship hulls and provides its validation against benchmark test cases. The VOF method is developed on the basis of RHRIC method developed by Park et al. that uses high resolution differencing schemes to algebraically preserve both the sharpness of interface and the boundedness of volume fraction. A finite volume method is used to solve the governing equations, while the realizable ${\kappa}-{\varepsilon}$ model is used for turbulence closure. The present numerical results of the resistance performance tests for DTMB5415 and KCS hull forms show a good agreement with available experimental data and those of other free surface methods.

Unsteady RANS Analysis of the Hydrodynamic Response for a Ship with Forward Speed in Regular Wave (규칙파중 전진하는 선박의 유체역학적 응답에 대한 비정상 수치해석)

  • Park, Il-Ryong;Kim, Kwang-Soo;Kim, Jin;Van, Suak-Ho
    • Journal of the Society of Naval Architects of Korea
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    • v.45 no.1
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    • pp.29-41
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    • 2008
  • The present paper provides a CFD analysis of diffraction problem for a ship with forward speed using an unsteady RANS simulation method, a WAVIS code. The WAVIS viscous solver adopting a finite volume method has second order accuracy in time and field discretizaions for the RANS equations. A two phase level-set method and a realizable ${\kappa}-{\varepsilon}$ turbulence model are adopted to compute the free surface and to meet the turbulence closure, respectively. To validate the capability of the present numerical methods for the simulation of an unsteady progressive regular wave, computations are performed for three grid sets with refinement ratio of ${\sqrt{2}}$. The main simulation is performed for a DTMB5512 model with a forward speed in a regular head sea condition. Validation of the present numerical method is carried out by comparing the present CFD results with available unsteady experimental data published in the 2005 Tokyo CFD Workshop: resistance, heave force, pitch moment, unsteady free surface elevations and velocity fields.