• 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.

• Journal of the Korean Society of Propulsion Engineers
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• v.2 no.3
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• pp.20-35
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• 1998

Characteristics of turbulent flow with wall transpiration is analyzed. The wall transpiration includes both of suction and injection and extends their range to 0~160 of absolute magnitude of Re$_{w}$ . Reynolds number based on inlet velocity also covers wide range of 3${\times}$$10^3$~8${\times}$$10^4$. The turbulent flow with wall transpiration induces change of wall boundary layer and rapid change of turbulent field. This, in turn, leads the change of whole flow field. For predicting this complicated flow field properly, newly modified $\kappa$-$\varepsilon$ model is utilized, which is formed by modifying dissipation rate equation. The modified $\kappa$-$\varepsilon$ model of Chien is also adopted for the comparison of model performance. Analysis shows the newly modified $\kappa$-$\varepsilon$ model is successfully able to reflect the characteristics of turbulent flow field with wall transpiration.ion.

• Journal of Advanced Marine Engineering and Technology
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• v.23 no.2
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• pp.216-225
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• 1999

Four turbulent $k-{varepsilon}$models(i.e standard model modified models with streamline curvature modification and/or preferential dissipation modification) are applied in order to analyze the tur-bulent flow of wall-attaching offset jet. The upwind numerical scheme was adopted in the present analyses. The streamline curvature modification results in slightly better prediction while the preferential dissipation modification does not. The obtained analytic results will be used as refer-ences for further study regarding Reynolds stress model. In addition this paper introduced a method of increasing nozzle outlet velocity gradually for numercal convergence. Even though the method was simple it was efficient in view of convergent speed CPU running time computer memory storage programming etc.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.3B
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• pp.247-257
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• 2009

In the present paper, turbulent open-channel flows with alternate vegetated zones are numerically simulated using threedimensional model. The Reynolds-averaged Navier-Stokes Equations are solved with the ${\kappa}-{\varepsilon}$ model. The CFD code developed by Olsen(2004) is used for the present study. For model validation, the partly vegetated channel flows are simulated, and the computed depth-averaged mean velocity and Reynolds stress are compared with measured data in the literature. Comparisons reveal that the present model successfully predicts the mean flow and turbulent structures in vegetated open-channel. However, it is found that the ${\kappa}-{\varepsilon}$ model cannot accurately predict the momentum transfer at the interface between the vegetated zone and the non-vegetated zone. It is because the ${\kappa}-{\varepsilon}$ model is the isotropic turbulence model. Next, the open channel flows with alternate vegetated zones are simulated. The computed mean velocities are compared well with the previously reported measured data. Good agreement between the simulated results and the experimental data was found. Also, the turbulent flows are computed for different densities of vegetation. It is found that the vegetation curves the flow and the meandering flow pattern becomes more obvious with increasing vegetation density. When the vegetation density is 9.97%, the recirculation flows occur at the locations opposite to the vegetation zones. The impacts of vegetation on the flow velocity and the water surface elevation are also investigated.

• Journal of the Korean Society of Propulsion Engineers
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• v.4 no.4
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• pp.59-69
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• 2000

A numerical analysis based on two-dimensional, incompressible and compressible Navier-Stokes equations was carried out for double circular arc compressor cascade and the results are compared with available experimental data. The incompressible code based on SIMPLE algorithm adopts pressure weighted method and hybrid scheme for the convective terms. The compressible code with preconditioning method involves a upwind-biased scheme for the convective terms and LU-SGS scheme for temporal integration. Several turbulence models are evaluated by computing the turbulent viscous flows; Baldwin-Lomax, standard $\kappa$ -$\varepsilon$, $\kappa$ -$\varepsilon$ Lam. Bremhorst, standard $\kappa$-$\omega$, $\kappa$ -$\omega$ SST model.

• Proceedings of the KSME Conference
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• 2000.04b
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• pp.25-30
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• 2000

Plate and shell heat exchanger(P&SHE) has been applied to the refrigeration and air conditioning systems as evaporators or condensers fur their high efficiency and compactness. The purpose of this study is to analyze the characteristics of pressure drop in plate and shell heat exchanger. An experiment for single phase (low pressure drop in plate and shell heat exchanger was performed. Also numerical work was conducted using the FLUENT code for $ {\kappa}-{\varepsilon}$ model. The dependence of friction factor on geometrical Parameters was numerically investigated. The study examines the internal flow and the pressure distribution in the channel of plate and shell heat exchanger. The results of CFD analysis compared with experimental data, and the difference of frictor factor in plate side and shell side are 10% and 12%, respectively. Therefore, the CFD analysis model is effectively predict the performance of plate and shell heat exchanger.

• Journal of computational fluids engineering
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• v.13 no.3
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• pp.1-7
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• 2008

A supersonic base flow is computed to investigate the effect of the eddy viscosity coefficient to the linear ${\kappa}-{\varepsilon}$ turbulence models. Slight modifications to the eddy viscosity coefficient, which are based on the realizability condition, are given to the Launder-Sharma turbulence model so that present models satisfy the realizability condition. Numerical results for supersonic base flow show that turbulence models with the weaky-nonlinear eddy viscosity coefficient can lead to reasonable enhancements in the prediction of the velocity and turbulent kinetic energy profiles.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.1
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• pp.82-91
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• 1999

A numerical analysis based on the three-dimensional Reynolds-averaged Navier-Stokes equation has been conducted to investigate the flow within a NASA rotor 67 transonic fan. General coordinate transformations are used to represent the complex blade geometry and an H-type grid is used. The governing equations are solved using implicit LU-SGS scheme for the time-marching integration and a standard ${\kappa}-{\varepsilon}$ model is used with wall functions for the turbulence modeling. The computations are compared with the experimental data and a detailed study of the flow structures near peak efficiency and near stall is presented. The calculated overall aerodynamic efficiency and three-dimensional shock system agree well with the laser anemometer data.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.8
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• pp.635-642
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• 2009

The three-dimensional turbulent wingtip vortex flows have been examined in the present study by using the commercial code FLUENT. The standard ${\kappa}-{\varepsilon}$ model is used as a closure relationship. The wing is constructed by using an elliptic body whose aspect ratio is 3.8 and the NACA 16-020 airfoil section. The simulations for various angle attack (${\alpha}=0^{\circ}$, $5^{\circ}$, and $10^{\circ}$) are carried out. The effect of Reynolds number is also investigated in this study. As the angle attack increases, the wingtip vortex becomes stronger. However, the relative vortex strength to inlet velocity decreases as Reynolds number increases.

• 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.