• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.8
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• pp.1114-1121
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• 2003

A mathematical model considering the air side and the frost layer is presented to predict the frost layer growth. The standard k-$\varepsilon$ model for the air flow and the diffusion and energy equations for the frost layer are employed. The numerical results are compared with experimental data to validate the present model, and agree well with experimental data within a maximum error of 10%. The present model predicts well the frost properties and heat and mass transfer with respect to the frosting time. The variation of total heat transfer strongly depends on the operating condition, and has a similar trend to that of the sensible heat transfer. The frost properties along the flow direction are also investigated.

• Polymer(Korea)
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• v.35 no.4
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• pp.325-331
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• 2011

Molecular simulations via the molecular dynamics method have been carried out to an equation of state of penetrable-sphere model fluids over a wide range of packing fraction ${\phi}$ and finite repulsive energy ${\varepsilon}^*$. The resulting simulation data are compared to theoretical predictions from the two limiting cases of high- and low-penetrability approximations available in the literature. A good agreement between theoretical and simulation results is observed ill the case of ${\varepsilon}^*$ <3.0. However, for the highly repulsive energy systems of ${\varepsilon}^*{\geqq}3.0$, where the potential energy barrier is more than two times higher than the particle kinetic energy, a poor agreement is found due to the clustering formation and the non-continuum size effects in the dense systems of ${\phi}{\geqq}0.7$ and ${\varepsilon}^*$=6.0.

• Journal of Mechanical Science and Technology
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• v.15 no.11
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• pp.1463-1471
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• 2001

The Kachanov and Rabotnov (K-R) creep damage model was interpreted and applied to type 316LN and HT-9 stainless steels. Seven creep constants of the model, A, B, $textsc{k}$, m, λ, ${\gamma}$, and q were determine d for type 316LN stainless steel. In order to quantify a damage parameter, the cavity was interruptedly traced during creep for measuring cavity area to be reflected into the damage equation. For type 316LN stainless steel, λ= $\varepsilon$R/$\varepsilon$* and λf=$\varepsilon$/$\varepsilon$R were 3.1 and increased with creep strain. The creep curve with λ=3.1 depleted well the experimental data to the full lifetime and its damage curve showed a good agreement when r=24. However for the HT-9 stainless steel, the values of λ and λf were different as λ=6.2 and λf=8.5, and their K-R creep curves did not agree with the experimental data. This mismatch in the HT-9 steel was due to the ductile fracture by softening of materials rather than the brittle fracture by cavity growth. The differences of the values in the above steels were attributed to creep ductilities at the secondary and the tertiary creep stages.

• 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 Korean Society of Coastal and Ocean Engineers
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• v.15 no.2
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• pp.80-85
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• 2003

This paper describes the generation and maximum run-up heights of cnoidal waves with varying periods by the numerical model. The model solves the Reynolds equations and the k-epsilon equations for the turbulent analysis. To track free surface displacements, the volume of fluid(VOF) method is employed. It is shown that profiles of the numerically generated cnoidal waves agree well with analytical solutions. The computed maximum run-up heights are compared with laboratory measurements and those of the boundary element method. The present model provides more agreeable results to laboratory measurements that the boundary element model.

• 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.

• Proceedings of the Korea Water Resources Association Conference
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• 1999.05a
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• pp.541-546
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• 1999

• Transactions of the Korean Society of Automotive Engineers
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• v.2 no.1
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• pp.38-50
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• 1994

To improve the efficiency of internal combustion engines, it is necessary to understand mixed air-fuel in-cylinder flow processes accurately at intake and compression strokes. There is experimental and numerical methods to analyse in-cylinder flow process. In numerical method, standard $k-{\varepsilon}$ model with wall function was mostly adopted in in-cylinder flow process. But this type model was not efficiently predicted in the near wall region. Therefore in the present study, low Reynolds number $k-{\varepsilon}$ model was adopted near the cylinder wall and standard $k-{\varepsilon}$ model in other region. Also QUICK scheme was used for convective difference scheme. This study takes axisymmetric reciprocating model engine motored at 200rpm with a centrally located valve, incorporated 60 degree seat angie, and flat piston surface excluding inlet port. Because in-cylinder flow processes are undergoing unsteady and compressible, averaged cylinder pressure and inlet velocity at arbitrary crank angle are determined from thermodynamic analytic method and incylinder states at that crank angle are iteratively determined from the numerical analytic method.

• 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.

• Journal of computational fluids engineering
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• v.3 no.2
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• pp.9-16
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• 1998

Predictive behaviors by the extended k-${\varepsilon}$ turbulence model and the standard k-${\varepsilon}$ turbulence model are compared. Grid dependency is tested with the H-type grid as well as the O-type grid. Computations have been performed on a circular-to-rectangular transition duct. The Reynolds number is 390,000 based on the bulk velocity at the inlet. The computed axial velocity contours, transverse velocity profiles, static pressure contours, peripheral skin friction coefficient, peripheral wall static pressure distributions and turbulence kinetic energy have been compared with experimental results. The computed results than those obtained with the standard k-${\varepsilon}$ turbulence model. Comparing to the computed results obtained with the H-type grid and O-type grid, those with H-type grid seem to agree well with experimental results.