• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.11
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• pp.1569-1580
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• 2001

A new nonlinear near-wall turbulence model is developed to predict turbulent flow and heat transfer in strongly nonequilibrium flows. The k-$\varepsilon$-f$\sub$${\mu}$/, model of Park and Sung$\^$(1)/ is extended to a nonlinear formulation. The stress-strain relationship is the thrid-order in the mean velocity gradients. The strain dependent coefficients are obatined from the realizability constraints and the singular behavior at large strains. An improved explicit heat flux model is proposed with the aid of Cayley-Hamilton theorem. This new model includes the quadratic effects of flow deformations. The near-wall asymptotic behavior is incorporated by modifying the f$\sub$λ/ function. The model performance is shown to be satisfactory.

• Journal of Korea Water Resources Association
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• v.45 no.4
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• pp.349-360
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• 2012

A horizontally curvilinear non-hydrostatic free surface model that was applicable to three-dimensional viscous flows was developed. The proposed model employed a top-layer equation to close kinematic free-surface boundary condition, and an isotropic k-${\varepsilon}$ model to close turbulence viscosity in the Reynolds averaged Navier-Stokes equation. The model solved the governing equations with a fractional step method, which solved intermediate velocities in the advection-diffusion step, and corrects these provisional velocities by accounting for source terms including pressure gradient and gravity acceleration. Numerical applications were implemented to the wind-driven currents in a two-dimensional closed basin, the flow in a steep-sided trench, and the flow in a strongly-curved channel accounting for secondary current by the centrifugal force. Through the numerical simulations, the model showed its capability that were in good agreement with experimental data with respect to free surface elevation, velocity, and turbulence characteristics.

• Proceedings of the Korea Water Resources Association Conference
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• 2015.05a
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• pp.395-395
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• 2015

하천 내 식생은 수리학, 지형학 및 생태학적으로 매우 중요하다. 식생은 하천 수생물들의 서식처를 제공할 뿐만 아니라 필터와 같은 역할을 함으로써 부유사에 의한 하천오염물의 퇴적을 유발하여 하천의 수질을 개선시킨다. 더욱이, 하천 내 흐름 및 난류구조를 변경시킴으로써 식생주변의 유사 퇴적량 및 분포에 크게 영향을 미치고, 결국 하천의 지형을 변화시킨다. 개수로의 식생에 대한 영향은 주로 실험 및 수치모델을 이용하여 연구되었고 전단면이 식재된 조건에서 식생의 항력계수, 식생역내의 부유사 및 확산에 관한 연구가 진행되어왔다. 이러한 연구를 통해 식생역 내의 전단력이 감소하여 부유사퇴적이 증가하고 식생역과 비식생역 사이의 운동량 교환에 의해 부유사 퇴적이 증가함을 보였다. 그러나 개수로에서 존재하는 유한한 크기의 식생에 의한 흐름 및 유사분포에 관한 연구는 아직 미흡하다. 이에 본 연구에서는 침수하지 않은 원형 식생 주변에서 발생하는 흐름특성을 수치모의 하였다. 침수하지 않은 원형식생 하류에서 발생하는 흐름을 계산하기 위해 2차원 수치모형을 적용하였다. 식생에 의한 저항을 고려하기 위해 운동량 방정식에 식생항을 추가하였고 $k-{\varepsilon}$ 난류모형을 적용하였다. 수치모의 조건은 Zong and Nepf (2012)의 수리실험을 참고하여 수로의 길이는 12 m, 폭은 1.2 m로 설정하였다. 0.13 m 수심을 갖는 개수로에 0.22 m 지름을 갖는 원형식생을 상류경계로부터 1.0 m 떨어진 곳에 설정하였다. 식생의 밀도($6{\sim}77m^{-1}$)를 변화시키면서 원형식생 하류의 흐름거동을 분석하였다. 식생밀도가 높은 경우에는 원형식생 양 측면에서 유발된 전단층들의 상호작용에 의해 하류에서 와류가 발생하였다. 와류가 발생하는 위치에서 난류강도가 가장 크게 나타났다. 그러나 식생밀도가 일정 값보다 낮아지면 와류가 발생하지 않는 것으로 나타났다.

• Nuclear Engineering and Technology
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• v.27 no.4
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• pp.491-502
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• 1995

In accurate prediction of cross-flow based on detailed knowledge of the velocity field in subchannels of a nuclear fuel assembly is of importance in nuclear fuel performance analysis. In this study, the low-Reynolds number k-$\varepsilon$ turbulence model has been adopted in too adjacent subchannels with cross-flow. The secondary flow is accurately estimated by the anisotropic algebraic Reynolds stress model. This model was numerically calculated by the finite element method and has been verified successfully through comparison with existing experimental data. Finally, with the numerical analysis of the velocity Held in such subchannel domain, an analytical correlation of the lateral loss coefficient is obtained to predict the cross-flow rate in subchannel analysis codes. The correlation is expressed as a function of the ratio of the lateral How velocity to the donor subchannel axial velocity, recipient channel Reynolds number and pitch-to-diameter.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.7 no.2
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• pp.148-155
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• 1995

For an accurate prediction of the temperature Held induced by surface discharge of heated water into an ambient cross-flow field. a three-dimensional near-field numerical model using k-$\varepsilon$ turbulence clousure is developed Rather restricted as it is, the numerical results of the model agree well with the experimental data. The developed model simulates quite adequately the stratification, gravitational lateral spreading, and upward entrainment of thermal jet which cannot be simulated by a depth-integrated two-dimensional numerical model, as well as the interaction with cross-flow.

• Journal of ILASS-Korea
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• v.1 no.3
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• pp.37-50
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• 1996

Computational fluid dynamics was used to optimize an A/C duct. Three dimensional flow analysis in an automotive A/C duct was performed computationally using various turbulence models and compared numerical predictions such as outlet flow split, surface pressure distribution along the duct to experimental data. Additionally, we studied the effect of location variation of 2nd branch on exit flow ratio and could find optimal location of 2nd branch. The design of an A/C duct was modeled and calculated to enhance the airflow distribution in each outlet using the STAR-CD computational fluid dynamics software. In results, modified $k-\varepsilon$ turbulence model allows a successful prediction of static pressure distribution particulary at around strong curvature but little improvement flow split. In the future, adoption of CFD to design an A/C duct with modified $k-\varepsilon$ model will bring benefits of producing more accurate prediction, and also give designers more detail information much more than now.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.8 no.3
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• pp.256-267
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• 1996

In estuarine 3-dimensional numerical modeling. it is very important to calculate vertical eddy viscosity accurately. Various turbulence models employing eddy viscosity concept were applied to the steady flow in an open-channel and the tidal flow in long tidal channel and compared. The evaluations include the verification tests against experimental data sets for steady and tidal flows. The simulation results have shown that the compared models are in good agreements with experimental data of steady flow while only $textsc{k}$-$\varepsilon$ model, $textsc{k}$-ι model, and 1-equation model with well-defined mixing length profile give good agreements with experimental data of tidal flow.

• Journal of Korea Water Resources Association
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• v.37 no.11
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• pp.949-958
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• 2004

A combined experimental and numerical effort is presented for investigation of reflection of irregular waves due to rectangular submerged breakwaters. In the numerical model, the Reynolds equations are solved by a finite difference method and k-$\varepsilon$ model is employed for the turbulence analysis. To track the free surface displacement, the volume of fluid method is employed. Numerical predictions of transmission and reflection coefficients are verified by comparing to laboratory measurements. Very reasonable agreements are observed. The reflection coefficients become stronger in proportion to numbers of submerged breakwaters.

• Journal of Ocean Engineering and Technology
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• v.11 no.2
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• pp.100-106
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• 1997

Viscous flow around an actual ship is calculated by an use of RANS(Reynolds-averaged Navier-Stokes) solver. Reynolds stress is modelled by using k-$\varepsilon$ turbulence model and the law of wall is applied near the body. Body fitted coordinates are introduced for the treatment of the complex boundary of the ship hull form. The transformed equations in the computational domain are numerically solved by an employment of FVM(Finite Volume Method). SIMPLE(Semi-Implcit Pressure Linked Equation) method is adopted in the calculation of pressure and the solution of the disssssssscretized equation is obtained by the line-by-line method with the use of TDMA(Tri-Diagonal Matrix Algorithme). The subject ship model of actual calculation is 4,410 TEU class container carrier. For 4 geosim models the calculated viscous resistancce values are compared with the model test results and analyzed on their componentss. The resistance performance of an actual ship is predicted very resonably, so this mothod may be utilized as a design tool of hull form.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.10
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• pp.1940-1954
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• 1992

Fine grid computations were attempted to analyze the turbulent flows in the near wall low Reynolds number region and the numerical analyses were incorporated by a finite-volume discretization with full find grid system and low Reynolds number k-.epsilon. model was employed in this region. For the improvement of low Reynolds number k-.epsilon. model, modification coefficient of eddy viscosity $f_{\mu}$ was derived as a function of turbulent Reynolds number $R_{+}$ and nondimensional length $y^{+}$ from the concept of two length scales of dissipation rate of turbulent kinetic energy. The modification coefficient $f_{\epsilon}$ in .epsilon. transport equation was also derived theoretically. In the turbulent kinetic energy equation, pressure diffusion term was added in order to consider low Reynolds number region effect. The main characteristics of this low Reynolds number k-.epsilon. model were founded as : (1) In high Reynolds number region, the present model has limiting behavior which approaches to the high Reynolds number model. (2) Present low Reynolds number k-.epsilon. model dose not need additional empirical constants for the transport equations of turbulent kinetic energy and dissipation of turbulent kinetic energy in order to consider wall effect. Present low Reynolds number turbulence model was tested in the pipe flow and obtained improved results in velocity profiles and Reynolds stress distributions compared with those from other k-.epsilon. models.s.s.