• Title, Summary, Keyword: RANS equation

### Numerical Analysis of Wind Turbine of Drag Force Type with change of Blade Number and Pitch Angle (수직항력식 터빈을 이용한 풍력발전 시스템의 형상 변화 및 피치각 변화에 관한 유동해석)

• Park C.;Park G. S.;Park W. G.;Yoon S. H.
• 한국전산유체공학회:학술대회논문집
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• pp.61-64
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• 2004
• To analyze the performance of Wind turbine of the drag force type, 3-D RANS equations were solved by the iterative time marching method on sliding multiblock grid system. The numerical flow simulations by changing blade number and pitch angle were carried out : blade number = 15, 20 circumferentially; pitch angle = $30^{\circ},\; 50^{\circ}$ radially. The torque coefficient was also calculated.

### Numerical investigation of Turbulent Flow in $270^{\circ}$ Bend using DES approaches (DES 모형을 이용한 270도 곡관 내 난류유동에 관한 수치해석)

• Seo, Jeong-Sik;Shin, Jong-Keun;Hong, Seong-Ho;Choi, Young-Don
• Proceedings of the SAREK Conference
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• pp.558-563
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• 2007
• Detached Eddy Simulation(DES) is performed for turbulent flow of the $270^{\circ}$ bend at a Reynolds number of 56,690. A Fine grid generation is used near a wall in order to satisfy the wall boundary condition of y+<1. Turbulence models adopted for DES and Reynolds Average Navier Stokes(RANS) simulation are SST(Shear Stress Transfort) model. Solutions for both streamwise and circumferential velocity components are compared with the experimental data by Lee for $270^{\circ}$ bend and by Chang for $180^{\circ}$ bend.

### Numerical Simulation of Turbulent Flow around 2-D Airfoils in Ground Effect (CFD에 의한 2차원 지면 효과익 주위의 난류유동계산)

• H.H. Chun;R.H. Chang;M.S. Shin
• Journal of the Society of Naval Architects of Korea
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• v.39 no.3
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• pp.28-40
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• 2002
• Turbulent flows around two-dimensional wing sections in ground effect are analysed by incompressible RANS equations and a finite difference method. The Baldwin-Lomax algebraic turbulence model is used to simulate high Reynolds number flows. The main purpose of this study is to clarify the two-dimensional ground effect and its flow characteristics due to different ground boundary conditions, i.e., moving and fixed bottom boundary. As a first step, to validate the present numerical code, the computational result of Clark-Y(t/C 11.7%) is compared with published numerical results and experimental data. Then, NACA4412 section in ground effect is calculated for various ground clearances with two bottom boundary conditions. According to the computational results, the difference in the lift and moment simulated with the two bottom boundary conditions is negligible, but the drag force simulated by the fixed bottom is to some extent smaller than that by the moving bottom. Therefore, it can be concluded that the drag force measured in a wind tunnel with the fixed bottom could be smaller than that with the moving bottom.

### Numerical simulation of cavitating flow past cylinders

• Park, Warn-Gyu;Koo, Tae-Kyoung;Jung, Chul-Min;Lee, Kurn-Chul
• 한국전산유체공학회:학술대회논문집
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• pp.327-333
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• 2008
• The cavitating flow simulation is of practical importance for many engineering systems, such as marine propellers, pump impellers, nozzles, injectors, torpedoes, etc. The present work has developed a base code for simulating cavitating flows past cylinders and hydrofoils. The governing equation is the Navier-Stokes equation based on homogeneous mixture model. The momentum and energy equation is in the mixture phase while the continuity equation is solved in liquid and vapor phase, separately. The solver employs an implicit preconditioning algorithm in curvilinear coordinates. The computations have been carried out for the cylinders with spherical, 1- and 0-caliber forebody and hydrofoil of ALE and NACA cross-section and, then, compared with experiments and other numerical results. Fairly good agreements with experiments and numerical results have been achieved. The present base code has shown the feasibility to solve the cavitating flow past supercavitating torpedo after the improvement for compressibility effects and interactions with hot exhaust gas of propulsive rocket.

### Numerical simulation of cavitating flow past cylinders

• Park, Warn-Gyu;Koo, Tae-Kyoung;Jung, Chul-Min;Lee, Kurn-Chul
• 한국전산유체공학회:학술대회논문집
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• pp.327-333
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• 2008
• The cavitating flow simulation is of practical importance for many engineering systems, such as marine propellers, pump impellers, nozzles, injectors, torpedoes, etc. The present work has developed a base code for simulating cavitating flows past cylinders and hydrofoils. The governing equation is the Navier-Stokes equation based on homogeneous mixture model. The momentum and energy equation is in the mixture phase while the continuity equation is solved in liquid and vapor phase, separately. The solver employs an implicit preconditioning algorithm in curvilinear coordinates. The computations have been carried out for the cylinders with spherical, 1- and 0-caliber forebody and hydrofoil of ALE and NACA cross-section and, then, compared with experiments and other numerical results. Fairly good agreements with experiments and numerical results have been achieved. The present base code has shown the feasibility to solve the cavitating flow past supercavitating torpedo after the improvement for compressibility effects and interactions with hot exhaust gas of propulsive rocket.

### Three-dimensional Numerical Analysis of Dam-break Waves on a Fixed and Movable Bed (고정상 및 이동상 수로에서 댐 붕괴파의 3차원 수치해석)

• Kim, Dae Geun;Hwang, Gun
• Journal of The Korean Society of Civil Engineers
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• v.31 no.4B
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• pp.333-341
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• 2011
• This study analyzed the propagation of dam-break waves in an area directly downstream of a dam by using 3D numerical modeling with RANS as the governing equation. In this area, the flow of the waves has three dimensional characteristics due to the instantaneous dam break. In particular, the dam-break flows are characterized by a highly unsteady and discontinuous flow, a mixture of the sharp flood waves and their reflected waves, a mixture of subcritical and supercritical flow, and propagation in a dry and movable bed. 2D numerical modeling, in which the governing equation is the shallow water equation, was regarded as restricted in terms of dealing with the sharp fluctuation of the water level at the dam-breaking point and water level vibration at the reservoir. However, in this 30 analysis of flood wave propagation due to partial dam breaking and dam-break in channels with $90^{\circ}$ bend, those phenomena were properly simulated. In addition, the flood wave and bed profiles in a movable bed with a flat/upward/downward bed step, which represents channel aggradation or degradation, was also successfully simulated.

### Computational modeling of the atmospheric boundary layer using various two-equation turbulence models

• Juretic, Franjo;Kozmar, Hrvoje
• Wind and Structures
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• v.19 no.6
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• pp.687-708
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• 2014
• The performance of the $k-{\varepsilon}$ and $k-{\omega}$ two-equation turbulence models was investigated in computational simulations of the neutrally stratified atmospheric boundary layer developing above various terrain types. This was achieved by using a proposed methodology that mimics the experimental setup in the boundary layer wind tunnel and accounts for a decrease in turbulence parameters with height, as observed in the atmosphere. An important feature of this approach is pressure regulation along the computational domain that is additionally supported by the nearly constant turbulent kinetic energy to Reynolds shear stress ratio at all heights. In addition to the mean velocity and turbulent kinetic energy commonly simulated in previous relevant studies, this approach focuses on the appropriate prediction of Reynolds shear stress as well. The computational results agree very well with experimental results. In particular, the difference between the calculated and measured mean velocity, turbulent kinetic energy and Reynolds shear stress profiles is less than ${\pm}10%$ in most parts of the computational domain.

### Free Surface Flow in a Trench Channel Using 3-D Finite Volume Method

• Lee, Kil-Seong;Park, Ki-Doo;Oh, Jin-Ho
• Journal of Korea Water Resources Association
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• v.44 no.6
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• pp.429-438
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• 2011
• In order to simulate a free surface flow in a trench channel, a three-dimensional incompressible unsteady Reynolds-averaged Navier-Stokes (RANS) equations are closed with the ${\kappa}-{\epsilon}$ model. The artificial compressibility (AC) method is used. Because the pressure fields can be coupled directly with the velocity fields, the incompressible Navier-Stokes (INS) equations can be solved for the unknown variables such as velocity components and pressure. The governing equations are discretized in a conservation form using a second order accurate finite volume method on non-staggered grids. In order to prevent the oscillatory behavior of computed solutions known as odd-even decoupling, an artificial dissipation using the flux-difference splitting upwind scheme is applied. To enhance the efficiency and robustness of the numerical algorithm, the implicit method of the Beam and Warming method is employed. The treatment of the free surface, so-called interface-tracking method, is proposed using the free surface evolution equation and the kinematic free surface boundary conditions at the free surface instead of the dynamic free surface boundary condition. AC method in this paper can be applied only to the hydrodynamic pressure using the decomposition into hydrostatic pressure and hydrodynamic pressure components. In this study, the boundary-fitted grids are used and advanced each time the free surface moved. The accuracy of our RANS solver is compared with the laboratory experimental and numerical data for a fully turbulent shallow-water trench flow. The algorithm yields practically identical velocity profiles that are in good overall agreement with the laboratory experimental measurement for the turbulent flow.

### Numerical Modeling of Hydrazine-Fueled Arcjet Thruster (하이드라진(N2H4) 아크젯 추력기의 수치적 모델링)

• Shin, Jae-Ryul;Lee, Dae-Sung;Oh, Se-Jong;Choi, J.-Y.
• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.9
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• pp.907-915
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• 2008
• The computational fluid dynamic analysis has been conducted for the thermo-chemical flow field in an arcjet thruster with mono-propellant Hydrazine (N2H4) as a working fluid. The Reynolds Averaged Navier-Stokes (RANS) equations are modified to analyze compressible flows with the thermal radiation and electric field. the Maxwell equation, which is loosely coupled with the fluid dynamic equations through the Ohm heating and Lorentz forces, is adopted to analyze the electric field induced by the electric arc. The chemical reactions of Hydrazine were assumed to be infinitely fast due to the high temperature field inside the arcjet thruster. The chemical and the thermal radiation models for the nitrogen-hydrogen mixture and optical thick media respectively, were incorporated with the fluid dynamic equations. The results show that performance indices of the arcjet thruster with 1kW arc heating are improved by amount of 180% in thrust and 200% in specific impulse more than frozen flow. In addition thermo-physical process inside the arcjet thruster is understood from the flow field results.

### Evaluation of Effective Wall Roughness for 3D Computational Analysis of Open Channel Flow (개수로 흐름의 3차원 전산해석을 위한 유효 벽면거칠기 산정)

• Choi, Junwoo;Baek, Un Il;Lee, Sang Mok;Yoon, Sung Bum
• Journal of The Korean Society of Civil Engineers
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• v.28 no.6B
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• pp.627-634
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• 2008
• In a numerical simulation of open channel turbulent flows using RANS (Reynolds averaged Navier-Stokes) equations model equipped with VOF (Volume of Fluid) scheme, the determination of wall roughness for wall function was studied. The roughness constant, based on the law-of-the-wall for flow on rough walls, obtained by experimental works for pipe flows is employed in general wall functions. However, this constant of wall function is the function of Froude number in open channel flows. Thus, the wall roughness should be determined by taking into account the effect of Froude number. In addition, the wall roughness should be corresponding to Manning's roughness coefficient widely used for open channels. In this study, the relation between wall roughness height as an input condition and Manning's roughness coefficient was investigated, and an equation for effective wall roughness height considering the characteristics of numerical models was proposed as a function of Manning's roughness coefficient.