• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.2192-2197
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• 2003

The purpose of this 3-D numerical simulation is evaluate the application of a commercial CFD code to predict 3-D flow and power characteristics of wind turbines. The experimental approach, which has been main method of investigation, appears to be its limits, the cost increasing with the size of the wind turbines, hence mostly limited to observing the phenomena on rotor blades. Therefore, the use of Computational Fluid Dynamics (CFD) techniques and Navier-Stokes solvers are considered a very serious contender. The flow solver CFX-TASCflow is employed in all computations in this paper. The 3-D flow separation and the wake distribution of 2 and 3 bladed Horizontal Axis Wind Turbines (HAWTs) are compared to Heuristic model and smoke-visualized experimental result by NREL(National Renewable Energy Laboratory). Simulated 3-D flow separation structure on the rotor blade is very similar to Heuristic model and the wake structure of the wind turbine is good consistent with smoke-visualized result. The calculated power of the 3 bladed rotor by CFD is compared with BEM results by TV-Delft. The CFD results of which is somewhat consist with BEM results, under an error less than 10%.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2006년도 추계 학술대회논문집
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• pp.193-198
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• 2006

It takes a lot of time and effort to generate grids for numerical analysis of problems with ground effect because the relative attitude and height of airfoil should be maintained to the ground as well as the inflow. A low Mach number preconditioned turbulent flow solver using the overlap grid technique has been developed and applied to the ground effect simulation. It has been validated that the present method using the multi-block grid gives us highly accurate solutions comparing with the experimental data of the RAE 101 airfoil in an unbounded condition. Present numerical method has been extended to simulate ground effect problems by using the overlapped grid system to avoid tedious work in generating multi-block grid system. An extended method using the overlapped grid has been verified and validated by comparing with results of multi-block method and experimental data as well. Consequently, the overlapped grid method can provide not only sufficiently accurate solutions but also the efficiency to simulate ground effect problems. It is shown that the pressure and aerodynamic centers move backward by the ground effect as the airfoil approaches to the ground.

• 한국전산유체공학회지
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• 제12권2호
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• pp.37-45
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• 2007

The hybrid Cartesian/immersed boundary method is applied to simulate fluid-structure interaction of a two-dimensional orbiting flexible foil. The elastic deformation of the flexible foil is modelled based on the dynamic equation of a thin-plate. At each time step, the locations and velocities of the Lagrangian control points on the flexible foil are used to reconstruct the boundary conditions for the flow solver based on the hybrid staggered/non-staggered grid. To test the developed code, the flow fields around a flapping elliptical wing are calculated. The time history of the vertical force component and the evolution of the vorticity fields are compared with recent other computations and good agreement is achieved. For the orbiting flexible foil, the vorticity fields are compared with those of the case without the deformation. The combined effects of the angle of attack and the orbit on the deformation are investigated. The grid independency study is carried out for the computed time history of the deformation at the tip.

• 한국전산유체공학회지
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• 제17권1호
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• pp.94-100
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• 2012

In this paper, the cavitating flows around a hydrofoil have been numerically investigated by using a 2-d multi-phase RANS flow solver based on pseudo-compressibility and a homogeneous mixture model on unstructured meshes. For this purpose, a vertex-centered finite-volume method was utilized in conjunction with 2nd-order Roe's FDS to discretize the inviscid fluxes. The viscous fluxes were computed based on central differencing. The Spalart-Allmaras one equation model was employed for the closure of turbulence. A dual-time stepping method and the Gauss-Seidel iteration were used for unsteady time integration. The phase change rate between the liquid and vapor phases was determined by Merkle's cavitation model based on the difference between local and vapor pressure. Steady state calculations were made for the modified NACA66 hydrofoil at several flow conditions. Good agreements were obtained between the present results and the experiment for the pressure coefficient on a hydrofoil surface. Additional calculation was made for cloud cavitation around the hydrofoil. The observation of the vapor structure, such as cavity size and shape, was made, and the flow characteristics around the cavity were analyzed. Good agreements were obtained between the present results and the experiment for the frequency and the Strouhal number of cavity oscillation.

• 한국전산유체공학회지
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• 제18권2호
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• pp.78-84
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• 2013

In this paper, numerical simulations of both low- and high-frequency buzz phenomena at the throttle ratios (T.R.) in Nagashima's experiment are performed. The dominant frequencies of the low-and high-frequency buzz in the experiment are about 109 Hz with T.R.=0.97 and 376 Hz with T.R.=0.55, respectively. An axisymmetric solver with the S-A turbulence model is used for the simulations, and DFT(Discrete Fourier Transform) on pressure histories is conducted for the buzz frequency analysis. In the present simulations, the free-stream Mach number and the Reynolds number based on the inlet diameter are 2 and $10^7$, respectively. Both the low- and high-frequency buzz phenomena are accomplished without the changes in the grid topology. The dominant frequency of the simulation is about 125 Hz with T.R.=0.97, while it is 399 Hz with T.R.=0.55.

• Journal of Advanced Marine Engineering and Technology
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• 제27권7호
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• pp.906-913
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• 2003

The purpose of this 3-D numerical simulation is to evaluate the application of a commercial CFD code to predict 3-D flow and power characteristics of wind turbines. The experimental approach, which has been main method of investigation, appears to be its limits, the cost increasing with the size of the wind turbines, hence mostly limited to observing the phenomena on rotor blades. Therefore. the use of Computational Fluid Dynamics (CFD) techniques and Navier-Stokes solvers are considered a very serious contender. The flow solver CFX-TASCflow is employed in all computations in this paper. The 3-D flow separation and the wake distribution of 2 and 3 bladed Horizontal Axis Wind Turbines (HAWTs) are compared to Heuristic model and smoke-visualized experimental result by NREL(National Renewable Energy Laboratory). Simulated 3-D flow separation structure on the rotor blade is very similar to Heuristic model and the wake structure of the wind turbine is good consistent with smoke-visualized result. The calculated power of the 3 bladed rotor by CFD is compared with BEM results by TU-Delft. The CFD results of which is somewhat consist with BEM results. under an error less than 10%.

• 대한기계학회논문집B
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• 제30권4호
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• pp.320-327
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• 2006

The computations of chemically reacting laminar and turbulent flows are performed using the preconditioned Navier-Stokes solver coupled with turbulent transport and multi-species equations. A low-Reynolds number $k-\varepsilon$ turbulence model proposed by Chien is used. The presence of the turbulent kinetic energy tenn in the momentum equation can materially affect the overall stability of the fluids-turbulence system. Because of this coupling effect, a fully coupled formulation is desirable and this approach is taken in the present study. Choi and Merkle's preconditioning technique is used to overcome the convergence difficulties occurred at low speed flows. The numerical scheme used for the present study is based on the implicit upwind ADI algorithm and is validated through the comparisons of computational and experimental results for laminar methane-air diffusion flame and $ H_2/O_2$ reacting turbulent shear flow. Preconditioning formulation shows better convergence characteristics than that of non-preconditioned system by approximately five times as much.

• 한국소음진동공학회논문집
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• 제24권3호
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• pp.205-212
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• 2014

이 연구에서 현대자동차의 단순실험모델(HSM)에 대한 썬루프 버페팅에 대한 수치해석이 수행되었다. 검증을 위하여 HSM 목부위의 경계층에 대한 속도분포 해석결과를 실험결과와 비교하였다. 썬루프 해석은 두 단계로 이루어졌다. 첫 번째로 난류 RANS 모델을 이용하여 정상상태 해석이 수행되었으며, 해석결과는 CAA++의 입력값으로 사용된다. 두 번째 단계는 유동속도에 대한 1차 최대 압력피크와 버페팅 주파수 해석을 위한 비정상상태 해석이 CAA++에서 이루어졌다. 주파수와 음향압력의 수치해석 결과는 타당한 물리적 현상을 보여주고 있으며, 현대 자동차의 실험결과와 잘 일치하는 결과를 보여주었다.

• 한국전산유체공학회지
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• 제17권3호
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• pp.25-32
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• 2012

This work was devoted to compare two different cavitation models to study the dependency of model constants. The cavitation model of Merkle et al.(2006) and Kunz et al.(2000) were used for the present computational study. The cavitation models were coupled with the incompressible unsteady Reynolds-Averaged Navier-Stokes solver to indicate the vaporization and condensation processes. For this purpose, a preconditioning method was added as the pseudo-time term to solve the unsteady stiffness problems. For the validation of the numerical simulation, the computation was performed for the cavitating flow in a converging-diverging channel. The present results show that Merkle's cavitation model is independent to the model constants, and the higher numerical accuracy over Kunz's cavitation model.

• 한국전산유체공학회지
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• 제19권2호
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• pp.99-107
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• 2014

The aim of the present study is to investigate the flow interference between two adjacent undulating fish-like body, and its effect on the undulating propulsion. For this purpose, unsteady two dimensional incompressible flow calculations were conducted using an unstructured mesh flow solver, coupled with an overset mesh technique. To deal with mesh deformation due to fish locomotion, spring analogy is utilized. The fish body used in the simulation is constructed from the NACA0012 airfoil. The study indicates that the propulsion of undulating fish is proportional to frequency and wavelength of the midline oscillation when there is no adjacent fish. It also reveals that average thrust was increased when the vortex shedding from the tail was conserved well and pressure difference between upper and lower sides of the fish was magnified due to flow interference. From this study, which relative position and phase difference of locomotion between two fishes can generate maximum thrust was known among six different cases.