• International Journal of Fluid Machinery and Systems
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• 제3권4호
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• pp.315-323
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• 2010

This work investigates the influence of water compressibility on pressure pulsations induced by rotor-stator interaction (RSI) in hydraulic machinery, using the commercial CFD solver ANSYS-CFX. A pipe flow example with harmonic velocity excitation at the inlet plane is simulated using different grid densities and time step sizes. Results are compared with a validated code for hydraulic networks (SIMSEN). Subsequently, the solution procedure is applied to a simplified 2.5-dimensional pump-turbine configuration in prototype with different speeds of sound as well as in model scale with an adapted speed of sound. Pressure fluctuations are compared with numerical and experimental data based on prototype scale. The good agreement indicates that the scaling of acoustic effects with an adapted speed of sound works well. With respect to pressure fluctuation amplitudes along the centerline of runner channels, incompressible solutions exhibit a linear decrease while compressible solutions exhibit sinusoidal distributions with maximum values at half the channel length, coinciding with analytical solutions of one-dimensional acoustics. Furthermore, in compressible simulation the amplification of pressure fluctuations is observed from the inlet of stay vane channels to the spiral case wall. Finally, the procedure is applied to a three-dimensional pump configuration in model scale with adapted speed of sound. Normalized Pressure fluctuations are compared with results from prototype measurements. Compared to incompressible computations, compressible simulations provide similar pressure fluctuations in vaneless space, but pressure fluctuations in spiral case and penstock may be much higher.

• 한국구조물진단유지관리공학회 논문집
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• 제6권3호
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• pp.201-209
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• 2002

Crib wall system은 일반적으로 segmental crib type의 옹벽형식으로 stretcher라는 전 후면 가로보와 세로 방향으로 header라는 버팀보를 연속적으로 쌓아 올리는 공법이다. 이때, stretcher와 header로 구성된 골격 내부에는 흙으로 속채움을 하여 다져, 본 구조체가 일체거동이 가능한 강성체(rigid body)를 형성시킴으로써 배면의 토압에 대하여 저항하는 구조물이다. 따라서, 안정성 해석은 일반적으로 기존의 철근콘크리트 옹벽과 같이 전체 옹벽이 하나의 강성체로 작용한다고 가정하여 토압이론에 의하여 평가하고 있다. 그러나, 변형문제에 있어서 본 구조물은 단순히 하나의 구조체로 가정하여 해석하기가 곤란하다. 왜냐하면, stretcher와 header는 일반 보강토옹벽의 전면 벽체 부재와는 달리 뒷채움재 내에 어떠한 인장 보강재도 삽입되지 않기 때문에 독립된 변위거동을 나타낸다. 또한, 각각의 독립부재로 구성된 재료와 내부채움재의 힘과 변형은 3차원적으로 거동하며 수평토압에 의하여 복합적인 상호거동을 일으키기 때문이다. 따라서, 본 연구는 Crib wall system의 변위경향을 보다 엄밀하게 해석하기 위하여 Brandl(1985)의 full scale 시험 결과를 바탕으로 수치해석 모델을 제시하였다.

• Journal of Mechanical Science and Technology
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• 제15권4호
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• pp.465-472
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• 2001

A numerical study is carried out to clarify the characteristics of flow fields and breaking phenomena around a high speed catamaran hull advancing on calm water. Computations are carried out for Froude numbers between 0.2 and 1.0 and for ratios of the distance between hulls to the catamaran length varying between 0.2 and 0.5 for a mathematically defined Wigley hull. A Navier-Stokes solver which includes the nonlinearities of free surface conditions is employed. Computations are performed in a rectangular grid system based on the Marker & Cell method. For validation, present computation results are compared with existing experimental results. As an application, the results of the displacement catamaran are used for the breaking analysis.

• Journal of Ship and Ocean Technology
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• 제5권2호
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• pp.50-61
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• 2001

In this paper, nonlinear interactions between water waves and a horizontally submerged circular cylinder are numerically simulated. In this case, the nonlinear interactions between them generated a wave breaking phenomenon. The wave breaking phenomenon plays an important role in the wave farce. Negative drifting forces are raised at shallow submerged cylinders under waves because of the wave breaking phenomenon. For the numerical simulation, a finite difference method based on the unsteady incompressible Navier-Stokes equations and the continuity equation is adopted in the rectangular grid system. The free surface is simulated with a computational simulation method of two-layer flow by using marker density. The results are compared with some existing computational and experimental results.

• Interaction and multiscale mechanics
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• 제1권2호
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• pp.191-209
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• 2008

In this paper, we model grain boundary evolution based on a multiple level set method. Grain boundary migration under a curvature-induced driving force is considered and the level set method is employed to deal with the resulting topological changes of grain structures. The complexity of using a level set method for modeling grain structure evolution is due to its N-phase nature and the associated geometry compatibility constraint. We employ a multiple level set method with a predictor-multicorrectors approach to reduce the gaps in the triple junctions down to the grid resolution level. A ghost cell approach for imposing periodic boundary conditions is introduced without solving a constrained problem with a Lagrange multiplier method or a penalty method. Numerical results for both uniform and random grain structures evolution are presented and the results are compared with the solutions based on a front tracking approach (Chen and Kotta et al. 2004b).

• 한국전산유체공학회지
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• 제9권2호
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• pp.62-69
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• 2004

The present work solved 3D incompressible RANS equations on a rotating, multi-blocked grid system to efficiently analyze ducted marine propulsor with the interaction of propeller guidevane and annular duct. To handle the interface boundary between the guidevane and the propeller, a sliding multiblock technique based on the cubic spline interpolation was applied. To validate the present code, a turbine flow was simulated and the time-averaged pressure coefficients were compared with experiment. After the code validation, the flowfield around a ducted marine propeller with pre-swirl guidevane was simulated.

• 한국해안해양공학회지
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• 제4권1호
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• pp.18-25
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• 1992

임의의 수심 및 흐름상의 풍파에 대한 성장, 박파 및 감쇠에 대한 이산형 스펙트럼 모형을 구성하였다. 변화되는 수심 및 흐름으로 인한 절대주파수의 변화를 포함한 파낭과 흐름간의 상호작용을 고려하였고 함장 및 감쇠과정은 Inoue의 방법을 사용했다. 전파기법은 2차의 정도를 가지며 굴절과 수심 및 흐름으로 인한 주파수의 변위도 2차의 기법을 사용하여 고정 격자에서 계산하였다.

• 한국유체기계학회 논문집
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• 제7권2호
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• pp.14-20
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• 2004

In this study, the overall performance and the local flow field of the centrifugal compressor with the refrigerant HFC-l34a were numerically studied using CFX-TASCflow. The thermodynamic and transport properties of the refrigerant gas were modeled by REFPROP which is NIST refrigerant properties database. The impacts of a grid qualify, discretization scheme, turbulent model and interaction between a impeller and a cascade diffuser were analyzed comparatively. The results were compared with experimental data and 1-D design results using COMPAL and agreed well with others. The numerical method and data obtained in this study can be applied to the design and modification of centrifugal compressors with real gases

• 한국해양공학회지
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• 제10권3호
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• pp.62-73
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• 1996

Numerical simulation is made of the three-dimensional wave breaking motion about a part of a floating offshore structure containing a circular cylinder mounted vertically onto a lower hull in regular periodic gravity wave generated by a numerical wave maker. TUMMAC-VIII finite-difference method is newly developed for such a problem. By use of density-function technique the three-dimensional wave breaking motion is approximately implenented in the framework of rectangular grid system. A porosity technique is devised for the implementation of the no-slip bydy boundary conditions. The generation of breaking waves by the interaction of incident waves with the structure is well simulated and interesting features of breaking waves are revealed with containing degree of quantitative and qualitative accuracy.

• 한국전산유체공학회지
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• 제6권1호
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• pp.47-55
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• 2001

The transient laminar reacting flows around fuel droplet have been numerically analyzed. The physical models used in this study can account for the variable thermophysical properties and the chemistry is represent by the one-step global reaction model. The present study is focused on the vaporization and ignition characteristics, flame structure including wake flame, transition flame and envelope flame, and interaction between droplets. Special emphasis is given to the triple flame structure and flame stabilization.