• Journal of Ocean Engineering and Technology
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• v.29 no.6
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• pp.427-435
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• 2015

This paper provides the numerical results of a simulation of the flow around a propeller working beneath the free surface. A finite volume method is used to solve the unsteady Reynolds averaged Navier-Stokes (URANS) equations, where the wave-making problem is solved using a volume-of-fluid (VOF) method. The numerical analysis focuses on the propeller wake structure affected by the free surface, where we consider another free surface boundary condition that treats the free surface as a rigid wall surface. The propeller wake under the effect of these two free surface conditions shows a reduction in the magnitude of the longitudinal and vertical flow velocities, and its vortical structures strongly interact with the free surface. The thrust and torque coefficient under the free surface effect decrease about 3.7% and 3.1%, respectively. Finally, the present numerical results show a reasonable agreement with the available experimental data.

• Journal of the Society of Naval Architects of Korea
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• v.45 no.1
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• pp.29-41
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• 2008

The present paper provides a CFD analysis of diffraction problem for a ship with forward speed using an unsteady RANS simulation method, a WAVIS code. The WAVIS viscous solver adopting a finite volume method has second order accuracy in time and field discretizaions for the RANS equations. A two phase level-set method and a realizable ${\kappa}-{\varepsilon}$ turbulence model are adopted to compute the free surface and to meet the turbulence closure, respectively. To validate the capability of the present numerical methods for the simulation of an unsteady progressive regular wave, computations are performed for three grid sets with refinement ratio of ${\sqrt{2}}$. The main simulation is performed for a DTMB5512 model with a forward speed in a regular head sea condition. Validation of the present numerical method is carried out by comparing the present CFD results with available unsteady experimental data published in the 2005 Tokyo CFD Workshop: resistance, heave force, pitch moment, unsteady free surface elevations and velocity fields.

• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.892-901
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• 2020

We used a numerical method to estimate the hydrodynamic maneuvering derivatives for the heave-pitch coupling motion of an underwater glider. It is very important to assess the hydrodynamic maneuvering characteristics of a specific hull form of an underwater glider in the initial design stages. Although model tests are the best way to obtain the derivatives, numerical methods such as the Reynolds-averaged Navier-Stokes (RANS) method are used to save time and cost. The RANS method is widely used to estimate the maneuvering performance of surface-piercing marine vehicles, such as tankers and container ships. However, it is rarely applied to evaluate the maneuvering performance of underwater vehicles such as gliders. This paper presents numerical studies for typical experiments such as static drift and Planar Motion Mechanism (PMM) to estimate the hydrodynamic maneuvering derivatives for a Ray-type Underwater Glider (RUG). A validation study was first performed on a manta-type Unmanned Undersea Vehicle (UUV), and the Computational Fluid Dynamics (CFD) results were compared with a model test that was conducted at the Circular Water Channel (CWC) in Korea Maritime and Ocean University. Two different RANS solvers were used (Star-CCM+ and OpenFOAM), and the results were compared. The RUG's derivatives with both static drift and dynamic PMM (pure heave and pure pitch) are presented.

• Wind and Structures
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• v.18 no.5
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• pp.529-548
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• 2014

This article presents the unsteady aerodynamic performance of crosswind stability obtained numerically for the ATM train. Results of numerical investigations of airflow past a train under different yawing conditions are summarized. Variations of occurrence flow angle from parallel to normal with respect to the direction of forward train motion resulted in the development of different flow patterns. The numerical simulation addresses the ability to resolve the flow field around the train subjected to relatively large yaw angles with three-dimensional Reynolds-averaged Navier-Stokes equations (RANS). ${\kappa}-{\varepsilon}$ turbulence model solved on a multi-block structured grid using a finite volume method. The massively separated flow for the higher yaw angles on the leeward side of the train justifies the use of RANS, where the results show good agreement with verification results. A method of solution is presented that can predict all aerodynamic coefficients and the wind characteristic curve at variety of angles at different speed.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.21 no.2
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• pp.182-190
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• 2009

For RANS equation model using VOF scheme Lin and Liu (1999) developed internal wave-maker method to generate target wave trains by using designed mass source functions of the continuity equation. By using this method studies on various numerical wave experiments has been achieved without the problem caused by wave reflection due to an external wave-maker. In this study, the wave-maker method to generate target wave trains by using a momentum source function was proposed. The computational results obtained by applying the mass and momentum source functions into FLUENT were compared with each other. To see its applicability, the hydraulic experiment of Luth et al. (1994) were numerically simulated and their measurements are compared with the computations, and the vertical variations of computed results were shown and investigated.

• Proceedings of the KSME Conference
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• 2000.04b
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• pp.704-708
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• 2000

A new efficient numerical method for computing unsteady, incompressible flows, DRANS (Decoupled Reynolds-Averaged Navier-Stokes), is presented. To eliminate the restriction of CFL condition, a fully-implicit time advancement in which the Crank-Nicolson method is used fer both the diffusion and convection terms. is adopted. Based on decomposition method, the velocity-turbulent quantity decoupling is achieved. The additional decoupling of the intermediate velocity components in the convection term is made for the fully-implicit time advancement scheme. Since the iterative procedures for the momentum, ${\kappa}\;and\;{\varepsilon}$ equations are not required, the components decouplings bring fourth the reduction of computational cost. The second-order accuracy in time of the present numerical algorithm is ascertained by computing decaying vortices. The present decoupling method is applied to turbulent boundary layer with local forcing.

• Journal of Ship and Ocean Technology
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• v.7 no.2
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• pp.40-47
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• 2003

LDV measurements in large cavitation tunnel around a propeller in operation are carried out to provide valuable information for more accurate wake-adapted propeller design and to study hull-propeller interactions. Effective velocities are computed by both the simplified vortex ring method and by RANS solver with the body force representing the propeller load. The former method uses the nominal velocities measured at the propeller plane as an input data of the numerical method and shows a better agreement with experimental data. The latter shows the qualitative agreement and may be used as an alternative design tools in the preliminary design stage.

• 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.

• Journal of Advanced Research in Ocean Engineering
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• v.4 no.2
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• pp.86-95
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• 2018

The fluid flow over structures has been widely investigated by many researchers because its extensive application in offshore structures, skyscrapers, chimneys and cooling towers, brides. In the viewpoint of reducing the drag for offshore structure, it becomes challenging problem in the field of hydrodynamic of offshore structure. The purpose of this study is to investigate a flow over a square cylinder with an attached splitter plate using RANS method. First, RANS turbulent models such as a standard $k-{\omega}$ model, SST $k-{\omega}$ model, RNG $k-{\varepsilon}$ model, realizable $k-{\varepsilon}$ model, standard $k-{\varepsilon}$ model were used for choosing suitable turbulent model which has the best agreement with available experimental result. Drag of single cylinder estimated by using standard $k-{\omega}$ has a good agreement with published experimental result. Therefore, the stand $k-{\omega}$ was selected for simulation for flow over a square cylinder with an attached plate. Second, the numerical results of drag of square cylinder with an attached splitter plate in various length of an attached plate were performed using RANS method in ANSYS Fluent. In this paper, the numerical simulations were conducted at a Reynolds number of 485 and the thickness of the splitter plate is chosen as a constant value about 10% of cylinder width. The numerical results of drag coefficient of square cylinder are compared with experimental result published by other researchers. Finally, the effect of the splitter plate attached to the rear side of the square cylinder has been investigated numerically with a focus on the drag coefficient and flow characteristic. As a result, the drag coefficient decreases with an increase in splitter plate length.

• 한국방재학회:학술대회논문집
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• 2007.02a
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• pp.159-162
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• 2007

The numerical efforts are presented for investigation of irregular waves passing a slit cassion and a warock block breakwater. 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(VOF) is employed. Numerical predictions of reflection and transmission coefficients are compared with those of the warock block breakwater with the slit caisson. Energy dissipation and seawater exchange rates of the slit caisson are better than those of the warock block breakwater.