• Journal of computational fluids engineering
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• v.19 no.4
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• pp.61-67
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• 2014

Since the most of the existing non-Newtonian models are not adequate to apply to the lattmce Boltzmann method, it is a challenging task from both the theoretical and the numerical points of view. In this research the hydro-kinetic model was modified and applied to the 3-D moving sphere in the circular channel flow and the characteristics of the shear thinning effect by the HK-model was evaluated and the condition of ${\Gamma}$ in the model was suggested for the stable simulation to generate non-trivial prediction in three dimension strong shear flows. On the wall boundaries of circular channel the curved wall surface treatment with constant velocity condition was applied and the bounceback condition was applied on the sphere wall to simulate the relative motion of the sphere. The condition is adequate at the less blockage than 0.7 but It may need to apply a multi-scale concept of grid refinement at the narrow flow region. to obtain the stable numerical results.

• Journal of computational fluids engineering
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• v.10 no.2
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• pp.7-14
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• 2005

In this study, the shallow water equations on the sphere is simulated by the proposed method which has high spatial resolution and is based on the CIP(Cubic Interpolated Pseudoparticle) method. The governing equations are approximated on the longitude-latitudinal coordinate system. To avoid the problems resulting from the convergence of the meridians toward high-latitude and singularities on the poles, the semi-Lagrangian and finite volume method are employed. in addition, the CIP method is employed to solve the advection equations and is extended to apply on the equations in the polar coordinate system. The mathematical formulation and numerical results are also described. To verify of the efficiency, accuracy and capability of proposed algorithm, the standard test cases proposed bv Williamson et. al are simulated and the results are compared with other results. As a result, it is found that the present scheme gives a good properties in preserving shapes of solution and settles the pole problems in solving the shallow water equations on the sphere.

• Journal of computational fluids engineering
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• v.12 no.1
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• pp.1-8
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• 2007

A code is developed using the hybrid Cartesian/immersed boundary method and it is applied to simulate flows around a three-dimensional deforming body. A new criterion is suggested to distribute the immersed boundary nodes based on edges crossing a body boundary. Velocities are reconstructed at the immersed boundary nodes using the interpolation along a local normal line to the boundary. Reconstruction of the pressure at the immersed boundary node is avoided using the hybrid staggered/non-staggered grid method. The developed code is validated through comparisons with other experimental and numerical results for the velocity profiles around a circular cylinder under the forced in-line oscillation and the pressure coefficient distribution on a sphere. The code is applied to simulate the flow fields around a plate whose tail is periodically flapping under a translation. The effects of the velocity and acceleration due to the deformation on the periodic shedding of pairs of tip vortices are investigated.

• 한국전산유체공학회:학술대회논문집
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• 2011.05a
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• pp.78-85
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• 2011

The Vortex-In-Cell(VIC) method combined with panel method is applied to the analysis of incompressible unsteady viscous flow. The dynamics of resulting flow is governed by the vorticity transport equation in Lagrangian form with vortex particle representation of the flow field. A regular grid which is independent to the shape of a body is used for numerical evaluation based on immersed boundary technique. With an introduction of this approach, the development and validation of the VIC method is presented with some computational results for incompressible viscous flow around two or three dimensional bodies such as wing section, sphere, finite wing and marine propeller.

• Journal of the Society of Naval Architects of Korea
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• v.45 no.4
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• pp.379-388
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• 2008

A hybrid Cartesian/immersed boundary code is expanded to simulate flow field around a three-dimensional body which undergoes large dynamic deformation. Immersed boundary nodes are automatically distributed based on the edges crossing triangles on body boundary. Velocity vectors are reconstructed at those immersed boundary nodes along local normal lines to the boundary. The reconstruction of pressure is avoided using the hybrid staggered/non-staggered grid method. The developed code is validated through comparisons with other results on laminar flow over a sphere. The code is applied to simulate flow around a foil which is attached to a body of revolution and rotates under periodic deformation. The periodic variation of the tip vortex is observed and the effects of the deformation on hydrodynamic force acting on the body are investigated.

• Steel and Composite Structures
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• v.46 no.2
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• pp.211-220
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• 2023

Welded hollow spherical joints are commonly used joints in space grid structures. An internal stiffener is generally adopted to strengthen the joints when large hollow spheres are used. To further strengthen it, external stiffeners can be used at the same time. In this study, axial compression tests are conducted on four full-scale 550 mm spherical joints. The failure modes and strengths of the tested joints are investigated. It shows that the external stiffeners are able to increase the strength of the joint up to 25%. A numerical model for large spherical joints with stiffeners is established and verified against the experimental results. Parametric studies are executed considering six main design factors using the verified model. It is found that the strength of the spherical joint increases as the thickness, height and number of the external stiffeners increase, and the hollow sphere's diameter has a neglectable effect on the enhancement caused by the external stiffeners. Based on the experimental and numerical results, a practical formula for the compressive bearing capacity of large welded hollow spherical joints with both internal and external stiffeners is proposed. The proposed formula gives a conservative prediction on the compressive capacity of large welded hollow spherical joints with both internal and external stiffeners.

• Journal of the Society of Naval Architects of Korea
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• v.51 no.5
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• pp.409-418
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• 2014

Simulation of flow past a complex marine structure requires a fine resolution in the vicinity of the structure, whereas a coarse resolution is enough far away from it. Therefore, a lot of grid cells may be wasted, when a simple Cartesian grid system is used for an Immersed Boundary Method (IBM). To alleviate this problems while maintaining the Cartesian frame work, we adopted an Adaptive Mesh Refinement (AMR) scheme where the grid system dynamically and locally refines as needed. In this study, We implemented a moving IBM and an AMR technique in our basic 3D incompressible Navier-Stokes solver. A Volume Of Fluid (VOF) method was used to effectively treat the free surface, and a recently developed Lagrangian Dynamic Subgrid-scale Model (LDSM) was incorporated in the code for accurate turbulence modeling. To capture vortex induced vibration accurately, the equation for the structure movement and the governing equations for fluid flow were solved at the same time implicitly. Also, We have developed an interface by using AutoLISP, which can properly distribute marker particles for IBM, compute the geometrical information of the object, and transfer it to the solver for the main simulation. To verify our numerical methodology, our results were compared with other authors' numerical and experimental results for the benchmark problems, revealing excellent agreement. Using the verified code, we investigated the following cases. (1) simulating flow around a floating sphere. (2) simulating flow past a marine structure.

• Journal of Ocean Engineering and Technology
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• v.26 no.6
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• pp.7-13
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• 2012

This paper presents a numerical study of the wave loads acting on offshore structures using a Cartesian-grid-based flow simulation method. Finite volume discretization with a volume-of-fluid (VOF) method is adopted to solve two-phase Navier-Stokes equations. Among the many variations of the VOF method, the CICSAM scheme is applied. The body boundary conditions are satisfied using a porosity function, and wave generation is carried out by using transient (wave or damping) zone approaches. In order to validate the present numerical method, three different basic offshore structures, including a sphere, Pinkster barge, and Wigley model, are numerically investigated. First, diffraction and radiation problems are solved using the present numerical method. The wave exciting and drift forces from the diffraction problems are compared with potential-based solutions. The added mass and wave damping forces from the radiation problems are also compared with the potential results. Next, the wave-induced motion responses of the structures are calculated and compared with the existing experimental data. The comparison results are fairly good, showing the validity of the present numerical method.

• 한국전산유체공학회:학술대회논문집
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• 2001.05a
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• pp.30-37
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• 2001

A Numerical simulation for the propulsion of axisymmetric body by contractive and dilative motion is carried out. The present analysis shows that a propulsive force can be obtained in highly viscous fluid by a contractive and dilative motion of axisymmetric body. An axisymmetric analysis code is developed with unstructured grid system for the simulation of complicated motion and geometry. The developed code is validated by comparing with the results of stokes approximation with the problem of uniform flow past a sphere in low Reynolds number($R_n=1$). The validated code is applied to the simulation of contractive and dilative motion of body. The simulation is extended to the analysis of waving surface with projecting part for finding out the difference of hydrodynamic performance according to the variation of waving surface configuration. The present study will be the basic research for the development of the propulsor of an axisymmetric micro-hydro-machine.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.60 no.5
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• pp.1005-1010
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• 2011

The accidents caused by dielectric instability have been increasing in power grid. It is important to enhance the dielectric reliability of a high voltage apparatus to reduce the damage from electrical hazards. To develop an electrically reliable high voltage apparatus, the experimental study on the electrical breakdown field strength is indispensable, as well as theoretical approach. In this paper, the lightning impulse breakdown characteristics considering utilization factors are studied for the establishment of insulation design criteria of an high voltage apparatus. The utilization factors are represented as the ratio of mean electric field to maximum electric field. Dielectric experiments are performed by using several kinds of sphere-plane electrode systems made of stainless steel. As a result, it is found that dielectric characteristics are affected by not only maximum electric field intensity but also utilization factors of electrode systems. The results are expected to be applicable to designing the air-insulated high voltage apparatuses.