• 소성∙가공
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• 제24권4호
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• pp.227-233
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• 2015

In this paper, the effect of flow stress, friction, temperature, and velocity on finite element predictions of metal flow lines after cylindrical upsetting is presented. An actual three-stage hot forging process involving an upsetting step is utilized and experimental metal flow lines are measured to study the effect of the various process variables. It was found that temperature and velocity for reasonable values of friction have little influence on metal flow lines especially those located deep within the cylinder but that flow stress has a direct influence on the flow lines. It was shown that a pure power law material model cannot reflect the real flow stress of hot material because it underestimates the flow stress especially around the dead-metal zone for the upsetting of a cylindrical specimen. It is thus recommended that a proper lower limit of flow stress be assumed to alleviate this issue.

• Korea-Australia Rheology Journal
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• 제16권1호
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• pp.47-54
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• 2004

Low Reynolds number, dilute, and surfactant-free bubble suspensions are prepared by mechanical mixing after introducing carbon dioxide bubbles into a Newtonian liquid, polyol. The apparent shear viscosity is measured with a wide-gap parallel plate rheometer by imposing a simple shear flow of capillary numbers(Ca) of the order of $10^{-2}$ ~ $10^{-1}$ and for various gas volume fractions ($\phi$). Effects of capillary numbers and gas volume fractions on the viscosity of polyol foam are investigated. At high capillary number, viscosity of the suspension increases as the gas volume fraction increases, while at low capillary number, the viscosity decreases as the gas volume fraction increases. An empirical constitutive equation that is similar to the Frankel and Acrivos equation is proposed by fitting experimental data. A numerical simulation for deformation of a single bubble suspended in a Newtonian fluid is conducted by using a newly developed two-dimensional numerical code using a finite volume method (FVM). Although the bubble is treated by a circular cylinder in the two dimensional analysis, numerical results are in good agreement with experimental results.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2005년도 추계 학술대회논문집
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• pp.117-124
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• 2005

Splitting algorithms of the incompressible Navier-Stokes equations using P1P1/P2P1 finite element formulation are newly proposed. P1P1 formulation allocates velocity and pressure at the same nodes, while P2P1 formulation allocates pressure only at the vertex nodes and velocity at both the vertex and mid nodes. For comparison of the elapsed time and accuracy of the two methods, they have been applied to the well-known benchmark problems. The three cases chosen are the two-dimensional steady and unsteady flows around a fixed cylinder, decaying vortex, and impinging slot jet. It is shown that the proposed P2P1 semi-splitting method performs better than the conventional P1P1 splitting method in terms of both accuracy and computation time.

• 한국소음진동공학회논문집
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• 제18권4호
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• pp.439-447
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• 2008

One of the key elements in efforts to minimize noise radiation from a powertrain is the knowledge of the main radiating component and the relation between the surface vibration of a powertrain and the sound pressure. In this research, the powertrain model is developed based on FEM(finite element method). This model is applied to the prediction of the vibration of a powertrain by using ADAMS and the radiation noise by using BEM(boundary element method). According to this numerical analysis, the surface vibration of a powertrain is investigated as a source of radiated noise. This surface vibration is caused by the 1st order natural vibration of the cylinder block and its mode shape is the torsion mode. Therefore, this mode shape is modified to reduce the surface vibration of the powertrain. The radiation noise of the modified powertrain is also reduced to $5{\sim}12\;dB$. This modification is very successful for the noise reduction based on the CAE technology.

• Wind and Structures
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• 제30권1호
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• pp.15-27
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• 2020

This paper reports the study on development and verification of numerical models and analyzes of flow at high speed around structural elements in the shape of a curved pipe (e.g., a fragment of a water slide). Possibility of engineering estimation of wind forces acting on an object in the shape of a helix is presented, using relationships concerning toroidal and cylindrical elements. Determination of useful engineering parameters (such as aerodynamic forces, pressure distribution, and air velocity field) is presented, impossible to obtain from the existing standard EN 1991-1-4 (the so-called wind standard). For this purpose, flow at high speed around a torus and helix, arranged both near planar surface and high above it, was analyzed. Analyzes begin with the flow around a cylinder. This is the simplest object with a circular cross-section and at the same time the most studied in the literature. Based on this model, more complex models are analyzed: first in the shape of half of a torus, next in the shape of a helix.

• 한국해양공학회지
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• 제24권6호
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• pp.16-22
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• 2010

In this paper, the diffraction problems for fixed offshore structures are solved using a hybrid scheme. In this hybrid scheme, potential-based solutions and the Navier-Stokes-based finite volume method (FVM) with a volume-of-fluid (VOF) method are combined. We introduce a buffer zone for efficient wave-making and damping. In this buffer zone, the near field solution from FVM-VOF is gradually changed to Stokes' 2nd order wave solutions. Three different models, including the truncated cylinder, sphere, and wigleyIII model, are numerically investigated in regular waves with a wave steepness of 1/30. The efficiency and accuracy of the hybrid scheme are numerically validated from results using different domain sizes and buffer zones. The wave exciting forces from the FVM-VOF simulations are compared with experiments and potential-based solutions from the higher-order boundary element method (HOBEM). This comparison shows good agreement between the hybrid scheme and potential-based solutions.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1993년도 정기총회 및 추계학술대회 논문집 학회본부
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• pp.169-171
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• 1993

In order to estimate space charging process in the corona charging apparatus which has been used to make polymer electret cell, the electrical properties of 30[${\mu}m$] thick polypropylene film were obtained from TSC measurement after corona charging between copper knife electrode and aluminum cylinder electrode with the voltage of -8, -7, -6, -5 (kV). And, the electrostatic contour and the electric field vector were calculated using Finite Element Method with the electrical properties obtained from TSC spectra analysis. The edge effect around the edge of knife electrode affects electrostatic contour on the surface of specimen and the electric field concentration inside the polymer. As a result the uneven charging state in the electret cell due to the mistake of design was calculated, and the optimal design of corona charging apparatus opprobriate to various specimen was come to be practicable.

• 한국해양공학회지
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• 제12권2호통권28호
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• pp.97-110
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• 1998

The numerical damping and dispersion error characteristics associated with difference schemes and a panel shift method used for the calculation of steady free surface flows by a panel method are an analysed in this paper. First, 12 finite difference operators used for the double model flow by Letcher are applied to a two dimensional cylinder with the Kelvin free surface condition and the numerical errors with these schemes are compared with those by the panel shift method. Then, 3-D waves due to a submerged source are calculated by the difference schemes, the panel shift method and also by a higher order boundary element method(HOBEM). Finally, the waves and wave resistance for Wigley's hull are calculated with these three schemes. It is shown that the panel shift method is free of numerical damping and dispersion error and performs better than the difference schemes. However, it can be concluded that the HOBEM also free of the numerical damping and dispersion error is the most stable, accurate and efficient.

• 대한기계학회논문집A
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• 제21권10호
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• pp.1598-1608
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• 1997

Based on the application of te theory of conjugate approximations and the Loubignac's iterative method in a local region, a method to improve the stress filed in a displacement-formulated finite element solution has been proposed. The validity of the proposed method has been tested through two examples : a thick cylinder under internal pressure loading and an infinite plate with a central circular hole subjected to uniaxial tension. As a result of analysis of the examples, it was found that the stress field obtained for the local region model by the proposed method approximates well for the whole domain model. In addition, it was found that because of a significant decrease in the computing time to obtain the improved stress field, the proposed method is efficient and useful for the detailed stress analysis in local regions.

• 한국자동차공학회논문집
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• 제17권1호
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• pp.80-86
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• 2009

Main bearing cap is one of the essential structural elements in internal combustion engine. Main bearing cap guides and holds the crankshaft, withstanding the full combustion and inertia loads of the engine. A seamless design methodology using FEA has been proposed to produce a reliable design of main bearing cap. A Levy's thick cylinder model was applied to calculate the contact pressure between bearing shell and housing bore. A calculated contact pressure at housing bore is within the allowed limit comparing with that from bearing shell model. An adequate FEA model was suggested to obtain reliable solutions for the durability of main bearing cap. 3D global model consists of engine bulkhead, main bearing cap, and bolts. Sub-model consisting of cap and part of bolts is used to get detailed solution of main bearing cap. A very careful contact modeling practice is needed to resolve the convergence problems frequently encountering during combined geometric and material non-linear problems. A proposed methodology has been applied to the main bearing cap model successfully and obtained reliable stress results and fatigue safety factors.