• Transactions of Materials Processing
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• v.18 no.7
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• pp.565-571
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• 2009

This paper is concerned with the analysis on the surface stress profiles of perfectly plastic material in backward extrusion process. Due to heavy surface expansion appeared usually in the backward extrusion process, the tribological conditions along the interface between the material and the punch land are very severe. In the present study, the analyses have focused to reveal the surface conditions at the contact boundary for various punch shapes in terms of surface expansion, contact pressure, and relative movement between punch and workpiece which consists of sliding velocity and distance, respectively. Punch geometries adopted in the analysis include concave, hemispherical, pointed and ICFG recommended shapes. Extensive simulation has been conducted by applying the rigid-plastic finite element method to the backward extrusion process under different punch geometries. The simulation results are summarized in terms of surface expansion, contact pressure, sliding velocity and sliding distance at different reduction in height, deformation patterns, and load-stroke relationship, respectively.

• Journal of the Society of Naval Architects of Korea
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• v.59 no.1
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• pp.18-28
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• 2022

The turbulence stimulation effect of studs for boundary layer over a flat plate was investigated through the flow measurement in KRISO cavitation tunnel. For the test, Laser Doppler Velocimetry (LDV) and three flat plate models were used: (1) flat plate without studs; (2) flat plate with one stud row; (3) flat plate with two stud rows. The dimension and location of stud rows and the inflow speed were selected considering test conditions for standard-sized model ships in KRISO towing tank. The boundary layer characteristics of test models were analyzed and compared in terms of mean velocity profiles, turbulence intensity profiles, boundary layer thickness, and shape factor. In the case of the flat plate without studs, transition from laminar to turbulent flow occurred around Re_x=3.83 ~ 5.19 × 10⁵. In the case of flat plates with stud rows, the flow rapidly changed into turbulent flow right after passing the first stud row. In the state where turbulence was already developed, the second stud row slightly increased the turbulence intensity near the top of the stud, but did not significantly affect the boundary layer characteristics such as mean velocity distribution, boundary layer thickness, and shape factor.

• The KSFM Journal of Fluid Machinery
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• v.13 no.2
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• pp.5-11
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• 2010

The nanofluidics is characterized by a large surface-to-volume ratio, so that the surface properties strongly affect the flow resistance. We present here the results showing that the effect of wetting properties and the surface roughness may considerably reduce the friction of fluid past the boundaries. For a simple fluid flowing over hydrophilic and hydrophobic surfaces, the influences of surface roughness are investigated by the nonequilibrium molecular dynamics (NEMD) simulations. The fluid slip at near a solid surface highly depends on the wall-fluid interaction. For hydrophobic surfaces, apparent fluid slips are observed on smooth and rough surfaces. The solid wall is modeled as a rough atomic sinusoidal wall. The effects on the boundary condition of the roughness characteristics are given by the period and amplitude of the sinusoidal wall. It was found that the slip velocity for wetting conditions at interface decreases with increasing effects of surface roughness. The results show the surface rougheness and wettability determines the slip or no-slip boundary conditions. The surface roughness geometry shows significant effects on the boundary conditions at the interface.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.8
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• pp.1097-1104
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• 2003

The current study analyzes Langmuir slip boundary condition theoretically and it is tested in practical numerical analysis for separation-associated flow. Slip phenomenon at the channel wall is properly implemented by various numerical slip boundary conditions including Langmuir slip model. Compressible backward-facing step flow is compared to other analysis results with the purpose of diatomic gas Langmuir slip model validation. The numerical solutions of pressure and velocity distributions where separation occurs are in good agreement with other numerical results. Numerical analysis is conducted for Reynolds number from 10 to 60 for a prediction of separation at T-shaped micro manifold. Reattachment length of flows shows nonlinear distribution at the wall of side branch. The Langmuir slip model predicts fairly the physics in terms of slip effect and separation.

• Journal of Mechanical Science and Technology
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• v.15 no.5
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• pp.555-561
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• 2001

A new approach to analyze the multi-domain acoustic system divided and enclosed by flexible structures is presented in this paper. The boundary element formulation of the Helmholtz integral equation is used for the internal fields and the finite element formulation for the structures surrounding the fields. We developed a numerical analysis program for the structural-acoustic coupling problems of the multi-domain system, in which boundary conditions such as the continuity of normal particle velocity and sound pressure in the structural interfaces between Field 1 and Field 2 are not needed. The validity of the numerical analysis program is verified by comparing the numerical results with the experimental ones. Example problems are included to investigate the characteristics of the coupled multi-domain system.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.1
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• pp.61-68
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• 1991

The system such as railway bridge can be modelled as the restrained beam with intermediate supports. This kind of structures are subject to the moving load, which has a great effect on dynamic stresses and can cause sever motions, especially at high velocities. Therefore, to analyze the dynamic characteristics of the system due to the moving load is very important. In this paper, the governing equation of motion of a restrained beam subjected to the moving load is derived by using the Hamilton's principle. The orthogonal polynomial functions, which are trial functions and satisfying the geometric and dynamic boundary conditions, are obtained through simple procedure. The dynamic response of the system subjected to the moving loads is obtained by using the Galerkin's method and the numerical time integration technique. The numerical tests for various constraint, velocity and boundary conditions were preformed. Furthermore, the effects of mass of the moving load are studied in detail.

• Ocean Systems Engineering
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• v.4 no.2
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• pp.83-97
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• 2014

Wave propagation in a three-dimensional (3D) fully nonlinear numerical wave tank (NWT) is studied based on velocity potential theory. The governing Laplace equation with fully nonlinear boundary conditions on the moving free surface is solved using the indirect desingularized boundary integral equation method (DBIEM). The fourth-order predictor-corrector Adams-Bashforth-Moulton scheme (ABM4) and mixed Eulerian-Lagrangian (MEL) method are used for the time-stepping integration of the free surface boundary conditions. A smoothing algorithm, B-spline, is applied to eliminate the possible saw-tooth instabilities. The artificial wave speed employed in MTF (multi-transmitting formula) approach is investigated for fully nonlinear wave problem. The numerical results from incorporating the damping zone (DZ), MTF and MTF coupled DZ (MTF+DZ) methods as radiation condition are compared with analytical solution. An effective MTF+DZ method is finally adopted to simulate the 3D linear wave, second-order wave and irregular wave propagation. It is shown that the MTF+DZ method can be used for simulating fully nonlinear wave propagation very efficiently.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.6
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• pp.619-627
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• 2004

In most industrial applications, the geometrical complexity is combined with the moving boundaries. These problems considerably increase the computational difficulties since they require, respectively, regeneration and deformation of the grid. As a result, engineering flow simulation is restricted. In order to solve this kind of problems the immersed boundary method was developed. In this study, the immersed boundary method is applied to the numerical simulation of stationary, rotating and oscillating cylinders in the 2-dimensional square cavity. No-slip velocity boundary conditions are given by imposing feedback forcing term to the momentum equation. Besides, this technique is used with a second-order accurate interpolation scheme in order to improve the accuracy of flow near the immersed boundaries. The governing equations for the mass and momentum using the immersed boundary method are discretized on the non-staggered grid by using the finite volume method. The results agree well with previous numerical and experimental results. This study presents the possibility of the immersed boundary method to apply to the complex flow experienced in the industrial applications. The usefulness of this method will be confirmed when we solve the complex geometries and moving bodies.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.3 no.3
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• pp.176-183
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• 1991

In this paper. boundary element method is applied to the analysis of nonlinear free surface wave. A particular concern is given to the treatment of the open boundaries at the in-flow boundary and out-flow boundary, which uses the mass-flux and energy-flux considering the continuity of fluid. By assuming the fluid to be inviscid and incompressible and the flow to be irrotational. the problem is formulated mathematically as a two-dimentional nonlinear problem in terms of a velocity potential. The equation(Laplace equation) and the boundary conditions are transformed into two boundary integral equations. Due to the nonlinearity of the problem. the incremental method is used for the numerical analysis. Numerical results obtained by the present boundary element method are compared with those obtained by the finite element method and also with experimental values.

• Transactions of the Korean hydrogen and new energy society
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• v.33 no.6
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• pp.776-785
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• 2022

Hydrogen combustion in modern gas-turbine engine is the cutting edge technology as carbon-free energy conversion system. Flashback of hydrogen flame, however, is inevitable and critical specially for premixed hydrogen combustion. Therefore, this experimental investigation is conducted to understand flashback phenomenon in premixed hydrogen combustion. In order to investigate flashback characteristics in premixed hydrogen (H₂)/air flame, we focus on pressure conditions and nozzle shapes. In general, quenching distance reduces as pressure of combustion chamber increases, causing flashback from boundary layer near wall. The flashback regime for reference and modified candidate configurations can broadly appear with increasing combustion chamber pressure. The later one can improve flashback-resist by compensating flow velocity at wall. Also, improved wall flow velocity profile of suggested contraction nozzle prevents entire flashback but causes local flashback at nozzle exit.