• Journal of the Society of Naval Architects of Korea
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• v.37 no.3
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• pp.37-44
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• 2000

An experimental study has been carried out at circulating water channel to investigate the viscous flow around breaking waves generated by a submerged hydrofoil(NACA0012). Detailed flow measurements were made at several critical points including an incipient wave-breaking point and a fully-developed wave breaker. Particle Image Velocimetry(PIV) was employed to visualize the flow field very close to the breaker as well as at the near- and far-wake of the breaker. Generation, development and decay of the wave breaker have been investigated. It is found that PIV technique could be well applied to the complex flow field, including the vortical structures near the free surface as well as the wake of the hydrofoil.

• Journal of the Society of Naval Architects of Korea
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• v.38 no.1
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• pp.1-8
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• 2001

The computations of the turbulent flow around the ship models with the free-surface effects were carried out. Incompressible Reynolds-Averaged Navier-Stokes equations were solved by using an explicit finite-difference method with the nonstaggered grid system. The method employed second-order finite differences for the spatial discretization and a four-stage Runge-Kutta scheme for the temporal integration. For the turbulence closure, a modified Baldwin-Lomax model was exploited. The location of the free surface was determined by solving the equation of the kinematic free-surface condition using the Lax-Wendroff scheme and a free-surface conforming grid was generated at each time step so that one of the grid boundary surfaces always coincides with the free surface. An inviscid approximation of the dynamic free-surface boundary condition was applied as the boundary conditions for the velocity and pressure on the free surface. To validate the computational method developed in the present study, the computations were carried out for beth Wigley and Series 60 $C_B=0.6$ ship model and the computational results showed good agreements with the experimental data.

• Journal of the Society of Naval Architects of Korea
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• v.33 no.2
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• pp.13-29
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• 1996

This paper presents calculated results of the free-surface flow around a Series 60($C_B=0.6$) model. Three-dimensional Navier-Sotkes equations are solved and Baldwin-Lomax algebraic turbulence model is adopted to simulate the high Reynolds-number flow. To reduce computational efforts, velocity components near the wall are extrapolated with a the solved by using the Implicit Approximate Factorization method[2]. The successive-over-relaxation method is used for solving pressure-Poisson equation when obtaining the pressure field projecting the divergence-free velocity field. To simulate the free-surface flows more precisely, the numerical scheme solving the equation for the kinematic boundary condition is very important. In this paper, there numerical schemes are employed and the results are compared with the available experimental data.

• Journal of the Korean Society for Nondestructive Testing
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• v.23 no.3
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• pp.254-262
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• 2003

In this study, acoustic emission (AE) signals due to surface cracking and moisture movement in the flat-sawn boards of oak (Quercus Variablilis) during drying under the ambient conditions were analyzed and classified using the principal component analysis. The AE signals corresponding to surface cracking showed higher in peak amplitude and peak frequency, and shorter in rise time than those corresponding to moisture movement. To reduce the multicollinearity among AE features and to extract the significant AE parameters, correlation analysis was performed. Over 99% of the variance of AE parameters could be accounted for by the first to the fourth principal components. The classification feasibility and success rate were investigated in terms of two statistical classifiers having six independent variables (AE parameters) and six principal components. As a result, the statistical classifier having AE parameters showed the success rate of 70.0%. The statistical classifier having principal components showed the success rate of 87.5% which was considerably than that of the statistical classifier having AE parameters.

• Journal of the Institute of Electronics Engineers of Korea SC
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• v.38 no.1
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• pp.45-52
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• 2001

The proximal isovelocity surface area (PISA) method is an effective way of measuring the regurgitant blood flow rate in the mitral valve. This method defines the modelling required to describe the geometry of the isotach of the PISA. In the normal PISA flow model, the flow rate is calculated assuming that the surface of the isotach is either hemispherical or non-hemispherical numerically. However, this paper evaluated the estimate flow rate using a direct surface subdivision flow model based on the height field after isotach extraction using a region-based scheme. To validate the proposed method, the various PISA flow models were compared using pusatile color Doppler images with flow rates ranging from $30\;cm^3/sec\;to\;60\;cm^3/sec$ flow rate. Whereas the hemispherical flow model had a mean value of $29\;cm^3/sec$ and underestimated the measured flow rate by 35%, the proposed model and non-hemispherical model produced a c;ame mean value of $45\;cm^3/sec$, moreover, both flow models produced a similar pulsatile flow rate.

• Journal of the Korean Society for Precision Engineering
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• v.22 no.11 s.176
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• pp.51-58
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• 2005

Laser surface hardening technologies have been used to improve characteristics of wear and to enhance the fatigue resistance for automotive parts. The objective of this research work is to investigate the influence of the process parameters, such as power of laser and defocused spot position, on the characteristics of laser heat treatment for the case of SM45C medium carbon steel. CW Nd:YAG laser is selected as the heat source. The optical lens with the elliptical profile is designed to obtain a wide heat treatment area with a uniform hardness. From the results of the experiments, it has been shown that the maximum hardness is approximatly 780 Hv when the power and the travel speed of laser are 1,095 W and 0.6 m/min, respectively. In addition, the hardening width using the elliptical lens was three time larger than that using the defocusing of laser beam.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2004.10a
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• pp.286-291
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• 2004

The objective of this research work is to investigate the influence of process parameters, such as power of laser, travel speed of laser and material thickness, on roughness and striation of the cut surface for the case of cutting of CSP 1N sheet using high power Nd:YAG laser with continuous wave(CW). In order to find the practical cutting region and the relationship between process parameters on the roughness and the striation, several laser cutting experiments are carried out. From the results of experiments, the allowable cutting region and an optimal cutting speed for each cutting condition have been obtained to improve the quality of the cut surface. In addition, it has been shown that the surface roughness is related to the number of striation and depth of valley of the cut surface.

• Journal of the Korean Society for Precision Engineering
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• v.24 no.4 s.193
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• pp.57-67
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• 2007

Laser surface hardening technologies have been used to improve characteristics of wear and to enhance the fatigue resistance fur mold parts. The objective of this research work is to investigate the influence of the process parameters, such as power of laser and defocused spot position, on the characteristics of laser surface hardening for the case of SKD61 steel. CW Nd:YAG laser is selected as the heat source. The optical lens with the elliptical profile is designed to obtain a wide surface hardening area with a uniform hardness. From the results of the experiments, it has been shown that the maximum average hardness is approximatly 780 Hv when the power, focal position and the travel of laser are 1,095 W, 0mm and 0.3 m/min, respectively. In samples treated with lower scanning speeds, some small carbide particles appear in the interdendritic regions. This region contains fine martensite and carbide in proportions which depend on the local thermal cycle.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.16 no.5 s.110
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• pp.441-451
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• 2006

It is known that ball bearings mounted in housing or on shaft are playing a key role to keep it running smoothly. The roundness of a housing bore on which bearing outer ring is mounted with interference has directly affected the running accuracy of bearing. The running accuracy of bearing, therefore, can extend the significant influence to the rotating machinery as well. In this paper, bearing's vibration and fatigue life considering raceway roundness of ball bearing before and after mounted in housing bore are theoretically estimated. To perform analysis, a simple three degrees of freedom model was proposed and then Newton-Raphson iterative method was introduced to be utilized in the analysis. The results show that the vibration magnitude of ball bearing fitted into housing unit is appeared considerably larger than the one of its pre-assembling. And theoretical $L_{10}$ life which ninety percent of the bearing population will endure decreased in about fifty percent.

• Journal of Ocean Engineering and Technology
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• v.33 no.3
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• pp.236-244
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• 2019

Internal waves occur at the interface between two layers caused by a seawater density difference. The internal waves generated by a body moving in a two-layer fluid are also related to the generation of surface waves because of their interaction. In these complex flow phenomena, the experimental measurements and experimental set-up for the wave patterns of the internal waves and surface waves are very difficult to perform in a laboratory. Therefore, studies have mainly been carried out using numerical analysis. However, model tests are needed to evaluate the accuracy of numerical models. In this study, the various experimental conditions were evaluated using CFD simulations before experiments to measure the wave patterns of the internal waves and surface waves in a stratified two-layer fluid. The numerical simulation conditions included variations in the densities of the fluids, depth of the two-layer fluid, and moving speed of the underwater body.