• Journal of Korean Society of Steel Construction
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• v.11 no.1 s.38
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• pp.33-42
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• 1999

In the present study, crack initiation criteria, static failure and tensile mode fatigue behavior for a rail steel are evaluated to assure the railway vehicle's safety. The transverse fissure, which is the most critical damage in the rail, is initiated by the maximum shear stress and its location is subsurface. In addition, the possibility of transition from the shear mode to the mixed mode increases with increasing the length of subsurface crack. Because of the brittleness by the welding, the fracture toughness of the welded part is lower than of the base metal. For low ${\Delta}K$, the stage II fatigue crack growth rates of the welded part is slower than of the base metal but, for high ${\Delta}K$, this different behavior for fatigue crack growth rate is nearly diminished. These trends are more remarkable for low stress ratio, R=0.1. It is believed that this behavior is caused by the change of the microstructure which that of the welded part is coarser than of base metal.

• Journal of the Society of Naval Architects of Korea
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• v.48 no.3
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• pp.222-232
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• 2011

This article examines the scale effect of the flow characteristics, resistance and propulsion performance on a 317k VLCC. The turbulent flows around a ship in both towing and self-propulsion conditions are analyzed by solving the Reynolds-averaged Navier-Stokes equation together with the application of Reynolds stress turbulence model. The computations are carried out in both model- and full-scale. A double-body model is applied for the treatment of free surface. An asymmetric body-force propeller is used. The speed performances including resistance and propulsion factors are obtained from two kinds of methods. One is to analyze the computational results in model scale through the revised ITTC' 78 method. The other is directly to analyze the computational results in full scale. Based on the computational predictions, scale effects of the resistance and the self-propulsion factors including form factor, thrust deduction fraction, effective wake fraction and various efficiencies are investigated. Scale effects of the streamline pattern, hull pressure and local flow characteristics including x-constant sections, propeller and center plane, and transom region are also investigated. This study presents a useful tool to hull-form and propeller designers, and towing-tank experimenters to take the scale effect into consideration.

• Journal of Digital Contents Society
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• v.16 no.6
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• pp.909-916
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• 2015

This paper presents a method to display the 3D vector fields by analyzing phase space. This method is based on the connections between ordinary differential equations and the topology of vector fields. The phase space analysis should be geometric interpolation of an autonomous system of equation in the form of the phase space. Every solution of it system of equations corresponds not to a curve in a space, but the motion of a point along the curve. This analysis is the basis of this paper. This new method is required to decompose the hexahedral cell into five or six tetrahedral cells for 3D vector fields. The critical points can be easily found by solving a simple linear system for each tetrahedron. The tangent curves can be integrated by finding the intersection points of an integral curve traced out by the general solution of each tetrahedron and plane containing a face of the tetrahedron.

• Journal of the Optical Society of Korea
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• v.20 no.6
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• pp.669-677
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• 2016

A miniaturized FTIR spectrometer based on lamellar grating interferometry is being developed for passive remote-sensing. Consisting of a pair of micro-mirror arrays, the lamellar grating can be fabricated using MEMS technology. This paper describes a method to compute the optical field in the interferometer to optimize the design parameters of the lamellar grating FTIR spectrometer. The lower limit of the micro-mirror width in the grating is related to the formation of a Talbot image in the near field and is estimated to be about $100{\mu}m$ for the spectrometer to be used for the wavelength range of $7-14{\mu}m$. In calculating the far field at the detection window, the conventional Fraunhofer equation is inadequate for detection distance of our application, misleading the upper limit of the micro-mirror width to avoid interference from higher order diffractions. Instead, the far field is described by the unperturbed plane-wave combined with the boundary diffraction wave. As a result, the interference from the higher order diffractions turns out to be negligible as the micro-mirror width increases. Therefore, the upper limit of the micro-mirror width does not need to be set. Under this scheme, the interferometer patterns and their FT spectra are successfully generated.

• Journal of Magnetics
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• v.11 no.2
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• pp.70-73
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• 2006

The magnetization angle and thickness dependence of magnetic anisotropy in the exchange-biased [Pd/Co]${\times}$5/FeMn multilayers with an out-of-plane anisotropy were investigated to determine the origin of perpendicular exchange biasing. As the Co thickness increased to 1.5 nm in the [Pd(0.8 nm)/Co(t)]${\times}$5/FeMn(120 nm) films, the hysteresis loops were converted from square loops at a thin Co (<0.4 nm) to complicated round ones at a thick Co. The irregularly asymmetric step (IAS) at the left top of the loop appeared in the loop of the 0.6-nm Co film due to an inhomogeneity in the exchange anisotropy. As the Pd thickness increased to 1.6 nm, the step disappeared, and the perpendicular magnetic anisotropy was maximized in the Co thickness between 0.6 and 0.9 nm. The conversion of the magnetization loop along the magnetization angle coincided with the equation $H_{(eff)}=H_o\;cos{\theta}$. The IAS of the 0.8-nm Pd film disappeared after thermal annealing up to $200^{\circ}C$ under an external magnetic field.

• Journal of Ocean Engineering and Technology
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• v.30 no.1
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• pp.25-31
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• 2016

In this study, hydrodynamic coefficients were obtained from a Rotating Arm (RA) test, which is one of the captive model tests used to provide accurate coefficients in the control motion equation of an underwater vehicle. The RA test was carried out at the RA facility of ADD (Agency for Defense Development), and the forces and moments acting on the underwater vehicle were measured using a six-axis waterproof gage. A multiple regression analysis was used in the analysis of the measured data. The experimental results were also verified by comparison with the theoretical values of the previous linear coefficients. In addition, the stability indices in the horizontal plane were calculated using the linear and nonlinear coefficients, and the dynamic stability of the underwater vehicle was estimated to have a good dynamic performance with a depth ratio of 6.0.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.9
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• pp.1262-1272
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• 2003

A two-dimensional direct numerical simulation is performed to investigate the dynamic behaviors of a single vortex in counter reacting and non-reacting flow field. A predictor-corrector-type numerical scheme with a low Mach number approximation is used in this simulation. A 16-step augmented reduced mechanism is adopted to treat the chemical reaction. The budget of the vorticity transport equation is examined to reveal a mechanism leading to the formation, destruction and transport of a single vortex according to the direction of vortex generation in reacting and non-reacting flows. The results show that air-side vortex has more larger strength than that of fuel-side vortex in both non-reacting and reacting flows. In reacting flow, the vortex is more dissipated than that in non-reacting flow as the vortex approach the flame. The total circulation in reacting flow, however, is larger than that in non-reacting flow because the convection transport of vorticity becomes much large by the increased velocity near the flame region. It is also found that the stretching and the convection terms mainly generate vorticity in non-reacting and reacting flows. The baroclinic torque term generates vorticity, while the viscous and the volumetric expansion terms attenuate vorticity in reacting flow. Furthermore, the contribution of volumetric expansion term on total circulation for air-side vortex is much larger than that of fuel-side vortex. It is also estimated that the difference of total circulation near stagnation plane according to the direction of vortex generation mainly attributes to the convection term.

• Journal of the Society of Naval Architects of Korea
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• v.48 no.2
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• pp.147-157
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• 2011

A RANS(Reynolds averaged Navier-Stokes) based numerical method is developed for the evaluation of ship resistance and self-propulsion performances. In the usability aspect of CFD for the hull form design, the field grid around practical hull forms is generated by solving a grid Poisson equation based on the hull surface grid generated from station offsets and centerline profile. A body force technique is introduced to model the effects of the propeller in which the propeller loads are obtained from potential flow analysis using an unsteady lifting surface method. The free surface is captured by using a two-phase level-set method and the realizable $k-{\varepsilon}$ model is used for turbulence closure. The hull attitude in vertical plane, i.e., trim and sinkage, is calculated by using a quasi-steady method and then considered in the computation by translating and rotating the grid system according to the values. For the validation of the proposed method, the numerical results of resistance tests for KCS, KLNG, and KVLCC1 and of self-propulsion test for KCS are compared with experimental data.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.18 no.9
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• pp.1023-1029
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• 2007

In this paper, we propose the method to solve the BLT equations using Laguerre polynomials in time domain. The solution of BLT equations is obtained by recursive, differential and integral properties of Laguerre polynomials. The verification of the proposed method is tested by applying it to the two-wired transmission line with resistors and capacitors, which is illuminated by the electromagnetic plane wave pulse. And the result is compared with the corresponding transient responses obtained from inverse fast Fourier transform(IFFT) of the frequency domain solutions of BLT equations.

• Journal of the Korea Concrete Institute
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• v.11 no.3
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• pp.81-88
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• 1999

In order to properly evaluate the shear strength of reinforced concrete circular columns due to the transverse shear reinforcement, the average of fractions of forces generated along the circular transverse hoops across the shear failure plane in the loading direction is calculated. For this, the center-to-center diameter of circular transverse hoops. spacing and the crack angle measured to the column longitudinal axis are considered. Using these variables, an equation representing the effective section area of circular transverse shear steel is proposed. The study result shows that the constant parameter. used for the calculation of the effective section area of circular hoops over the last 10 years, should not universally be applied any more. The use of the constant parameter may not seriously do harm to the evaluation of shear strength for circular columns with non-seismically designed transverse hoop reinforcement, since it gives slightly conservative results. However. for well-confined circular columns with close spacing or circular steel jacketing. it gives about 20% overestimation of the shear capacity contributed by the transverse hoop steel.