• 한국전산유체공학회:학술대회논문집
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• 2011.05a
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• pp.414-419
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• 2011

Missile am fighter aircraft have been challenged by low restoring nose-down pitching moment at high angle of attach. The consequence of weak nose-down pitching moment can be resulting in a deep stall condition. Especially, the pressure oscillation has a huge effect on noise generation, structure damage, aerodynamic performance and safety, because the flow has strong unsteadiness at high angle of attack. In this paper, the unsteady aerodynamics coefficients were analyzed at high angle of attack up to 60 degrees around two dimensional NACA0012 airfoil. The two dimensional unsteady compressible Navier-Stokes equation with a LES turbulent model was calculated by OHOC (Optimized High-Order Compact) scheme. The flow conditions are Mach number of 0.3 and Reynolds number of $10^5$. The lift, drag, pressure distribution, etc. are analyzed according to the angle of attack. The results at a low angle of attack are compared with other results before a stall condition. From a certain high angle of attack, the strong vortex formed by the leading edge are flowing downstream as like Karman vortex around a circular cylinder. Unsteady velocity field, periodic vortex shedding, the unsteady pressure distribution on the airfoil surface, and the acoustic fields are analyzed. The effects of these unsteady characteristics in the aerodynamic coefficients are analyzed.

• International Journal of Aeronautical and Space Sciences
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• v.16 no.1
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• pp.8-18
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• 2015

A numerical simulation for a nonslender BWB UCAV configuration with a rounded leading edge and span of 1.0 m was performed to analyze its aerodynamic characteristics. Numerical results were compared with experimental data obtained at a free stream velocity of 50 m/s and at angles of attack from -4 to $26^{\circ}$. The Reynolds number, based on the mean chord length, is $1.25{\times}106$. 3D multi-block hexahedral grids are used to guarantee good grid quality and to efficiently resolve the boundary layer. Menter's shear stress transport model and two transition models (${\gamma}-Re_{\theta}$ model and ${\gamma}$ model) were used to assess the effect of the laminar/turbulent transition on the flow characteristics. Aerodynamic coefficients, such as drag, lift, and the pitching moment, were compared with experimental data. Drag and lift coefficients of the UCAV were predicted well while the pitching moment coefficient was underpredicted at high angles of attack and influenced strongly by the selected turbulent models. After assessing the pressure distribution, skin friction lines and velocity field around UCAV configuration, it was found that the transition effect should be considered in the prediction of aerodynamic characteristics of vortical flow fields.

• Journal of the Korean Magnetics Society
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• v.5 no.5
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• pp.900-904
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• 1995

We have developed a new torque magnetometric method for measuring the M-H hysteresis loop of a spheroid-shape magnetic material having a uniaxial anisotropy. Our torque magnetometric method gives the saturation magnetization as well as the remnant magnetization, simultaneously. A torque magnetometer having the torque sensitivity of $10^{-6}$ dyn cm could give the extreamly high sensitivity of $10^{-9}$ emu in measuring the magnetic moment, which is high enough to measure the magnetic moment of a monolayer Ni film. The accuracy of the present method was negligibly affected even at the applied fields near the coercivity where magnetization was inhomogeneous.

• Progress in Superconductivity and Cryogenics
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• v.23 no.1
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• pp.12-16
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• 2021

The magnetic properties of heat treated O-series high temperature superconducting (HTS) GdBCO coated conductor (CC) tapes which were formed of Ag/GdBCO/Buffer-layers/Stainless Steel (SS), were investigated by employing a Quantum Design PPMS-14. Using a modified Bean model, the critical current density J_c values have been estimated from the 𝚫m_irr(H) data, which are obtained by measuring the magnetic moment m(H) loops. For a range of intermediate fields, which are interacting or collective flux pinning area, the magnetic flux behaviors were investigated from the relationship J_c ∝ H^-β. In addition, the changes of irreversibility magnetic field H_irr line of heat-treated O-series HTS GdBCO CC tapes were analyzed, according as the annealing temperature under oxygen flowing increases. Both weak and strong break-downs were found by examining the changes of irreversibility magnetic field H_irr lines.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.15 no.2
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• pp.65-70
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• 2015

In this paper, the solutions of TE (transverse electric) scattering problems by a conductive strip grating over grounded two dielectric layers are analyzed by applying the PMM (point matching method) known as a numerical method of electromagnetic fileld. The boundary conditions are applied to obtain the unknown field coefficients, the scattered electromagnetic fields are expanded in a series of Floquet mode functions, and the conductive boundary condition apply to analysis of conducting strip. The most normalized reflected powers of the sharp variations in minimum values are scattered in direction of the other angles except incident angle. The numerical results for normalized reflected power are analyzed by according as the width and spacing of conductive strip, the relative permittivity and thickness of the two dielectric layers, and incident angles. The numerical results of present numericl analysis are shown in good agreement compared to those of the existing papers using FGMM (fourier galerkin moment method).

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.6
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• pp.4158-4163
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• 2015

In this paper, the solutions of TE(transverse electric) scattering problems by a condutive strip grating over a dielectric layer are analyzed by using the FGMM(fourier galerkin moment method) and PMM(point matching method) known as a numerical method of electromagnetic fileld. The scattered electromagnetic fields are expanded in a series of floguet mode functions, the boundary conditions are applied to obtain the unknown field coefficients, and the conductive boundary condition is used for the relationship between the tangential electric field and the induced surface current density on the strip. The numerical results for the reflected and transmitted power of zeroth mode analyzed by according as the width and spacing of conductive strip, the relative permittivity and thickness of dielectric layer, and incident angles. Generally, according to the relative permittivity of dielectric layer increased, also the normalized reflected power of zeroth mode increased. To examine the accruacy of this paper, the numerical results of FGMM shown in good agreement compared to those of PMM.

• The korean journal of orthodontics
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• v.35 no.5 s.112
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• pp.361-370
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• 2005

Fiber reinforced composite (FRC) has been widely used in operative and prosthetic fields of dentistry and its use is expanding into the orthodontic field. The purpose of this study was to examine the changes of flexural properties of FRC reinforced with silica glass fiber (FibreKor, Jeneric/Pentron Inc.. Wallingford. U.S.A.) according to the duration of water absorption. Specimens were grouped according to their shape as round and rectangular cross sections, and were immersed in distilled water at room temperature $(23^{\circ}C)$ for 0 hour 1 hour 1 week. 15 days, 1 mouth and 3 mouths. The number of specimens was 5 for each duration and bending test was done using a torque tester The flexural stiffness after 24 hour water immersion was reduced to 59% for round specimens and 25% for rectangular specimens and after 3 mouths of water immersion it was reduced to 29% and 19% stiffness of the 0 hour-specimen respectively Yield flexural moment after 24 hour water immersion was reduced to 45%for round specimens and 76% for rectangular specimens and after 3 months of water immersion it was reduced to 29% and 60% stiffness of the 0 hour-specimen respectively Ultimate flexural moment after 24 hour water immersion was reduced to 35% for round specimens and 76% for rectangular specimens and after 3 mouths of water immersion it was reduced to 25% and 37% stiffness of 0 hour-specimen respectively. Those results suggested that the flexural stiffness of FibreKor decreased greatly after initial water immersion. Consequently, further research for the maintenance of strength against water will be necessary

• Journal of Electrical Engineering and Technology
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• v.6 no.6
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• pp.799-805
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• 2011

This study presents three different magnetization models for identifying unknown magnetization of the ferromagnetic thin plate of a ship. First, the forward problem should be solved to accurately predict outboard magnetic fields due to the magnetization distribution estimated at a certain time. To achieve this, three different modeling methods for representing remanent magnetization (i.e., magnetic charge method, magnetic dipole array method, and magnetic moment method) were utilized. Material sensitivity formulas containing the first-order gradient information of an objective function were then adopted for an efficient search of an optimum magnetization distribution on the hull. The validity of the proposed methods was tested with a scale model ship, and field signals predicted from the three different models were thoroughly investigated with reference to the experimental data.

• The Proceeding of the Korean Institute of Electromagnetic Engineering and Science
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• v.6 no.3
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• pp.68-74
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• 1995

In this paper, E-polarized electromagnetic scattering by a conducting strip grating with 2 dielectric layers is analyzed to calculate the geometrical reflected and transmitted power by applying the Fourier-Galerkin moment mothed. The induced current density is expanded in a Fourier series using a simple exponential func- tion, and we applied the boundary conditions to the electromagnetic fields at the boundary planes. The sca- ttered electromagnetic fields are expanded in a series of Floquet mode functions. To examine the accuracy of the present method, the geometrically reflected and transmitted power are evaluated and compared with those of the existing papers, and then the numerical results are found in good agreement with those of the existing paper.

• Advances in aircraft and spacecraft science
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• v.8 no.1
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• pp.53-67
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• 2021

The increasing interest in the exploration of Mars stimulated the authors to study aerodynamic problems linked to space vehicles. The aim of this paper is to evaluate the aerodynamic effects of a flapped wing in collaborating with parachutes and retro-rockets to reduce velocity and with thrusters to control the spacecraft attitude. 3-D computations on a preliminary configuration of a blunt-cylinder, provided with flapped fins, quantified the beneficial influence of the fins. The present paper is focused on Aerodynamics of a wing section (NACA-0010) provided with a trailing edge flap. The influence of the flap deflection was evaluated by the increments of aerodynamic force and leading edge pitching moment coefficients with respect to the coefficients in clean configuration. The study was carried out by means of two Direct Simulation Monte Carlo (DSMC) codes (DS2V/3V solving 2-D/3-D flow fields, respectively). A DSMC code is indispensable to simulate complex flow fields on a wing generated by Shock Wave-Shock Wave Interaction (SWSWI) due to the flap deflection. The flap angle has to be a compromise between the aerodynamic effectiveness and the increases of aerodynamic load and heat flux on the wing section lower surface.