• Journal of KSNVE
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• v.6 no.2
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• pp.233-244
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• 1996

In this paper chaotic mothions of a straight pipe conveying oscillatory flow and being subjected to external forces such as earthquake are theoretically investigated. The nonlinear partial differential equation of motion is derived by Newton's method. In this equation, the nonlinear curvature of the pipe and the thermal expansion effects are contained. The nonlinear ordinary differential equation transformed from that partial differential equation is a type of Hill's equations, which have the parametric and external exciation term. This original system is transfered to the averaged system by the averaging theory. Bifurcation curves of chaotic motion of the piping system are obtained in the general case of the frequency ratio, n by applying Melnikov's method. Numerical simulations are performed to demonstrate theorectical results and show strange attactors of the chaotic motion.

• Journal of Ocean Engineering and Technology
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• v.11 no.4
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• pp.102-110
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• 1997

A two dimensional shallow-water flow around a cavity driven by a sinusoidally oscillating external flow was studied numerically with an Ekman pumping model. A container model of "T" shape was constructed in the numerical computation for comparison with the experimental observation. The material transport in the external region is in good agreement with the experimentally recorded particle trajectories. It turns out that two large coherent vortices situated in the exterior region of the cavity are responsible for clockwise and counterclockwise drift motions, in large scale, of particles. The Ekman pumping model suggested in this study was found to be satisfactory.isfactory.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.16 no.10
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• pp.931-937
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• 2003

The principle of the superconducting vortex flow transistor (SVFT) is based on control of the Abrikosov vortex flowing along a channel. The induced voltage is controlled by a bias current and a control current, instead of external magnetic field. The device is composed of parallel weak links with a nearby current control line. We explained the process to get an I-V characteristic equation and described the method to induce the external and internal magnetic field by the Biot-Savarts law in this paper. The equation can be used to predict the I-V curves for fabricated device. From the equation we demonstrated that the current-voltage characteristics were changed with input parameters. I-V characteristics were simulated to analyze a SVFT with multi-channel by a computer program.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.18 no.7
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• pp.578-586
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• 2006

The present study has been conducted to examine the effect of obstacles around a cooling tower and an air-guide to prevent recirculation. In order to analyze the interaction between external flow and cooling tower exit flow, the external region as well as the cooling, tower are included in computational domain. Two dimensional analysis is performed using the finite volume method with non-orthogonal and unstructured grid system. The standard ${\kappa}-{\varepsilon}$ turbulence model is used. To investigate the recirculation phenomena, flow and temperature fields are calculated with three approaches such as, the distance between cooling tower and obstacle, the allocated geometrical type, and the effect of height of obstacle. In addition, the air-guide is considered in the current computation. The mean recirculation rate increases with the height of obstacle. The effect of air-guide to reduce the mean recirculation rate is obviously observed.

• The Transactions of the Korean Institute of Electrical Engineers
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• v.41 no.5
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• pp.445-458
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• 1992

As one of the forced cooling method of the underground power transmission system, external water cooled system with trough in tunnel was investigated. This study is performed on thermal analysis for a standard condition to determine the cable transmission current of the underground power transmission system about the cooling facility. A parametric study was performed for the inlet water temperatures, flow rates, the inlet air velocities, flow rates and the cooling spans. This study shows that the cable transmission current varies within the allowable limitation in compliance with the variation of inlet water temperatures and flow rates. It exhibits little variations for the most intervals in compliance with the variation of inlet air temperatures and flows. But, the cable transmission current fast reduces for a specified interval and consequently affects the underground transmission system. As a result, when the actual forced cooling system is designed, the design conditions of inlet air have to be considered as the most important parameters in determination of the cable transmission current.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.16 no.5
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• pp.424-428
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• 2003

The channel of a superconducting flux flow transistor has been fabricated with plasma etching method using a inductively coupled plasma etching. The ICP conditions then were ICP Power of 450 W, rf chuck power of 150 W, the pressure in chamber of 5 mTorr, and Ar : Cl$_2$=1:1. Especially, over the 5 mTorr, the superconducting thin films were not etched. The channel etched by plasma gas showed the critical temperature over 85 K. The critical current of the SFFT was altered by varying the external applied current. As the external applied current increased from 0 to 12 mA, the critical current decreased from 28 to 22 mA. Then the obtained trans-resistance value was smaller than 0.1 $\Omega$ at a bias current of 40 mA.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1995.11a
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• pp.69-83
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• 1995

A great number of experimental data indicating shock-induced separation(SIS) in internal or external supersonic flows were reviewed to make clear the mechanism of SIS and to present the criterion of turbulent boundary layer separation. The interesting conclusions were obtained for the considerably wide range of flow geometries that the incipient separation is almost independent of the flow geometries, and that it is relatively unaffected by changes in gas specific heat, and boundary layer Reynolds number, Furthermore, the pressure rise necessary to separate boundary layer in external flows was found to be applicable to SIS in overexpanded propulsion nozzles. This is due to the fact that the SIS phenomenon caused by the interaction between shock waves and turbulent boundary layers is processed through a supersonic deceleration. This is, the SIS in almost all of interacting flow fields is governed by the concept of free interaction, and criterion of SIS is only a Function of upstream Mach number.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.11a
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• pp.427-430
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• 2008

This study considered the heat exchanger of bleed air from engine. The computional fluid analysis was performed considering the external flow and internal flow on heat exchanger. Using the CFD results, the external configuration and internal flow path of heat exchanger were designed. And also the performance test is conducted and the results of tests were compared with the analysis resutls.

• Nuclear Engineering and Technology
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• v.53 no.1
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• pp.322-336
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• 2021

This paper presents a numerical prediction of the transient hydraulic loads acting on the tubes and external supports of a pressurized water reactor (PWR) steam generator (SG) during blowdown following a sudden feedwater line break (FWLB). A simplified SG model was used to easily demonstrate the prediction. The blowdown discharge flow was treated as a flashing flow to realistically simulate the transient flow fields inside the SG and the connected broken feedwater pipe. The effects of the SG initial pressure or the broken feedwater pipe length on the intensities or magnitudes of transient hydraulic loads were investigated. Then predictions of the decompression pressure wave-induced impulsive pressure differential loads on SG tubes and the transient blowdown loads on SG external supports were demonstrated and the general aspects of transient responses of such transient hydraulic loads to the FWLB were discussed.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.19 no.1
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• pp.36-43
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• 2023

Lots of researches have been conducted on in-vessel retention (IVR) to prevent or mitigate severe accident in nuclear power plants. Various methodologies were proposed and the external reactor vessel cooling was selected as a part of promising IVR strategy. In this study, the strategy is strengthened by enhancing the natural circulation performance through the adoption of insulation in the reactor cavity. A thermal analysis was carried out based on an assumed accident scenario and its results were used as boundary conditions for subsequent seven flow analysis cases. By comparing the natural circulation performance, effects of annular gaps and insulation shapes on the mass flow rate and flow velocity were quantified. The improvement in cooling performance can be reflected in actual design via detailed assessment.