• Journal of Mechanical Science and Technology
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• 제17권12호
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• pp.2066-2077
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• 2003

The time-dependent behavior of unsteady condensation of moist air through the Ludwieg tube is investigated by using a computational fluid dynamics (CFD) work. The two-dimensional, compressible, Navier-Stokes equations, fully coupled with the condensate droplet growth equations, are numerically solved by a third-order MUSCL type TVD finite-difference scheme, with a second-order fractional time step. Baldwin-Lomax turbulence model is employed to close the governing equations. The predicted results are compared with the previous experiments using the Ludwieg tube with a diaphragm downstream. The present computations represent the experimental flows well. The time-dependent unsteady condensation characteristics are discussed based upon the present predicted results. The results obtained clearly show that for an initial relative humidity below 30% there is no periodic oscillation of the condensation shock wave, but for an initial relative humidity over 40% the periodic excursions of the condensation shock occurs in the Ludwieg tube, and the frequency increases with the initial relative humidity. It is also found that total pressure loss due to unsteady condensation in the Ludwieg tube should not be ignored even for a very low initial relative humidity and it results from the periodic excursions of the condensation shock wave.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1990년도 하계학술대회 논문집
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• pp.11-14
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• 1990

The Complex Harmonic Balance Finite Element Method CHBFEM ) is dicussed for the time - periodic magnetic field with saturation characteristics. And Jw - method which is used for analyzing liner system with sinusoidal voltage input can be generalized in nonlinear time-periodic magnetio field system. The CHBFEM enables us to calculate the each harmonic magnetic flux ditribution and the distortion of currents resulting from material at an AC voltage source and to save calculating time, the number of calculation and computer memory.

• 대한기계학회논문집
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• 제18권4호
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• pp.1031-1038
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• 1994

Characteristics of pulsatile flow and heat transfer have been studied numerically in the constant heat flux curved tube with periodic pressure gradient. As the Womersley number increases, the phase difference between the pressure gradient and the cross section averaged axial velocity becomes larger. In case of the Womersley number $\beta = 2$, when cross section averaged axial velocity reaches periodic state with time, the reverse and the natural flow coexist at phase angle, $\lambda = 1.44\pi$ and $\lambda =1.96\pi$. For all the Womersley numbers of present investigation, the time variation of wall temperature near inner wall is higher than that of near outer wall, independent of phase angle.

• 제어로봇시스템학회논문지
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• 제5권6호
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• pp.734-743
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• 1999

In this paper, a real-time communication method using a PICNET-NP(Plant instrumentation and Control Network for Nuclear Power plant) is proposed with an analysis of the control network requirements of DCS(Distributed Control System) in nuclear power plants. The method satisfies deadline in case of worst data traffics by considering aperiodic and periodic real-time data and others. In addition, the method was used to analyze the data characteristics of the DCS in existing nuclear power plant. The result shows that use of this method meets the response time requirement(100ms).

• 한국해양공학회지
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• 제10권3호
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• pp.50-61
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• 1996

The breaking phenomenon of regular periodic waves generated by a numerical wave maker is simulated by finite-difference method which can cope with strong interface motions. The air and water flows are simultaneously solved in the time-marching solution procedure for the Navier-Stokes equation. A density-function technique is devised for the implemenation of the interface conditions. The accuracy is examined and applied to the simulation of two-dimensional breaking phenomena of periodic gravity waves.

• 품질경영학회지
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• 제20권2호
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• pp.33-43
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• 1992

Items are assumed to fail by degradation. An appropriate stochastic model of such item is a cumulative process in which an item can fail only when the total amount of wear exceeds a prespecified failure level. This paper presents replacement policy in which an item is replaced at a certain level of wear before failure or at failure, whichever occurs first. Yet, when measuring the item wear level is very expensive, destructive or time-consuming, it may be economical to use substitutive characteristics that are correlated with the item wear level and relatively inexpensive to measure. The item's wear level could usually be estimated by monitoring such substitutive characteristics only except for a breakdown, which may be observed immediately at its occurrence. The purpose of this paper is to find an optimal periodic replacement policy based on such substitutive characteristics that balance the cost of replacement with the cost of failure and result in a minimum total long-run average cost per unit time. The optimal level of substitutive characteristics to replace the item is obtained. Numerical example illustrate how the model can be used to determine the optimal replacement policy.

• 한국산학기술학회논문지
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• 제21권4호
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• pp.446-451
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• 2020

전열관군으로 이루어진 열교환기에서 유동에 따른 진동은 전열 관군 배관의 파손을 유발할 수 있어서 열교환기에서 유동 유발 진동 특성을 규명할 필요가 있다. 본 연구는 전열 관군의 원관에서 입구의 유속이 일정한 경우와 주기적인 변동이 있는 경우에 대하여 시간에 따라 전열 관군 원관 1, 10 그리고 마지막 19번 원관에서 와류의 시간 변동 특성을 살펴보고 양력의 시간 변화 특성과 PSD 특성을 분석하여 전열 관군 원관에서 전방류의 주기적인 속도 변동에 따른 유동 특성을 규명하였다. 일정 입구 유속의 경우는 칼만 와류가 후류에 있는 원관의 유동에 영향을 미치고 있고 후류의 전열관군과 전방의 전열관군에서의 와류는 다소 시간적인 차이를 보여주지만 같은 주기의 칼만 와류를 발생하고 있음을 관찰할 수 있었다. 주기적인 입구 유속의 경우는 전열 관군에서 와류가 강한 유동이 흐르다가 유속이 줄어들 때는 와류가 약한 유동이 흐르는 것이 반복됨을 알 수 있다. 일정 입구 유속의 경우는 양력의 PSD로 살펴본 결과 주파수는 37.25Hz이며 19번 원관의 경우는 18.63Hz와 50Hz 근방에서 주파수가 관찰 되었다. 주기적인 입구 유속의 경우는 37.25Hz와 속도 주기인 18.63Hz에서 주된 주파수 특성을 보여주었다. 마지막 원관인 19번 원관은 20Hz에서 50Hz 사이에서 많은 피크 주파수를 관찰할 수 있었다.

• 동력기계공학회지
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• 제3권3호
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• pp.14-21
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• 1999

The wave nature of heat conduction has been developed in situations involving extreme thermal gradients, very short times, or temperatures near absolute zero. Under the excitation of a periodic surface heating in a finite medium, the hyperbolic and parabolic heat conduction equations and the damped wave equations in heat flux are presented for comparative analysis by using the Green's function with the integral transform technique. The Kummer transformation is also utilized to accelerate the rate of convergence of these solutions. On the other hand, the temperature distributions are obtained through integration of the energy conservation law with respect to time. For hyperbolic heat conduction, the heat flux distribution does not exist throughout all the region in a finite medium within the range of very short times(${\xi}<{\eta}_l$). It is shown that due to the thermal relaxation time, the hyperbolic heat conduction equation has thermal wave characteristics as the damped wave equation has wave nature.

• 한국정보전자통신기술학회논문지
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• 제10권1호
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• pp.77-84
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• 2017

회전하는 기계시스템에서는 일반적으로 시변 진동신호가 발생되며, 회전기에 고장이 있는 경우 이 신호에 잡음이 포함된다. 본 논문에서는 잡음이 포함된 시변 진동신호를 분해하기 위한 적응예측기와 이진트리구조 필터뱅크로 구성되는 시스템을 제안한다. 그리고 이 시스템에서 분석된 진동신호 결과를 회전기 고장진단에 활용할 수 있음을 보인다. 제안 시스템 적응 예측기는 주기신호성분을 예측하며, 필터뱅크 시스템에서는 입력신호와 예측된 주기신호와의 차 신호를 부밴드 대역으로 분해한다. 각 부밴드 신호에는 고장에 따른 잡음 신호성분이 포함되므로, 이를 이용 회전기의 고장진단이 가능하다. 제안한 신동신호분석 방법의 타당성을 시뮬레이션에서 보이며, 시뮬레이션에서는 주기신호 성분이 제거된 진동신호를 32 부밴드로 분해하여 고장관련 신호 특성을 분석한다.

• Smart Structures and Systems
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• 제23권6호
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• pp.641-651
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• 2019

The stochastic stability control of the parameter-excited vibration of an inclined stay cable with multiple modes coupling under random and periodic combined support disturbances is studied by using the direct eigenvalue analysis approach based on the response moment stability, Floquet theorem, Fourier series and matrix eigenvalue analysis. The differential equation with time-varying parameters for the transverse vibration of the inclined cable with control under random and deterministic support disturbances is derived and converted into the randomly and deterministically parameter-excited multi-degree-of-freedom vibration equations. As the stochastic stability of the parameter-excited vibration is mainly determined by the characteristics of perturbation moment, the differential equation with only deterministic parameters for the perturbation second moment is derived based on the $It{\hat{o}}$ stochastic differential rule. The stochastically and deterministically parameter-excited vibration stability is then determined by the deterministic parameter-varying response moment stability. Based on the Floquet theorem, expanding the periodic parameters of the perturbation moment equation and the periodic component of the characteristic perturbation moment expression into the Fourier series yields the eigenvalue equation which determines the perturbation moment behavior. Thus the stochastic stability of the parameter-excited cable vibration under the random and periodic combined support disturbances is determined directly by the matrix eigenvalues. The direct eigenvalue analysis approach is applicable to the stochastic stability of the control cable with multiple modes coupling under various periodic and/or random support disturbances. Numerical results illustrate that the multiple cable modes need to be considered for the stochastic stability of the parameter-excited cable vibration under the random and periodic support disturbances, and the increase of the control damping rather than control stiffness can greatly enhance the stochastic stability of the parameter-excited cable vibration including the frequency width increase of the periodic disturbance and the critical value increase of the random disturbance amplitude.