• 대한기계학회논문집B
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• 제30권7호
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• pp.656-662
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• 2006

The effects of concave hemispherical surface with inclined angle on the local heat transfer from a turbulent round jet impinging were experimentally investigated using transient liquid crystal method. This method suddenly exposes a preheated wall to an impinging jet and then the video system records the response of liquid crystals for the measurement of the surface temperature. The Reynolds numbers were used 11000, 23000 and 50000, nozzle-to-surface distance ratio from 2 to 10 and the surface angles $\alpha=0^{\circ},\;15^{\circ},\;30^{\circ}\;and\;40^{\circ}$. Correlations of the stagnation point Nusselt number according to Reynolds number, jet-to-surface distance ratio and dimensionless surface angle are investigated. In the stagnation point, in term of $Re^n$, n ranges from 0.43 in case of $2{\leq}L/d\leq6$ to 0.45 in case of $6. The maximum Nusselt number occurs in the direction of upstream. The displacement of the maximum Nusselt number from the stagnation point increases with increasing surface angle or decreasing nozzle-to-surface distance. The maximum displacement is about 0.7 times of the jet nozzle diameter.

• 한국전자파학회논문지
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• 제14권5호
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• pp.458-465
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• 2003

본 논문에서는 3차원 유전체로부터의 전자기 과도 응답을 해석하기 위하여 시간 영역 전장 적분방정식을 이용한 새로운 해법을 제안한다. 이를 위하여 공간 및 시간 시험 과정으로 분리한 갤러킨 방법을 적용한다. 3차원임의 형태의 유전체 표면을 삼각형으로 분할한 다음, 공간에 대한 등가 전류의 전개 및 시험 함수로서 삼각형 벡터 함수를 사용한다. 시간 영역의 미지 계수를 라게르 함수로부터 유도된 기저함수로 근사하며, 이 함수를 시간 영역의 시험 함수로도 사용한다 제안된 방법에 의하여 계산된 등가 전류 및 원거리장의 수치 결과들을 제시한다.

• 한국전산유체공학회지
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• 제7권3호
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• pp.35-43
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• 2002

This paper addresses a numerical method for predicting transient temperature distributions in the wall of a curved pipe subjected to internal laminar thermally-stratified flow. A simple and convenient numerical method of treating the unsteady conjugate heat transfer in non-orthogonal coordinate systems is presented. Numerical calculations are performed for the transient evolution of thermal stratification in two curved pipes, where one has thick wall and the other has so thin wall that its presence can be negligible in the heat transfer analysis. The predicted results show that the thermally stratified flow and transient conjugate heat transfer in a curved pipe with a finite wall thickness can be satisfactorily analyzed by the present numerical method, and that the neglect of wall thickness in the prediction of pipe wall temperature distributions can provide unacceptably distorted results for the cases of pipes with thick wall such as safety related-piping systems of nuclear power plant.

• Journal of Electrical Engineering and Technology
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• 제11권1호
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• pp.135-142
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• 2016

Generally, closed slots are adopted to reduce the water friction loss in both the stator and the rotor of water filling submersible motor due to the special environment of operation. One of the obvious differences between the traditional induction motors and water filling submersible motors is that the submersible motors only need relatively smaller starting torque. This paper aims to analyze the slot leakage reactance of water filling submersible motor during starting transient operation. An improved analytical method which considered the magnetic saturation of the slot bridge and the skin effect of rotor bars is proposed. The slot permeance factor which has a direct impact on the slot leakage reactance is calculated. Then finite element models with different stator slot types are constructed and search coils are introduced to measure the slot flux linkage. Moreover, the starting performances of the models with two typical stator slots are compared and the flux leakage characteristics are obtained. Finally, the results obtained by finite element method are very close to the results obtained by analytical method.

• 전산구조공학
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• 제10권3호
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• pp.217-223
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• 1997

본 논문은 집중 질량 구조물의 천이응답에 대한 시간영역 민감도 해석의 기본 개념을 설명한다. 외부 가진에 따른 구조물의 응답에 미치는 설계변수 변화의 영향을 구하기 위해 시간영역 민감도 함수를 구하는 방법을 제시하였다. 시간영역에서 구조물의 설계변수 민감도는 1차 표준 민감도 함수와 백분율 민감도 함수를 통해 확인하였다. 이러한 민감도 함수와 그 계산은 설계변수에 대한 시스템 상태변수의 편미분에 의한 것이다. 또한, 직접 미분법에 의한 해석적 방법의 편미분 결과와 수치적 방법에 의한 결과를 비교하였다.

• 한국전자파학회논문지
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• 제14권10호
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• pp.1096-1103
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• 2003

본 논문에서는 시간영역 PMCHW 적분방정식을 이용하여 3차원 유전체로부터의 전자기 과도 응답을 해석하기 위한 새로운 해법을 제안한다. 이를 위하여 공간 및 시간 시험 과정으로 분리한 갤러킨 방법을 적용한다. 3차원 임의 형태의 유전체 표면을 삼각형으로 분할한 다음, 공간에 대한 등가전류의 전개 및 시험 함수로서 삼각형 벡터 함수를 사용한다. 시간영역의 미지 계수를 라게르 함수로부터 유도된 기저함수로 근사하며, 이 함수를 시간영역의 시험함수로도 사용한다. 제안된 방법에 의하여 계산된 유전체의 등가전류 및 원거리장의 수치 결과들을 보인다.

• 대한기계학회논문집B
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• 제22권3호
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• pp.373-385
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• 1998

The local Nusselt numbers have been measured for a round turbulent jet impinging on the convex surface with and without rib. Liquid crystal/transient method was used to determine the Nusselt number distributions along the surface. The temperature on the surface was measured to within .+-.0.25 deg. C accuracy using liquid crystal and a digital color image processing system. The experiments were made for the jet Reynolds number (Re) 23,000, the dimensionless nozzle-to-surface distance (L/d) from 6 to 10, the dimensionless surface curvature (d/D) 0.056, and the various rib types (height(d$_{1}$) from 1 to 2 mm, pitch (p) from 6 to 32 mm). It was found that the average Nusselt numbers on the convex surface with rib are higher than those without rib, mainly due to an increase in the turbulent intensity caused by flow separation, recirculation and reattachment on the wall surface. In addition, we compared the results by the steady-state method using the gold-film Intrex with those by the transient method.

• Structural Engineering and Mechanics
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• 제31권2호
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• pp.211-223
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• 2009

Transient response analysis can be conducted either in the time domain, or via the frequency domain. Sometimes a frequency domain method (FDM) has advantages over a time domain method. A practical issue in the FDM is to find out an appropriate extended period, which may be affected by several factors, such as the excitation duration, the system damping, the artificial damping, the period of interest, etc. In this report, the extended period of the FDM based on the Duhamel's integral is investigated. This Duhamel's integral based FDM does not involve the unit impulse response function (UIRF) beyond the period of interest. Due to this fact, the ever-lasting UIRF can be simply set as zero beyond the period of interest to shorten the extended period. As a result, the preferred extended period is the summation of the period of interest and the excitation duration. This conclusion is validated by numerical examples. If the extended period is too short, then the front portion of the period of interest is more prone to errors than the rear portion, but the free vibration segment is free of the wraparound error.

• KIEE International Transactions on Power Engineering
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• 제11A권4호
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• pp.15-20
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• 2001

Power system stability is correlated with system structure, disturbances and operating conditions, and power flows on transmission lines are closely related with those conditions. This paper proposes a methodology to identify correlative power flows for power system transient and small-signal stability prediction. In transient stability sense, the Critical Clearing Time is used to select some dominant contingencies, and Transient Stability Prediction index is proposed for the quantitative comparison. For small-signal stability discusses a methodology to identify crucial transmission lines for stability prediction by introducing a sensitivity factor based on eigenvalue sensitivity technique. On-line monitoring of the selected lines enables to predict system stability in real-time. Also, a procedure to make a priority list of monitored transmission lines is proposed. The procedure is applied to a test system, and it shows capabilities of the proposed method.

• 대한기계학회논문집A
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• 제21권1호
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• pp.133-143
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• 1997

Piezoelastic materials have recently attracted considerable attention because of their potential use in intelligent structural systems. In this paper, we treat a transient piezothermoelastic problem in a hexagonal plate of crystal class 6mm subjected to antisymmetric heating temperature. We analyze this problem by use of the potential function method. Numerical calculations are carried out for a cadmium selenide solid, and the results are presented graphically in comparison with those derived from the similar problem in a cadmium selenide plate subjected to symmetric heating temperature for a symmetry transient problem.