• E2M - 전기 전자와 첨단 소재
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• 제10권7호
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• pp.659-667
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• 1997

The influence of SiO$_2$on the microstructure and electrical properties of zinc oxide varistor was investigated. Zn$_2$SiO$_4$third phase in the sintered body was found at grain boundaries, multiple grain junctions, and occasionally within ZnO grains. This phase acted as a grain growth inhibitor, which retard the grain growth of the ZnO matrix by impeding migration on the grain boundaries. As SiO$_2$ addition increases, average grain size decreased from 40.6${\mu}{\textrm}{m}$ to 26.9${\mu}{\textrm}{m}$ due to the pinning effect by Zn$_2$SiO$_4$ and drag effect by Si segregation at grain boundaries, the breakdown voltage consequently increased. When SiO$_2$ addition is increased, interface state density decreased, however, the barrier height increased by decrease of donor concentration, as a result, the nonlinear exponent increased and leakage current decreased. While, as SiO$_2$ addition increase, it was found that the apparent dielectric loss factor shows a tendency of decrease. Wholly, electrical properties of zinc oxide varistor can be said to be improved by SiO$_2$addition.

• Wind and Structures
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• 제23권3호
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• pp.253-273
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• 2016

Ice accretions on stay cables may result in the instable vibration of galloping, which would affect the safety of cable-stayed bridges. A large number of studies have investigated the galloping vibrations of transmission lines. However, the obtained aerodynamics in transmission lines cannot be directly applied to the stay cables on cable-stayed bridges. In this study, linear and nonlinear single degree-of-freedom models were introduced to obtain the critical galloping wind velocity of iced stay cables where the aerodynamic lift and drag coefficients were identified in the wind tunnel tests. Specifically, six ice shapes were discussed using section models with geometric scale 1:1. The results presented obvious sudden decrease regions of the aerodynamic lift coefficient for all six test models. Numerical analyses of iced stay cables associated to a medium-span cable-stayed bridge were carried out to evaluate the potential galloping instability. The obtained nonlinear critical wind velocity for a 243-meter-long stay cable is much lower than the design wind velocity. The calculated linear critical wind velocity is even lower. In addition, numerical analyses demonstrated that increasing structural damping could effectively mitigate the galloping vibrations of iced stay cables.

• Journal of Mechanical Science and Technology
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• 제16권12호
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• pp.1561-1575
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• 2002

A reduced-order linear feedback controller, which is used to control the linear disturbance in two-dimensional plane Poiseuille flow, is applied to a boundary layer flow for stability control. Using model reduction and linear-quadratic-Gaussian/loop-transfer-recovery control synthesis, a distributed controller is designed from the linearized two-dimensional Navier-Stokes equations. This reduced-order controller, requiring only the wall-shear information, is shown to effectively suppress the linear disturbance in boundary layer flow under the uncertainty of Reynolds number. The controller also suppresses the nonlinear disturbance in the boundary layer flow, which would lead to unstable flow regime without control. The flow is relaminarized in the long run. Other effects of the controller on the flow are also discussed.

• 한국해양공학회지
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• 제1권1호
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• pp.49-56
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• 1987

The dynamic behaviour of a marine riser was studied theoretically and experimentally. In linear analysis, the natural frequencies and mode shapes of the riser were obtained from the experiment and they were found to be in good agreement with theoretical results by using a simple asymptotic formula. In nonlinear ananlysis including viscous drag and large displacement, a numerical-perturbation technique based on the derived linear asymptotic solutions is used to predict the displacements and stresses of the riser in harmonic motion. These results were also compared with experimental data and found to be in general in good agreement.

• 한국항공우주학회지
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• 제45권11호
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• pp.967-974
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• 2017

반작용휠은 저속구간에서 마찰로 인해서 비선형 토크 응답을 보인다. 따라서 이 구간에서는 위성의 정밀한 자세제어를 달성하기 어렵다. 기존 연구들은 마찰력 보상이나 디더명령을 인가하는 방법을 사용하여 본 문제를 해결하려 하였다. 하지만 마찰력 모델링의 어려움이나 휠속도의 빈번한 영점 교차 때문에, 이러한 방법을 실제 위성 자세제어에 적용하기에는 어려움이 있다. 이를 해결하기 위해서, 자세오차에 따라서 자세제어기의 이득값을 조절하는 방법을 제안한다.

• 대한토목학회논문집
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• 제5권2호
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• pp.1-9
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• 1985

본 논문에서는 해저석유개발에 사용되는 고정해양구조물의 불규칙파랑하중에 대한 해적기법으로 주파수영역해법과 시간영역해법에 대하여 연구하였다. 주파수영역해법에서는 파동의 비선형 점성저항력을 선형화한 후 파고스펙트럼으로부터 구조물의 응답스펙트럼을 구하였으며, 시간영역해법에서는 불규칙파의 유동속도와 가속도의 시계열을 파고스펙트럼으로부터 Monte Carlo 기법으로 시뮬레이션한 후 이를 이용하여 구조물의 거동을 해석하였다. 수심이 다른 지점에 설치된 2개의 구조물을 예로 택하여, 여러 가지 파고조건에 대한 구조물의 예상최대변위를 구한 후, 두 해법에 의한 결과를 비교분석하였다.

• 한국생산제조학회지
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• 제7권3호
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• pp.29-36
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• 1998

In recent year, as the demand about low vibration and noise vehicle is increased constantly. automobile companies try to a lot of things to achieve this demand. Gear rattle vibration become an emergency problem to be cured at idling. There are two kinds method to reduce idle gear rattle vibration One is optimization of clutch damper design parameters(stiffness, hysteresis torque, preload, length of lst stage) the other is system parameters modification(inertia, drag torque, backlash, etc) But these methods are impossible to estimate influence rate of each gearpair on the idle gear rattle vibration. In this study, 14degrees of freedom nonlinear model is developed to analyze influnce rate of each gearpair on the idle gear rattle vibration and the counterplan to reduce the gear noise is suggeted through the shift system modification.

• 대한조선학회논문집
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• 제42권6호
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• pp.601-607
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• 2005

This paper describes a hydrodynamic modelling study based on the Feldman's equation to predict the nonlinear and coupled maneuvering characteristics of high speed submarine. The hydrodynamic coefficients set is obtained from the modeling of the cross flow drag force and sail induced vorticity, and the captive model experiments(VPMM and RA test) results used to improved the accuracy. The results contained in this paper will be helpful to predict the behavior of tight turn maneuver and to improve the SOE(Safety Operational Envelope) analysis in case of emergency maneuver.

• Wind and Structures
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• 제2권1호
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• pp.41-49
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• 1999

The flutter of a bridge is induced by self-excited force factors such as lift, drag and aerodynamic moment. These factors are associated with flutter derivatives in the analysis of wind engineering. The flutter derivatives are the function of structure configuration, wind velocity and response circular frequency. Therefore, the governing equations for the interaction between the wind and dynamic response of the structure are complicated and highly nonlinear. Herein, a numerical algorithm through graphical technique for the solution of wind at flutter is presented. It provides a concise approach to the solution of wind velocity at flutter.

• 반도체디스플레이기술학회지
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• 제22권4호
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• pp.1-6
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• 2023

In CO₂ laser surface machining of plastic films in modern display manufacturing, scattering of fume particles could be a major source of well-recognized film surface contamination. This computational fluid dynamics research investigates the suction air flow patterns over a film surface as well as the dispersion of micron-sized fume particles with low-Reynolds number particle drag model. The numerical results show the recirculatory flow patterns near laser machining point on film surface and also over the surface of vertical suction slot, which may hinder the efficient removal of fume particles from film surface. The dispersion characteristics of fume particles with various particle size have been tested systematically under different levels of suction flow intensity. It is found that suction removal efficiency of fume particles heavily depends on the particle size in highly nonlinear manners and a higher degree of suction does not always results in more efficient particle removal.