• Title/Summary/Keyword: divergence instability

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On the development of succesive finite element code for semiconductor devices analysis (유한요소법(有限要素法)에 의한 반도체(半導體) 소자(素子) 해석(解析)의 안정화(安定化)에 관한 연구(硏究))

  • Choi, Kyung
    • Journal of Industrial Technology
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    • v.9
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    • pp.109-117
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    • 1989
  • In the finite element analysis of semiconductor devices analysis, the solution often be diverged due to the numerical instability of discretized equations. To overcome this problems, a noble finite element code which guarantees a successful convergence is developed. The factor of divergence in the current continuity equation of semiconductor governing equations is derived using stability test and an adaptive mesh refine scheme is introduced to eliminates the divergence properties. A test calculation of GaAs MESFET model reveals that the proposed scheme has a robust self-convergence property and is suitable for the semiconductor devices analysis.

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Flow-induced Instability of Multi-wall Carbon Nanotubes for Various Boundary Conditions (경계조건에 따른 다중벽 탄소나노튜브의 유체유발 불안정성 변화)

  • Yun, Kyung-Jae;Song, Oh-Seop
    • Transactions of the Korean Society for Noise and Vibration Engineering
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    • v.20 no.9
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    • pp.805-815
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    • 2010
  • This paper studies the influence of internal moving fluid and flow-induced structural instability of multi-wall carbon nanotubes conveying fluid. Detailed results are demonstrated for the variation of natural frequencies with flow velocity, and the flow-induced divergence and flutter instability characteristics of multi-wall carbon nanotubes conveying fluid and modelled as a thin-walled beam are investigated. Effects of various boundary conditions, Van der Waals forces, and non-classical transverse shear and rotary inertia are incorporated in this study. The governing equations and three different boundary conditions are derived through Hamilton's principle. Numerical analysis is performed by using extended Galerkin's method which enables us to obtain more exact solutions compared with conventional Galerkin's method. This paper also presents the comparison between the characteristics of single-wall and multi-wall carbon nanotubes considering the effect of van der Waals forces. Variations of critical flow velocity for different boundary conditions of two-wall carbon nanotubes are investigated and pertinent conclusion is outlined.

Using Largest Lyapunov Exponent to Confirm the Intrinsic Stability of Boiling Water Reactors

  • Gavilan-Moreno, Carlos J.;Espinosa-Paredes, Gilberto
    • Nuclear Engineering and Technology
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    • v.48 no.2
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    • pp.434-447
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    • 2016
  • The aim of this paper is the study of instability state of boiling water reactors with a method based in largest Lyapunov exponents (LLEs). Detecting the presence of chaos in a dynamical system is an important problem that is solved by measuring the LLE. Lyapunov exponents quantify the exponential divergence of initially close state-space trajectories and estimate the amount of chaos in a system. This method was applied to a set of signals from several nuclear power plant (NPP) reactors under commercial operating conditions that experienced instabilities events, apparently each of a different nature. Laguna Verde and Forsmark NPPs with in-phase instabilities, and Cofrentes NPP with out-of-phases instability. This study presents the results of intrinsic instability in the boiling water reactors of three NPPs. In the analyzed cases the limit cycle was not reached, which implies that the point of equilibrium exerts influence and attraction on system evolution.

Effect of boundary conditions on the stability of beams under conservative and non-conservative forces

  • Marzani, Alessandro;Viola, Erasmo
    • Structural Engineering and Mechanics
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    • v.16 no.2
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    • pp.195-217
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    • 2003
  • This paper, which is an extension of a previous work by Viola et al. (2002), deals with the dynamic stability of beams under a triangularly distributed sub-tangential forces when the effect of an elastically restrained end is taken into account. The sub-tangential forces can be realised by a combination of axial and tangential follower forces, that are conservative and non-conservative forces, respectively. The studied beams become unstable in the form of either flutter or divergence, depending on the degree of non-conservativeness of the distributed sub-tangential forces and the stiffness of the elastically restrained end. A non-conservative parameter ${\alpha}$ is introduced to provide all possible combinations of these forces. Problems of this kind are usually, at least in the first approximation, reduced to the analysis of beams according to the Bernoulli-Euler theory if shear deformability and rotational inertia are negligible. The equation governing the system may be derived from the extended form of Hamilton's principle. The stability maps will be obtained from the eigenvalue analysis in order to define the divergence and flutter domain. The passage from divergence to flutter is associated with a noticeable lowering of the critical load. A number of particular cases can be immediately recovered.

Nonlinear aerodynamic stability analysis of orthotropic membrane structures with large amplitude

  • Zheng, Zhoulian;Xu, Yunping;Liu, Changjiang;He, Xiaoting;Song, Weiju
    • Structural Engineering and Mechanics
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    • v.37 no.4
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    • pp.401-413
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    • 2011
  • The aerodynamic stability of orthotropic tensioned membrane structures with rectangular plane is theoretically studied under the uniform ideal potential flow. The aerodynamic force acting on the membrane surface is determined by the potential flow theory in fluid mechanics and the thin airfoil theory in aerodynamics. Then, based on the large amplitude theory and the D'Alembert's principle, the interaction governing equation of wind-structure is established. Under the circumstances of single mode response, the Bubnov-Galerkin approximate method is applied to transform the complicated interaction equation into a system of second order nonlinear differential equation with constant coefficients. Through judging the stability of the system characteristic equation, the critical divergence instability wind velocity is determined. Finally, from different parametric analysis, we can conclude that it has positive significance to consider the characteristics of orthotropic and large amplitude for preventing the instability destruction of structures.

Structual Stability Analysis According to the Lumped Mass of High Speed Vehicles in Underwater (집중질량 변화에 따른 수중 고속 운동체의 구조 안정성 해석)

  • Oh, Kyung-Won;Sur, Joo-No;Cho, Byung-Gu;Ryu, Si-Ung;Kong, Gong-Duk
    • Journal of Ocean Engineering and Technology
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    • v.23 no.1
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    • pp.54-59
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    • 2009
  • In this paper, the effect of the position and size of a lumped mass on the structural stability of a high speed underwater vehicle is presented. For simplicity, a real vehicle was modeled as a follower force subjected beam that was resting on an elastic foundation, and the lumped mass effect was simplified as an elastic intermediate support. The stability of the simplified model was numerically analyzed based on the Finite element method (FEM). This numerical simulation revealed that flutter type instability or divergence type instability occurs, depending on the position and stiffness of the elastic intermediate support, which implies that the instability of the real model is affected by the position and size of the lumped mass.

Dynamic Instability of Elastically Restrained Beams under Distributed Tangential Forces (분포접선력을 받는 탄성지지된 보의 동적 불안정)

  • 류봉조;김인우;이규섭;임경빈;최봉문
    • Journal of the Korean Society for Precision Engineering
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    • v.15 no.10
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    • pp.140-147
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    • 1998
  • The dynamic behavior of elastically restrained beams under the action of distributed tangential forces is investigated in this paper. The beam, which is fixed at one end, is assumed to rest on an intermediate spring support. The governing equations of motion are derived from the energy expressions, and the finite element formulation is employed to calculate the critical distributed tangential force. Jump phenomena for the critical distributed tangential force and instability types are presented for various spring stiffnesses and support positions. Stability maps are generated by performing parametric studies to show how the distributed tangential forces affect the frequencies and the stability of the system considered. Through the numerical simulations, the following conclusioils are obtained: (i) Only flutter type instability exists for the dimensionless spring stiffness K $\leq$ 97, regardless of the position of the spring support. (ii) For the dimensionless spring stiffness K $\leq$ 98, the transition from flutter to divergence occurs at a certain position of the spring support, and the transition position moves from the free end to the free end of the beam as the spring stiffness increases. (iii) For K $\leq$ 10$^{6}$ the support condition can be regarded as a rigid support condition.

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Influence of Torque Fluctuation on the Stability of a Rotating Disk (토크 하중의 변동이 회전원판의 안정성에 미치는 영향)

  • Shin, Eung-Soo
    • Journal of the Korean Society of Manufacturing Technology Engineers
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    • v.24 no.1
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    • pp.110-116
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    • 2015
  • This study investigates the whirling stability of a rotating shaft-disk system under parametric excitation using periodically varying torque. The equations of motion were derived using a lumped-mass model, and the Floquet method was employed to find the effects of torque fluctuation, internal and external damping, and rotational speed on whirling stability. Results indicated that the effect of torque fluctuation was considerable on the instability around resonance, but minimal on supercritical instability. Stability diagrams were sensitive to the parametric excitation frequency; critical torque decreased upon increasing excitation frequency, with faster response convergence or divergence. In addition, internal and external damping had a considerable effect on unstable regions, and reduced the effects of the parametric excitation frequency on critical torque and speed. Results obtained from the Floquet approach were in good agreement with those obtained by numerical integration, except for some cases with Floquet multipliers very close to unity.

Stability Analysis of Pipe Conveying Fluid with Crack (크랙을 가진 유체유동 파이프의 안정성 해석)

  • Son, In-Soo;Ahn, Tae-Su;Yoon, Han-Ik
    • Transactions of the Korean Society for Noise and Vibration Engineering
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    • v.17 no.1 s.118
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    • pp.10-16
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    • 2007
  • In this paper, the dynamic stability of a cracked simply supported pipe conveying fluid is investigated. In addition, an analysis of the flutter and buckling instability of a cracked pipe conveying fluid due to the coupled mode(modes combined) is presented. Based on the Euler-Bernouli beam theory, the equation of motion can be constructed by using the Galerkin method. The crack section is represented by a local flexibility matrix connecting two undamaged pipe segments. The stiffness of the spring depends on the crack severity and the geometry of the cracked section. The crack is assumed to be in the first mode of fracture and to be always opened during the vibrations. This results of study will contribute to the safety test and a stability estimation of the structures of a cracked pipe conveying fluid.