• Wind and Structures
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• 제7권4호
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• pp.251-264
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• 2004

The effects of a class of nonlinear Tuned Mass Dampers on the aeroelastic behavior of SDOF systems are investigated. Unlike classical linear TMDs, nonlinear constitutive laws of the internal damping acting between the primary oscillator and the TMD are considered, while the elastic properties are keept linear. The perturbative Multiple Scale Method is applied to derive a set of bifurcation equations in the amplitude and phase and a parametric analysis is performed to describe the postcritical scenario of the system. Both cubic- and van der Pol-type dampings are considered and the dependence of the limit-cycle amplitudes on the system parameters is studied. These new results, compared with the previously obtained bifurcation scenario of a SDOF aeroelastic oscillator equipped with a linear TMD, show a detrimental effect on the maximum limit-cycle amplitude reduction of the nonlinear TMD. However, the analyses evidence that in the parameter region away from the perfect tuning condition the nonlinear connection can be used to tune the system with an enhancement of the limit-cycle amplitude reduction.

• Structural Engineering and Mechanics
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• 제66권6호
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• pp.729-736
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• 2018

This paper studies thermal buckling and post-buckling behaviors of functionally graded materials (FGM) tubes subjected to a uniform temperature rise and resting on elastic foundations via a refined beam model. Compared to the Timoshenko beam theory, the number of unknowns of this model are the same and no correction factors are required. The material properties of the FGM tube vary continuously in the radial direction according to a power function. Two ends of the tube are assumed to be simply supported and in-plane boundary conditions are immovable. Energy variation principle is employed to establish the governing equations. A two-step perturbation method is adopted to determine the critical thermal buckling loads and post-buckling paths of the tubes with arbitrary radial non-homogeneity. Through detailed parametric studies, it can be found that the tube has much higher buckling temperature and post-buckling strength when it is supported by an elastic foundation.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 2005년도 ICCAS
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• pp.265-270
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• 2005

This paper presents a new attitude stabilization and control of an unmanned helicopter based on neural network compensation. A systematic derivation on the dynamics of an unmanned small-scale helicopter is performed. Combined rotor-fuselage-tail dynamics is derived in body-fixed reference frame with its origin at the C.G. of the helicopter. And the resulting nonlinear equation of motion consists of 6-DOF air vehicle dynamics as well as the rotor flapping and engine torque equations. A simulation model was modified using the existing simulator for an unmanned helicopter dynamic model, which reflects the unmanned test helicopter(CNUHELI). The dynamic response of the refined model was compared with the flight test data. It can be shown that a good coincidence was accomplished between the real unmanned helicopter system and the mathematical model. This dynamic model was linearized for classical controller design using small perturbation method. A Neuro-PD control system was designed for both longitudinal and lateral flight modes, and the results were compared with the PD-only control response. Simulation results show that the proposed Neuro-PD control system demonstrates better performance.

• International Journal of Control, Automation, and Systems
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• 제5권5호
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• pp.547-558
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• 2007

This paper proposes a simpler solution to the stabilization problem of a special class of nonlinear underactuated mechanical systems which includes widely studied benchmark systems like Inertia Wheel Pendulum, TORA and Acrobot. Complex internal dynamics and lack of exact feedback linearizibility of these systems makes design of control law a challenging task. Stabilization of these systems has been achieved using Energy Shaping and damping injection and Backstepping technique. Former results in hybrid or switching architectures that make stability analysis complicated whereas use of backstepping some times requires closed form explicit solutions of highly nonlinear equations resulting from partial feedback linearization. It also exhibits the phenomenon of explosions of terms resulting in a highly complicated control law. Exploiting recently introduced Dynamic Surface Control technique and using control Lyapunov function method, a novel nonlinear controller design is presented as a solution to these problems. The stability of the closed loop system is analyzed by exploiting its two-time scale nature and applying concepts from Singular Perturbation Theory. The design procedure is shown to be simpler and more intuitive than existing designs. Design has been applied to important benchmark systems belonging to the class demonstrating controller design simplicity. Advantages over conventional Energy Shaping and Backstepping controllers are analyzed theoretically and performance is verified using numerical simulations.

• 한국환경과학회지
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• 제7권3호
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• pp.349-360
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• 1998

Effects of uniform flow on a two-dimensional mesoscale horizontal convection were investigated by using the vorticity and thermodynamic equations. For thins purpose, We simulated properties of a thermal convection m a stably stratified Boussinesq flued caused by partial heating at the center of a lower boundary If we don't consider effects of the uniform flow, the convection takes the form of aidsymmetrlc with respect to the z-alds. But when uniform flow Is strong, velocity field and temperature field consist of a sin91e cell structure which spreads upstream side of the partial heating area. The flow pattern for strong uniform flows takes the form of positive temperatue near the ground and negative temperature perturbation soft over the partial heating area, and downward motion directly over the upwind portion of the partial heating area and upward motion on the downstream side. The downstream edge of the upstream cell Is shifted in the downstream direction with the Increase of uniform flow almost linearly.

• ETRI Journal
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• 제21권4호
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• pp.65-82
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• 1999

We present a calculation model for an improved quantitative theoretical analysis of electronic and optical properties of strained-piezoelectric[111] InGaAs/GaAs superlattices (SLs). The model includes a full band-coupling between the four important energy bands: conduction, heavy, light, and spin split-off valence bands. The interactions between these and higher lying bands are treated by the k ${\cdot}$ p perturbation method. The model takes into account the differences in the band and strain parameters of constituent materials of the heterostructures by transforming it into an SL potential in the larger band-gap material region. It self-consistently solves an $8{\times}8$ effective-mass $Schr{\ddot{o}}dinger$ equation and the Hartree and exchange-correlation potential equations through the variational procedure proposed recently by the present first author and applied to calculate optical matrix elements and spontaneous emission rates. The model can be used to further elucidate the recent theoretical results and experimental observations of interesting properties of this type of quantum well and SL structures, including screening of piezoelectric field and its resultant optical nonlinearities for use in optoelectronic devices.

• 대한기계학회논문집
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• 제14권6호
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• pp.1487-1494
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• 1990

본 연구에서는 공기분자간 평균 자유비형거리(molecular mean free path)를 고려한 수정된 레이놀즈 방정식을 공기막 두께의 미소 교란항에 대하여 전개하여 비선 형 정적 평형방정식과 교란 미분방ㄹ정식을 구하였다. 비선형 정적 평형방정식을 슬 라이더의 정량적인 거동형태를 표시하므로 이를 이용하여 슬라이더의 정적특성을 구할 수 있다. 이에 반하여, 동적 교란미분 방정식은 슬라이더의 간극함수에 대한 각종 교란에 의하여 유발되는 반발압력을 정성적으로 나타내므로, 슬라이더의 외부교란에 대한 응답특성 및 자기복원특성 등을 구할 수 있다. 이러한 특성을 서스펜션에 부착 된 헤드 시스템의 운동방정식에 함께 고려하여 시스템의 동적 특성을 해석하고 슬라이 더의 설계변수가 이에 미치는 영향을 고찰하고저 한다.

• Nuclear Engineering and Technology
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• 제44권1호
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• pp.43-52
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• 2012

To compute a permeability coefficient along a rough fracture that takes into account the fracture geometry, this study performed detailed measurements of fracture roughness using a confocal laser scanning microscope, a quantitative analysis of roughness using a spectral analysis, and a homogenization analysis to calculate the permeability coefficient on the microand macro-scale. The homogenization analysis is a type of perturbation theory that characterizes the behavior of microscopically inhomogeneous material with a periodic boundary condition in the microstructure. Therefore, it is possible to analyze accurate permeability characteristics that are represented by the local effect of the facture geometry. The Cpermeability coefficients that are calculated using the homogenization analysis for each rough fracture model exhibit an irregular distribution and do not follow the relationship of the cubic law. This distribution suggests that the permeability characteristics strongly depend on the geometric conditions of the fractures, such as the roughness and the aperture variation. The homogenization analysis may allow us to produce more accurate results than are possible with the preexisting equations for calculating permeability.

• 수산해양기술연구
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• 제34권3호
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• pp.328-338
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• 1998

It is main objective of this approach to present a method to analyse stochastic design sensitivity for problems of structural dynamics with randomness in design parameters. A combination of the adjoint variable approach and the second oder perturbation method is used in the finite element approach. An alternative form of the constant functional that holds for all times is introduced to consider the time response of dynamic sensitivity. The terminal problem of the adjoint system is solved using equivalent homogeneous equations excited by initial velocities. The numerical procedures are shown to be much more efficient when based on the fold superposition method : the generalized co-ordinates are normalized and the correlated random variables are transformed to uncorrelated variables, where as the secularities are eliminated by the fast Fourier transform of complex valued sequences. Numerical algorithms have been worked out and proved to be accurate and efficient : they codes whose element derivative matrices can be explicitly generated. The numerical results of two cases - 2-dimensional portal frame and 3/4-cylindrical shell structure - for the deterministic and stochastic sensitivity analysis illustrates in this paper.

• 물과 미래
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• 제24권2호
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• pp.81-95
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• 1991

본 연구는 조석과 하천류의 상호작용을 살펴보기 위해 지배방정식을 섭동근사댑을 이용하여 해석 해를 구하였다. 또한 여기에 이류항과 비선형 마찰항을 포함 하였다. 하천류와 조석분조들과의 복잡한 상호작용을 각각의 분조와의 상호작용으로 단순화 시키기 위해 조화분석을 실시하였다 조석의 무차원 매개변수, 특히 감쇄율을 각각의 해에 대하여 구하였다. 평가결과 해석해로부터 구한 감쇄율은 조화분석으로부터 구한 측정치보다 낮게 평가되어 그들의 상관관계를 설정하여 보정 하였다. 해석해를 금강의 감조구간에 적용하여 관측치와 비교한 결과 잘 일치하였다.