• Advances in aircraft and spacecraft science
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• v.3 no.4
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• pp.379-397
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• 2016

In this manuscript, free vibrations of a unidirectional composite orthotropic Timoshenko beam based on finite strain have been studied. Using Green-Lagrange strain tensor and comprising all of the nonlinear terms of the tensor and also applying Hamilton's principle, equations of motion and boundary conditions of the beam are obtained. Using separation method in single-harmonic state, time and locative variables are separated from each other and finally, the equations of motion and boundary conditions are gained according to locative variable. To solve the equations, generalized differential quadrature method (GDQM) is applied and then, deflection and cross-section rotation of the beam in linear and nonlinear states are drawn and compared with each other. Also, frequencies of carbon/epoxy and glass/epoxy composite beams for different boundary conditions on the basis of the finite strain are calculated. The calculated frequencies of the nonlinear free vibration of the beam utilizing finite strain assumption for various geometries have been compared to von Karman one.

• Structural Engineering and Mechanics
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• v.41 no.6
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• pp.735-750
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• 2012

This paper presents a comparison between two different procedures to deal with the geometric nonlinear analysis of space trusses, considering its structural stability aspects. The first nonlinear formulation, called positional, uses nodal positions rather than nodal displacements to describe the finite elements kinematics. The strains are computed directly from the proposed position concept, using a Cartesian coordinate system fixed in space. The second formulation, called corotational, is based on the explicit separation between rigid body motion and deformed motion. The numerical examples demonstrate the performances and the convergence of the responses for both analyzed formulations. Two numerical examples were compared, including a lattice beam with postcritical behavior. Despite the two completely different approaches to deal with the geometrical nonlinear problem, the results present good agreement.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.8
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• pp.1097-1104
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• 2003

The current study analyzes Langmuir slip boundary condition theoretically and it is tested in practical numerical analysis for separation-associated flow. Slip phenomenon at the channel wall is properly implemented by various numerical slip boundary conditions including Langmuir slip model. Compressible backward-facing step flow is compared to other analysis results with the purpose of diatomic gas Langmuir slip model validation. The numerical solutions of pressure and velocity distributions where separation occurs are in good agreement with other numerical results. Numerical analysis is conducted for Reynolds number from 10 to 60 for a prediction of separation at T-shaped micro manifold. Reattachment length of flows shows nonlinear distribution at the wall of side branch. The Langmuir slip model predicts fairly the physics in terms of slip effect and separation.

• Transactions on Control, Automation and Systems Engineering
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• v.2 no.2
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• pp.104-109
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• 2000

In this paper, the output feedback stabilizing problem is solved using any given state feedback control law. Compared to the linear systems is not so straightforward for nonlinear systems. We briefly explain the intrinsic obstructions for this problem and provide new output feedback scheme which achieves the semi-global stabilization with the high-gain state observer. THe overall uniform observability of the plant. Therefore, the result can be regarded as an extension of the separation principle for linear systems in some aspect.

• Proceedings of the KIEE Conference
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• 1998.07c
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• pp.976-978
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• 1998

This paper presents the stability proof of a nonlinear feedback linearization-reduced order observer/sliding mode controller (NFL-ROO/SMC). The closed-loop stability is proved by a Lyapunov function candidate using an addition form of the sliding surface vector and the estimation error.

• Steel and Composite Structures
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• v.13 no.3
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• pp.277-293
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• 2012

The interaction between steel tube and concrete core is the key design considerations for concrete-filled steel tube columns. In a concrete-filled steel tube (CFST) column, the steel tube provides confinement to the concrete core which permits the composite action among the steel tube and the concrete. Due to construction faults and plastic shrinkage of concrete, the debonding separation at the steel-concrete interface weakens the confinement effect, and hence affects the behaviour and bearing capacity of the composite member. This study investigates the axial loading behavior of the concrete filled circular steel tube columns with debonding separation. A three-dimensional nonlinear finite element model of CFST composite columns with introduced debonding gap was developed. The results from the finite element analysis captured successfully the experimental behaviours. The calibrated finite element models were then utilized to assess the influence of concrete strength, steel yield stress and the steel-concrete ratio on the debonding behaviour. The findings indicate a likely significant drop in the load carrying capacity with the increase of the size of the debonding gap. A design formula is proposed to reduce the load carrying capacity with the presence of debonding separation.

• Journal of Institute of Control, Robotics and Systems
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• v.10 no.2
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• pp.112-124
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• 2004

A neural network based adaptive reconfigurable flight controller is presented for a class of discrete-time nonlinear flight systems in the presence of variations of aerodynamic coefficients and control effectiveness decrease caused by control surface damage. The proposed adaptive nonlinear controller is developed making use of the backstepping technique for the angle of attack, sideslip angle, and bank angle command following without two time separation assumption. Feedforward multilayer neural networks are implemented to guarantee reconfigurability for control surface damage as well as robustness to the aerodynamic uncertainties. The main feature of the proposed controller is that the adaptive controller is developed under the assumption that all of the nonlinear functions of the discrete-time flight system are not known accurately, whereas most previous works on flight system applications even in continuous time assume that only the nonlinear functions of fast dynamics are unknown. Neural networks learn through the recursive weight update rules that are derived from the discrete-time version of Lyapunov control theory. The boundness of the error states and neural networks weight estimation errors is also investigated by the discrete-time Lyapunov derivatives analysis. To show the effectiveness of the proposed control law, the approach is i]lustrated by applying to the nonlinear dynamic model of the high performance aircraft.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2002.11b
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• pp.914-920
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• 2002

Nonlinear flow-induced vibration characteristics of a generic missile wing (or control surface) are investigated in this study. The wing model has freeplay structural nonlinearity at its pitch axis. Nonlinear aerodynamic flows with unsteady shock waves are considered in the transonic flow region. To practically consider the effects of freeplay structural nonlinearity, the fictitious mass method (FMM) is applied to structural vibration analysis based on a finite element method (FEM). A computational fluid dynamics (CFD) technique is used for computing the nonlinear unsteady aerodynamics of all-movable wings. The aerodynamic analysis is based on the efficient transonic small-disturbance aerodynamic equations of motion using the potential-flow theory. To solve the nonlinear aeroelastic governing equations including the freeplay effect, a modal-based computational structural dynamic (CSD) analysis technique based on fictitious mass method (FMM) is used in time-domain. In addition, CSD and unsteady CFD techniques are simultaneously coupled to give accurate computational results. Various aeroelastic computations have been performed for a generic missile wing model. Linear and nonlinear aeroelastic computations have been conducted and the characteristics of flow-induced vibration are introduced.

• Journal of Institute of Control, Robotics and Systems
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• v.9 no.2
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• pp.99-106
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• 2003

A new reconfigurable model following flight control method based on direct adaptive scheme is presented. Using the timescale separation principle, both the inner-loop and the outer-loop states are controlled simultaneously. For the timescale separation assumption to be satisfied, the inner-loop model dynamics is set to be fast whereas the outer-loop model dynamics is set to be relatively slow. The stability and convergence of the proposed control law is proved by Lyapunov theorem. One of the merits of the proposed reconfigurable controller is that the FDI process and the persistent input excitation are not necessary, which is suitable for the flight control system. To evaluate the reconfiguration performance of the proposed control method, numerical simulation is performed using six degree-of-freedom nonlinear dynamics.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.20 no.7
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• pp.677-684
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• 2010

Backlashes of gears provide gears for good lubrication and for removal of the interference between teeth by the wear and manufacturing errors. The backlash is the strong nonlinear factor to gears. This study deals with nonlinear modeling of helical gears with backlash. Excitation of helical gears comes from torque variation, the tooth surface error, and the periodical change of mesh stiffness. To study the effect of torque fluctuation, equation of motion for the single degree of freedom torsional model of helical gears with the periodical change of mesh stiffness and the backlash was derived. The Newmark beta method and the Newton-Raphson method were used to obtain the nonlinear behaviors of mesh forces of helical gears. All excitation frequencies initially caused the tooth separation and single-sided impacts of the gear pair and eventually led to the normal tooth contact. However, some special excitation frequencies caused the single-sided impacts in the entire time as well as the initial time. Damping increase reduced the duration of single-sided impacts, and the backlash increase caused those in the entire time domain.