Robust Aeroelastic Control of Very Flexible Wings using Intrinsic Models

This paper explores the robust control of large exible wings when their dynamics are written in terms of intrinsic variables, that is, velocities and stress resultants. Assuming 2-D strip theory for the aerodynamics, the resulting nonlinear aeroelastic equations of motion are written in modal coordinates. It is seen that a system which experiences large displacements can nonetheless be accurately described by a system with only weak nonlinear couplings in this description of the wing dynamics. As result, a linear robust controller acting on a control surface is able to effectively provide gust load alleviation and flutter suppression even when the wing structure undergoes large deformations. This is numerically demonstrated on various representative test cases. © 2013 by Yinan Wang, Andrew Wynn and Rafael Palacios

Vibration based performance assessment of concrete-concrete composite bridges

Includes bibliographical references (leaves 142-148).Concrete composites consisting of precast pre-stressed standardized beams and a cast in-situ deck slab have been used for the construction of short to medium span bridges for the past four decades in South Africa and worldwide. The pre-cast beams and cast in-situ slab are commonly connected using shear connectors. Failure of these connectors would compromise the composite action of the structure, thus reducing the load carrying capacity and hence its efficiency. This study seeks to assess the integrity of such shear connectors using dynamic testing and Finite Element (FE) analysis. The main objective of the work is to assess the practicality of vibration-based techniques to detect damaged shear connectors using experimental and analytical modal data. A scaled bridge model was constructed and 10 mm bolts connected the beams and slab to simulate shear connectors in the prototype bridge. Different damage scenarios were introduced by loosening some of the connectors and vibration testing was done to detect the artificial damage. An FE model of the system was also developed. The shear connectors were modelled as non-linear spring elements capable of simulating the composite action between the slab and beams. Damage of shear connectors was simulated by reducing the spring stiffness. The updating of the FE model was done manually by adjusting appropriate spring stiffnesses. The experimental and analytical results show that the natural frequencies are sensitive to this damage. The frequencies dropped from undamaged to severe damaged structure. Very little information was deduced from the damping ratios, modal assurance criteria (MAC) and coordinate modal assurance criteria (COMAC) values. The experimental and analytical first bending, torsion and transverse modes were sensitive to the damaged shear connectors. 65% of damaged connectors were located using these modes. Using experimental modal data, the mode curvatures and flexibility changes were able to locate the damaged region when more than 35% of shear connectors were loosened. However, using numerical data, the mode curvatures and flexibility changes were able to localize the damaged region for 6% damage introduced. 75% of the loosened connectors were identified. The stiffness change technique could only identify less than 10% of damaged shear connectors using experimental modal data. The same technique was applied on analytical data and over 75% of damaged shear connectors were located. The FE modelling of shear connectors used in this work was applied on an existing bridge. Van der Kloof bridge (South Africa) was constructed using precast pre-stressed beams and a cast in-situ slab. Extended beam web stirrups were used as shear connectors. The main aim was to develop a robust FE model for this bridge that could be used in future to investigate the condition of shear connectors. Using 6-0 non-linear spring elements to model the shear connectors, a maximum difference of 0.98% was observed between the measured and theoretical frequencies after manual updating. This is quite a small difference. This model could therefore be used as a true representative of the physical structure for future investigations

The design and development of a method for investigating the properties of materials in plastic torsion

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CEAS/AIAA/ICASE/NASA Langley International Forum on Aeroelasticity and Structural Dynamics 1999

The proceedings of a workshop sponsored by the Confederation of European Aerospace Societies (CEAS), the American Institute of Aeronautics and Astronautics (AIAA), the National Aeronautics and Space Administration (NASA), Washington, D.C., and the Institute for Computer Applications in Science and Engineering (ICASE), Hampton, Virginia, and held in Williamsburg, Virginia June 22-25, 1999 represent a collection of the latest advances in aeroelasticity and structural dynamics from the world community. Research in the areas of unsteady aerodynamics and aeroelasticity, structural modeling and optimization, active control and adaptive structures, landing dynamics, certification and qualification, and validation testing are highlighted in the collection of papers. The wide range of results will lead to advances in the prediction and control of the structural response of aircraft and spacecraft

Micro-Resonators: The Quest for Superior Performance

Microelectromechanical resonators are no longer solely a subject of research in university and government labs; they have found a variety of applications at industrial scale, where their market is predicted to grow steadily. Nevertheless, many barriers to enhance their performance and further spread their application remain to be overcome. In this Special Issue, we will focus our attention to some of the persistent challenges of micro-/nano-resonators such as nonlinearity, temperature stability, acceleration sensitivity, limits of quality factor, and failure modes that require a more in-depth understanding of the physics of vibration at small scale. The goal is to seek innovative solutions that take advantage of unique material properties and original designs to push the performance of micro-resonators beyond what is conventionally achievable. Contributions from academia discussing less-known characteristics of micro-resonators and from industry depicting the challenges of large-scale implementation of resonators are encouraged with the hopes of further stimulating the growth of this field, which is rich with fascinating physics and challenging problems

Modeling, Analysis, and Optimization Issues for Large Space Structures

Topics concerning the modeling, analysis, and optimization of large space structures are discussed including structure-control interaction, structural and structural dynamics modeling, thermal analysis, testing, and design

Multi-physics modelling of a grid connected diesel engine driven synchronous generator set for the analysis of transient low voltage ride through performance

Understanding the electromechanical behaviour of synchronous generators during severe low voltage fault conditions is vital to measure the potential performance impact and damage that may occur as a result to both connected equipment and the generator set. The work outlined in this paper addresses the process of developing a multi-physics model of a grid connected synchronous generator utilising 2D transient electromagnetic modelling with co-simulation of a 1D torsional beam element model. A 3D finite element model of a synchronous generator rotor is created and analysed using the ANSYS modal solver to obtain the orthotropic elastic properties of the laminated generator rotor core using a parametric modal analysis technique. The first 8 modes are obtained with a mean error of 1.3%, proving the accuracy of the method as the first contribution to knowledge. The dynamically validated rotor model is used to obtain the torsional stiffness values for an equivalent 1D torsional model which is coupled to a 2D transient magnetic model of the generator (ANSYS Maxwell and Simplorer), completing the multi-physics generator set model. Three phase short circuit simulation results are compared with experimental test results to validate the model for use in low voltage ride through simulations. The relationship between the relative angular position of the rotor and stator fields and the peak electromagnetic torque transient is obtained. In the case of an unprotected generator, a peak torque of 16.5 pu is observed with a relative angular shift of 360o. Five distinct peak torques were discovered over the 360o range that was simulated, occurring in steps of 90o owing to the four-pole configuration of the rotor. The discovery of this relationship provides the second contribution to knowledge in this research. Finally, three modifications are made to the generator set model subjected to the worst-case transient, an increase in rotor inertia, inclusion of controlled series braking resistors and a clutch to reduce the angular displacement of the flexible coupling between the two machines. The peak electromagnetic torque is reduced by 0.88 kNm (32.2%) and 4.17 kNm (73.8%) respectively. Ultimately, the peak stress in the generator shaft keyway is reduced by 77.4%. The relative impact of the modifications on low voltage fault ride through performance represents the third contribution to knowledge

Detecção e controle de trincas transversais em máquinas rotativas

In this dissertation, crack detection and crack control of rotating machinery are addressed. The objectives of this study involves building and validating a numerical model capable of simulating the dynamic behavior of a real rotating machine. Numerical and experimental results for crack detection based on the shaft vibration signals are obtained. Besides, numerical and experimental results from an active control method, capable of suppressing 2X and 3X super-harmonics are obtained, excited by the crack presence. The innovation of this thesis lays on the fact of merging crack detection and crack control techniques in one single study. It is worth mentioning that the subject of crack control is quite new in the literature. The rotor is modeled by the finite element (FE) method, considering Timoshenko beam elements with circular cross section and constant radius. The Mayes model is addressed for simulating the breathing behavior of a transverse crack in a progressive way. The linear fracture mechanics theory is applied to correlate the crack depth with the corresponding additional shaft flexibility. The so-called modal state observer (MSO) technique was chosen to verify the existence of a transverse crack on a rotating machine. Moreover, the PID active control technique was addressed to perform crack control, suppressing crack signatures over the rotor frequency spectrum. Both numerical and experimental results highlight the possibility of detecting the existence of a crack and also how to decrease its effects (through active control) of an operating rotating machine. In this way, inspections requiring full stop of the machine can be performed less often while keeping the rotating machine safety.CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorTese (Doutorado)Esta tese considera detecção e controle de uma trinca transversal em máquinas rotativas. Os objetivos deste estudo envolvem a obtenção e o ajuste de um modelo matemático que represente o comportamento dinâmico de uma máquina rotativa real, a aplicação numérica e experimental de um método de detecção de falhas baseado no sinal de vibração do rotor e de um método de controle ativo de trincas que reduza os níveis dos super-harmônicos 2X e 3X excitados pela presença da trinca no espectro de frequência do rotor. A novidade desta tese está no fato de se realizar detecção e controle de trinca em um único projeto de pesquisa. Além disso, o controle ativo de trincas ainda é um assunto novo nesta área. A modelagem matemática do rotor utiliza o método dos Elementos Finitos (FE) e considera elementos de viga de Timoshenko, com seção circular e raio constates. O comportamento dinâmico da trinca transversal é modelado pelo modelo de Mayes, que considera abertura e fechamento da trinca de maneira progressiva. Mecânica da fratura linear é aplicada, a fim de relacionar a profundidade da trinca com o aumento da flexibilidade do eixo. O observador de estado modal (MSO) foi utilizado nesta tese quando do estudo do problema de detecção de trincas. Já para o controle ativo da trinca, a técnica de controle PID foi aplicada. Os resultados evidenciam a possibilidade de se diagnosticar uma trinca e diminuir seus efeitos (através do controle ativo) sobre uma máquina rotativa em operação. Desta forma, manutenções que exigem a parada da máquina podem ser realizadas com menor frequência e, mesmo assim, a segurança da máquina rotativa é garantida