Jae-Sung Bae

Improved Concept and Model of Eddy Current Damper

When a conductive material experiences a time-varying magnetic field, eddy currents are generated in the conductor. These eddy currents circulate such that they generate a magnetic field of their own, however the field generated is of opposite polarity, causing a repulsive force. The time-varying magnetic field needed to produce such currents can be induced either by movement of the conductor in the field or by changing the strength or position of the source of the magnetic field. In the case of a dynamic system the conductor is moving relative to the magnetic source, thus generating eddy currents that will dissipate into heat due to the resistivity of the conductor. This process of the generation and dissipation of eddy current causes the system to function as a viscous damper. In a previous study, the concept and theoretical model was developed for one eddy current damping system that was shown to be effective in the suppression of transverse beam vibrations. The mathematical model developed to predict the amount of damping induced on the structure was shown to be accurate when the magnet was far from the beam but was less accurate for the case that the gap between the magnet and beam was small. In the present study, an improved theoretical model of the previously developed system will be formulated using the image method, thus allowing the eddy current density to be more accurately computed. In addition to the development of an improved model, an improved concept of the eddy current damper configuration is developed, modeled, and tested. The new damper configuration adds significantly more damping to the structure than the previously implemented design and has the capability to critically damp the beam s first bending mode. The eddy current damper is a noncontacting system, thus allowing it to be easily applied and able to add significant damping to the structure without changing dynamic response. Furthermore, the previous model and the improved model will lie applied to the new damper design and the enhanced accuracy of this new theoretical model will he proven.

Aerodynamic and Static Aeroelastic Characteristics of a Variable-Span Morphing Wing

The morphing concept for unmanned aerial vehicles is a topic of current research interest in aerospace engineering. One concept of morphing is to change the wing configuration during flight to allow for multiple flight regimes. A particular approach to planform morphing is a variable-span morphing wing to increase wingspan to reduce induced drag and increase range and endurance. The wing area and the aspect ratio of the variable-span morphing wing increase as the wingspan increases. This means that the total lift increases while the induced drag is reduced, whereas the wing-root bending moment increases, thus, requiring a larger bending stiffness of the wing structure. Therefore, a study of the variable-span morphing wing requires not only aerodynamic analysis but also an investigation of the aeroelastic characteristics of the wing. The aerodynamic characteristics of the variable-span morphing wing are investigated, and a static aeroelastic analysis is performed.

Modeling and flight control of large-scale morphing aircraft

As morphing is an emerging topic of interest in aircraft research, the following article provides a review of the subject, with specific focus on modeling and flight control of large-scale planform altering flight vehicles. Our discussion proceeds in a fundamental manner to demonstrate that, although design methods for rigid aircraft have become highly developed, the consideration of morphing necessitates further investigation into the typically disparate fields of dynamic modeling, aerodynamic theory, and flight control theory. To clarify these points, the equations of atmospheric flight are derived in a general form, methods of integrating the aerodynamic forces are examined, and we distinguish between various approaches and methods of flight control.

Eddy Current Damping in Structures

When a conductive material is subjected to a time-varying magnetic flux, eddy currents are generated in the conductor. These eddy currents circulate inside the con- ductor generating a magnetic field of opposite polarity as the applied magnetic field. The interaction of the two magnetic fields causes a force that resists the change in magnetic flux. However, due to the internal resistance of the conductive material, the eddy currents will be dissipated into heat and the force will die out. As the eddy currents are dissipated, energy is removed from the system, thus producing a damp- ing effect. There are several different methods of inducing a time-varying magnetic field, and from each method arises the potential for a different type of damping system. There- fore, the research into eddy current and magnetic damping mechanisms has led to a diverse range of dampers, many of which are detailed in this paper. The majority of the research in eddy current damping has taken place in the area of mag- netic braking. A second topic that has received significant interest is the use of eddy current dampers for the suppres- sion of structural vibrations. However, much of this research is not concentrated in one area, but has been applied to a variety of different structural systems in a number of distinct ways. In this paper, we review the research into various types of eddy current damping mechanisms and we discuss the future of eddy currents with some potential uses that have not yet been studied.

Aerodynamic and Aeroelastic Considerations of A Variable- Span Morphing Wing

Morphing concepts for air vehicles such as unmanned air vehicles (UAVs) have been a topic of current research interest in aerospace engineering. A morphing wing is a bird-like wing that has the ability of adapting to obtain better flight performance. One concept of morphing is the variable-span morphing wing (VSMW) in which the wingspan is varied to accommodate multiple flight regimes. In the present study, the advantages and disadvantages of a VSMW for a cruise type missile are discussed. The aerodynamic characteristics and the range of this morphing wing are analyzed as its wingspan is changed. The results of the analysis demonstrate an improvement in the aerodynamic characteristics of the VSMW, in the form of a reduction in the induced drag, resulting in an increase in range. As further discussed, the VSMW also provides an alternative method of controlling the roll motions of a bank-to-turn cruise missile. Compared to conventional roll control the variable span method provides an increase in roll control authority. Unfortunately, the aeroelastic characteristics of the VSMW become worse because the wing-root bending moment increases due to the wingspan increase.

Aeroelastic Considerations on Shape Control of an Adaptive Wing

In this work, the aeroelastic effects on the shape control of an adaptive camber wing are studied through numerical simulation. The adaptive camber concept consists of a compliant beam, actuated by a series of piezoelectric actuators, embedded within the rib to deform the wing in an effort to produce a desired lift. Piezoelectric laminated plate theory is used to model the actuation force and the resulting camber line shape of the wing. The aerodynamic forces are calculated using two-dimensional airfoil theory and a feedback shape control algorithm is applied to obtain the desired camber. The results of the simulations demonstrate that the deformations due to aerodynamic forces are large enough to significantly alter the total lift produced by the wing as compared with the lift produced solely through actuation. Using an adaptive control algorithm, aeroelastic shape control of the adaptive camber wing is successfully achieved. Moreover, it is shown that, due to aerodynamic forces, the power requirements of shape control might be reduced.

Extension of Flutter Prediction Parameter for Multimode Flutter Systems

This research was supported by the Agency for Defense Development and was partially supported Ministry of Science and Technology (National Research Laboratory Program) in the Republic of Korea. This support is gratefully acknowledged. Authors express thanks to the associate editor Franklin Eastep, and to reviewers for many valuable comments and suggestions. Also, the authors appreciate the review and comment of Henry A. Sodano of Virginia Polytechnic Institute and State University about this paper.

Aeroelastic characteristics of linear and nonlinear piezo-aeroelastic energy harvester

In the present study, a piezo-aeroelastic energy harvester using nonlinear aeroelastic behaviors is proposed, and their characteristics and performance are investigated. The energy harvester is modeled by a two-dimensional typical section airfoil. The nondimensional parameters of the harvester are introduced, and the nondimensional piezo-aeroelastic equations are formulated. For the piezo-aeroelastic analysis, the root-locus method and time-integration method are used, and the present method is verified with experimental and analytical results. The iterative method is introduced to calculate the frequency response functions of a nonlinear piezo-aeroelastic energy harvester. The aeroelastic characteristics of a linear piezo-aeroelastic energy harvester and the effects of parameters are investigated. The results show that the linear piezo-aeroelastic energy harvester can be used to generate electricity only at the vicinity of flutter speed. It is assumed that the nonlinear piezo-aeroelastic energy harvester has free play and cubic hardening in pitch. For free play, nonlinear aeroelastic results show that stable limit cycle oscillations are observed in the wide range of air speed below flutter speed when the frequency ratio is 1.3, and unstable limit cycle oscillations are observed at air speeds over flutter speed when the frequency ratio is 0.3. For cubic hardening, unstable limit cycle oscillations are observed at air speeds below flutter speed when the frequency ratio is 0.3, and stable limit cycle oscillations are observed at air speeds over flutter speed when the frequency ratio is 1.3. Finally, the authors discussed how to use these aeroelastic responses for piezo-aeroelastic energy harvesting.

Modeling and Application of Eddy Current Damper for Suppression of Membrane Vibrations

Inflatable space-based structures have become increasingly popular over the past three decades due to their minimal deployed mass and launch volume. To facilitate packaging of the satellite in the shuttle bay, the optical or antenna surface is in many cases a thin-film membrane. Additionally, because the structure holding the membrane is a lightweight and flexible inflated device, the membrane is subjected to a variety of dynamic loadings. For the satellite to perform optimally, the membrane structure must be free of vibration. However, due to the extreme flexibility of the membrane, the choice of applicable sensing and actuation methods to suppress the vibration is limited. The present study investigates the use of an eddy current damper to passively suppress the vibration of a thin membrane. Eddy currents are induced when a nonmagnetic conductive material is subjected to a timechanging magnetic flux. As the eddy currents circulate inside the conductor they are dissipated, causing energy to be removed from the system and thus allowing the system to function as a type of viscous damper. Using this concept, the ability to generate sufficient damping forces in the extremely thin-film membranes used in space is studied. First, a theoretical model of the interaction between the eddy current damper and the membrane is developed. The model is then validated through experiments carried out at both ambient and vacuum pressures. The results show that the model can accurately predict the damping of the first mode as the distance between the magnet and membrane is varied. Furthermore, the results of the experiments performed on the membrane at vacuum conditions show the functionality of the damping mechanism in space and indicate damping levels as high as 30% of critical at ambient pressure and 25% of critical at vacuum pressure.

Vibration Suppression of a Large Beam Structure Using Tuned Mass Damper and Eddy Current Damping

For a few decades, various methods to suppress the vibrations of structures have been proposed and exploited. These include passive methods using constrained layer damping (CLD) and active methods using smart materials. However, applying these methods to large structures may not be practical because of weight, material, and actuator constraints. The objective of the present study is to propose and exploit an effective method to suppress the vibration of a large and heavy beam structure with a minimum increase in mass or volume of material. Traditional tuned mass dampers (TMD) are very effective for attenuating structural vibrations; however, they often add substantial mass. Eddy current damping is relatively simple and has excellent performance but is force limited. The proposed method is to apply relatively light-weight TMD to attenuate the vibration of a large beam structure and increase its performance by applying eddy current damping to a TMD. The results show that the present method is simple but effective in suppressing the vibration of a large beam structure without a substantial weight increase.