2,125 research outputs found

    BROADBAND VIBRATION CONTROL THROUGH PERIODIC ARRAYS OF LOCALLY RESONANT INCLUSIONS

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    openDottorato di ricerca in Ingegneria industriale e dell'informazioneopenZientek, Michal Wladysla

    Suppression of Limit Cycle Oscillations using the Nonlinear Tuned Vibration Absorber

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    The objective of the present study is to mitigate, or even completely eliminate, the limit cycle oscillations in mechanical systems using a passive nonlinear absorber, termed the nonlinear tuned vibration absorber (NLTVA). An unconventional aspect of the NLTVA is that the mathematical form of its restoring force is not imposed a priori, as it is the case for most existing nonlinear absorbers. The NLTVA parameters are determined analytically using stability and bifurcation analyses, and the resulting design is validated using numerical continuation. The proposed developments are illustrated using a Van der Pol-Duffing primary system

    Limits of flexural wave absorption by open lossy resonators: reflection and transmission problems

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    The limits of flexural wave absorption by open lossy resonators are analytically and numerically reported in this work for both the reflection and transmission problems. An experimental validation for the reflection problem is presented. The reflection and transmission of flexural waves in 1D resonant thin beams are analyzed by means of the transfer matrix method. The hypotheses, on which the analytical model relies, are validated by experimental results. The open lossy resonator, consisting of a finite length beam thinner than the main beam, presents both energy leakage due to the aperture of the resonators to the main beam and inherent losses due to the viscoelastic damping. Wave absorption is found to be limited by the balance between the energy leakage and the inherent losses of the open lossy resonator. The perfect compensation of these two elements is known as the critical coupling condition and can be easily tuned by the geometry of the resonator. On the one hand, the scattering in the reflection problem is represented by the reflection coefficient. A single symmetry of the resonance is used to obtain the critical coupling condition. Therefore the perfect absorption can be obtained in this case. On the other hand, the transmission problem is represented by two eigenvalues of the scattering matrix, representing the symmetric and anti-symmetric parts of the full scattering problem. In the geometry analyzed in this work, only one kind of symmetry can be critically coupled, and therefore, the maximal absorption in the transmission problem is limited to 0.5. The results shown in this work pave the way to the design of resonators for efficient flexural wave absorption

    On the use of the wave finite element method for passive vibration control of periodic structures

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    International audienceIn this work, a strategy for passive vibration control of periodic structures is proposed which involves adding a periodic array of simple resonant devices for creating band gaps. It is shown that such band gaps can be generated at low frequencies as opposed to the well known Bragg scattering effects when the wavelengths have to meet the length of the elementary cell of a periodic structure. For computational purposes, the wave finite element (WFE) method is investigated, which provides a straightforward and fast numerical means for identifying band gaps through the analysis of dispersion curves. Also, the WFE method constitutes an efficient and fast numerical means for analyzing the impact of band gaps in the attenuation of the frequency response functions of periodic structures. In order to highlight the relevance of the proposed approach, numerical experiments are carried out on a 1D academic rod and a 3D aircraft fuselage-like structure

    A summary of recent NASA/Army contributions to rotorcraft vibrations and structural dynamics technology

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    The requirement for low vibrations has achieved the status of a critical design consideration in modern helicopters. There is now a recognized need to account for vibrations during both the analytical and experimental phases of design. Research activities in this area were both broad and varied and notable advances were made in recent years in the critical elements of the technology base needed to achieve the goal of a jet smooth ride. The purpose is to present an overview of accomplishments and current activities of govern and government-sponsored research in the area of rotorcraft vibrations and structural dynamics, focusing on NASA and Army contributions over the last decade or so. Specific topics addressed include: airframe finite-element modeling for static and dynamic analyses, analysis of coupled rotor-airframe vibrations, optimization of airframes subject to vibration constraints, active and passive control of vibrations in both the rotating and fixed systems, and integration of testing and analysis in such guises as modal analysis, system identification, structural modification, and vibratory loads measurement

    Active Metastructures for Light-Weight Vibration Suppression

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    The primary objective of this work is to examine the effectiveness of metastructures for vibration suppression from a weight standpoint. Metastructures, a metamaterial inspired concept, are structures with distributed vibration absorbers. In automotive and aerospace industries, it is critical to have low levels of vibrations while also using lightweight materials. Previous work has shown that metastructures are effective at mitigating vibrations but does not consider the effects of mass. This work considers mass by comparing a metastructure to a baseline structure of equal mass with no absorbers. The metastructures are characterized by the number of vibration absorbers, the mass ratio, and the natural frequencies of the vibration absorbers. The metastructure and baseline structure are modeled using a lumped mass model and a distributed mass model. The lumped mass model allows for mass and stiffness parameters to be varied independently without the need to consider geometry constraints. The distributed mass model is a more realistic representation of a physical structure and uses relevant material properties. The steady-state and transient time responses of the structure are obtained. The results of these models examine how the performance of the structure varies with respect to the number of vibration absorbers and the mass ratio. Additionally, the stiffness and mass distributions of the vibration absorbers are considered. When the ratio of stiffness over mass varies linearly, the absorbers create broad-band suppression. Overall, these results show it is possible to obtain a favorable vibration response without adding additional mass to the structure. The distributed vibration absorbers are realized through geometry modifications on the centimeter scale. To obtain the complex geometry needed for these structures, the metastructures are typically manufactured using 3D printers, specifically the Objet Connex 3D printer. To better understand the damping properties of the materials used by the Objet Connex, the viscoelastic properties are characterized. These properties are characterized by measuring the frequency and temperature dependent complex modulus values using a dynamic mechanical analysis (DMA) machine. The material properties are incorporated into the Golla-Hughes-McTavish (GHM) model to capture the damping effect. Using the time-temperature equivalence, the material properties are transformed to various temperatures, allowing the response of the structures to be modeled at various temperatures. A 3D printed metastructure is experimentally tested and compared to the GHM model. These results show the GHM model can accurately predict the natural frequencies of the vibration absorbers. Lastly, the concept of adding active vibration control to a metastructure to get additional vibration suppression is explored. This is done by attaching piezoelectric materials to the metastructure and utilizing the positive position feedback (PPF) control law to further reduce vibrations. Two active vibration absorber designs are explored; the first uses a stack actuator to control the position of a single absorber and the second design bonds PZT patches in a bimorph cantilevered configuration to the beam of one absorber. This work shows that the active vibration absorber design utilizing a stack actuator is not practical, but the PZT bimorph configuration is capable of further reducing vibrations. Due to the metastructure design, each mode corresponds to the oscillation of a single absorber. When a single vibration absorber is active, the controller can control the corresponding mode. Overall, this shows that integrating active vibration control into a metastructure design can provide additional performance improvements.PHDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/144044/1/reichl_1.pd

    Dynamics of digitally controlled forced vibration of suspended cables

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    Dynamics of suspended cables with active vibration control is studied. The control device is an electrical vibration absorber that is driven by a motor and that may be fixed at any position along the cable. The absorber applies a control force that reduces vibration amplitude at the position where it is placed. The methodology is efficient for attenuating high-frequency, low-amplitude vibration due to periodic excitation that may consider wind effect. The dynamic behavior is described by a mechanical model of the absorber and the cable at the location where the absorber is attached. The model takes into account such practical problems as time delay and backlash at the driving, which lead to limitation in the applicability of control. Time delay occurs in digital control, because samples of data are taken at discrete time intervals and response is provided after the sampling delay. Backlash influences control when the direction of control force changes, since the control force is not transmitted in the small domain of backlash. The present research examines the effects of time delay and backlash on the local control of cable vibration, and assesses the range of time delay and backlash when the control can be applied successfully. Moreover, the presence of time delay and backlash together results in a motion with some irregularity what justifies the detailed study of the dynamic behavior in order to evaluate the types of motion that may arise in such systems
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