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Published Articles

The Volume 12, No 2, June 2007

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Active Nonlinear Vibration Absorber Design for Flexible Structures

Lei Chen, Colin H. Hansen, Fangpo He and Karl Sammut


An active Vibration Clamping Absorber (VCA) technique designed for vibration suppression in flexible structures is proposed and investigated in this paper. The technique uses a Quadratic-Modal-Positive- Position-Feedback strategy to design a simple second-order nonlinear controller that is capable of suppressing structural vibrations at various resonances. The VCA can effectively transfer vibration energy from the main structure to another sacrificial absorber so that large amplitude vibrations in the main structure can be clamped within tolerable limits. The effectiveness of the VCA design is demonstrated through single-mode and multiplemode control on a flexible cantilever beam system using one sensor/actuator pair. The simulation and experimental results reveal that the proposed design can be used for real-time control of vibration in large flexible structures.

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Instabilities of Compressible Fluid Flow over a Plate

Sanjay Dharmavaram, Anirvan DasGupta


This paper considers the linearised dynamics of flow of a compressible fluid over a plate. In particular, we study the stability of the flow, and identify the types of instability that occur. The zones of stability/instability in the Mach number-velocity ratio plane have been obtained. Two regions, namely convectively unstable and absolutely unstable, are observed to exist. Thus, the flow is always unstable. It is found that low values of Mach number and velocity ratio yield a convectively unstable flow. At high Mach number and/or velocity ratio, the system becomes absolutely unstable.

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Performance of Heuristic Optimisation Methods in the Characterisation of the Dynamic in the Characterisation of the Dynamic

Klaus H. Hornig and George T. Flowers


Of the fundamental dynamic properties (mass, damping, and stiffness), damping is usually the most difficult to quantify. This is perhaps particularly true for composite materials which tend to have substantially higher damping than comparable isotropic materials and therefore having an accurate representation is correspondingly more important. Accordingly, some heuristic optimisation techniques for the identification of the dynamic characteristics of honeycomb-core sandwich composite materials have been suggested, such as Particle Swarm Optimisation (PSO) and Genetic Algorithms (GA). Experimental measurements have been made of the dynamic responses (in the form of hysteresis loops) of a simply-supported beam and a simplified semi-empirical mathematical model has been developed for such a system when it is excited at its midpoint by sinusoidal displacement waves. The hysteresis loops that were obtained are for several frequencies and excitation amplitudes around the first mode of vibration. The basic model contains four unknown system parameters that must be identified. The performances of both optimisation methods are compared when used with computer-generated and experimental hysteresis loops. In addition, the effect of noise contamination in the signals has been studied in order to assess the search accuracy of the optimisation algorithms under such conditions.

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Recent Findings on Our Auditory System: It Is Highly Sensitive Owing to the Motility of Sensory Cells

Hiroshi Wada


Even though the amplitude of tympanic membrane vibrations is only a few nanometres when we speak in a low voice, we can clearly understand what is being said. This is speculated to be due to cochlear amplification. The origin of this amplification is believed to be the motility of mammalian outer hair cells (OHCs), which are located in the organ of Corti of the cochlea. These are the main focuses of this paper. However, firstly, peripheral anatomy is overviewed. Then, the acoustic properties of the outer and middle ears are interpreted. An explanation of the cochlear function follows. Finally, the origin of the motility of OHCs, i.e., the motor protein ?prestin,? is discussed.

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