Simple modeling of self-oscillation in Nano-electro-mechanical systems

We present here a simple analytical model for self-oscillations in nano-electro-mechanical systems. We show that a field emission self-oscillator can be described by a lumped electrical circuit and that this approach is generalizable to other electromechanical oscillator devices. The analytical model is supported by dynamical simulations where the electrostatic parameters are obtained by finite element computations.Comment: accepted in AP

Performance of field-emitting resonating carbon nanotubes as radio-frequency demodulators

International audienceWe report on a systematic study of the use of resonating nanotubes in a field emission (FE) configuration to demodulate radio frequency signals. We particularly concentrate on how the demodulation depends on the variation of the field amplification factor during resonance. Analytical formulas describing the demodulation are derived as functions of the system parameters. Experiments using AM and FM demodulations in a transmission electron microscope are also presented with a determination of all the pertinent experimental parameters. Finally we discuss the use of CNTs undergoing FE as nanoantennae and the different geometries that could be used for optimization and implementation. © 2011 American Physical Society

Effect of length on the instability of hanging pipes

International audienceThe flutter instability of a hanging fluid-conveying pipe is investigated, as its length is increased. Experiments show that there exists a critical length above which the flow velocity necessary to cause flutter becomes independent of the pipe length. The fluid-structure interaction is thus modelled by following the work of Bourrières and of Païdoussis. Computations using a standard Galerkin method confirm this evolution. A short pipe model is then considered, where gravity plays a negligible role. Transition between this short length model and the asymptotic situation is found to occur where a local stability criterion is satisfied at the upstream end of the pipe. For longer pipes, a model is proposed where the zone of stable waves is totally disregarded. Comparison of these models with experiments and computations show a good agreement over all ranges of mass ratios between the flowing fluid and the pipe

Flow energy harvesting by fluttering slender bodies in axial currents

International audienceCoupled fluid-solid instabilities offer promising perspectives for the development of new technologies to harvest energy from water currents. A fundamental study of these instabilities and of the impact of the damping induced by the energy harvesting on the fluid-solid system is essential to identify promising configurations, quantify energy harvesting potential and characterize possible design optimization. In this work, we focus on the dynamics of long flexible cylinders placed in axial currents. It is well established that above a critical velocity threshold the flexible cylinder's rest position becomes unstable to flutter and self-sustained limit-cycle oscillations can develop. A fraction of the solid kinetic energy can then be converted into electrical form, acting as a dissipative mechanism on the fluid-solid system. The non-linear dynamics of the dissipative system are studied in this paper using a reduced-order model of the deformable cylinder in the form of a bi-articulated system of two rigid cylinders with energy harvesting at each articulation. The impact of energy extraction on the system's properties and the optimal placement of energy harvesters are then analyzed and discussed. It is shown that optimal energy harvesting only involves a single harvester at the upstream end, away from the region of useful curvature that drives the instability mechanism at the origin of the self-sustained oscillations. Copyright © 2011 by ASME

Vortex-induced waves along cables

International audienceA low-order model for transverse vortex-induced vibrations of cables in stationary uniform flow is analyzed. The near wake vortex street is modeled by a continuous distribution of non-linear van der Pol oscillators, arranged along the spanwise extent of the structure and interacting by diffusion and stiffness. The structure, described as a classical tensioned cable, is forced by the fluid via a linear fluctuating lift model and reacts on the fluid through different linear actions. The model dynamics is investigated analytically and discussed with respect to the choice of the coupling terms, then verified by numerical simulations, in comparison with literature data. Discussion follows on the effectiveness of the linear inertial action of the structure on the fluid in describing, qualitatively and quantitatively, the main features of vortex-induced vibrations phenomenology, and standing versus traveling wave behavior