107 research outputs found
Edge flutter of long beams under follower loads
The linear instability of a beam tensioned by its own weight is considered.
It is shown that for long beams, in the sense of an adequate dimensionless
parameter, the characteristics of the instability caused by a follower force do
not depend on the length. The asymptotic regime significantly differs from that
of short beams: flutter prevails for all types of follower loads, and flutter
is localized at the edge of the beam. An approximate solution using matched
assymptotic expansion is proposed for the case of a semi-infinite beam. Using a
local criterion based on the stability of waves, the characteristics of this
regime as well as its range of application can be well predicted. These results
are finally discussed in relation with cases of flow-induced instabilities of
slender structures.Comment: to appear in Journal of Mechanics of Materials and Structure
Dissipation effect on local and global stability of fluid-conveying pipes
International audienceIn this article, the effect of dissipation on local and global stability of fluid conveying pipes is analyzed. The local approach refers to an infinite medium and uses wave propagation analyses without taking boundary conditions into account. The global approach refers to the same medium, but with finite length and associated with a given set of boundary conditions. The finite length system is generally studied by calculating its eigenmodes and eigenfrequencies. Criteria for local instability are derived in the first part of this paper, and dissipation is found to significantly affect local stability. Moreover, dissipation is found to have a stabilizing or destabilizing effect, depending on the other parameters. Next, numerical computations are presented for finite-length systems and results are analyzed and compared with local stability properties of the corresponding media. When the system is sufficiently long, it is found that critical velocity for global instability tends to a local criterion which can be that of local stability of the damped medium or a local transition criterion of the undamped medium, which is not necessarily the local instability criterion. Finally, a reasoning based on lengthscale ratios is developed. It allows to know which criterion is able to predict the global stability for long systems
Influence and optimization of the electrodes position in a piezoelectric energy harvesting flag
Fluttering piezoelectric plates may harvest energy from a fluid flow by
converting the plate's mechanical deformation into electric energy in an output
circuit. This work focuses on the influence of the arrangement of the
piezoelectric electrodes along the plate's surface on the energy harvesting
efficiency of the system, using a combination of experiments and numerical
simulations. A weakly non-linear model of a plate in axial flow, equipped with
a discrete number of piezoelectric patches is derived and confronted to
experimental results. Numerical simulations are then used to optimize the
position and dimensions of the piezoelectric electrodes. These optimal
configurations can be understood physically in the limit of small and large
electromechanical coupling.Comment: To appear in Journal of Sound and Vibratio
Energy harvesting efficiency of piezoelectric flags in axial flows
International audienceSelf-sustained oscillations resulting from fluid-solid instabilities, such as the flutter of a flexible flag in axial flow, can be used to harvest energy if one is able to convert the solid energy into electricity. Here, this is achieved using piezoelectric patches attached to the surface of the flag, which convert the solid deformation into an electric current powering purely resistive output circuits. Nonlinear numerical simulations in the slender-body limit, based on an explicit description of the coupling between the fluid-solid and electric systems, are used to determine the harvesting efficiency of the system, namely the fraction of the flow kinetic energy flux effectively used to power the output circuit, and its evolution with the system's parameters. The role of the tuning between the characteristic frequencies of the fluid-solid and electric systems is emphasized, as well as the critical impact of the piezoelectric coupling intensity. High fluid loading, classically associated with destabilization by damping, leads to greater energy harvesting, but with a weaker robustness to flow velocity fluctuations due to the sensitivity of the flapping mode selection. This suggests that a control of this mode selection by a careful design of the output circuit could provide some opportunities to improve the efficiency and robustness of the energy harvesting process
Interactions entre fluides et structures actives : Instabilités, contrôle et récupération d'énergie
Ce mémoire traite dans sa première partie des phénomènes d'instabilité en interaction fluide-structure. La première partie y est entièrement consacrée. Les problèmes spécifiques qui sont abordés sont le plus souvent fondamentaux~: les plaques en flottement en présence d'un écoulement axial (instabilité du drapeau), le flottement du tuyau d'arrosage, la dynamique d'oscillateurs masse-ressort sous écoulement. Ces systèmes sont représentatifs de nombreux phénomènes vibratoires rencontrés dans la vie courante ou l'industrie~: les vibrations de structures sous écoulement dans l'industrie nucléaire, le flottement de panneaux souples en aéronautique, les vibrations de structures soumises au vent dans le génie civil, les oscillations du papier défilant à grande vitesse entre les tambours d'imprimerie, la vibration de la glotte à l'origine du ronflement ou la vibration des cordes vocales.D'autres problématiques sont ensuite abordées en deuxième partie, qui concernent le cas des structures actives en interaction avec un fluide au repos. Il s'agit notamment des vibrations de plaques piézoélectriques pour la reproduction sonore ou les oscillations structures en alliage à mémoire de forme forcées, dont il est envisagé l'utilisation pour amortir les vibrations en génie civil. L'extension naturelle de l'ensemble de ces travaux consiste finalement à s'atteler à des problèmes d'interaction entre écoulements et structures actives. Un premier projet sur ce thème est présenté dans ce manuscrit~: la récupération d'énergie du flottement de plaques piézoélectriques
Inductive effects on energy harvesting piezoelectric flag
National audienceInteraction between a flexible flag and a flow leads to a canonical fluid–structure instability whichproduces self-sustained vibrations, from which mechanical energy could be converted to electrical energythrough piezoelectric materials covering the flag and thus being deformed by its motion. We study thepossibility of harvesting this energy, especially the effect of an inductive circuit on the energy harvestingprocess. A destabilization of the coupled system is observed after adding an inductance. In the nonlinearcase, the harvesting efficiency increases significantly at lock–in between the frequencies of the flutteringflag and the electrical circuit.L'interaction d'un drapeau flexible avec un écoulement est connue pour donner lieu à une vibration auto-entretenue, dont l’énergie mécanique peut être convertie en énergie électrique par le biais des matériaux piézoélectriques qui couvrent le drapeau et ainsi se déforment avec celui-ci. On étudie la possibilité de récupérer cette énergie, et en particulier l'effet d'un circuit inductif sur le processus de récupération. Dans l’étude linéaire, une déstabilisation du système est observée par l'ajout d'une inductance. Une méthode numérique, basée sur une description explicite entre le couplage fluide–solide–électrique, est utilisée pour la simulation non-linéaire du système. En régime non-linéaire, l'efficacité de récupération augmente significativement lors de l'accrochage entre les fréquences de battement du drapeau et du circuit électrique
Measurement of flow separation in a human vocal folds model
International audienceThe paper provides experimental data on flow separation from a model of the human vocal folds. Data were measured on a four times scaled physical model, where one vocal fold was fixed and the other oscillated due to fluid-structure interaction. The vocal folds were fabricated from silicone rubber and placed on elastic support in the wall of a transparent wind tunnel. A PIV system was used to visualize the flow fields immediately downstream of the glottis and to measure the velocity fields. From the visualizations, the position of the flow separation point was evaluated using a semiautomatic procedure and plotted for different airflow velocities. The separation point position was quantified relative to the orifice width separately for the left and right vocal folds to account for flow asymmetry. The results indicate that the flow separation point remains close to the narrowest cross-section during most of the vocal fold vibration cycle, but moves significantly further downstream shortly prior to and after glottal closure
Effect of coupling with internal and external fluids on the mechanical behavior of aerostats
International audienceIn the context of the simulation of aerostats in flight, we are interested here in the coupling between a deformable structure, the fluid contained inside and the fluid flow outside. To study the dynamic stability of such systems, the fluid-structure coupled equations are linearized around an equilibrium position and, by assuming that the fluid flow perturbations are potential, the loads exerted by the fluids on the moving structure can be decomposed in terms proportional, respectively, to the displacement, velocity and acceleration fields of the structure, representing what are generally called the added stiffness, damping and mass effects of the fluid on the structure. In this work, a focus is made on the added mass because, for such lightweight structures, its effect is of prime importance. A Boundary Element Method (BEM) is proposed to compute the fluid added mass operators, for external and internal fluids, and for any structure deformation field. Numerical and experimental validations are conducted on an axisymmetric ellipsoid mockup immerged in water and subject to rigid motions. Variations of the imposed movement amplitude and velocity have also helped to evaluate the validity domain of this model
Forces exerted on a cylinder in near-axial flow
International audienceThis study investigates the flow around a cylinder in a near-axial flow at a Reynolds number of 27,000. Both computational fluid dynamics (CFD) calculations and experiments are performed. Time-mean values of lift force coefficient are investigated against the inclination of the cylinder in the domain of low inclinations (<15 deg). A pressure distribution and flow profiles are also measured and extracted from the CFD calculation results for a characteristic inclination α = 5 deg. Numerical results for force and pressure show fair agreement with experiments for inclination below 5 deg and reveal that at low angles, the lift force is proportional to the angle. In the framework of a quasi-static approach, the instantaneous damping force exerted on a cylinder oscillating in axial flow is equivalent to the normal force exerted on a cylinder placed in an oblique flow
- …