Electro-hydrodynamic synchronization of piezoelectric flags

Hydrodynamic coupling of flexible flags in axial flows may profoundly influence their flapping dynamics, in particular driving their synchronization. This work investigates the effect of such coupling on the harvesting efficiency of coupled piezoelectric flags, that convert their periodic deformation into an electrical current. Considering two flags connected to a single output circuit, we investigate using numerical simulations the relative importance of hydrodynamic coupling to electrodynamic coupling of the flags through the output circuit due to the inverse piezoelectric effect. It is shown that electrodynamic coupling is dominant beyond a critical distance, and induces a synchronization of the flags' motion resulting in enhanced energy harvesting performance. We further show that this electrodynamic coupling can be strengthened using resonant harvesting circuits.Comment: 14 pages, 10 figures, to appear in J. Fluids Struc

Fluid-solid-electric lock-in of energy-harvesting piezoelectric flags

The spontaneous flapping of a flag in a steady flow can be used to power an output circuit using piezoelectric elements positioned at its surface. Here, we study numerically the effect of inductive circuits on the dynamics of this fluid-solid-electric system and on its energy harvesting efficiency. In particular, a destabilization of the system is identified leading to energy harvesting at lower flow velocities. Also, a frequency lock-in between the flag and the circuit is shown to significantly enhance the system's harvesting efficiency. These results suggest promising efficiency enhancements of such flow energy harvesters through the output circuit optimization.Comment: 8 pages, 8 figures, to appear in Physical Review Applie

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

Synchronized flutter of two slender flags

The interactions and synchronization of two parallel and slender flags in a uniform axial flow are studied in the present paper by generalizing Lighthill's Elongated Body Theory (EBT) and Lighthill's Large Amplitude Elongated Body Theory (LAEBT) to account for the hydrodynamic coupling between flags. The proposed method consists in two successive steps, namely the reconstruction of the flow created by a flapping flag within the LAEBT framework and the computation of the fluid force generated by this nonuniform flow on the second flag. In the limit of slender flags in close proximity, we show that the effect of the wakes have little influence on the long time coupled-dynamics and can be neglected in the modeling. This provides a simplified framework extending LAEBT to the coupled dynamics of two flags. Using this simplified model, both linear and large amplitude results are reported to explore the selection of the flapping regime as well as the dynamical properties of two side-by-side slender flags. Hydrodynamic coupling of the two flags is observed to destabilize the flags for most parameters, and to induce a long-term synchronization of the flags, either in-phase or out-of-phase.Comment: 14 pages, 10 figures, to appear in J. Fluid Mec

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

Self-propulsion of pure water droplets by spontaneous Marangoni stress driven motion

We report spontaneous motion in a fully bio-compatible system consisting of pure water droplets in an oil-surfactant medium of squalane and monoolein. Water from the droplet is solubilized by the reverse micellar solution, creating a concentration gradient of swollen reverse micelles around each droplet. The strong advection and weak diffusion conditions allow for the first experimental realization of spontaneous motion in a system of isotropic particles at sufficiently large P\'eclet number according to a straightforward generalization of a recently proposed mechanism. Experiments with a highly concentrated solution of salt instead of water, and tetradecane instead of squalane, confirm the above mechanism. The present swimming droplets are able to carry external bodies such as large colloids, salt crystals, and even cells.Comment: 5 pages, 5 figure

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

Flow field around a confined active droplet

International audienc

L’intervention publique paysagère comme processus normatif

Notre analyse interroge la production sociale de normes concomitante à la définition et à la mise en place d’interventions publiques paysagères à l’échelle locale. Elle montre que cette production normative s’accompagne d’une évolution dans la façon de nommer certains objets matériels. Cette nouvelle dénomination, souvent instable, témoigne de conflits normatifs, tant certaines normes peuvent être difficiles pour certains acteurs à concilier avec celles appliquées jusque-là. Notre étude montre également que ces conflits de normes autour des objets matériels sont autant de signes des usages alternatifs ou combinés de ces objets. On voit de la sorte que les politiques paysagères ont pour effet de reconnaître le « multi-usages » de l’espace rural en renforçant les pratiques liées aux loisirs (randonnées, visites). Elles légitiment en cela l’appropriation de l’espace par le regard, appropriation spécifique qui accompagne ces pratiques.landscape, local public action, norm, rural area