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

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

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

A modal-based approach to the nonlinear vibration of strings against a unilateral obstacle:Simulations and experiments in the pointwise case

International audienceThis article is concerned with the vibration of a stiff linear string in the presence of a rigid obstacle. A numerical method for unilateral and arbitrary-shaped obstacles is developed, based on a modal approach in order to take into account the frequency dependence of losses in strings. The contact force of the barrier interaction is treated using a penalty approach, while a conservative scheme is derived for time integration, in order to ensure long-term numerical stability. In this way, the linear behaviour of the string when not in contact with the barrier can be controlled via a mode by mode fitting, so that the model is particularly well suited for comparisons with experiments. An experimental configuration is used with a point obstacle either centered or near an extremity of the string. In this latter case, such a pointwise obstruction approximates the end condition found in the tanpura, an Indian stringed instrument. The second polarisation of the string is also analysed and included in the model. Numerical results are compared against experiments, showing good accuracy over a long time scale

Dynamique non linéaire d'un oscillateur à mémoire de forme

Nous étudions les réponses forcées d'un oscillateur reproduisant le comportement pseudo-élastique d'un alliage à mémoire de forme. Le modèle est dérivé d'une loi de comportement tridimensionnelle prenant en compte les couplages entre la thermique, la mécanique et les changments de phase solide-solide du matériau. Les réponses forcées montrent un comportement assouplissant dès que la transformation martensitique est activée, ainsi que l'existence de zones chaotiques. Nous présenterons aussi des comparaisons calcul/essai réalisées sur des fils en torsion

Instabilités locales et globales en interaction fluide-structure

PALAISEAU-Polytechnique (914772301) / SudocSudocFranceF

Geometric optimization of dielectric elastomer electrodes for dynamic applications

Dielectric elastomers are soft actuators, made of an elastomer membrane sandwiched by compliant electrodes. Because of their high energy density and quick response, they are well suited for dynamic applications such as loudspeakers. Thanks to progress in the manufacturing process of dielectric elastomer actuators, the electrode shape can be patterned to very diverse shapes. In this study, we focus on the relation between the electrode shape and the dynamical and acoustical behavior of a dielectric elastomer loudspeaker. By using a finite element model of the loudspeaker, an optimization algorithm is set up to compute optimal electrode shapes according to chosen objectives, such as maximizing or minimizing the contribution of an eigenmode to the radiated sound. The optimal designs are then tested experimentally, and the efficiency of the optimization procedure is assessed. It is shown that the frequency response of dielectric elastomer loudspeakers can be tuned by optimizing the shape of the electrodes, and simulations suggest that the directivity can also be controlled. Finally, perspectives of the proposed optimization method are briefly discussed. (C) 2021 Elsevier Ltd. All rights reserved