Flapping states of an energy harvesting device have been investigated by means of experiments, numerical simulations and a phenomenological model. The main aim is to predict the geometrical/physical properties of the system allowing sustained flapping limit cycles to emerge. These latter regimes are interesting when the system is used to harvest energy from flows. The main argument to identify flapping states is based on a simple resonance condition between the characteristic (elastic) time of the system and the flow time-scale. Similar arguments have been successful in other fields of fluid dynamics and fluid-structure interactions including turbulent flows of dilute polymer solutions and interactions between the wake originated by bluff bodies and elastic structures. The predictions of the geometrical/physical properties associated to critical conditions (i.e. those separating stable stages from flapping regimes) have been compared against the results of experiments, numerical simulations and a phenomenological model based on a set of ordinary differential equations. Results clearly confirm the expectations from the resonance condition. Discussions on how to extend our analysis in situations where the extraction stage is taken into account are also provided: this latter is indeed expected to influence the flapping stage and thus the critical conditions for flapping

Recent developments of low-power microprocessors open to new applications such as wireless sensor networks (WSN) with the consequent problem of autonomous powering. For this purpose, a possible strategy is represented by energy harvesting from wind or other flows exploiting fluid–structure interactions. In this work, we present an updated picture of a flutter-based device characterized by fully passive dynamics and a simple constructive layout, where limit cycle oscillations are undergone by an elastically bounded wing. In this case, the conversion from mechanical to electrical energy is performed by means of an electromagnetic coupling between a pair of coils and magnets. A centimetric-size prototype is shown to harvest energy from low wind velocities (between 2 and 4 m s−1), reaching a power peak of 14 mW, representing a valuable amount for applications related to WSN. A mathematical description of the nonlinear dynamics is then provided by a quasi-steady phenomenological model, revealing satisfactory agreement with the experimental framework within a certain parametric range and representing a useful tool for future optimizations.

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Abstract:The development of the Internet of Things infrastructure requires the deployment of millions of heterogeneous sensors embedded in the environment. The powering of these sensors cannot be done with wired connections, and the use of batteries is often impracticable. Energy harvesting is the common proposed solution, and many devices have been developed for this purpose, using light, mechanical vibrations, and temperature differences as energetic sources. In this paper we present a novel energy-harvester device able to capture the kinetic energy from a fluid in motion and transform it in electrical energy. This device, named FLEHAP (FLuttering Energy Harvester for Autonomous Powering), is based on an aeroelastic effect, named fluttering, in which a totally passive airfoil shows large and regular self-sustained motions (limit cycle oscillations) even in extreme conditions (low Reynolds numbers), thanks to its peculiar mechanical configuration. This system shows, in some centimeter-sized configurations, an electrical conversion efficiency that exceeds 8% at low wind speed (3.5 m/s). By using a specialized electronic circuit, it is possible to store the electrical energy in a super capacitor, and so guarantee self-powering in such environmental conditions.

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