Final Report Summary - FLOWENERGY (Flow energy harvesting in assemblies of vibrating solids: stability analysis and non-linear coupled dynamics.) The limited availability and environmental impact of fossile fuels motivate the development of new renewable energy technologies to exploit geophysical flows such as winds, river, tidal and oceanic currents. Flow-induced vibrations and fluid-solid instabilities offer promising perspectives as they generate sponta- neous, self-sustained and large-amplitude solid vibrations, which can be used to produce electricity when coupled to electric generators. Such systems become particularly attractive in the operating conditions where traditional wind- or water-turbines are not efficient (e.g. low power applications) or to power remote or portable devices.Fluid-solid instabilities have been traditionally studied, including at LadHyX, from the point of view of reducing or controlling such vibrations that arise in a large variety of industrial applications (e.g. vibrations of heat exchanger tubes in a steam generator and vibrations of cables and risers in offshore platform). The point of view followed by the FLOWENERGY project is opposite : in order to maximize how much energy can be produced from a given steady fluid flow, the solid’s vibrations must be promoted and enhanced.After his 5-year stay in the United States, the Marie Curie International Reintegration Grant support to the FLOWENERGY project enabled Dr. Michelin to establish his research activity in the European research community by giving him the possibility to initiate a fundamental and original research activity at LadHyX on the application of fluid-solid instabilities to flow energy harvesting from an incoming fluid flow.The support of the European Community to the FLOWENERGY project enabled several important advances in the understanding of the energy exchange mechanisms in such vibrating fluid-solid systems, and in the determination of their energy harvesting potential. First and foremost, a particular attention was dedicated to the impact and feedback of the extraction mechanism itself on the dynamics : conversion of mechanical energy into electrical form effectively removes a significant amount of energy from the vibrating solid, thereby impacting its vibration properties (instability threshold, amplitude and frequency). The research carried out during this project showed that, despite the general trend to reduce vibration amplitude, energy extraction could in fact in some cases take advantage of some of the fundamental pro- perties of these fluid-solid systems to instead promote vibrations at lower flow velocity and with larger flapping amplitudes.Going beyond this first result, the full fluid-solid-electric interactions between the fluid flow, the vibrating solid and the electric output circuit was studied on a model system (a piezoelectric flag). It was shown that the property of the circuits can in fact significantly impact the harvesting efficiency by promoting phenomena such as tuning or lock-in, when the typical time-scales of the circuit and of the vibrations are similar.The second objective of the FLOWENERGY project was to consider complex assemblies of such vibrating solids and determine the relation between local arrangement and vibration properties. This is a critical question for the design of energy harvesting farms or assemblies of flow energy harvesters. To this end, an experimental study was carried out on irregular arrays of vibrating cylinders. A simple model was proposed, based on the experimental data, to determine the vibration amplitude of the different cylinders as a function of their local arrangement.Finally, energy extraction and propulsion represent two dual problems : in the former, a fluid flow is exploited to produce a vibration that can power a generator, while in the latter, a generator (e.g. a muscle) is used to deform a solid body in order to create a net fluid flow or net propulsive force on the fluid. During this project, several collaborations were established to study the problem of hydrodynamic interactions in assemblies of self-propelled systems. This objective of the project enabled to rejoin two of the main research topics of Dr. Michelin and establish his activity at his new institution.The research carried out by Dr. Michelin and his colleagues during this project provides some important fundamental results on the problem of energy harvesting mechanisms that should be most useful to assess the potential efficiency of currently proposed prototypes. It also identifies several routes for optimization of such devices, some of them currently being investigated during ongoing projects led by Dr. Michelin at LadHyX.
