When an elastic solid lies in a flowing fluid, its static equilibrium state can become unstable once a critical value of the flow velocity is reached. This can result in strong oscillations are prone to severely damage the structure. This phenomenon is encountered in various engineering applications such as nuclear engineering, aeronautics, civil engineering or biomechanics. It is known as "flutter".
Among the various geometries for which studies are necessary, I am particulary interested in slender structures interacting with an axial flow. Well known examples are the fluid-conveying pipe (see image) and the fluttering flag. These systems may develop unstable flexural waves (local approach) as well as unstable modes (global approach). In particular, my aim is to predict the critical flow velocities for flutter instability and to characterize of the role of various physical parameters (dissipation) and geometry (flow confinement, length). Recently, progresses in the comprehension of unexpected effect of destabilization through addition of damping were obtained. This effect can be attributed to a mechanism of energy transfer between flow and waves which only exists when dissipation is present in the system. Effects of confinement and three-dimensionality of flow on critical flow velocities are also part of my concerns.
Vocal folds are also examples of structures prone to oscillate when a flowing fluid reaches a critical velocity. At UME, we study numerically and experimentally the interaction between a flow and oscillating structures modelling vocal folds in the phonation regime. We in particular seek to quantify the position of the separation point of the jet at exit of the vocal folds, whose knowledge is crucial in the theoretical models used at present.
This research project focus the attention on mechanisms able to produce self-sustained vibrations of a solid substrate (like flutter) on one hand and to convert the corresponding mechanical energy into electrical power on the other. It is motivated by the environmental impact and limited resources of fossile fuel energies.
Slender structures in axial flow are the main systems I am interested in. As mentionned above, these systems are able to present self-sustained oscillations which mechanical energy may be converted to electrical energy.
Among the methods that may be used to harvest energy, I mainly focus on piezoelectricity. Piezoelectric materials are known to produce electrical charge displacement in response to mechanical strain. This allows to produce electricity. On the modelization side, it is necessary to derive fully coupled fluid-solid-electrical conservation equations to predict the system behavior and compute estimates of the harvested electrical power.
Piezoelectric materials are particular materials that are able to convert mechanical strain into electricity, a phenomenon referred to as "direct piezoelectric effect". The "inverse piezoelectric effect" also exists: these materials are able to deform themselves in response to an electrical field forcing.
These materials are part of my concerns when I investigate energy harvesting from unstable oscillations or when I investigate active control of these oscillations.
The dynamical behaviour of shape-memory alloys (SMA) has received great attention in the last years for their potential use for mitigating undesirable vibrations. As a large amount of energy can be dissipated due to the hysteretic pseudo-elastic martensitic transformation, these alloys are natural candidates for designing innovative efficient passive vibration isolation devices.
The nonlinear dynamical response of structures made of these materials are then studied.