[MUSIC] Hello my name is Sebastien Michelin. I'm an Associate Professor in mechanics at Ecole polytechnique. My research focuses on flow-induced vibrations and how we can use them to produce energy. Fluid solid interactions and instabilities are responsible for spontaneous vibrations of structures. Often, this sets in motion something we did not want to be moving. Therefore, research focuses traditionally on the control and mitigation of such instabilities because they can cause damage to engineering structures. For example, the Tacoma Narrows Bridge failure. However, these vibrations also represent great opportunity. Because we can use them to harvest energy from the flow such as wind or river current. In a classical wind turbine, mechanical energy is extracted from the rotation of the blades resulting from the wind forcing. The fraction of the solid kinetic energy is converted into electrical form by a generator that powers an output circuit. The mechanical efficiency of the device is then defined as the ratio of the extracted power to the available mechanical power. Here, this available power is simply the flux of kinetic energy in the fluid, through the cross-section occupied by the device. The same principle applies to an energy harvester based on flow-induced vibrations. For example, a flapping flag. When electromagnetic or electric generator, converts a fraction of the vibrations' mechanical energy into electricity. The critical parameter in the problem is the conversion intensity. Which can be understood as how much of the solid energy is converted into electricity. If this intensity is low, the solid's dynamics are marginally impacted. Basically, the solid does not see that you are taking energy out, and vibrations amplitudes remain large. However, the harvested energy and efficiency are very limited. Let's look at the opposite limit. If the conversion intensity is large, vibrations are damped out, which greatly reduces the available energy and therefore the system's efficiency. The optimal regime therefore exist between these two limits. Where the vibrations are modified due to the energy extraction but not completely mitigated. Understanding and quantifying this peak is a major challenge and objective of our research on flow-induced vibration and energy harvesting. Vortex-induced vibrations are classical example of spontaneous and self-sustained flow-induced vibrations. They result from the coupling of the bluff body's dynamics to its unsteady vortex wave. When the structure is a flexible cable, vortex-induced waves develop and carry the energy extracted from the flow, along the lengths of the cable. This energy can then be harvested by placing a generator at the top of the cable. We modeled this generator experimentally by attaching to the top end of a hanging cable a small rod that moves in viscous fluid mimicking the energy transfer and dissipation introduced by the generator. The viscosity of that fluid and the length of the immersed rod can be changed to tune the conversion intensity. Using video and laser measurements, the cables dynamics and the harvesting efficiency are monitored. The results demonstrate that there is indeed a region of maximum efficiency for intermediate damping intensity. These results were also confirmed numerically using a double oscillator model. One for the structure, and one for the wake. These are shown by the solid line and are in agreement with our experimental measurements. Our second example is a piezoelectric flag, placed in an actual wind, the flexible membrane undergoes spontaneous, periodic and large amplitude oscillations characterized by the propagation of bending waves along the structure. This is the so called flag instability problem. Piezoelectric patches positioned on the flag's top and bottom surfaces convert this deformation waves into an electric current and as such, act as an electric generator. In return the voltage in the circuit applies an additional mechanical stress on the structure, that modifies its dynamics. The system allows for a complete description of the interaction between the fruit solid system on one hand and the electrical circuit on the other. In recent years, we have made much progress in understanding this coupled dynamics. We showed, that a careful design of the circuit can lead to significant efficiency increases, even for simple passive circuits, such as an inductive resistive loop. For such systems, the circuit properties and the piezoelectric coupling can lock the flapping frequency onto the electric resonant frequency. This enables a forcing of the circuit exactly at resonance, leading to large deformation amplitudes and large efficiency peaks in an extended range of operating conditions. To learn more about this, you can visit my website where you will also find a publication on this topic. Thank you. [MUSIC]
