Research report : Energy harvesting

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From the course by École Polytechnique

Fundamentals of Fluid-Solid Interactions

155 ratings

École Polytechnique

Fundamentals of Fluid-Solid Interactions

155 ratings

What is fluid-solid interactions ? It is what happens when the motions of a fluid and of a solid are somehow coupled. This happens all the time, around you when leaves flutter in the wind, inside you when your heart beats, above you when wings of a plane vibrate, under the sea... The idea behind this MOOC is to give you the basic tools to be able to predict and eventually mitigate things called flutter, galloping, sloshing, vortex-induced vibrations, added mass, to cite a few. We are going to consider any possible domains of applications such as civil engineering, aerospace engineering, nuclear engineering , ocean engineering, biomechanics and even food processing ! This is why we called the course “Fundamentals of Fluid Solid Interactions ”. There are so many phenomena and so many models that we need to work together on the basic mechanisms . If you want to see how fluid-solid interactions work, and be able to use that knowledge, join us ! A first session of the course was run in early 2016, with learners from over 100 countries. It is now available with subtitles, in English and now in Chinese. See the video at http://goo.gl/YKSMnD

From the lesson

Coupling with any flow

6.1 The garden hose instability8:31

6.2 The fluid-conveying pendulum12:16

6.3 A model for all fluid velocities6:22

6.4 The fluid-conveying bi-pendulum9:31

Experiments : Fluid-conveying pipes3:59

6.5 Vortex-induced vibrations10:25

6.6 Lock-in10:12

Research report : Energy harvesting5:44

A case of wind engineering (in French with English subtitles)5:26

Meet the Instructors

Emmanuel de Langre
Professor
Mechanics
Xavier Amandolese
Lecturer
Mechanics
Olivier Doaré
Professor
IMSIA, ENSTA Paristech

[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]

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