Can a piezoelectric create dynamic pressure in oil fluid for energy harvesting?

[javs]

Would it be possible to do a experimental setup where I create a dynamic pressure (10-500 Hz) of around 0.1-2 MPa, using a high force piezoelectric pushing on a piston on a Small volume of oil fluid.

There is this paper, where the authors create a pressure wave of around 170 KPa using a piezoelectric. I want to do something similar but with higher ∂P.

Well, I work on electronics, (energy harvesting) . So I need to provide a pressure disturbance to my test subject, the pressure amplitude should be around 0-2 MPa and the precision is not important. I can take 3-4 harmonics. I'll be sensing the pressure with a piezoelectric pressure sensor @ 5khz. I wanted to know if the ∂P will follow the relation of a compressible fluid : ∂P/B=∂V/V, where B: bulk modulus of the oil. Or it will follow another behaviour. I have zero knowledge in fluid dynamics, been reading about it, but cannot find a good answer. Thanks for the help

 

[Baluncore]

Welcome to PF.

If you generate a sinusoidal pressure variation, and if your hydraulic fluid is at atmospheric pressure, (100 kPa absolute), you will be limited to a peak pressure of about +200 kPa absolute. That is because, at the minimum of the pressure wave, –100kPa gauge = zero absolute), you will get cavitation. To get an overpressure of 2 MPa you will need a confining pressure of at least 1 MPa ≈ 10 atmospheres = 150 psi.

The paper that reported 170 kPa was probably operating at 100 kPa atmospheric confining pressure.

When the piezo element applies a force to the fluid, the movement of the element will be limited by the bulk modulus of the fluid compared with the bulk modulus of the piezo element. You need to find bulk modulus data on the piezo element and the hydraulic fluid. Do you have that information?

Depending on the shape of your test vessel, you can expect standing waves that may confuse the results by generating twice the pressure or cavitation. You will need to terminate your test tank in a lossy matched acoustic impedance.

 

[javs]

Welcome to PF.

If you generate a sinusoidal pressure variation, and if your hydraulic fluid is at atmospheric pressure, (100 kPa absolute), you will be limited to a peak pressure of about +200 kPa absolute. That is because, at the minimum of the pressure wave, –100kPa gauge = zero absolute), you will get cavitation. To get an overpressure of 2 MPa you will need a confining pressure of at least 1 MPa ≈ 10 atmospheres = 150 psi.

The paper that reported 170 kPa was probably operating at 100 kPa atmospheric confining pressure.

When the piezo element applies a force to the fluid, the movement of the element will be limited by the bulk modulus of the fluid compared with the bulk modulus of the piezo element. You need to find bulk modulus data on the piezo element and the hydraulic fluid. Do you have that information?

Depending on the shape of your test vessel, you can expect standing waves that may confuse the results by generating twice the pressure or cavitation. You will need to terminate your test tank in a lossy matched acoustic impedance.

Thanks for the great answer. So let see if I get you right, if I want a 2 MPa over pressure I will need a static pressure of 1 MPa to avoid cavitation?. What is the relation between static pressure and cavitation due to dynamic disturbance?. For the paper, the author reported a static pressure of 1000 kPa with a 170 kPa peak-peak dynamic pressure, so the dynamic pressure was ~20% of the static pressure.

For the bulk modulus of the piezo and fluid, I will be using a Noliac piezo with stiffness $k_{Piezo}= 4.5x10^6 \frac{N}{m}$ & $Bulk_{Piezo}=69 GPa$, the bulk modulus of the oil is not reported but taking as example similar fluids we can assume is around 0.9-1.2 GPa @ 30ºC.

If I'm right the displacement of the piezo will be given by:

$$ \Delta L = L \frac{F}{k_{fluid}+k_{piezo}}$$ : where $k_{piezo}$ and $k_{fluid}$ are the stiffness of the actuator and the stiffness of the oil enclosed in the fluid ( given by $k_{fluid}= \frac{ A^2 B}{V}$) and $F$ is the blocking force of the actuator (around ~9000 N). $L$ is the "no load" displacement,. If this relation is true, as long as the stiffness of the volume of the fluid is low compared to the actuator, I should get a decent displacement.

I am worried about the damping, wave cancellation and thermic effects in the volume, i.e., not getting any controllable pressure disturbance. I'm planning to have a small volume. i.e., a cylinder 10mm diameter x70mm length. Where can I refer to find a good approximation to find the relation between geometry and wave cancellation/superposition, it might be interesting to be able to obtain wave amplification with a small volume change (superposition). Do you think that the sinousoidal pressure change will have a normal response in the band from 10-500 Hz?

thanks a lot for the help. I don't want to start building stuff before having everything clear.

 

[Baluncore]

What is the relation between static pressure and cavitation due to dynamic disturbance?

Cavitation occurs when the absolute pressure falls below the vapour pressure of the fluid.

A symmetrical sinewave has equal pressure variations above and below the average confining pressure. Short pulses with a very low duty cycle have asymmetric pressure excursions. It comes down to the Fourier transform of the waveform being transmitted. The amplitude and phase of the harmonics will decide the pressure excursion symmetry.