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pressure wave generation due to thermal expansion in fluid

Posted Apr 26, 2015, 5:11 p.m. EDT Structural Mechanics Version 5.0 5 Replies

Yuankang Lu
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Hi all,

I'm a beginner of COMSOL, and I m working on a laser-ultrasonic model. But I have encountered a problem and hope someone can help ;)

My model is relatively simple. a small block of metal is immerged in water and a laser is used to heat this metal. The metal will go through an thermo expansion because of the laser heating and generate pressure wave in water.

I split the model in two parts. In the first part, I use the module " Thermal stress" to simulate the thermal expansion of the metal. In the second part, I use an acoustic module "acoustic-solid interaction " to simulate the pressure wave generation and propagation in water.

The output of the first part, Thermal Stress, is displacement, while the input (or boundary conditions) of acoustic model is always pressure. I have some difficulty in setting the boundary conditions for the second part of my simulation.

Does anybody have some ideas? Any suggestions are welcomed ;) Thx
5 Replies Last Post Nov 24, 2015, 10:17 p.m. EST
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Hello Yuankang Lu

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Rijil Thomas
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Posted: 9 years ago Nov 24, 2015, 6:36 a.m. EST
Hi,
May I ask, were you able to solve this problem? Even I have a similar problem and I wish to know how did you manage to solve it?
Especially, the boundary conditions you used for the first part to find displacement field.
Hi, May I ask, were you able to solve this problem? Even I have a similar problem and I wish to know how did you manage to solve it? Especially, the boundary conditions you used for the first part to find displacement field.
Nagi Elabbasi Facebook Reality Labs
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Posted: 9 years ago Nov 24, 2015, 12:35 p.m. EST
Hi Rijil,

That’s an interesting problem. Thermal expansion (and heating) is usually slow enough that it does not create a significant pressure wave in a surrounding fluid.

Nagi Elabbasi
Veryst Engineering
Hi Rijil, That’s an interesting problem. Thermal expansion (and heating) is usually slow enough that it does not create a significant pressure wave in a surrounding fluid. Nagi Elabbasi Veryst Engineering
Rijil Thomas
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Posted: 9 years ago Nov 24, 2015, 1:11 p.m. EST
Hi Nagi,
Correct me if I am wrong.
Isn't the same principle of heating and thermal expansion that is being obeyed in photoacoustics?
According to photoacoustics, from what I read, a sudden heating due to a very short laser pulse will cause heating and expansion producing pressure to surrounding medium.
Hi Nagi, Correct me if I am wrong. Isn't the same principle of heating and thermal expansion that is being obeyed in photoacoustics? According to photoacoustics, from what I read, a sudden heating due to a very short laser pulse will cause heating and expansion producing pressure to surrounding medium.
Nagi Elabbasi Facebook Reality Labs
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Posted: 9 years ago Nov 24, 2015, 1:57 p.m. EST
Thanks Rijil, you are right. I was not aware of applications involving such a fast rate of change in temperature! So, going back to the original questions. The thermal boundary conditions for the thermal-structural part should in my opinion be a boundary heat flux (spatially varying if necessary) on the surface where the laser is applied to the metal. The structural boundary conditions depend on how the block is held in place. The surface of the solid block should be automatically detected by COMSOL as a shared structural-acoustic boundary. Therefore, COMSOL will set up the correct interface conditions relating water pressure to displacements/velocities/accelerations of the metal block. Depending on the geometry/loading/material properties there may be substantial heating of the water as well, in which case you need a thermoacoustic model of the water as well. COMSOL should again handle the thermal interface conditions between block and water. There is a good COMSOL Model Gallery example called “Photoacoustic Resonator” that you have probably seen already. It has some good details.

Good luck.

Nagi Elabbasi
Veryst Engineering
Thanks Rijil, you are right. I was not aware of applications involving such a fast rate of change in temperature! So, going back to the original questions. The thermal boundary conditions for the thermal-structural part should in my opinion be a boundary heat flux (spatially varying if necessary) on the surface where the laser is applied to the metal. The structural boundary conditions depend on how the block is held in place. The surface of the solid block should be automatically detected by COMSOL as a shared structural-acoustic boundary. Therefore, COMSOL will set up the correct interface conditions relating water pressure to displacements/velocities/accelerations of the metal block. Depending on the geometry/loading/material properties there may be substantial heating of the water as well, in which case you need a thermoacoustic model of the water as well. COMSOL should again handle the thermal interface conditions between block and water. There is a good COMSOL Model Gallery example called “Photoacoustic Resonator” that you have probably seen already. It has some good details. Good luck. Nagi Elabbasi Veryst Engineering
Rijil Thomas
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Posted: 9 years ago Nov 24, 2015, 10:17 p.m. EST
Thank you Nagi for your response.
Can I clarify few more things?
In my case, it is a nanoparticle suspended in a medium. And I know the heat absorbed by the particle. So I modeled the particle in heat transfer physics as a heat source (W/m^3). Do I also need to give the heat flux which is in W/m^2 ? If so, how to convert heat source to heat flux?
Furthermore, it is suspended freely in medium. So how will the structural boundary condition be?
Thank you Nagi for your response. Can I clarify few more things? In my case, it is a nanoparticle suspended in a medium. And I know the heat absorbed by the particle. So I modeled the particle in heat transfer physics as a heat source (W/m^3). Do I also need to give the heat flux which is in W/m^2 ? If so, how to convert heat source to heat flux? Furthermore, it is suspended freely in medium. So how will the structural boundary condition be?

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