Ultrasound Reflection & Transmission At Water-Gold Boundary Strange Behavior

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I am trying to simulate ultrasound propagation from a low intensity pulsed ultrasound (LIPUS) transducer in a water tank with a slab of solid gold placed in the middle of the tank. The goal is to develop an understanding of what the ultrasound pressure field should look like when I introduce large pieces of gold & eventually gold nanoparticles into the simulation. Based on the acoustic impedance mismatch of gold vs water, I have calculated that there should be ~95.5% reflection of the ultrasound waves at the water-gold interface, however in my results I am consistently seeing high transmission of the ultrasound waves into the gold block as well as some strange reverberation artifacts inside the gold.

I am using the frequency domain pressure acoustics module & defining my LIPUS transducer as a flat line with normal displacement. I defined the water & gold mediums in the pressure acoustics module with a user defined attenuation (COMSOL mph file attached as well). From my understanding, the pressure acoustics module should have the ability to compare acoustic impedance of the gold and water material domains and define reflection / transmission coefficients based on the acoustic impedance mismatch, however that seems to be failing in this simulation.

What's strange, is that if I swap the gold and water mediums with each other (slab of water in a gold tank), I have almost full reflection of the ultrasound waves at the gold-to-water boundry. It looks like for some reason I have the expected reflection only when travelling from domains of high acoustic impedance to low acousic impedance, but not low acoustic impedance to high acoustic impedance.

Does anyone know what is going on here?



3 Replies Last Post Jan 15, 2021, 5:58 AM EST
Edgar J. Kaiser Certified Consultant

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Posted: 2 weeks ago Jan 12, 2021, 3:19 PM EST

Hi Tyler,

for your modeling task you need to use the acoustic-structure interaction. Gold is a solid material that cannot be appropriately modeled by pressure acoustics.

Cheers Edgar

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Hi Tyler, for your modeling task you need to use the acoustic-structure interaction. Gold is a solid material that cannot be appropriately modeled by pressure acoustics. Cheers Edgar

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Posted: 2 weeks ago Jan 15, 2021, 3:58 AM EST

Hey Edgar,

Thank you for your helpful response. I have added in the solid mechanics module and the acoustic-structure interaction in multiphysics. The 2D pressure field now shows the correct reflected waves but no transmission. Is there a specific expression that I can plot in the solid mechanics module to show wave transmission (calculated as ~5%) through the gold slab?

I am also still curious about the capabilities of the pressure acoustics module. If I replace the gold slab with a theoretical fluid of high acoustic impedance, I still see the previously mentioned interactions taking place. To properly model reflection & transmission at a boundry between two fluids of high acoustic impedance mismatch, are there specific impedance boundry conditions that I need to use with the pressure acoustics module?

Hey Edgar, Thank you for your helpful response. I have added in the solid mechanics module and the acoustic-structure interaction in multiphysics. The 2D pressure field now shows the correct reflected waves but no transmission. Is there a specific expression that I can plot in the solid mechanics module to show wave transmission (calculated as ~5%) through the gold slab? I am also still curious about the capabilities of the pressure acoustics module. If I replace the gold slab with a theoretical fluid of high acoustic impedance, I still see the previously mentioned interactions taking place. To properly model reflection & transmission at a boundry between two fluids of high acoustic impedance mismatch, are there specific impedance boundry conditions that I need to use with the pressure acoustics module?

Edgar J. Kaiser Certified Consultant

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Posted: 2 weeks ago Jan 15, 2021, 5:58 AM EST

Hi Tyler,

you could use the displacement magnitude in the solid domain or some energy or power related quantity. For two different pressure acoustics domains no specific boundary settings should be needed to see the transmission. Could it be that you see a visualization effect? Try to plot the logarithm of the pressure, that helps to see fields with large differences in range.

Cheers Edgar

-------------------
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Hi Tyler, you could use the displacement magnitude in the solid domain or some energy or power related quantity. For two different pressure acoustics domains no specific boundary settings should be needed to see the transmission. Could it be that you see a visualization effect? Try to plot the logarithm of the pressure, that helps to see fields with large differences in range. Cheers Edgar

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