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Negative pressure on the surface of my model

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Posts: 15
 Shey
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(@shey)
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Joined: 3 years ago

Hello everyone,

I have a model that consists of a thin shell layer (skull) and a soft solid inside of that (brain). These parts are connected to each other using shared nodes; so there is no contact defined. I applied pressure-time history curve with peak overpressure about 300 kPa as my loading on half part of my skull. The problem is that I get negative pressures on the surface of my skull in post-processing. Anyone has any idea about the reason of that? I think it could be wave reflection.

I also added damping to my shell to eliminate lots of oscilations. When I apply a high damping factor, it removes oscilations and also make the pressure on skull surface positive. The problem here is that in this situation, the pressure in the center of my solid part gets the same as the pressure on the surface of the solid where it is connected to the shell. I am not sure if this approach is correct.

Any help or comment would be appreciated.

 

Thanks,

Sheida

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Negative Volume
Posts: 641
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(@negativevolume)
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Joined: 5 years ago

Hi @sheida, 

How are you prescribing the pressure? And how are you visualizing the pressure in post-processing? Are the values near what you would expect and just a polarity issue?

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 Shey
(@shey)
Joined: 3 years ago

Paid Intern
Posts: 15

Hi @negativevolume,

In my ASCII output (*elout) I get negative pressure on the same region that I applied positive pressure. Also, the absolute value of its peak is much higher than what I initially applied as my loading. Although when I add damping (Damping_Part_Mass) with high damping factor, it gives me the positive peak overpressure very close to what I applied as my liading first. I applied my pressure loading using *Load_Segment_set in a way that each wave enters perpendicular to each element. I did not know how to apply the pressure waves so that they enter to the region as parallel waves.

Thanks,

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Negative Volume
Admin
(@negativevolume)
Joined: 5 years ago

CEO
Posts: 641

@sheida have you taken into account that your prescribed load is applied to each segment in your set? In some applications, this might lead to higher than anticipated loads. 

Have you looked into using *Load_mask? You can use it to apply a distributed pressure load over a 3D shell part. If you have solid elements, you'll need to cover them with null shells. 

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 Shey
(@shey)
Joined: 3 years ago

Paid Intern
Posts: 15

Hi @negativevolume,

I did not know about *Load_Mask; I will try that and say if it make changes. Thank you.

I've also uploaded my prescribed loading here to better visualize it. I applied this pressure loading on the right side of my shell segment.

 

Thank you,

Sheida

 pressure time
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