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strange behavior of pressure

Posted Jun 13, 2012, 1:19 p.m. EDT Structural Mechanics Version 4.3 4 Replies

Jessica
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Hi,

I have a rubber-like material cylinder whose inner surface pressure is supposed to change as a function of time like p=1000*t. The load type that I have used is "Pressure" on that boundary. When I plot the deformation with time I can see the expansion of the cylinder, which is expected, but there are two issues:

1) when I "Probe" the value of the pressure with time on that boundary, the pressure is not following p=1000*t at all. Even the pressure changes non-linearly with time. I am probing "solid.p". Is this the correct variable to select? May be I am probing unrelated variable. My assumption is that that is the same as "Pressure" that I selected as the load type. I plotted the Surface Plot of solid.p and I noticed that the value on the inner surface is almost equal to the correct value (1000*t). But I don't know what is the definition of COMSOL for pressure within a DOMAIN, because stresses are defined to supposedly take care of the inner parts of a domain.

2) the values that the Probe gives me are negative, which do not make sense. I don't know but maybe because the program is treating it like a compressive stress!!, which make it negative. Of course as I mentioned earlier, the cylinder is expanding, which means that pressure is active as a positive value.

Any comments about these issues are appreciated.

Jessica

4 Replies Last Post Jun 15, 2012, 2:25 p.m. EDT
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Jun 13, 2012, 1:35 p.m. EDT
Two issues. Pressure is positive when compressive, so it makes sense that you’re getting negative (tensile) values. Also you are probing the wrong quantity. An applied pressure on a boundary should be equal to the projection of the stress tensor on the boundary (called surface traction in COMSOL) not the pressure.

Nagi Elabbasi
Veryst Engineering
Two issues. Pressure is positive when compressive, so it makes sense that you’re getting negative (tensile) values. Also you are probing the wrong quantity. An applied pressure on a boundary should be equal to the projection of the stress tensor on the boundary (called surface traction in COMSOL) not the pressure. Nagi Elabbasi Veryst Engineering
Jessica
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Posted: 1 decade ago Jun 13, 2012, 2:58 p.m. EDT

Two issues. Pressure is positive when compressive, so it makes sense that you’re getting negative (tensile) values. Also you are probing the wrong quantity. An applied pressure on a boundary should be equal to the projection of the stress tensor on the boundary (called surface traction in COMSOL) not the pressure.

Nagi Elabbasi
Veryst Engineering


Hi Nagi,

Thank you very much for your comments.
In fact my pressure is compressing the cylinder, so it should be positive based on what you mentioned and as I expected, but it is negative.
Surface Traction worked fine and I got the correct results. But not sure what solid.p is.

Thank you again,
Jessica
[QUOTE] Two issues. Pressure is positive when compressive, so it makes sense that you’re getting negative (tensile) values. Also you are probing the wrong quantity. An applied pressure on a boundary should be equal to the projection of the stress tensor on the boundary (called surface traction in COMSOL) not the pressure. Nagi Elabbasi Veryst Engineering [/QUOTE] Hi Nagi, Thank you very much for your comments. In fact my pressure is compressing the cylinder, so it should be positive based on what you mentioned and as I expected, but it is negative. Surface Traction worked fine and I got the correct results. But not sure what solid.p is. Thank you again, Jessica
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Jun 13, 2012, 4:33 p.m. EDT
Hi Jessica, glad I could help.
The pressure is the negative of the first stress invariant:
solid.p = -(solid.sx+solid.sy+solid.sz)/3
Hi Jessica, glad I could help. The pressure is the negative of the first stress invariant: solid.p = -(solid.sx+solid.sy+solid.sz)/3
Jessica
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Posted: 1 decade ago Jun 15, 2012, 2:25 p.m. EDT
Thank you.
Thank you.

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