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question in the blood vessel tutorial

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In the attached tutorial, on page 15, the force (boundary load) in the x,y,z directions is specified with a negative sign. I am wondering if someone can explain what the negative sign signifies. I am asking because when I look under the fluid properties, the stress vectors are defined as spf.T_stressx , spf.T_stressy and spf.T_stressz and not -spf.T_stressx,-spf.T_stressy and -spf.T_stressz.

Thanks for the help.

Sirisha


6 Replies Last Post May 10, 2016, 5:10 a.m. EDT

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Posted: 1 decade ago May 25, 2011, 12:10 p.m. EDT
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Posted: 1 decade ago Jan 27, 2012, 7:50 a.m. EST
Hi,

I am also involved in implementing the similar problem. I also want to have clarification especially when implementing such relation to other problems.

Thanks
Hi, I am also involved in implementing the similar problem. I also want to have clarification especially when implementing such relation to other problems. Thanks

Nagi Elabbasi Facebook Reality Labs

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Posted: 1 decade ago Jan 29, 2012, 3:23 p.m. EST
The minus sign is because the traction or force per unit area on the solid surfaces is the negative of the same quantity on the matching fluid surfaces.

Nagi Elabbasi
Veryst Engineering
The minus sign is because the traction or force per unit area on the solid surfaces is the negative of the same quantity on the matching fluid surfaces. Nagi Elabbasi Veryst Engineering

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Posted: 10 years ago Dec 29, 2012, 3:05 a.m. EST
I think the the stress (spf.T_stressx , spf.T_stressy and spf.T_stressz ) is acted on the fluid , while the force acted on the artery walls is -spf.T_stressx , -spf.T_stressy and -spf.T_stressz. They are a pair of acting force and reacting force.
I think the the stress (spf.T_stressx , spf.T_stressy and spf.T_stressz ) is acted on the fluid , while the force acted on the artery walls is -spf.T_stressx , -spf.T_stressy and -spf.T_stressz. They are a pair of acting force and reacting force.

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Posted: 7 years ago Dec 7, 2015, 12:41 p.m. EST
in (r,z) coordinate system, these would be spf.T_stressr , spf.T_stressz right?
in (r,z) coordinate system, these would be spf.T_stressr , spf.T_stressz right?

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Posted: 7 years ago May 10, 2016, 5:10 a.m. EDT
www.comsol.com/blogs/how-do-i-compute-lift-and-drag/;
May this will help you , the minus sign would represent the force to "the fluid", if integral the force to the wall or boundary , "(spf.T_stressx , spf.T_stressy and spf.T_stressz )" would be the right choice.
https://www.comsol.com/blogs/how-do-i-compute-lift-and-drag/; May this will help you , the minus sign would represent the force to "the fluid", if integral the force to the wall or boundary , "(spf.T_stressx , spf.T_stressy and spf.T_stressz )" would be the right choice.

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