[Ari]

Hi everyone, does anyone know what is a "stress free" boundary condition? and how do you set that in FLUENT? I'm going to simulate the blood flow in a model of thoracic aorta. I've read some papers in which they talk about this stress free outflow BC to impose at the outlet. I've searched around but could't figure out what stress free boundary condition exactly is and how to impose it on fluent.
Any help would be really appreciated! thanks!

[FMDenaro]

Originally Posted by Ari

Hi everyone, does anyone know what is a "stress free" boundary condition? and how do you set that in FLUENT? I'm going to simulate the blood flow in a model of thoracic aorta. I've read some papers in which they talk about this stress free outflow BC to impose at the outlet. I've searched around but could't figure out what stress free boundary condition exactly is and how to impose it on fluent.
Any help would be really appreciated! thanks!

it is often equivalent to set to zero the normal derivative of the velocity components

[Ari]

Originally Posted by FMDenaro

it is often equivalent to set to zero the normal derivative of the velocity components

Thanks for your reply!
I also found this kind of explanation, but it still not clear to me how I'm supposed to impose this boundary condition at the outlet of my domain. I found some people talking about the use of symmetry condition in fluent but this is not clear to me since it seems to be more related to the no-slip condition at the wall and not at the outlet...

[hunt_mat]

Originally Posted by FMDenaro

it is often equivalent to set to zero the normal derivative of the velocity components

from a mathematical perspective, does this come from:
[LaTeX Error: Syntax error]
I have a sheet of compressible elastic material that is resting on one edge, and the others are free to move.

I know that there is going to be no stress on the boundary and no shear. Would the above BC cover the conditions?

[LuckyTran]

Latex formula showing but,

Normal derivative equal to zero is the impenetrable BC for a slip-wall. If you have a free surface of an elastic solid (which I suspect you do from your bajillion other posts) then you need the entire stress tensor going to zero. For the butted end, normal derivative of velocity going to zero is okay because that is also the impenetrable BC.

[hunt_mat]

Originally Posted by LuckyTran

Latex formula showing but,

Normal derivative equal to zero is the impenetrable BC for a slip-wall. If you have a free surface of an elastic solid (which I suspect you do from your bajillion other posts) then you need the entire stress tensor going to zero. For the butted end, normal derivative of velocity going to zero is okay because that is also the impenetrable BC.

So set the ENTIRE tensor to be zero at the boundary? I need to think about how I derive the stress condition for the part resting on the floor.

Cheers.

[LuckyTran]

Your case is 1D, your "entire" stress tensor is only one derivative

[hunt_mat]

Originally Posted by LuckyTran

Your case is 1D, your "entire" stress tensor is only one derivative

The 1D case is done, and I'm looking to 2D and 3D. I have seen that stress-free should really mean "traction free"? That is the vector . I've seen this around the internet, and it makes sense, as it can be thought of as a force.

I'm thinking about a 2D compressible sheet with a free surface. I know that on that free surface there is no stress(or traction?) and I have the usual free surface equation derived by saying that particles on the free surface remain on the free surface.