# What is the meaning of stress free boundary condition?

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 March 16, 2013, 22:11 What is the meaning of stress free boundary condition? #1 Senior Member   Govindaraju Join Date: Apr 2010 Posts: 207 Rep Power: 10 Dear all I have come across stress free boundary condition at the outlet in a pressure drop analysis in many journals ( Transient analysis). I have confused with 0 Pa setup at the outlet with stress free boundary condition. for stress free boundary condition, I assume that the initial condition pressure setup and outlet boundary condition must be the same. Please correct me if I am wrong. Thank you Regards Govind

 March 18, 2013, 11:26 #2 Senior Member   OJ Join Date: Apr 2012 Location: United Kindom Posts: 475 Rep Power: 13 From what I understand, the stress-free boundary condition must be "shear-stress free" boundary condition which can be symmetry or free slip wall Analogically, since three is no change in velocity perpendicular to the boundary (unlike wall where we have significant velocity gradient), the shear stress value should be zero/negligible. Though, you may want to confirm this with a bit of literature search. OJ

March 19, 2013, 02:57
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Govindaraju
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 Originally Posted by oj.bulmer From what I understand, the stress-free boundary condition must be "shear-stress free" boundary condition which can be symmetry or free slip wall Analogically, since three is no change in velocity perpendicular to the boundary (unlike wall where we have significant velocity gradient), the shear stress value should be zero/negligible. Though, you may want to confirm this with a bit of literature search. OJ

so, stress free boundary condition doesn't mean that outlet pressure equals to zero . Am I correct?

I know the symmetry boundary condition where all the normal component should be zero. consider a pipe , how do you apply symmetry boundary condition at the outlet face?
if so what does it mean?

when we apply free slip boundary condition at the outlet face?

Thank you

Regards

Govind

Last edited by kmgraju; March 19, 2013 at 03:21.

 March 19, 2013, 05:31 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,991 Rep Power: 107 This question has been asked a few times on the forum. Do a search of the forum for more details. I recall the stress free BC is zero normal gradient at the outlet. CFX does not implement the outlet boundary exactly this way, read the documentation for what it does do. And implementing a zero stress BC is going to be difficult, and frankly why bother when the built in outlet boundary is better in many ways.

March 19, 2013, 05:41
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OJ
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 so, stress free boundary condition doesn't mean that outlet pressure equals to zero . Am I correct?
The stress free boundary (or symmetry) condition should have tangential (shear) stresses and normal velocity as zero. This is what precisely the characteristic of symmetry.

Ref: Houseman, G. A. "Boundary Conditions and Efficient Solution Algorithms For the Potential Function Formulation of the 3‐D Viscous Flow Equations." Geophysical Journal International 100.1 (1990): 33-38.

Quote:
 I know the symmetry boundary condition where all the normal component should be zero. consider a pipe , how do you apply symmetry boundary condition at the outlet face? if so what does it mean?
Whether stress-free boundary condition is appropriate for you or not, depends on your flow physics. You can have a model with symmetry boundary condition and outlet pressure as zero. Except, you can't specify symmetry at the outlet of your pipe. If you do that, the fluid can't flow out, nor can it flow parallel to outlet towards pipe walls as there won't be any motivation for it. The whole situation would be nonphysical.

Quote:
 when we apply free slip boundary condition at the outlet face?
Free slip wall condition is different than symmetry. Here, there can be normal components of gradients. Moreover, you can have curved surface as free slip, unlike symmetry where you need planar surfaces. Consequently, you can't have outlet, with free-slip wall, because fluid can't go out!!

OJ

March 10, 2016, 16:44
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 Originally Posted by oj.bulmer The stress free boundary (or symmetry) condition should have tangential (shear) stresses and normal velocity as zero. This is what precisely the characteristic of symmetry. Ref: Houseman, G. A. "Boundary Conditions and Efficient Solution Algorithms For the Potential Function Formulation of the 3‐D Viscous Flow Equations." Geophysical Journal International 100.1 (1990): 33-38. Whether stress-free boundary condition is appropriate for you or not, depends on your flow physics. You can have a model with symmetry boundary condition and outlet pressure as zero. Except, you can't specify symmetry at the outlet of your pipe. If you do that, the fluid can't flow out, nor can it flow parallel to outlet towards pipe walls as there won't be any motivation for it. The whole situation would be nonphysical. Free slip wall condition is different than symmetry. Here, there can be normal components of gradients. Moreover, you can have curved surface as free slip, unlike symmetry where you need planar surfaces. Consequently, you can't have outlet, with free-slip wall, because fluid can't go out!! OJ
Would you mind explain why Free slip wall condition is different than symmetry?

March 10, 2016, 18:17
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Glenn Horrocks
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 Free slip wall condition is different than symmetry. Here [free slip], there can be normal components of gradients. Moreover, you can have curved surface as free slip, unlike symmetry where you need planar surfaces.

August 11, 2017, 12:33
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can you have dirichlet and neumann boundary condition at same boundary at the same time?

 August 12, 2017, 06:35 #9 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,991 Rep Power: 107 Have a think about what they mean mathematically - the answer is no. You can define the boundary to have, in general, either a prescribed value at the boundary or a prescribed gradient, but you can't define it to have both a prescribed value and gradient.

August 24, 2017, 11:06
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 Originally Posted by ghorrocks Have a think about what they mean mathematically - the answer is no. You can define the boundary to have, in general, either a prescribed value at the boundary or a prescribed gradient, but you can't define it to have both a prescribed value and gradient.
Isn't setting up the reference value (for example pressure) is somewhat like a dirichlet boundary condition?

 August 24, 2017, 18:17 #11 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,991 Rep Power: 107 No. It just offsets the entire pressure field. It has nothing to do with defining boundary conditions.

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