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Understanding Boundry Conditions
Posted 18 nov. 2015, 14:33 UTC−5 Structural Mechanics Version 5.1 4 Replies
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Hi there,
I am completely new to COMSOL and am trying to learn as i go along. I am trying to do a very simple stress simulation with a simple 2D geometry. I want to apply a small load to a few of the edges of my geometry and have the rest of the edges "free", then see the resulting strain in the centre but if i do not allocate at least 1 fixed constraint/roller constraint somewhere in the geometry i get " Failed to find a solution. The relative residual (0.28) is greater than the relative tolerance.
Returned solution is not converged."
Do i have to have at least one fixed constraint/roller constraint for this to work or am i missing something? Like i said i very new to this to sorry for my lack of understanding.
Thanks
I am completely new to COMSOL and am trying to learn as i go along. I am trying to do a very simple stress simulation with a simple 2D geometry. I want to apply a small load to a few of the edges of my geometry and have the rest of the edges "free", then see the resulting strain in the centre but if i do not allocate at least 1 fixed constraint/roller constraint somewhere in the geometry i get " Failed to find a solution. The relative residual (0.28) is greater than the relative tolerance.
Returned solution is not converged."
Do i have to have at least one fixed constraint/roller constraint for this to work or am i missing something? Like i said i very new to this to sorry for my lack of understanding.
Thanks
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4 Replies Last Post 24 nov. 2015, 13:17 UTC−5
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Posted:
9 years ago
Hello Waseem,
Indeed, you need to specify constraints sufficient to prevent all rigid body motions. This question pops up from time to time, see
www.comsol.com/community/forums/general/thread/46191
www.comsol.com/community/forums/structural-mechanics/thread/45963
etc .
In 2D there are only 3 rigid body motions, and adding three constraints is enough to make the problem well posed. For instant you can imposed a fixed constraint to point 19 and a y displacement of 0 at point 37.
Of course, this is only one possibility. You need to chose constraints that reflect the actual physical situation you are modeling.
Best,
Jeff
Indeed, you need to specify constraints sufficient to prevent all rigid body motions. This question pops up from time to time, see
www.comsol.com/community/forums/general/thread/46191
www.comsol.com/community/forums/structural-mechanics/thread/45963
etc .
In 2D there are only 3 rigid body motions, and adding three constraints is enough to make the problem well posed. For instant you can imposed a fixed constraint to point 19 and a y displacement of 0 at point 37.
Of course, this is only one possibility. You need to chose constraints that reflect the actual physical situation you are modeling.
Best,
Jeff
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Posted:
9 years ago
Thanks for your quick reply Jeff!
I am trying to place a load at each of the fan arm boundaries (13,14,18,20,29,30,43,44) using the coordinate system "Boundary System" and study the resulting stress in the centre of the geometry. Where would you suggest placing other necessary constraints to best reflect this?
Thanks in advance
I am trying to place a load at each of the fan arm boundaries (13,14,18,20,29,30,43,44) using the coordinate system "Boundary System" and study the resulting stress in the centre of the geometry. Where would you suggest placing other necessary constraints to best reflect this?
Thanks in advance
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Posted:
9 years ago
You need to pick how to constrain the model based on how the actual system is held in place. Sorry I can't be more specific, but it's a modeling decision that only someone familiar with the actual apparatus can make.
Jeff
Jeff
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Posted:
9 years ago
Maybe if i explain what i am trying to achieve that would help?
In theory i am trying to model a germanium structure (layer) grown onto a layer of silicon and introduce biaxial strain as a result. I suppose the structure would constrained from the bottom but there is no option to select this, just boundaries. I was told i would not need to add a silicon layer to simulate this, i.e. just add a boundary load to act as the cause of strain.
In theory i am trying to model a germanium structure (layer) grown onto a layer of silicon and introduce biaxial strain as a result. I suppose the structure would constrained from the bottom but there is no option to select this, just boundaries. I was told i would not need to add a silicon layer to simulate this, i.e. just add a boundary load to act as the cause of strain.