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Constraints to deformation
Posted 19.04.2018, 04:25 GMT-4 3 Replies
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Hello, I am simulating a 3D FSI model of an aortic valve inside a fluid domain. The valve consist of 3 leaflets which deform symmetricaly but i am simulating only the one of them. Since normally the leaflets are very close to each other, the deformation of the one leaflet i am modeling has to be so such the leaflet does not penetrate the other two hypothetical adjacent leaflets. How can i make this restriction? In other words i dont want the leaflet to deform in the direction where the other hypothetical leaflets are, but start from its initial position demorming to the outward direction radially. Can anyone suggest me an option? I attached some photos of my model to be helpful. Thankyou
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Hi Elen,
Depending on exactly how your model is set up, and which version you are running, there are two different possible approaches.
The first is to actually do a contact analysis. To do that, add a block which has one face in the symmetry plane. You can mesh it with a single brick element. Use a Fixed Constraint on the whole block. Now you can set up a contact problem with the face of this block as source boundary.
An example of this technique is https://www.comsol.com/model/prestressed-bolts-in-a-tube-connection-185 where there is a symmetry plane perpendicular to the pipe.
The second method is to impose your own 'penalty contact' condition. To do this, you can add a Spring Foundation node to the boundary that may come into contact. Then you write an expression for the spring force which is such that there is no stiffness when the boundary is inside the allowed region, and very high when it is outside. Alternatively, you can add a Boundary Load, which has the same type of behavior; a large force acts when passing the symmetry plane.
Regards, Henrik
-------------------Henrik Sönnerlind
COMSOL
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Thank you for replying Henrik it was really helpful.
I have tried the boundary load but since i have a curved and rotated geometry i had to find the exact coordinates of the symmetry plane to determine the acting force. I tried it but unfortunately the solving got stuck on a time step and didnt move on. About using the contact method, i am using a form union geometry. Does it need to be a form assembly? Also i am a begginer in comsol and i am not sure how to use this method. I think i cant add a block since it is a FSI method and the domains have to be continous.
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Hi Elen,
Using assembly mode is not necessary for doing contact analysis. It is mainly useful to avoid that the contacting objects are adjacent in the initial configuration.
The solver problem can be related to that there may be a need for a smooth transition from 'n contact' to high contact force. You could use an expression for the force like exp(-k*gap)
But there is another potential issue: The deforming mesh in the fluid cannot change its topolgy. Thus , you cannot completely close a gap. Often, leaving a small gap is good enough to reduce the the flow down to an acceptable level. If not, a trick that can be used is to increase the viscosity locally when the gap is small.
By the way, please check out this blog post: https://www.comsol.com/blogs/modeling-fluid-structure-interaction-in-a-heart-valve/
Regards,
Henrik
Henrik Sönnerlind
COMSOL