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Old   September 20, 2011, 03:13
Default mesh stiffness of FSI
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Dear All,

I am trying to run vibration of plate using fsi (to find critical flutter velocity)

I have a big problem about folded mesh. I have been searched this problem in here and google but I cannot solve it.

I did control mesh stiffness of increase near boundary and increase near small volume. I used 1 or 10.

But every running case has folded mesh.

I really have no idea how to control foled mesh.

Please welcome any ideas for this problem

Thanks in advance
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Old   September 20, 2011, 04:06
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Matthias Voß
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hey,

you need to create an expression for the stiffening around the corners. smth. like (sqrt((x-x0)^2+(y-y0)^2))^4 with x0/y0 being the coordinates of the current location of one corner e.g. (itīs a circle function to the power of 4 to make it really hard at the pref. point).

Create an expression for every point or even for smth. like a line function and overlay them by summation in a final expression for the stiffness.

The tricky thing is to update the x0/y0 every timestep. There is a solution for this ... smth. dealing with monitorpoints.... i donīt remember exactly.

neewbie
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Old   September 20, 2011, 04:10
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Quote:
Originally Posted by neewbie View Post
hey,

you need to create an expression for the stiffening around the corners. smth. like (sqrt((x-x0)^2+(y-y0)^2))^4 with x0/y0 being the coordinates of the current location of one corner e.g. (itīs a circle function to the power of 4 to make it really hard at the pref. point).

Create an expression for every point or even for smth. like a line function and overlay them by summation in a final expression for the stiffness.

The tricky thing is to update the x0/y0 every timestep. There is a solution for this ... smth. dealing with monitorpoints.... i donīt remember exactly.

neewbie
Thanks you your response. I am not familiar with expression formula.
Do I just put the above equation in the mesh stiffness term of CFX?
Could you tell me a little bit detail about how to create it?

This is wind-induced vibration problem so how to define x0 and y0?

Sorry to bother you.

Thanks.
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Old   September 20, 2011, 04:14
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take a look at the tutorials ... creating the expressions is straight forward.
create an expression stiff1, stiff2 with the app. expressions for x1,y1,x2,y2 and combine them to letīs say: stiffness=stiff1+stiff2
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Old   September 20, 2011, 04:28
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Quote:
Originally Posted by neewbie View Post
take a look at the tutorials ... creating the expressions is straight forward.
create an expression stiff1, stiff2 with the app. expressions for x1,y1,x2,y2 and combine them to letīs say: stiffness=stiff1+stiff2
Thanks.. I will try to do that. I really appreciate your help
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Old   September 20, 2011, 06:18
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Quote:
Originally Posted by neewbie View Post
... (sqrt((x-x0)^2+(y-y0)^2))^4
it has to be \sqrt{(x-x_{0})^{2}+(y-y_{0})^{2}}^{-4}
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Old   September 20, 2011, 18:03
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Quote:
Originally Posted by neewbie View Post
it has to be \sqrt{(x-x_{0})^{2}+(y-y_{0})^{2}}^{-4}
Very appreciate it. I tried to make CEL expression but I do not have no idea how to define x0 and y0.
Does this x0 and y0 indicate the coordinate of edge of plate?
if you help me more,. thank you very much.

Last edited by colopolo; September 20, 2011 at 18:38.
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Old   September 20, 2011, 18:47
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Originally Posted by colopolo View Post
Very appreciate it. I tried to make CEL expression but I do not have no idea how to define x0 and y0.
Does this x0 and y0 indicate the coordinate of edge of plate?
if you help me more,. thank you very much.

I changed more simple tetra mesh and attached a mesh image just before error.

I supposed the error occur near the plate but, in my case, large mesh deformation occur between inlet and plate.

Does anyone has any ideas?

I am still trying to make CEL which you mentioned.
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Old   September 21, 2011, 09:28
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just guessing... but wouldnīt smth. like C*f(x) for the stiffness solve this similar to the idea above? Isnīt there a "stiff near bc" option?
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Old   September 21, 2011, 19:43
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Quote:
Originally Posted by neewbie View Post
just guessing... but wouldnīt smth. like C*f(x) for the stiffness solve this similar to the idea above? Isnīt there a "stiff near bc" option?
Thanks for ur response. There is "stiff near bc'' option. I also controlled 'stiff near bc' as 1, 10, and 100. but nothing changed.
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Old   September 22, 2011, 03:52
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Leave the exponent value at 10. Looking at your pic, the mesh in the farfield is skewing. Try enlarging the domain or have a coarser mesh in the farfield which will allow more deformation overall
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Old   September 22, 2011, 04:15
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Quote:
Originally Posted by Darren Leong View Post
Leave the exponent value at 10. Looking at your pic, the mesh in the farfield is skewing. Try enlarging the domain or have a coarser mesh in the farfield which will allow more deformation overall

Thanks !! I will try to do that and leave the result in this thread..

Very appreciate it

Also, I have another problem. the plate moves at x -direction even if the plate is constrained in x-dir.

I onle let it free moving at y-dir and rotating at z-dir but it moves also x-dir.
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Old   September 22, 2011, 21:50
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Quote:
Originally Posted by Darren Leong View Post
Leave the exponent value at 10. Looking at your pic, the mesh in the farfield is skewing. Try enlarging the domain or have a coarser mesh in the farfield which will allow more deformation overall

It looks ok. but Force can not converge after some time stpes.
Force is increasing more and more.. how to control force convergence?
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