# mesh stiffness of FSI

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September 20, 2011, 03:13
mesh stiffness of FSI
#1
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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

Attached Images
 grid.jpg (18.1 KB, 53 views)

 September 20, 2011, 04:06 #2 Senior Member   Matthias Voß Join Date: Mar 2009 Location: Berlin, Germany Posts: 448 Rep Power: 13 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

September 20, 2011, 04:10
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Quote:
 Originally Posted by neewbie 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.

 September 20, 2011, 04:14 #4 Senior Member   Matthias Voß Join Date: Mar 2009 Location: Berlin, Germany Posts: 448 Rep Power: 13 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

September 20, 2011, 04:28
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Quote:
 Originally Posted by neewbie 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

September 20, 2011, 06:18
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Quote:
 Originally Posted by neewbie ... (sqrt((x-x0)^2+(y-y0)^2))^4
it has to be

September 20, 2011, 18:03
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Quote:
 Originally Posted by neewbie it has to be
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.

September 20, 2011, 18:47
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Quote:
 Originally Posted by colopolo 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.
Attached Images
 ANSYS.jpg (56.1 KB, 46 views)

Last edited by colopolo; September 20, 2011 at 20:20.

 September 21, 2011, 09:28 #9 Senior Member   Matthias Voß Join Date: Mar 2009 Location: Berlin, Germany Posts: 448 Rep Power: 13 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?

September 21, 2011, 19:43
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Quote:
 Originally Posted by neewbie 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.

 September 22, 2011, 03:52 #11 Member   Darren Leong Join Date: Dec 2010 Posts: 63 Rep Power: 8 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

September 22, 2011, 04:15
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Quote:
 Originally Posted by Darren Leong 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.

September 22, 2011, 21:50
#13
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Quote:
 Originally Posted by Darren Leong 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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