
[Sponsors] 
September 20, 2011, 03:13 
mesh stiffness of FSI

#1 
Member
^^
Join Date: Aug 2011
Posts: 70
Rep Power: 7 
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 

September 20, 2011, 04:06 

#2 
Senior Member
Matthias Voß
Join Date: Mar 2009
Location: Berlin, Germany
Posts: 447
Rep Power: 13 
hey,
you need to create an expression for the stiffening around the corners. smth. like (sqrt((xx0)^2+(yy0)^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 

#3  
Member
^^
Join Date: Aug 2011
Posts: 70
Rep Power: 7 
Quote:
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 windinduced 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: 447
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 

#5 
Member
^^
Join Date: Aug 2011
Posts: 70
Rep Power: 7 
Thanks.. I will try to do that. I really appreciate your help


September 20, 2011, 06:18 

#6 
Senior Member
Matthias Voß
Join Date: Mar 2009
Location: Berlin, Germany
Posts: 447
Rep Power: 13 

September 20, 2011, 18:03 

#7 
Member
^^
Join Date: Aug 2011
Posts: 70
Rep Power: 7 
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 

#8  
Member
^^
Join Date: Aug 2011
Posts: 70
Rep Power: 7 
Quote:
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. 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: 447
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 

#10 
Member
^^
Join Date: Aug 2011
Posts: 70
Rep Power: 7 
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 

#12  
Member
^^
Join Date: Aug 2011
Posts: 70
Rep Power: 7 
Quote:
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 xdir. I onle let it free moving at ydir and rotating at zdir but it moves also xdir. 

September 22, 2011, 21:50 

#13  
Member
^^
Join Date: Aug 2011
Posts: 70
Rep Power: 7 
Quote:
It looks ok. but Force can not converge after some time stpes. Force is increasing more and more.. how to control force convergence? 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
FSI Mesh stiffness  Andrew.Coughtrie  CFX  3  July 29, 2011 03:50 
external flow with snappyHexMesh  chelvistero  OpenFOAM  11  January 15, 2010 20:43 
Convergence moving mesh  lr103476  OpenFOAM Running, Solving & CFD  30  November 19, 2007 15:09 
Mesh Stiffness Option for Mesh Deformation  Ste_Lakey  CFX  3  January 19, 2006 17:33 
How to control Minximum mesh space?  hung  FLUENT  7  April 18, 2005 09:38 