problem in solving fsi simulation

siavash · July 9, 2009, 05:31

Hello every one,
in a FSI problem a solid circle moves in water.i use ansys & cfx11.0 for this simulation but it stops with errors.(negative element volume & folded mesh).i change the size of meshes and velocities many times but it doesn't work.a simple image is attached.
thanks,

ckleanth · July 9, 2009, 05:53

what type of mesh do you use? - I wouldn't be surprised if you used tetra with cfx mesh as its very easy to distort your mesh and have the problems you just mentioned.

if this is the case the solution is:
a) use a different meshing tool to create a purely hexa mesh if your simulation is 2d as in your picture.
b) if the above is not available create a better mesh around the circle stop, remesh and restart your simulation depending of the movement of the mesh in your problem.
c) upgrade in v12 as the meshing tools are better and a lot more functions are available to use in cfx

michael_owen · July 9, 2009, 10:11

Is the cylinder constrained from touching the wall in some fashion (like spring mounting it)? Unless it is, the code will inevitably crash when the cylinder approaches the wall.

Other than that, the two main things in problems like this are to reduce the timestep and increase the number of convergence iterations for the mesh displacement, on the equation class settings tab of the Solver Controls from the default 5.

Another problem you could run into, even if the cylinder cannot touch the walls, is that if it moves a significant distance down the channel, it can stretch the elements until they fold because of the curvature of the front of the cylinder. A technique that I would use here would be to cut out a section of the channel around the cylinder, and move the ENTIRE block parallel to the channel, and the cylinder within the block transverse to the channel. Obviously this would require some advanced CEL.

Or switch to R12.

ghorrocks · July 9, 2009, 19:35

Hi,

This should be fairly easy in the immersed solid feature and 6DOF solver in V12. I would recommend you upgrade to V12, it will be much easier.

Glenn Horrocks

siavash · July 10, 2009, 12:54

as you wrote i used tetra mesh!
if i create hexa mesh can i generate a "gtm" file?
thanks for your answer.

siavash · July 10, 2009, 12:59

thanks for your answer.

July 9, 2009, 05:53		#2
ckleanth Senior Member George Join Date: Mar 2009 Location: Birmingham, UK Posts: 257 Rep Power: 18	what type of mesh do you use? - I wouldn't be surprised if you used tetra with cfx mesh as its very easy to distort your mesh and have the problems you just mentioned. if this is the case the solution is: a) use a different meshing tool to create a purely hexa mesh if your simulation is 2d as in your picture. b) if the above is not available create a better mesh around the circle stop, remesh and restart your simulation depending of the movement of the mesh in your problem. c) upgrade in v12 as the meshing tools are better and a lot more functions are available to use in cfx __________________ Top 4 tips 1. Knowledge is everything and Ignorance is dangerous. 2. Understand your limitations and try to eliminate them. 3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window. 4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials

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July 9, 2009, 10:11		#3
michael_owen Senior Member Michael P. Owen Join Date: Mar 2009 Posts: 196 Rep Power: 17	Is the cylinder constrained from touching the wall in some fashion (like spring mounting it)? Unless it is, the code will inevitably crash when the cylinder approaches the wall. Other than that, the two main things in problems like this are to reduce the timestep and increase the number of convergence iterations for the mesh displacement, on the equation class settings tab of the Solver Controls from the default 5. Another problem you could run into, even if the cylinder cannot touch the walls, is that if it moves a significant distance down the channel, it can stretch the elements until they fold because of the curvature of the front of the cylinder. A technique that I would use here would be to cut out a section of the channel around the cylinder, and move the ENTIRE block parallel to the channel, and the cylinder within the block transverse to the channel. Obviously this would require some advanced CEL. Or switch to R12.

July 9, 2009, 19:35		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	Hi, This should be fairly easy in the immersed solid feature and 6DOF solver in V12. I would recommend you upgrade to V12, it will be much easier. Glenn Horrocks

July 10, 2009, 12:54		#5
siavash New Member siavash ghassemi Join Date: Jul 2009 Posts: 11 Rep Power: 17	as you wrote i used tetra mesh! if i create hexa mesh can i generate a "gtm" file? thanks for your answer.

July 10, 2009, 12:59		#6
siavash New Member siavash ghassemi Join Date: Jul 2009 Posts: 11 Rep Power: 17	thanks for your answer.