Timestep with mesh deformation

rbarrett · July 19, 2011, 12:44

Hi all,

I am attempting to simulate the compression of a piston within a cylinder. The model is axisymmetric and I have used Ansys Meshing to generate the mesh. Mesh deformation is employed through the use of CEL expressions, coupled together with the use of 'Specified Displacement' mesh motion.

The problem that I am having is related to convergence 11.6 ms into a 16.6 ms compression time. Up to this point, relatively good convergence is obtained and then the solution diverges. I have tried reducing the size of the time step, but reducing this value beyond a certain point (3e-04 [s]), the model fails due to a negative volume error at multiple locations throughout the deforming zone.

I would greatly appreciate any help on this matter, particularly in relation to the size of the timestep employed.

ghorrocks · July 19, 2011, 20:22

Highly compressing a mesh using moving mesh is difficult, you frequently get negative volume errors. You might be able to improve things using mesh weighting factors, but the biggest step forwards in this area is in CFX V14 where you have much more control over the mesh motion diffusion.

rbarrett · July 21, 2011, 12:40

Hi ghorrocks. Thanks for your reply. Unfortunately I have only access to Ansys v12.1. Are there any other workarounds within CFX itself that may allow for more accurate modeling of a large compression, i.e. inputting a new mesh and interpolating a results file to give the initial values?

stumpy · July 21, 2011, 15:49

Glenn, out of interest, what's in v14 that's new for mesh motion diffusion?

ghorrocks · July 21, 2011, 20:25

For the V14 new stuff check out the ANSYS Community portal. One thing which caught my eye in this area is the ability to define an anisotropic mesh diffusion for the mesh smoothing step. The demo case using it compared a mesh being compressed which distorted using normal isotropic diffusion, but the anisotropic diffusion kept the mesh nice as the mesh was squashed.

Richard - if you want to model a large compression and want to stay on V12 then you have two options: Either use fortran to define the mesh motion, and then you can control it directly; or put a remeshing step in there.

July 19, 2011, 12:44	Timestep with mesh deformation	#1
rbarrett New Member Richard Barrett Join Date: Jun 2011 Posts: 14 Rep Power: 15	Hi all, I am attempting to simulate the compression of a piston within a cylinder. The model is axisymmetric and I have used Ansys Meshing to generate the mesh. Mesh deformation is employed through the use of CEL expressions, coupled together with the use of 'Specified Displacement' mesh motion. The problem that I am having is related to convergence 11.6 ms into a 16.6 ms compression time. Up to this point, relatively good convergence is obtained and then the solution diverges. I have tried reducing the size of the time step, but reducing this value beyond a certain point (3e-04 [s]), the model fails due to a negative volume error at multiple locations throughout the deforming zone. I would greatly appreciate any help on this matter, particularly in relation to the size of the timestep employed.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
[ICEM] Unstructure Meshing Around Imported Plot3D Structured Mesh ICEM	kawamatt2	ANSYS Meshing & Geometry	17	December 20, 2011 12:45
Mesh reload and deformation	Attesz	CFX	3	March 20, 2011 22:19
Mesh deformation using user fortran	matled	CFX	3	February 18, 2010 17:49
CFX mesh deformation of fine elements	Ji Ke	CFX	2	January 30, 2008 04:38
How to control Minximum mesh space?	hung	FLUENT	7	April 18, 2005 10:38

July 19, 2011, 20:22		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Highly compressing a mesh using moving mesh is difficult, you frequently get negative volume errors. You might be able to improve things using mesh weighting factors, but the biggest step forwards in this area is in CFX V14 where you have much more control over the mesh motion diffusion.

July 21, 2011, 12:40		#3
rbarrett New Member Richard Barrett Join Date: Jun 2011 Posts: 14 Rep Power: 15	Hi ghorrocks. Thanks for your reply. Unfortunately I have only access to Ansys v12.1. Are there any other workarounds within CFX itself that may allow for more accurate modeling of a large compression, i.e. inputting a new mesh and interpolating a results file to give the initial values?

July 21, 2011, 15:49		#4
stumpy Senior Member Join Date: Apr 2009 Posts: 531 Rep Power: 21	Glenn, out of interest, what's in v14 that's new for mesh motion diffusion?

July 21, 2011, 20:25		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	For the V14 new stuff check out the ANSYS Community portal. One thing which caught my eye in this area is the ability to define an anisotropic mesh diffusion for the mesh smoothing step. The demo case using it compared a mesh being compressed which distorted using normal isotropic diffusion, but the anisotropic diffusion kept the mesh nice as the mesh was squashed. Richard - if you want to model a large compression and want to stay on V12 then you have two options: Either use fortran to define the mesh motion, and then you can control it directly; or put a remeshing step in there.