# Time Step and CEL expressions for Mesh Deformation

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 LinkBack Thread Tools Display Modes
 July 1, 2011, 04:10 Time Step and CEL expressions for Mesh Deformation #1 New Member   Richard Barrett Join Date: Jun 2011 Posts: 14 Rep Power: 8 Hi all, I am relatively new to CFX and I was just wondering if anyone could help me with a few problems that I have been having with a mesh deformation problem. I am modelling the compression of a piston within a cylinder. The actual geometry consists of two pistons within one cylinder, so there is a line of symmetry at mid chamber location. My model is also axisymmetric. The problem that I am having is that the simulation deformation is over predicting the applied displacement and I was wondering if there is an issue with (i) my CEL expressions or (ii) my time step. (i) The CEL expressions that I am using are ('WeightFunction' is a linear weighting function and 'Li' is the total length of the compression chamber): Disp = sDisp*WeightFunction Li = 0.178 [m] WeightFunction = 1-((z-sDisp)/(Li-sDisp)) sDisp = velCV*t velCV = 8 [m/s] (ii) The time step that I am using is 5.2e-4 s for a total compression time of 16.6 ms. Any help with this issue is greatly appreciated.

 July 2, 2011, 06:50 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,210 Rep Power: 110 You seem to be describing a strange motion. Doesn't your function describe a displacement proportional to z height? That means the piston will pivot, not slide. The best way to debug mesh motion stuff is to do a test run with all the solvers turned off using the expert parameters, and saving a result file (including the mesh!) every time step. Then you can quickly run the mesh motion and check you are getting the motion you intend.

 July 4, 2011, 09:16 #3 Senior Member   Join Date: Apr 2009 Posts: 532 Rep Power: 14 There might be a problem with your weighting function here. Instead of using sDisp in WeightFunction use a callback to get the piston position, e.g areaAve(zGlobal)@movingend. The callback is lagged to the previous timestep, but sDisp is not, which may be causing the unexpected behaviour.

 July 6, 2011, 10:13 #4 New Member   Richard Barrett Join Date: Jun 2011 Posts: 14 Rep Power: 8 The issue was related to sDisp in the weighting function. I have now replaced this with a function based on t-tStep and the model is deforming as desired. Thanks a million for your help.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post xujjun CFX 9 June 9, 2009 07:59 Jervds CFX 0 March 4, 2008 11:03 xujjun CFX 1 January 15, 2008 17:46 new@cfx CFX 3 September 30, 2005 08:03 joseph CFX 1 April 5, 2002 23:06

All times are GMT -4. The time now is 09:19.

 Contact Us - CFD Online - Privacy Statement - Top