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Time Step and CEL expressions for Mesh Deformation

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Old   July 1, 2011, 05:10
Default Time Step and CEL expressions for Mesh Deformation
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Richard Barrett
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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.
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Old   July 2, 2011, 07:50
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Glenn Horrocks
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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.
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Old   July 4, 2011, 10:16
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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.
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Old   July 6, 2011, 11:13
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Richard Barrett
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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.
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