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Old   October 12, 2014, 18:42
Default Post Processing Tetrahedral mesh
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Ali Madayen
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Hi everyone

I'm studying a very complex geometry which I cannot generate structured mesh for it. Hence, I used ICEM to mesh it with commonly tetrahedral elements (more than 90%)
Checking the grid independency with number of the elements varying from 300K to 1000K, results are independent from the mesh.
But the problem is, the chart that I draw for checking independency is not smooth, I've uploaded an image of it.
Is this due to poor mesh quality, or it is an intrinsic characteristic of the unstructured meshes? I should mention that the solution is well converged (Residuals 1e-5 and imbalance 1e-3.
Is there a way to make these graphs smooth?
I have not worked with unstructured grids that much so sadly i'm not familiar with them

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Ali
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Old   October 12, 2014, 19:05
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Glenn Horrocks
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Firstly - a mesh sensitivity sweep from 300k to 1M nodes is unlikely to be wide enough to capture anything useful. The parameter you should be sweeping is the element edge length. So a halving of the element edge length will result in approximately 2^3 = 8 times the number of nodes.

Your graph shows a parameter which should be smoothish but looks bumpy. You are correct in saying:
* Yes, this can be due to poor mesh quality.
* Yes, unstructured meshes frequently do this.

But mesh refinement should smooth it out more and reduce the wobblyness.
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Old   October 13, 2014, 06:59
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Ali Madayen
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Thanks for your help.
I've checked the 2.2M grid but the problem still exists.
I've extracted geometry from CT angio imaging by Mimics software, and I doubt smoothness of the surface. I plotted velocity in cross section for various meshes and they were identical.
Is this parameters for the mesh acceptable?
I couldn't find any suitable range for edge-length ratio, but for face angle values more than 10 is said to be acceptable.

60<Face Angle<160
1<Edge Length Ratio<13
Connectivity number: 2-42
volume ratio: 1-125
(I guess high volume ratio is due to the coarse-mesh entrance volume that i've placed in the inlet for having fully developed flow at the main volume inlet)

I would appreciate if you could comment on these parameters.
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Old   October 13, 2014, 07:02
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Glenn Horrocks
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Before considering any of the issues you discuss, have a look at the geometry you are modelling? How smooth is it? If it is rough - especially with roughness of the same length scale as your mesh - then no matter what you do you are going to get a lumpy results curve.
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Old   October 13, 2014, 07:11
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Ali Madayen
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I could plot this. X coordinate on the surface polyline.
I guess for a grid element size of 2.1e-4 these oscillations are not suitable.
Is there any better way to determine the smoothness?
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Old   October 13, 2014, 17:24
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Glenn Horrocks
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I have no idea what that graph is showing. You would have to show an image of your geometry for that to make sense.

Have a look at the wall boundary facets. Is the wall being resolved as a smooth curve, or does each facet have its own angle?
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