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More errors with the coupled heat/fluid transient solver

6 REPLIES 6
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Message 1 of 7
bjorn_fallqvist
1030 Views, 6 Replies

More errors with the coupled heat/fluid transient solver

Sorry for posting yet another topic about this, but I am encountering a massive amount of error messages when running analyses with the transient coupled heat/fluid solver.

 

This is the error message shown in the log file:

 

 **** Error: No active fluid flow group.

 

What does this mean? I have checked, and re-checked the element type, element material (fluid: air), re-meshed the entire model, but nothing helps.

 

This is how the summary file looks like:

 

 Autodesk Algor Thermal Decoder for AGSDB
 Version 2011.00.00.0111-W64/X64 12-MAR-2010
 Copyright (c) 2010, Autodesk, Inc. All rights reserved.


** Model [C:\Users\Emil Olofsson\Documents\Algor tests\radiator_air.ds_data\1\ds] opened

   --- 84231 nodes
   --- 2 element parts ...


*** Warning *** Elements with zero jacobian have been removed from analysis. 3
   --- Part(2): 285858 type 39 elements, done.
   --- Part(1): 39676 type 39 elements, done.

** Model [C:\Users\Emil Olofsson\Documents\Algor tests\radiator_air.ds_data\1\ds] done

 Autodesk Algor Fluid Flow Analysis Decoder for AGSDB
 Version 2011.00.00.0111-W64/X64 12-MAR-2010
 Copyright (c) 2010, Autodesk, Inc. All rights reserved.

 Keep all tables from heat transfer...
 Start to modify model...
 Start to make Egroup(2) [type=63] ...
 Made 285861 element tables


**** ERROR could not make C:\Users\Emil Olofsson\Documents\Algor tests\radiator_air.ds_data\1\ds ****
  unknown error (16):
  Input data may have wrong boundary mesh or illegal elements.
 Autodesk Algor Surface-to-Surface Radiation Management
 Version 2011.00.00.0111-W64/X64 12-MAR-2010
 Copyright (c) 2010, Autodesk, Inc. All rights reserved.

 Model [C:\Users\Emil Olofsson\Documents\Algor tests\radiator_air.ds_data\1\ds] opened for checking/generating radiation data...

 No Body-to-Body radiation enclosure enabled.

 

"Wrong boundary mesh or illegal elements"? That doesn't exactly help pinpoint where the problem lies...

 

Any input would be appreciated.

 

Thanks

Björn

6 REPLIES 6
Message 2 of 7

Bjorn,

 

The first message, "Error: No active fluid flow group", indicates that the software did not find any parts with an element type of "Fluid 3-D". Do you have any parts whose element type is "Fluid"?

 

The second message, "Warning *** Elements with zero jacobian have been removed from analysis." indicates that a few elements are highlight distorted and are being removed from the analysis. It is not a concern unless they are in the fluid part which most likely would leave a hole for fluid to leak. If you view the "Mesh > View Mesh Results" and look at the Solid Mesh Statistics, the discarded elements most likely came from the part with the largest "Max. length ratio". See the page "Autodesk Algor Simulation > Meshing Overview > Mesh Overview > Meshing CAD Solid Models > Model Mesh Settings > Solid > Nonpositive Jacobian Errors or Distorted Elements" in the Help.

 



John Holtz, P.E.

Global Product Support
Autodesk, Inc.


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Message 3 of 7

John,

 

I have checked and double-checked, and one part is consisting of fluid 3D-elements.

 

Also, the problem in the summary file is not with the warning (warnings doesn't stop the analysis from running, as I understand), but with this:

 

"Input data may have wrong boundary mesh or illegal elements."

 

Illegal elelements or wrong boundary mesh? What does this mean? Is the boundary mesh compromised somehow?

 

Message 4 of 7

I forgot to add: I checked, and there is one element removed from the fluid body also. Does this cause problems with the solver itself, or only with the accuracy of the results? What kind of boundary conditions are used in the connecting elements, if a fluid element is removed? None at all, so the fluid leaks out, as you said, as opposed to the "wall" nodes where constraints are applied automatically as wall conditions? Interesting.

 

Thanks,

Björn

Message 5 of 7
Joey.X
in reply to: bjorn_fallqvist

the error message for "Input data may have wrong boundary mesh or illegal elements." indicates that the model has at least one illegal element, i.e, element does not existed physically, they are lots of combination of physicallt non-existed element, typical exampels include wrong node order in an element, nagative jacobian (i.e, negative volume detected).

 

- this message is from fluid decoder.

- it's not necessary to be an boundary layer element if user didn't not defined, but this problem often happens in boundary layer elements.

 

Jianhui Xie, Ph.D
Principal Engineer
MFG-Digital Simulation
Message 6 of 7
bjorn_fallqvist
in reply to: Joey.X

Hi, Jianhui

 

Do you know of a systematic way to try to fix this in the mesh, or have any general tips for how to deal with it? How come Algor mislabels the nodes?

Message 7 of 7
Joey.X
in reply to: bjorn_fallqvist

Björn,

 

Here are some general tips, but the ultimate solution is the better meshing engine.

- Depress CAD tiny features, which are trivial to CFD result, but make meshing hard (and produce illegal elements)

- Refine mesh in the place of complicated  geometry, (negative: this makes model bigger)

- Try tetrahedral mesh instead of hybrid mesh, tetrahedral mesh usually can avoid illegal element, but makes model bigger

- Turn setting in boundary layer mesh if you turned it on

 

Jianhui

 

Jianhui Xie, Ph.D
Principal Engineer
MFG-Digital Simulation

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