Pro/Mechanica Thermal for high aspect ratio structure

I'm trying to do some thermal analysis on some very high aspect ratio
structures. For example, I'm considering a circular plate that is 10
microns thick and a diameter of 2cm subject to a fixed temperature
along the outer radius and a point heat load somewhere on the plate.

When I start the thermal analysis I get a warning that says: "The
highlighted point heat loads can cause singular temperature
gradients." Then Pro/Mechanica never seems to complete the "Creating
solid elements..." (I've let it run somewhere around 5 hours and
finally gave up.) Below is a listing of the run status.

In the status it does say that the max allowable aspect ratio is 30.0.
I assume this is in reference to the aspect ratio of the individual
elements. Is there anyway that I can change this?

If the plate is made thicker (say 500 microns) it works great. Also,
if I remove the point heat load and use two different prescribed
temperatures it worked.

Thanks for any help!

Mike


------------------------------------------------------------
Pro/MECHANICA THERMAL Version 24.8(823)
Log for Design Study "test"
Wed Dec 10, 2003 11:25:14
------------------------------------------------------------

Begin Creating Database for Design Study
Wed Dec 10, 2003 11:25:14
Elapsed Time (sec): 0.61
CPU Time (sec): 0.24
Memory Usage (kb): 24924
Work Dir Disk Usage (kb): 0
___________________________________________________________

Pro/MECHANICA External Database Interface -
MNF_TO_MDB Translator
Copyright(c) 2002PTC (tm) -
All Rights Reserved

___________________________________________________________


Global Information:

Model Name: .\test\test
Product Name Undefined
Product Version 24.8

Creation Date 12/10/03
Creation Time 11:25:14
Author UNKNOWN



Header Information:

MNF Version 24.8
MNF Author mechxfce
Date of MNF Creation Not available
Import Mode ptc
Name of Original Model Not available
Comments Not available


Error / Warning Reports :

Translation Error/Warning Summary



Total errors: 0
Total warnings: 0
Total informational messages: 0


**** Entity Creation Summary ****

----- Summary Information -----



Description Entities Entities
Found Created

Groups 3 3
Coordinate Systems 2 2
Parts 1 1
Points 3 3
Curves 11 11
Topological Edges 6 0
Surfaces 4 4
Surface Curves 12 0
Surface Regions 4 4
Volumes 1 1



Translation Elapsed Time

*** 0 seconds ***


Step Elapsed Time (sec): 0.13
Step CPU Time (sec): 0.13

Begin Integrated Mode Error Checking
Wed Dec 10, 2003 11:25:14
Elapsed Time (sec): 0.74
CPU Time (sec): 0.36
Memory Usage (kb): 33188
Work Dir Disk Usage (kb): 0
Checking the model before creating elements...
These checks take into account the fact that AutoGEM will
automatically create elements in volumes with material
properties, on surfaces with shell properties, and on curves
with beam section properties.


Step Elapsed Time (sec): 0.05
Step CPU Time (sec): 0.05

Begin Generating Elements
Wed Dec 10, 2003 11:25:14
Elapsed Time (sec): 0.79
CPU Time (sec): 0.41
Memory Usage (kb): 33188
Work Dir Disk Usage (kb): 0

Copying elements from an existing mesh file ...
The meshed study model is geometrically
incompatible with the current model.
Only copied 64% of the elements. The model must be remeshed.
Element copy failed. Pro/MECHANICA will automatically remesh.


____________________________________________________________

AutoGEM Settings

Isolate Surface Features:

[x] Reentrant Corners
[x] Point Loads
[x] Point Constraints
[x] Point Heat Loads
[x] Point Prescribed Temperatures
[x] Point Convection Conditions

------------------------------------------

[x] Insert Points
[x] Create Links Where Needed
[x] Move or Delete Existing Points
[ ] Modify or Delete Existing Elements
[ ] Detailed Fillet Modeling
[x] Display AutoGEM Messages
[ ] Automatic Interrupt

------------------------------------------

Shell Element Type: Quad and Tri
Solid Element Type: Tetra

Allowable Angles (Degrees):
Edge Min: 5.0 Max: 175.0
Face Min: 5.0 Max: 175.0

Max Allowable Edge Turn (Degrees): 95.0

Max Allowable Aspect Ratio: 30.0

____________________________________________________________

Working on 1 volume(s) ...
Pre-processing features for 4 surfaces ...
Searching for existing solid elements inside the volume ...
Found 0 existing solid element(s) inside the volume.
Pre-processing boundary features ...
Optimizing boundary point locations ...
Detecting thin features on the volume ...
Pre-processing volume features ...
Creating solid elements ...
58.8% complete after 2.0 min (4875 elements).
64.7% complete after 4.0 min (5694 elements).

 

Hello Mike,

You should be able to change the element properties by using the
AutoGEM option in the main thermal menu, and changing from the default
values there (under AutoGEM>Settings>AutoGEM, I think). It might be
worth playing with all the values in there, not only aspect ratio. As
far as I understand, tightening the controls (i.e. reducing aspect
ratio) will increase the number of elements, but should lower the
element order required to get convergance, so there is some trade-off
going on.

Hope that helps,

Gwilym

 

: I'm trying to do some thermal analysis on some very high aspect ratio
: structures. For example, I'm considering a circular plate that is 10
: microns thick and a diameter of 2cm subject to a fixed temperature
: along the outer radius and a point heat load somewhere on the plate.
:
Have you checked your part accuracy? This ratio of thickness to diameter is
1:2000. You would need an accuracy setting of, at most, .0005 (and making it .0002
wouldn't hurt a bit) for the solver to work correctly. Higher part accuracy will
allow smaller elements and higher aspect ratio.

David Janes

 

Well thanks for the suggestions. Here is the update.

I figured out how to increase the aspect ratio of the elements through
the AutoGEM settings. However, it still seems to take forever while,
"Creating solid elements..." even when I set the aspect ratio to 3000.
(With a thick structure, it runs in about 30 seconds or so.)

I also tried changing the accuracy of the part. It is tough to say
exactly what effect that has on the simulation.

Thanks again,
Mike

 

This reminds me of some ancient issues I had with Mechanica in meshing
aluminum extrusions... the AutoGem would always take way too long and
give WAY too many elements. The solution was to mesh one surface with
shells and extrude it into solid elements. Then it was very
controllable. This should still exist in independent mode... Not so sure
you can do this in integrated mode.

Us ancient Mechanica guys are not so sure about all this "Independent
Mode is going away talk"

-D