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Author Topic: ANSYS: Varying temperatures between load sets  (Read 1613 times)

JanPio

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ANSYS: Varying temperatures between load sets
« on: April 16, 2018, 08:41:31 AM »
Hello,
we currently use HyperSizer 7.3.50 with ANSYS 18.2 to evaluate various designs of a Reusable Space Launch Vehicle with metallic materials.

First, our understanding of the thermal behavior when connecting HS to ANSYS: The help page on “Thermal Loads” says that “ANSYS thermal loads are not supported.” However, as HyperSizer only imports the resultant element forces from the ANSYS results file and BF-commands are also included in the newly created "*_i.cdb", thermal strains/stresses  seem to be considered. Therefore, the only relation HyperSizer cannot build up is which material property to use at which region of the structure, because the current temperature is not known. Is this right?

Now to our problem: Different parts of the structure, such as the tank or the wings, have different temperatures assigned in a wide range. Additionally, these temperatures should change for different load sets, such as liftoff or reentry. Thermal strains/stresses seem to be included in the results, but we cannot change the loads between load sets.

Workaround: For considering temp-dependent material properties in the parts of the structure we created separate materials for different temperatures, such as "Alu 20K" and "Alu 293K". But this obviously neglects temperature changes between load sets, because there is no possibility to change these materials between load sets.

Questions: Is our understanding of the use of temperatures in HS/ANSYS correct? Is there another workaround to include variable thermal setups in load sets?

Thank you in advance,
Jan

Brent

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Re: ANSYS: Varying temperatures between load sets
« Reply #1 on: April 16, 2018, 10:38:10 AM »
Hi Jan,

You are correct in your assumption. Since HyperSizer cannot import temperatures from ANSYS, it does not know where to apply the temperature dependent material properties.

The work around we suggest is the following:

1. Import your thermal load sets as mechanical. (Skip this step if the thermal loads are already combined to mechanical loads in your FEM)
2. Assign a unique reference temperature to each "Thermal" load set.
3. Assign a different material to each part of the structure that would have a constant applied temperature.
4. Assign reference temperatures to each material system that is consistent with the "applied temperatures" of the "thermal" load set. 

EXAMPLE:
1. When sizing a single component for one load case, HyperSizer will use the temperature dependent properties associated with the reference temperature assigned to the load set.

2. For one "thermal" load case with multiple components that have different applied temperatures, the different material systems for these components will have different temperature dependent properties for the same reference temperature.

Careful attention must be used when managing the various temperature dependent properties and reference temperatures for each material system on each structural location. Supporting ANSYS temperature loads will be evaluated for future software releases. 

Question: Will you need to update the FEM with the temperature dependent properties unique to each thermal case? Or, will the room temperature properties for used in the FEA runs?

Hope this helps!

Brent 
 

JanPio

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Re: ANSYS: Varying temperatures between load sets
« Reply #2 on: April 23, 2018, 03:53:53 AM »
Hi Brent,
thanks for your reply. Your suggestions sounds promising, we are currently applying it to a test model. The first problem:
Quote
1. Import your thermal load sets as mechanical. (Skip this step if the thermal loads are already combined to mechanical loads in your FEM)
In fact the loads are a combination of mechanical and thermal already, otherwise thermal expansion/contraction would not be considered in the ANSYS FEA. Consequently, the load steps are imported as "thermal" load sets in HS, as described on the manual page "Importing Load Sets". But this prevents me from changing the reference temperature (as well as LL / UL factors) in the Load Sets Tab of the Project Setup Form. The input values vanish when pressing enter. Removing thermal loads (i.e., having mechanical load sets) enables me again to edit LL, UL and Reference Temperature.
How can we then change the Ref-Temps while keeping thermal loads in the FEA? I could not find anything in the manual.

Thanks,
Jan

JanPio

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Re: ANSYS: Varying temperatures between load sets
« Reply #3 on: May 02, 2018, 11:24:57 AM »
After trying some solutions we still face the dilemma that we lose in either way one of the needed features:
removing temperatures from the ANSYS-FEA:
+PRO+ load sets are imported as mechanical, we can use the reference temperature function (load set tab) in HS which enables us to use your workaround for temp.-dependent material properties, which sounded really promising
-CON- we don't take any thermal strains/stresses into account

keeping temperatures in the ANSYS-FEA:
+PRO+ we take thermal strains/stresses into account
-CON- the load sets are imported as "thermal": reference temperatures can neither be typed in nor are they recognized via the TREF command in the *.cdb & *.s0i files

Using separate mechanical and thermal load sets in both ANSYS and HS would remove the temp.-dependence of the material properties in the mechanical FEA load sets, so no solution either.

Or is there any possibility to keep thermal load sets and assign a reference temperature?

Thanks,
Jan