[STRUC] Thermal load with cyclic region - non-axisymmetric unphysical stresses
Dear all,
I am using Ansys Mechanical 17.0 academic to simulate a double interference fit. The geometry of interest has a cyclical symmetry around the z axis. The loading condition includes a thermal load (uniform temperature). The problem I encounter is that the cyclic region boundaries are not free to "slide" radially from the center point (r=0) with the thermal expansion/contraction, with the result that unnatural compression stresses arise in the tangential direction between the two faces of the cyclic symmetry region under increased temperature.
In order to debug the problem, I have setup a simpler, completely axisymmetric problem using both a 2D axisymmetric model and a 3D quarter model of only two concentric layers of different materials. In both cases, the program correctly outputs an axisymmetric solution. However, isolating just one sector of the 3D model and setting up a cyclic region again results in a non-axisymmetric solution: the problem seems to appear just with the cyclic symmetry. The amount of stresses is also directly linked to the sector angle, with smaller angles leading to higher stresses, as if the two cyclic boundaries were acting as "walls".
I tried reading the manual on cyclic symmetry and researching online resources as well as asking the people around me, but I was unable to find a solution for a correct modelling of thermal loads with cyclic symmetry condition. I am not experienced with the APDL commands, so there might be a quick fix which I didn't notice because of my poor understanding of the scripting possibilities.
I figured the kind of problem should be straighforward to solve, given that so many engineering applications benefit from cyclic symmetry also with thermal loads (turbomachines, etc...). I am basically looking for a way to tell the program that every thermal induced displacement should be originating from the center r=0 axis.
Might there be a way to calculate first the thermal displacement field and then superimpose it with the other loads? Given the fact that the temperature field is constant this would be an overkill solution or would it just be simpler?
If anyone reading this is aware of a possible solution or documentation on the topic that I missed, I will be grateful for any help.
PS: This is my first question to the group, I hope I am not breaking any rules of the netiquette.
Emile Piccoli
M.Sc. Student in Mechanical Engineering
Ecole Polytechnique Fédérale de Lausanne - EPFL (Switzerland)
Emile,
How are you setting up the cyclic symmetry? It is obviously generating improper constraints. If running in ANSYS classic, the cyclic command is generally used for modal cyclic symmetry to simulate various nodal diameter responses. This is not really applicable to your problem, although it should work.
What you want to do is rotate all the nodes on the cyclic boundaries to a cylindrical coordinate system (NROTATE), then couple matching nodes from one face to the other in all DOF. You can use offset node coupling (CPCYC). You still need to add a rigid body motion constraint in the axial and tangential directions.
Good luck.
Joseph T Metrisin
Structures Lead
Florida Turbine Technologies, Inc
1701 Military Tr. Suite 110
Jupiter, FL 33458 U.S.A.
+1 (561)427-6346 Office
(561)427-6191 Fax
JMetrisin@fttinc.com
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-----Original Message-----
From: Xansys-temp [mailto:xansys-temp-bounces@xansystest.info] On Behalf Of Piccoli Émile
Sent: Monday, November 14, 2016 12:35 PM
To: xansys-temp@xansystest.info
Subject: [Xansys] [STRUC] Thermal load with cyclic region - non-axisymmetric unphysical stresses
Dear all,
I am using Ansys Mechanical 17.0 academic to simulate a double interference fit. The geometry of interest has a cyclical symmetry around the z axis. The loading condition includes a thermal load (uniform temperature). The problem I encounter is that the cyclic region boundaries are not free to "slide" radially from the center point (r=0) with the thermal expansion/contraction, with the result that unnatural compression stresses arise in the tangential direction between the two faces of the cyclic symmetry region under increased temperature.
In order to debug the problem, I have setup a simpler, completely axisymmetric problem using both a 2D axisymmetric model and a 3D quarter model of only two concentric layers of different materials. In both cases, the program correctly outputs an axisymmetric solution. However, isolating just one sector of the 3D model and setting up a cyclic region again results in a non-axisymmetric solution: the problem seems to appear just with the cyclic symmetry. The amount of stresses is also directly linked to the sector angle, with smaller angles leading to higher stresses, as if the two cyclic boundaries were acting as "walls".
I tried reading the manual on cyclic symmetry and researching online resources as well as asking the people around me, but I was unable to find a solution for a correct modelling of thermal loads with cyclic symmetry condition. I am not experienced with the APDL commands, so there might be a quick fix which I didn't notice because of my poor understanding of the scripting possibilities.
I figured the kind of problem should be straighforward to solve, given that so many engineering applications benefit from cyclic symmetry also with thermal loads (turbomachines, etc...). I am basically looking for a way to tell the program that every thermal induced displacement should be originating from the center r=0 axis.
Might there be a way to calculate first the thermal displacement field and then superimpose it with the other loads? Given the fact that the temperature field is constant this would be an overkill solution or would it just be simpler?
If anyone reading this is aware of a possible solution or documentation on the topic that I missed, I will be grateful for any help.
PS: This is my first question to the group, I hope I am not breaking any rules of the netiquette.
Emile Piccoli
M.Sc. Student in Mechanical Engineering
Ecole Polytechnique Fédérale de Lausanne - EPFL (Switzerland)
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