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2D Cylinderical coordinate system
Posted 3 ago 2011, 17:07 GMT-4 Parameters, Variables, & Functions, Studies & Solvers, Structural Mechanics Version 4.0, Version 4.1, Version 4.2 1 Reply
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In my 2D cylindrical structural mechanics model, I am going to simulate almost a quarter of a rubber-like (hyperelastic) material tube (please see the attached picture) and stretch it circumferentially to make it a half tube. I have successfully done this in Cartesian coordinate, however, I prefer to do that in cylindrical system too.
In my V4.2 COMSOL, I have defined a cylindrical coordinate system, but I have problem correctly defining the boundary conditions. In the boundary condition menu, first I selected the Cylindrical coordinate system. For boundary # 3, the displacement in r-direction should be “r-R”, where r is the final radius (unknown) and R is initial radius, which is known. Of course I enter “sys2.r – R” as my Cyl. Coord. Sys. is sys2.
When I enter the expression “sys2.r – R”, the program gives me a warning indicating that R is not a defined variable. So I decided to replace R with sqrt(X^2+Y^2). This resolves the error, but the solution does not converge, which I believe may be due to this expression rather than other solution parameters that may improve the convergence. I have two issues:
1- Although I do not get a warning when I use X and Y when using cyl. coord. sys, I hesitate to use sqrt(X^2+Y^2) because I have already selected Cyl. Coord system, while X and Y are defined in Global coord. Sys.
2- I wonder if I need to use sqrt(x^2+y^2) (i.e. use spatial coord sys) instead of sqrt(X^2+Y^2) (reference coord. Sys), because the frame type was selected Spatial when I was defining the cyl. coord sys. If I didn’t select Spatial, the cyl. Coord. Sys. wouldn’t be available in the physics menu. However, I believe that the reference coord system should be used (i.e. R=sqrt(X^2+Y^2) ).
I appreciate if you give me your opinion about the above issues.
Thanks
J.W.
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I beleive you are getting confused with the frames (I too get them always mixed up so I must recheck the convention regularly ;) when you define your cylindrical coordinates, you define the frame it applies on: Material frame the original geoemtry" or the spatial frame i.e. the deformed geoemtry, check carefully the doc.
then define your variables, you can make a "Definitions" variable my_R =sqrt(X^2+Y^2) and my_r=sqrt(x^2+y^2) then when you write "my_r-my_R" for any boundary or domain you will get the difference between the deformed and the undeformed shape (for a linear case it should also be close to "sqrt(u^2+v^2)" )
The easiest to use learn how to use these frames is to take a cube in "solid", attach one boundary, and apply a gravity load. Define the material as something "soft" E=1[MPa] so it changes drastically its shape under the gravity load. Then integrate the volume over the spatial and the material frames and compare the values.
final note, my suggestions get hands on the lates version with patch, the early v4.0 was not really well "ironed out", and I found it rather frustrating to use for serious work
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Good luck
Ivar