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Stability in a Rotating Machinery models

Posted 30 gen 2012, 14:17 GMT-5 Version 4.2, Version 4.2a 3 Replies

John R Puentes
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I am running a model in the rotating machinery module of COMSOL. Anytime I increase the time of my model, I am obtaining different answers. Does anybody know if the rotating machinery models become stable in some point, or because of the rotational nature, there is always a time dependency?


Thanks
3 Replies Last Post 11 feb 2012, 04:24 GMT-5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 30 gen 2012, 15:09 GMT-5
Hi
I'm not sure I fully understand your question. If you have a rotary model, as time goes on the geometry will "rotate" and if you have put accelerating loads on it it will even turn faster and faster, until your time stepping might interact with the process, as you need several time steps per 2*pi rotation to resolve the displacements correctly

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Good luck
Ivar
Hi I'm not sure I fully understand your question. If you have a rotary model, as time goes on the geometry will "rotate" and if you have put accelerating loads on it it will even turn faster and faster, until your time stepping might interact with the process, as you need several time steps per 2*pi rotation to resolve the displacements correctly -- Good luck Ivar
Trevor Munroe
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Posted: 1 decade ago 10 feb 2012, 22:46 GMT-5
I ran into a similar problem. This is because as you increase your time the time steps under free time stepping option become larger and this has a tendency of changing your results. My recommendation is to fix your maximum time step or control your time steps using the strict option.
I ran into a similar problem. This is because as you increase your time the time steps under free time stepping option become larger and this has a tendency of changing your results. My recommendation is to fix your maximum time step or control your time steps using the strict option.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 11 feb 2012, 04:24 GMT-5
Hi
Indeed one need to study in detail the solver log for time steppings, there one sees when COMSOl calculates and when it interpolates to give your solution, if you have severa "out" following each other without any calculation steps in between, you should ask yourself how the solution (and your input BCs) is expected to change in between these 2 solver steps, if you expect an assymptotical endency towards a given result, then it's mostly OK, if your BC or output is expected to oscillate in between, then mstly the results are wrong, as the COMSOL interpolation of the two (or more for higher order fitting) last points will not give a representative results. Then the "Free" (default automatic) time stepping should be changed to "intermediate" (at least one solver sequence in between all your prescribed points) or "strict" at least solving at youre time steps, perhaps also (if solvers not happy with the convergence) in betwen, in addition

--
Good luck
Ivar
Hi Indeed one need to study in detail the solver log for time steppings, there one sees when COMSOl calculates and when it interpolates to give your solution, if you have severa "out" following each other without any calculation steps in between, you should ask yourself how the solution (and your input BCs) is expected to change in between these 2 solver steps, if you expect an assymptotical endency towards a given result, then it's mostly OK, if your BC or output is expected to oscillate in between, then mstly the results are wrong, as the COMSOL interpolation of the two (or more for higher order fitting) last points will not give a representative results. Then the "Free" (default automatic) time stepping should be changed to "intermediate" (at least one solver sequence in between all your prescribed points) or "strict" at least solving at youre time steps, perhaps also (if solvers not happy with the convergence) in betwen, in addition -- Good luck Ivar

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