modal analysis, shell vs solid
modal analysis, shell vs solid
(OP)
Hi all,
i'm performing a modal analysis on a piping system. i discovered that natural frequency values change if i use shell elements instead of solid.
Is this possible?
I checked everything: from material used, till constrain and contacts.
First prototype built (shell elements used), had problem of resonance even if the value of the natural frequencies for pipes were far away from excitation of the force, calculation tell me 35.8 Hz while force is near 47Hz.
Just for curiosity i changed my model using solid elements for the pipes instead if shell, and i discovered that the pipes have a frequency in the field of the frequency of the force, 45.8Hz.
the only thing i see different is that while with shell elements bonded contact are generated between edge, with solid elements i'm supposed to put contact between faces arising the whole stiffness, is this thought correct or should i look for other reasons?
thanks in advance
i'm performing a modal analysis on a piping system. i discovered that natural frequency values change if i use shell elements instead of solid.
Is this possible?
I checked everything: from material used, till constrain and contacts.
First prototype built (shell elements used), had problem of resonance even if the value of the natural frequencies for pipes were far away from excitation of the force, calculation tell me 35.8 Hz while force is near 47Hz.
Just for curiosity i changed my model using solid elements for the pipes instead if shell, and i discovered that the pipes have a frequency in the field of the frequency of the force, 45.8Hz.
the only thing i see different is that while with shell elements bonded contact are generated between edge, with solid elements i'm supposed to put contact between faces arising the whole stiffness, is this thought correct or should i look for other reasons?
thanks in advance





RE: modal analysis, shell vs solid
more specific answers will require response from those more involved with the calculation methods.
RE: modal analysis, shell vs solid
Tobalcane
"If you avoid failure, you also avoid success."
RE: modal analysis, shell vs solid
Tata
RE: modal analysis, shell vs solid
Yes, solid element size is about 3mm along thickness of the pipe that is 0,7mm: so this lead to say that the resonance value i calculated is more probably a coincidence.
Furthermore the ratio thickness by lenght is about 0.7/500 minimum. So shell elements seem to remain the correct one to use.
But nothing about contact? i mean, don't they play a relevant role on FEM analysis?
thanks again for your support
RE: modal analysis, shell vs solid
RE: modal analysis, shell vs solid
RE: modal analysis, shell vs solid
Contact between what? different parts? Are you saying you have contact between the surface of the pipe and some other part? If so, how specifically have you modelled it?
RE: modal analysis, shell vs solid
Any idiot can finish a test if he's not worried about getting right answers. More time processing is the least of your worries if your results aren't right.
Run again with a mesh refined by a factor of about 1.5. If your results are comparable, you're OK. If not, keep refining.
RE: modal analysis, shell vs solid
If you're only interested in the pipes then why have the rest of the structure there as relatively speaking it must be fairly rigid compared to the pipes and could be replaced by imposing fixed restraints, perhaps?
Tata
RE: modal analysis, shell vs solid
"so this lead to say that the resonance value i calculated is more probably a coincidence."
I'll have more faith in hand calcs than the FEA. IMHO, FEA are the pretty pics you show to management to communicate what you calculated by hand. Don't become complacent with FEA, if you can not do the hand calcs, you shouldn't do the FEA. In any case, you should have a good idea what the frequencies and mode shape (stress, strain, deflections...etc) should be before you do the FEA.
Tobalcane
"If you avoid failure, you also avoid success."
RE: modal analysis, shell vs solid
Tobalcane
"If you avoid failure, you also avoid success."