COSMOS - Negative Natural Frequencies
COSMOS - Negative Natural Frequencies
(OP)
I have been conducting a modal analaysis of a long tube hanging vertically. The tube is constrained at one end and free at the other.
For lengths less than 2m, the natural frequencies obtained from COSMOS seem reasonable. However, at lengths greater than 2m, I am getting a negative natural frequency for the 1st mode shape. Can anybody comment on this?
I suspect that the L/D ratio of the geometry is getting too large for the FEA code to solve it properly. The outer diameter of the tube is only 17.3mm with a 2.1mm wall thickness.
See the attached powerpoint for an example of the model. Any help would be appreciated.
For lengths less than 2m, the natural frequencies obtained from COSMOS seem reasonable. However, at lengths greater than 2m, I am getting a negative natural frequency for the 1st mode shape. Can anybody comment on this?
I suspect that the L/D ratio of the geometry is getting too large for the FEA code to solve it properly. The outer diameter of the tube is only 17.3mm with a 2.1mm wall thickness.
See the attached powerpoint for an example of the model. Any help would be appreciated.





RE: COSMOS - Negative Natural Frequencies
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: COSMOS - Negative Natural Frequencies
RE: COSMOS - Negative Natural Frequencies
What you are describing sounds unbelievable to me, can you do a diff between the input files for a good run and a bad run to check that nothing odd has changed?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: COSMOS - Negative Natural Frequencies
Vibrations was never one of my stronger subjects. Could you direct me to the equation that I can verify the frequencies with by hand cals?
RE: COSMOS - Negative Natural Frequencies
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: COSMOS - Negative Natural Frequencies
M
--
Dr Michael F Platten
RE: COSMOS - Negative Natural Frequencies
He's using the embedded Cosmos solver bundled with Solidworks, i.e. only allows auto-meshing in solid elements. Would guess that the mesh is becoming quite distorted if the automesher is not reset/re-run from case to case as the modelled length is varied.
RE: COSMOS - Negative Natural Frequencies
I tried to figure out what type of elements I am using and it stated that they are solids. I varied the number of points up to 16 and get the exact same answer.
So, I'm at a loss at this point as to what might be going on. Something is clearly wrong because the hand calcs using a cantilevered beam give me way different answers.
RE: COSMOS - Negative Natural Frequencies
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: COSMOS - Negative Natural Frequencies
3.7 Hz
23.4 Hz
65.6 Hz
128.3 Hz
RE: COSMOS - Negative Natural Frequencies
RE: COSMOS - Negative Natural Frequencies
What are the nodes of the mode shape?
Fe
RE: COSMOS - Negative Natural Frequencies
I have the full package so maybe COSMOS express isn't as robust.
Ryan Marks
Mechanical Engineer
http://www.stryker-tech.com/