Abaqus Natural Frequencies Extraction
Abaqus Natural Frequencies Extraction
(OP)
Hello everybody!
I would like to have some help with abaqus concerning the extraction of natural frequencies. I would like to understand the outputs of Abaqus.
I analysing a gas tank made of steel plates. I'm using shell elements S4R to model the tank and I performed a linear perturbation frequency analysis to extract the natural frequencies (eigenvalues).
First, I'm obtaining in a range of 0 - 2Hz a lots (thousands) of modes, which I assume the majority are either local modes either "incorrect" solution of Abaqus. I wanted to analyse the mass participating in the modes, and for that I found the Effective Mass (EMi) for each degree of freedom on Output History of Abaqus. Though, I do not uderstand the values because either they are very small (which can be the sign of local mode) either very high (much higher than the mass of the tank). So, I'm confused. What is this Effective Mass (EMi) that Abaqus provides? What are the units? (If for example I work in kg, N, mm).
Thanks for any help.
Further information concerning the model:
height of the tank =+/- 33m
radius =+/- 29m
thickness of the wall plate = 5mm
thickness of the roof plate = 10mm
Eignesolver: Lanczos
Best regards,
Alexandre
I would like to have some help with abaqus concerning the extraction of natural frequencies. I would like to understand the outputs of Abaqus.
I analysing a gas tank made of steel plates. I'm using shell elements S4R to model the tank and I performed a linear perturbation frequency analysis to extract the natural frequencies (eigenvalues).
First, I'm obtaining in a range of 0 - 2Hz a lots (thousands) of modes, which I assume the majority are either local modes either "incorrect" solution of Abaqus. I wanted to analyse the mass participating in the modes, and for that I found the Effective Mass (EMi) for each degree of freedom on Output History of Abaqus. Though, I do not uderstand the values because either they are very small (which can be the sign of local mode) either very high (much higher than the mass of the tank). So, I'm confused. What is this Effective Mass (EMi) that Abaqus provides? What are the units? (If for example I work in kg, N, mm).
Thanks for any help.
Further information concerning the model:
height of the tank =+/- 33m
radius =+/- 29m
thickness of the wall plate = 5mm
thickness of the roof plate = 10mm
Eignesolver: Lanczos
Best regards,
Alexandre





RE: Abaqus Natural Frequencies Extraction
Abaqus does not have "units"*. It should state the total (movable) mass of the model in the .dat file that is created. Typically, the effective modal mass is presented as a percent of total mass. So, for example, your movable mass is 10, and the effective mass in the x-direction for a certain mode is 1, then you have 10% for that mode. If you take "enough modes" into account the x,y,z sums should asymptotically approach the total movable mass.
I don't think it is possible to exceed the total movable mass in an effective modal mass calculation, so maybe you are confusing the actual mass of the model with the constrained "movable" mass of the model.
Also, something to take into consideration if you have any degree of torsional vibration is the rotational degrees of freedom. I believe that these "effective masses" are based on the MOIs.
*typically in my work, I have seen tonnes used as the base mass unit in Abaqus, mm used as the base length unit and Newton as the base force unit (giving stress results in MPa). But, I have no idea what you are using in your material data files and drawings.
All of this is from memory, but I can check some of this out if you're still having issues.
RE: Abaqus Natural Frequencies Extraction
I don't agree that the effective mass is in % because in same cases is clearly above 100% whcih is not logical. I found then that I had a mistake in copying the effective mass results and I found that the mass is for example in my case in kg, as I introduced the material density in kg/mm3. Now the results in terms of effective mass are in my view coherent. I have now to check why so many modes at frequencies close to 0Hz.
RE: Abaqus Natural Frequencies Extraction
Could you attach the .dat file? I still don't think it is possible to exceed the unconstrained mass, and am interested to see this
RE: Abaqus Natural Frequencies Extraction
RE: Abaqus Natural Frequencies Extraction
RE: Abaqus Natural Frequencies Extraction
Thank you.
RE: Abaqus Natural Frequencies Extraction