Modal effectctive Mass
Altair EmployeeFrom the below results, how can we calculate the percentage of mass contribution in a particular natural frequency?
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Answers
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Hi,
Can you share the entire .out file?
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I would like to know this as well. I am using PARAM, EFFMAS to calculate modal effective mass and I am getting similar results, but I don't know how to calculate the effective mass of each mode with given data in the out file. I would be happy to know how I can obtain modal effective masses using Optistruct.
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Altair Forum User said:
I would like to know this as well. I am using PARAM, EFFMAS to calculate modal effective mass and I am getting similar results, but I don't know how to calculate the effective mass of each mode with given data in the out file. I would be happy to know how I can obtain modal effective masses using Optistruct.
I would like to remind you my inquiry. I didn't want to start a separate topic since this one is on the exact same subject. Thank you for your time.
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Sorry for missing this out.
could you please let me know what the use case is?
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I have a relatively large flexbody model and I want to make sure that I include the absolute minimum number of flexible modes which will still be sufficient to model the system's dynamics. I want to use modal effective mass approach and include a number of modes such that those modes' total modal effective mass corresponds to about 90% of the system's total mass.
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Hello Prakash,
I have tried both in 2017 and 2017.2. They're generating the same results in the .out file. Results from a sample for effmass is attached. It seems to be working but my problem is I can't see any entries for modal effective mass in this output.
Thank you
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Hi,
Can you try this:
please use the output
PARAM,EFFMAS,YES
and
OUTPUT,HGEFFMASS and then the option where to write it
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Hello Prakash,
I added the OUTPUT card with HGEFFMASS selected, however I don't see any option asking me where to write it. I checked the docs on HGEFFMASS and it says:
Effective mass is output as a HyperGraph bar chart via the “BAR” option (default), or as a HyperGraph XY plot via “XY” option.
and says *.mass and *_mass.mvw files are used to store modal effective masses. I do get these files so I used HyperGraph to plot the .mass file Optistruct generated. There is indeed some data in it but they aren't labeled properly so I don't know which data represents which property. I would like more help on this.
Also, the entries in out file are the same as I posted above.
ADDITION: I have seen the doc pages for .mass and _mass.mwv files. Attached is my .mass file. I have understand that the rows are for each requested mode and columns are for subcases. However, I don't know the unit of these values. The total mass of my test system is 3.14e-5 tonnes, or 31.4 grams. The magnitudes in the .mass file are very large compared to my total mass if .mass file gives modal effective masses in my units. Could you tell me what unit .mass file uses?
Thank you
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Yes it will write to HG file which you can plot in HyperGraph
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Thanks Prakash. As I said, I am able to output the results now. However they don't add up to the total mass of the system so I would you to share with me more information on the units used in the .mass file. Thank you
I have added a screenshot of hypergraph plots. X and Y axes are not labeled so I'm unable to interpret the plot.
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I would like to remind you my last inquiry. As I said, I am able to print the results to the HG file, however I am unable to interpret the results. My model uses metric tons for mass unit, how can I interpret the results and see if they add up to 90% of total mass or not? Thank you
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HI,
The unit system written is same as your input.
If your unit system used for solving is tonnes for mass, then the output is also Tonnes.
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Altair Forum User said:
HI,
The unit system written is same as your input.
If your unit system used for solving is tonnes for mass, then the output is also Tonnes.
Hey Prakash,
if you check the .mass file I shared in my previous posts, you'll see that the numbers don't add up to 3.14e-5 tons, which is the total mass of my test structure. In fact, the numbers are way higher than the total mass of my system. Attached is the .fem file for my test structure and modal analysis. I would be grateful if you could share more information with me on this. Thank you for your time.
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OK, I will check and update you soon.
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After some debugging, I got the MEM equal to total mass for components.
Please request 10 modes and don't start with V1=1
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Hello Prakash,
I have set V1 = 0 and requested 10 modes. .out file now gives the total mass (3.14e-5) as you have also obtained in your results. Though, there are small differences in individual modal masses between your and my results, but I don't think that's an issue. Thank you very much for your help.
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I have a question about this topic, can you please explain further -
When you are reading the output file under the 'modal effective mass' by the 'subcase total' do you add the column up going left to right until you get a total mass greater then 90% when you convert it to a decimal number?
Or is it possible to request just the total mass number from hypermesh and have it wrote to the outfile?
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If you look at the above .out file from one of the posts, the sum of mass for all X-Trans modes is equal to the total mass.
The same goes for Y and Z...
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Hi @Prakash,
I just wanted to confirm I understand the results correctly, your saying the sum total for each axis of the X,Y,Z - Trans has to be equal to or at least 90% of the mass of 3.14000E-005 correct? Then what frequency would I use for my V1 and V2 (from the normal modes), the one in the list that has the most mass?
Or to word it better how can I tell which one is my upper and lower limit for freq?
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