Identification of global and local mode in modal analyis

gopal_rathore

Hi,

I am working on a modal analysis and need to find the global and local modes from it.

Can anyone let me know general threshold values kept for parameters like mass participation factor etc. to consider it s as global mode 

 

Or any other way to identify the same

Altair Forum User

Altair Forum User said:

Hi,

I am working on a modal analysis and need to find the global and local modes from it.

Can anyone let me know general threshold values kept for parameters like mass participation factor etc. to consider it s as global mode 

 

Or any other way to identify the same

90+% effective mass..?

 

OEMs use different effective mass numbers...

gopal_rathore

Hi,

I have doe modal analysis allowing first 6 rigid modes also i.e with v1=0 . These first 6 modes being rigid modes have almost zero natural frequency but the mas contribution is very high when compared to the modes starting from 7th. This is expected as the complete system is moving as a single rigid body in space.

Now., I have extracted 20 modes and the summation of modal effective mass is coming out to be the total mass of the system for x, y and z translation with major contribution from the first 6 rigid modes.

I am not able to interpret using this which mode is local and which mode is global as in this case the first 6 rigid modes are very dominant with respect to mass and those frequencies are not of interest.

I have shared excel file for the parameters obtained through secure box.

 

 

 

 

 

Altair Forum User

Hi Gopal,

 Modal effective mass indicates the mass participation of each mode in each of the 6 rigid body motions (T1< -> R3). It's only useful under constrained conditions (no Rigid Body modes).

 

So, “Effective mass is more relevant to constrained conditions.”

gopal_rathore

Hi,

Yes, how shall I interpret it by going on free- free run analysis.The requirement is to get the results interpreted based on free- free run 

 

 

Also on seeing the .out file it shows:

 

 MODAL EFFECTIVE MASS FOR SUBCASE        1
 RIGID BODY MODES BASED ON REFERENCE POINT AT ORIGIN OF BASIC COORDINATE SYSTEM

 

This reference point is somewhere in the structure . How does this influence the parameters coming in .out files like eff. mass, modal participation factor etc.

 

 

Thank you

Altair Forum User

Altair Forum User said:

MODAL EFFECTIVE MASS FOR SUBCASE        1
 RIGID BODY MODES BASED ON REFERENCE POINT AT ORIGIN OF BASIC COORDINATE SYSTEM

 

This reference point is somewhere in the structure . How does this influence the parameters coming in .out files like eff. mass, modal participation factor etc.

I am not completely sure about the reference point, I will check and update you soon,

Altair Forum User

Altair Forum User said:

Yes, how shall I interpret it by going on free- free run analysis.The requirement is to get the results interpreted based on free- free run 

 

Usually, for free- free run, I think the first six modes will be global modes and rest will be local modes. 

gopal_rathore

Altair Forum User said:

Usually, for free- free run, I think the first six modes will be global modes and rest will be local modes. 

Hi,

Yes in comparison to rigid body modes the modes after 6 would be local only as in rigid mode complete system will participate as the system is behaving as a rigid body

I am neglecting those first 6 modes and now want to analyse the first global mode after 6 i.e from 7

Is there any other parameter apart from the EFFMASS to understand this.

gopal_rathore

Hi,
Anything on above points

Altair Forum User

Hi Gopal,

 

I couldn't find anything apart from that and I wrote to experts on the same. 

gopal_rathore

OK thank you

gopal_rathore

Hi,

There is one tool in 17 version in Hyperview> Preferences>NVH utilities to understand the identification of global and local modes. It talks about setting a criteria based on percentage range for identification. I wanted to know how it is setting up this criteria 

 

I have attached image for reference

<?xml version="1.0" encoding="UTF-8"?>

Rahul Rajan_21763

Please refer attached pdf file for Global Modes.

Unable to find an attachment - read this blog