Frequency response Function : Mobility for somepoints

Altair Forum User
Altair Forum User
Altair Employee
edited October 2020 in Community Q&A

Hallo Group

 

I want to have value of frequency response function Mobility defined as (Normal velocity/ force) at some points on my structure.

I am doing direct frequency response analysis with optistruct. How do I go about it.

 

Regards

Pratik

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Answers

  • Altair Forum User
    Altair Forum User
    Altair Employee
    edited May 2015

    Hi Pratik,

     

    Please refer to Frequency response function tutorial on our help documents.

     

    OS-1300: Direct Frequency Response Analysis of a Flat Plate

    OS-1305: Modal Frequency Response Analysis of a Flat Plate

     

    In addition refer to RLOAD2 card (used in above mentioned tutorials) to learn how to apply velocity as a freq function.

  • Altair Forum User
    Altair Forum User
    Altair Employee
    edited May 2015

     Step 8: Create a Set of Frequencies to be used in the Response Solution
     

    can you elaborate this steps? does is it calculate the response for only this 49 freqencies (20 hz -1000 hz)

     

    Modal frequency response : does it used some Modal matrix reduction method? any  literature  for optistruct about this method?

     

    Can i use EIGRA instead to EIGRL for Mode frequency extraction

  • Altair Forum User
    Altair Forum User
    Altair Employee
    edited May 2015

    Hi Pratik,

     

    Yes, response is calculated from 20-1000Hz.

     

    You can use EIGRA for modal FRA. You can refer to OptiStruct user guide form more information on FRF analysis.

     

    Altair OptiStruct > User's Guide > Structural Finite Element Analysis > Solution Sequences:

  • Altair Forum User
    Altair Forum User
    Altair Employee
    edited May 2015

    Hello Prakash

    In Step 5:  Create a Unit Load at a Point on the Flat Plate

       7 Uncheck all the dof's; except dof3, and click the = to the right of dof3 and enter a value of 20.

     

    what dies the value 20 stand for? Isn,t it unit load?

     

    Regards

    Pratik

  • Altair Forum User
    Altair Forum User
    Altair Employee
    edited May 2015

    Pratik,

     

    that is the load/exitation you are giving in Z-direction.

     

    If you are giving load type as enforced displacement for instance, then the node will be displaced by 20 units.

  • Altair Forum User
    Altair Forum User
    Altair Employee
    edited May 2015
    Altair Forum User said:

     

    Hello Prakash

     

    so what exactly is Modal Frequency Response , how does it differ from drect frequency response

     

    for example  If i want velocity response between excitation frequency between say 1000 to 2000 hz , but i want response only at eigen frequncies in this range  should i do Modal Frequency response analysis?

  • Rahul_P1
    Rahul_P1
    Altair Employee
    edited May 2015
    you will have to provide the excitation frequencies for which you need the response,

     

    The direct method solves the coupled equation of motion in terms of the excitation frequency.

     

    The modal method uses the mode shape of the structure to uncouple the equations of motion and the solution for a particular excitation frequency is obtained by summation of individual modal responses or modal superposition.

     

    see help 

    OptiStruct > User's Guide > Structural Analysis:



    Frequency Response Analysis

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