PSD analysis setup in Optistruct

swapnil s
swapnil s Altair Community Member
edited October 2020 in Community Q&A

Hello,

 

I am having PSD data for mounting locations for frequency range of (1-50 Hz) from 60 hour shaker test.

I am facing problem setting up the model in OptiStruct.

 

should I go for FRF first and then PSD? 

or can I apply direct PSD data?

 

please help me to understand and setting up the model for PSD analysis.

I have attached the PSD data.

 

 

thanks,

Swapnil

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Answers

  • Rahul Rajan_21763
    Rahul Rajan_21763 New Altair Community Member
    edited April 2019

    Random response analysis is a transfer function of Frequency Response. 

    Random response analysis is treated as a data reduction procedure that is applied to the results of a frequency response analysis.

    First, the frequency response analysis is performed for sinusoidal loading conditions, for each separate subcase, at a sequence of frequencies .

    Normal data reduction procedures are then applied to the output of the frequency response analysis module, resulting in a set of output quantities , corresponding to an output and subcase.

     

    You can refer this topic in free eBook of Dynamic analysis with OptiStruct using below link.

    https://altairuniversity.com/free-ebook-learn-dynamic-analysis-with-altair-optistruct/

  • swapnil s
    swapnil s Altair Community Member
    edited May 2019

    Hi Rahul,

    thanks for reply.

     

    please tell me that approach I have used below for PSD analysis is correct or not:

    FRF Setup:

    >I have created unit load(acceleration) case in FRF setup in X,Y and Z direction simultaneously. using SPCD card.

    >Given frequency range and increment using TABLED1 and FREQ1

    >Excited the unit load through Rload2 cards with type as acceleration

    >SPC card is given at mounting location

    PSD Setup:

    >Given frequency vs G2/Hz as input to TABRND1 card for X,Y and Z direction.

    >Created combined RANDPS load collector for X,Y and Z direction TABRND1

     

    Load Steps:

    >Created 3 Direct Frequency Response load steps for X, Y and Z  using (SPC, DLOAD:RLOAD, FREQ1)

     

    Thanks,

    Swapnil

     

  • Rahul Rajan_21763
    Rahul Rajan_21763 New Altair Community Member
    edited October 2020

    You can refer the model shared in below post.

    https://community.altair.com/community?id=community_question&sys_id=73564cf61b2bd0908017dc61ec4bcb16

    So, you define the excitation (SPCD / Force) in the FRF step. For example if you want to apply SPCD in X, Y and Z directions, you will have

    1. 3 number of Subcases with 3 DLOAD

    2. 3 RLOAD cards corresponding to DLOADs

    3. 3 SPCD cards

    4. one or more TABLED1 card

    Now, the loading will be applied in FRF step and the results are outputted for those loads.

    When it comes to Random part, you need to define Random Input for all 3 direction using same ID. 'All RANDPS entry with same ID will be added together'.

    RANDPS,101, 11, 11, 1.0, 0.0, 11

    RANDPS,101, 12, 12, 1.0, 0.0, 12

    RANDPS,101, 13, 13, 1.0, 0.0, 13

    With 3 TABRND1 cards ( 11,12 and 13)

    The additive results of Random are then multiplied to the FRF results to get PSD / RMS outputs.

     

    Hja and Hjb are the results from FRF and Sab from Random (PSD)

     Please refer attached to pdf for the two scenarios based on Random input.

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

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