dm/m too high!

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

Hi everyone, i'm doing a roof crash analysis with an imposed displacement on a rigid plane. I set the minimum timestep to 1e-7 and during the first cycle it prints:

'  T= 0 DT= 5.67E-07 ERR= 0% DM/M= 3.5526E+00'

If my tmin is less than the actual dt, why do i have a so large dm/m?

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Answers

  • Pranav Hari
    Pranav Hari Altair Community Member
    edited October 2020

    Hi Solosono

     

    DM/M shows the mass error. When a user impose a time step in the model, mass will be added to the system. And DM/M shows the mass added to the system.

    At time T=0 you are having a time step of 5.67E-07,  but since time steps varies throughout the simulations this value of time step at time T=0 is not  necessary to be constant as time proceeds.

    Here since you have imposed a time step, after time T=0, it will take the imposed time step value (1e-7), thereby adding mass to the node/model

    As a good practice the mass error in the model should be less than .05 (5%)

     

    Please refer this forum post also to know about Advanced mass scaling.

    https://community.altair.com/community?id=community_question&sys_id=424600b61b2bd0908017dc61ec4bcb19

    Please share the entire .OUT file to get a better idea of the simulation.

     

     

    Thanks

  • Simon Križnik
    Simon Križnik Altair Community Member
    edited December 2018

    Hi @Solosono

     

    actually, the timestep printed at a first cycle is a natural timestep (without scaling) and no mass is added so dm/m should be 0.

    The only reason mass error is not 0 at the start if there are type 2 tied contact interfaces with sptflag=1. From Radioss help:

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

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