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When a consistency check reveals a difference in results, which result is true?

    • csw5046
      Subscriber

      Hi.
      I performed analysis on the same k file on different computers.
      I confirmed that the results were different depending on the CPU of the computer, and I found out that this could be resolved through consistency checking.
      However, I am curious as to which computer's results are true when the consistency checking is not performed.
      Additionally, can you briefly explain how consistency checking works?

    • Pedram Samadian
      Ansys Employee

      Hello,

      Thank you for contacting us. The difference between the predictions is related to the randomness of numerical round-off in finite element simulations, which refers to small, non-deterministic variations introduced when representing real numbers in a computer.

      Computers have finite precision and thus cannot represent every possible decimal value exactly. Instead, they use an approximation, which leads to small round-off errors. In complex simulations, these round-off errors can sometimes create seemingly random effects, particularly in highly sensitive computations.

      In the LS-DYNA software, such an issue can be raised when running simulations in the shared memory parallel mode. To address it, the use of the *CONTROL_PARALLEL card with the consistency option on is recommended. In any given problem with the consistency option off (i.e. CONIST = 2) in the *CONTROL_PARALLEL card, summations in calculations occur in a different order from run to run, so the round-off becomes random. However, with the included consistency flag (i.e. CONST = 1), all contributions to global vectors are enforced to be summed in a precise order, resulting in identical results (or nearly so) regardless of the number of processors used.

      Further information about the consistency checking in LS-DYNA can be found in the remark section of the *CONTROL_PARALLEL card in the LS-DYNA User Manual Vol I (URL: LSDYNA Manuals).

      Based on the provided information, the predictions with the consistency check should be more accurate; however, comparison with the experimental results or analytical calculations is recommended. 

      Thank you,
      Pedram

       

       

       

       

       

       

       

       

       

       

    • csw5046
      Subscriber

      Thanks for the answer.

      But I have one more question.

      Then why is it necessary to turn off Consistency checking?

      Thanks.
      steven

    • Pedram Samadian
      Ansys Employee

       

      Hi Steven,

      No problem. I recommended to turn on the consistency flag (CONST = 1) if you are running in the SMP mode, which helps summations in calculations to occur in a precise order from run to run and lead to identical results (or nearly so) independent of the number of processors.

      You might be asking when we can turn off the consistency flag. The answer is with the consistency option off (CONST = 2), the computation time will be less, and the solution is achieved faster (Check out the variable “CONST” in *CONTROL_PARALLEL card in the LS-DYNA User Manual Vol I (URL: LSDYNA Manuals)). You can check how different the predictions are when using CONST = 1 and CONST = 2 and then decide if you prefer to save more time at the expense of losing some degree of accuracy or to have more accurate predictions at the expense of more computation time.

      I hope this answer helps.

      Thanks,

      Pedram

       

    • csw5046
      Subscriber

      I appreciate your clear answer, it cleared up my curious. Thank you.

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