General Mechanical

General Mechanical

Error in Time history analysis in acoustic elements

    • pala_fito
      Subscriber

      In ansys workbench 19.2 I have a water storage model developed with acoustic elements and shell elements, at the same time I have it supported by combin40 spring elements. The modal analysis works ("Modal"), since the results were compared with mechanical models and the frequency response coincides, however, there are errors in the transient model "Transcient Structural" (time history analysis), when applying acceleration in the basis (time/acceleration) in the transient module, the model runs but the results are not consistent, the von misses voltage is extremely low and the displacements in the basis are also very very low, for example the values of acceleration applied in the base increases with time and the displacement response is reduced. For this example I applied an acceleration in a time of 3 seconds.

      I tried to apply acceleration based on modulus
      "Static Structural", but an error appears that the image is attached and I don't know how to solve it.

      Please, any suggestion or help on how to do it is welcome.

       

      Structural Transcient

      Acceleration example 3 second

       

      Desplacement error

       

       

       

    • Sheldon Imaoka
      Ansys Employee

      Hi pala_fito,

      When you have a free surface, it can be a little cumbersome to apply a base acceleration because the free surface assumes that the acceleration field is gravity.  (Both "Acceleration" and "Standard Earth Gravity"' are the same inertial load, and one overwrites the other.)  Unfortunately, you can't set up the model in the manner that you have.

      If you can convert your base acceleration to an imposed Displacement time-history, then you can keep gravity for the Free Surface and use Displacement to impose the base motion.  An alternative is to add a large mass (via Remote Point), then apply a Remote Force; since F=ma and if the large mass is 'large' (>> mass of your structure), then your applied {F} will be defining an acceleration time-history {a} to your base that is tied via Remote Point to the large mass [m].

      For a static analysis, the acoustic elements do not calculate an acoustic response.  Since acoustic waves are dynamic by nature, they can't be calculated in a static analysis - acoustic fluid elements are supported in a vibroacoustic static analysis only for purposes of a prestressed modal analysis for the structural portion.  (You can't solve a purely acoustic problem in a static manner since there are no waves generated if everything is time-independent.)  That is why the error message explains morphing - in a static analysis with both the fluid and structure (with structure undergoing large deflections), the fluid must morph to the new shape of the structure.

      I hope that the above may help clarify the situation,
      Sheldon

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