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General Mechanical

General Mechanical

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DMPRAT command

    • October 4, 2017 at 12:20 pm
      vganore
      Ansys Employee

      In a full harmonic analysis, DMPRAT did not actually produce a uniform ratio to critical damping value. For a harmonic analysis based on a single mode the damping it produces is greater than the ratio to critical for frequencies below resonance and less than the ratio to critical for frequencies above resonance. You can see this in Equation 14-22 of the MAPDL Theory Manual. The term (2/OMEGA)mj is the material dependent form of DMPRAT. As you can see it is divided by the driving frequency, not by the natural frequency as it would be if it was truly a ratio to the critical frequency. So it gets smaller as OMEGA gets larger and is only equal to the ratio to the critical frequency when OMEGA = omega at resonance for a single mode analysis. To avoid the impression that DMPRAT produces a ratio to the critical frequency we undocumented it. What we do now with DMPSTR is consistent with other codes and common terminology.

    • October 4, 2017 at 2:39 pm
      pgl
      Ansys Employee

      Why is the very commonly used DMPRAT command no longer supported for a full harmonic analysis?

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