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Modal Analysis

  1. Jul 17, 2010 #1
    Hi all,

    If we do free free modal analysis in ANSYS shall we need constrain the model or not? Where can I see the first 6 modes which have frequency equal to zero. I am confused can anybody please help?


    Regards
    Shankar
     
  2. jcsd
  3. Jul 17, 2010 #2

    minger

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    What does free-free mean? Means nothings holding it right?

    Also, by this terminology I assume that you're trying some sort of rotordynamic analysis? If so, I have a rather long post somewhere on here about how to do it. It's not quite as easy for complex geometries as you might think (i.e. you cannot use axisymmetric elements).
     
  4. Jul 18, 2010 #3
    Sir, you are rirht but how can we find modes in ANSYS without holding. As a rigid body we have 6-modes fequency almost null, is that right?

    where can we see these modes (rigid modes)?
     
  5. Oct 19, 2011 #4
    Hi,
    I am not sure about the performance of free-free modal analysis in ANSYS.
    If I wanted to get the natural frequencies of a box-like structure with four wheels (at each corner of the bottom surface) by means of FE-analysis, should it be performed as constrained or unconstrained ?
    Indeed, the wheels are not bounded to the ground, so please, could you help me?
    Thank you very much
     
  6. Oct 19, 2011 #5

    AlephZero

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    Right.
    "Free-free" is used to describe any type of dynamic analysis, not just rotordynamics. It is often useful to correlate a free-free vibration model with test data (for example supporting the structure on very flexible springs or bungee ropes) to get the mass and striffness properties of the model correct, independent of figuring out how to model the constraints.

    There are also some mathematical techniques for using free-free models constructed entirely from measured data, to combine them with models of other components (either measured or FE) models, constrain them in different ways, etc.
     
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