Reply To: Negative frequencies during eigenvalue analysis

[Reno Genest]

Hello Tonmoy,

The negative eigenvalues are the rigid body modes. You can ignore those. Even if the parts are moving in the XY plane or around a node, they are still moving as a rigid body (rigid body translations and rotations). 

Here is more information from our knowledge database:

"

First of all, always use a double precision executable when doing an eigenvalue analysis.

Eigenvalues near zero (negative or positive) are associated with rigid body modes.    For confirmation, animate the modes using the d3eigv database.    If you see no motion, then plot velocity vectors, which is a trick we use to identify nodal rotation.  

To eliminate rigid body modes, one would normally define (additional) constraints in the model.   If you're unconcerned about the rigid body modes, then do nothing.
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Computation of the eigenvalues occur in floating point arithmetic.
You should ignore the sign associated with near zero eigenvalues and
just treat them as zero.  An eigenvalue of value -1.0e-6 should just be recognized as an eigenvalue of 0.0.
The difficulty we have is deciding what is "near zero" as the we do not have the units of the model.  Changing
the units in which the model is described will change the tolerance for what is near zero.  Also whether the
analysis is performed in single or double precision.  I strongly recommend that all eigenvalue computation
be performed in double precision to accurate computation of the zero eigenvalue modes. "

Let me know how it goes.

 

Reno.