Reply To: Negative frequencies during eigenvalue analysis

[Reno Genest]

Hello Tonmoy,

Is your model fully constrained? Or do you have parts that can move as a rigid body in space (not fully constrained)? The negative and zero frequency modes are probably due to rigid body modes. You can animate the modes and see what deformation they produce. You should be able to see the rigid body modes if you have any.

To get rid of the rigid body modes, you need to use *BOUNDAY_SPC or *BOUNDARY_PRESCRIBED_MOTION or other constrains (joints, tied contacts, etc.) so that no parts can fly away. This should eliminate the zero or negative frequencies.

If you don't want to fully constrain your model, you can ignore the negative or zero frequency modes and look at the first positive non zero frequency. Animate the mode to make sure it is the real first mode of vibration (lowest stiffness of your system).

Let me know how it goes.

 

Reno.