Evaluating rigid body modes in modal analysis

what can be expected from modal?analysis of natural frequency and mode shape which describe dynamic characteristic of system?

Running modal analysis to check if the contacts are correctly configured for non-uniform gaps. When there are no contacts for the rigid body like satellite, ship, airplane. Rigid body modes are solved by free-free modal analysis.?

In free-free modal analysis 1st six natural frequency represent rigid body modes in 3 translational direction and 3 rotational direction. Other rigid body mode include when grounding exists in the system. Here grounding refers to artificial constraints preventing movement of the system. If grounding exists it means modal result of system is not accurate because grounding can effect stiffness matrix of structure. Identify grounding in system can be done by suppressing all support or boundary condition and running modal analysis. It can be verify by the rigid body mode has non zero frequency in 1st six mode of frequency. And higher frequency with constraint will also get affected. Grounding can be done by bonded contact with large non uniform gaps and multiple constraints formulation can also be used. Solution to eliminate grounding is to switch between different contact formulation to make sure that 0 frequency values are there for rigid body modes.

Original model with grounding / fix support at top edge. As the frequency of mode 4 to 6 shows values which is not close to zero so we have to review the bonded contact.

Using MPC in bonded contact connections (without fix support)

During rigid body motion, the evaluation of participation factor becomes less meaningful due to dominance of rigid body motion.

After comparing ,

Comparing both mode shapes result we can se considerable difference in mode 3 so evaluating the Total deformation of mode 3 results we can see that mode 3 is in the direction of contact which explains the considerable difference in mode 3.