Rotordynamics: unbalance loads due to displacement - FEA

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SUMMARY

This discussion focuses on the impact of off-axis displacement on unbalance loads in Finite Element Analysis (FEA) for rotordynamics, specifically using a Jeffcott rotor model. The user inquires about how commercial FEA codes account for additional unbalance forces resulting from the displacement of the center of gravity (CG) during analysis. Key considerations include the updating of the mass matrix and the implications of non-linear updates, such as those seen in tangent stiffness matrices. The conversation highlights the need for clarity on how these factors influence load generation in FEA simulations.

PREREQUISITES
  • Understanding of rotordynamics and Jeffcott rotor modeling
  • Familiarity with Finite Element Analysis (FEA) principles
  • Knowledge of mass matrix updates in dynamic simulations
  • Concept of non-linear updates in structural analysis
NEXT STEPS
  • Research how commercial FEA codes like ANSYS or Abaqus handle unbalance forces in rotordynamics
  • Learn about the mathematical formulation of mass matrix updates in dynamic systems
  • Explore the concept of geometric stiffening and softening in FEA
  • Investigate the effects of off-axis displacement on rotor dynamics and stability analysis
USEFUL FOR

Mechanical engineers, rotordynamics specialists, and FEA analysts looking to deepen their understanding of unbalance forces and their implications in rotor modeling.

tricha122
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Hi,

Im wondering if / how commercial FEA codes take into account additional unbalance forces due to the off-axis displacement of masses.

For instance, if I am modeling a jeffcott rotor with an unbalance, I would traditionally represent this unbalance with a force that increases with speed^2. However during the analysis, the jeffcott rotor moves off axis (whirls) and should have an additional unbalance force equal to the mass of the disk times the displacement of the CG times the speed^2.

I am not sure how commercial FEA codes account for this force (if they do) - would this come about due to mass matrix being updated? Even if the mass matrix were updated, how would it take into account the new locations (displacements) of the disk into the generation of the new load? Mass matrix is just multiplied by acceleration... unless the mass is fictitiously increased in a non-linear update (a la tangent stiffness matrix for geometric stiffening/softening)

Anyone that could shed some light on the situation would be greatly appreciated! Thanks!
 
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