Computational Tradeoff in Modal Characteristics of Complex Rotor Systems Using FEM

Modal analysis of complex rotor-bearing systems using the finite element method (FEM) may become controversial when considering the tradeoff between geometric complexity and elastodynamic integrity of the FEM model. In this paper, the modal characteristics of an actual multi-impeller rotor-bearing system with complicated geometry are estimated and verified using two modeling schemes. The first scheme invokes a developed finite element elastodynamic model that accounts for gyroscopic effects, torsional-bending inertia coupling, internal material damping, shear and anisotropic bearings. In this model, the intricate details of the impeller were ignored, and only the inertial properties of the impeller disk are considered. The second scheme employs a general-purpose finite element code, wherein the complicated impeller geometry is included in lieu of ignoring some of the rotational effects and inertia coupling effects. The obtained results shed the light on the tradeoffs involved in modal analysis of complicated rotor systems using the FEM. Comparisons with experimental values showed that the first scheme, which adopts simplified impeller geometry while accounting for all the rotational effects is more accurate in estimating the modal characteristics of such complex rotor-bearing systems.