Rotordynamic Behavior of Fan-Type Rotors: Role of Impeller Flexibility and Structural Idealization

Fan-type rotor dynamics arise from the coupled behavior of the shaft, impeller, bearings, and support structure, with the impeller’s required structural detail depending on the frequency range and response quantities of interest. This study examines the role of impeller flexibility in the global rotordynamic behavior of an overhung fan-type rotor when local blade dynamics lie outside that range. The impeller is represented globally as a flexible annular disk—a continuum model for examining rotating-component flexibility against the rotor–support system—and compared with an inertia-equivalent rigid representation matched in mass, polar and diametral inertia, attachment, bearings, operating conditions, and excitation, to assess the effect of further idealization. The numerical framework couples a Timoshenko-beam shaft, an MITC4 flexible disk, linear bearings, and a mass–stiffness–damping foundation. Modal characteristics, Campbell behavior, critical-speed crossings, modal kinetic-energy distributions, synchronous unbalance response, and transmitted bearing forces are evaluated across variations in disk bending rigidity, disk-to-shaft inertia, bearing stiffness, and foundation properties. Model selection is assessed through modal proximity, internal disk deformation, and agreement of response-relevant flexible and rigid families. The foundation study is a secondary investigation of absolute system dynamics, complemented by paired flexible–rigid calculations showing that the discrepancy decreases as the foundation softens. This study thus provides a system-level framework for evaluating structural idealization of impellers in fan-type rotordynamic models where blade-local dynamics are not governing.