Mistuned Higher-Order Mode Forced Response of an Embedded Compressor Rotor: Part I — Steady and Unsteady Aerodynamics

This paper is the first part of a two-part paper that presents a comprehensive study of the higher-order mode mistuned forced response of an embedded rotor blisk in a multistage axial research compressor. The resonant response of the second-stage rotor (R2) in its first chordwise bending (1CWB) mode due to the second harmonic of the periodic forcing of its neighboring stators (S1 and S2) is investigated computationally and experimentally at three steady loading conditions in the Purdue Three-Stage Compressor Research Facility. State-of-the-art numerical methods applicable in an industrial design environment are used to construct a 1.5-stage stator/rotor/stator configuration for the prediction of the aerodynamic forcing function of the rotor. The time-averaged component of these simulations provides a good prediction of the compressor performance, rotor tip leakage flow (TLF), and characteristics of the stator aerodynamic disturbances. The contribution of the rotor TLF on the rotor forcing function is small, responsible for less than 5% of the total modal force in amplitude. Moreover, the individual contributions of the upstream and downstream stators to the rotor modal force are separated via a linear forcing decomposition approach. It is shown that the upstream stator provides the dominant forcing function with an amplitude almost 6 times that of the downstream stator, and is mostly due to the impulse-like appearance of the upstream stator wakes which have significant higher-harmonic (including the second-harmonic) contents. An excellent prediction of the tuned 1CWB resonant response amplitudes is achieved with only 35%, 4%, and 7% difference to the measured values at three loading conditions.