Impact of Upstream and Downstream Excitation and Multi-Row Interaction on the Mistuned Forced Response Behavior of an Embedded Compressor Rotor: Higher-Order Mode Aeromechanics

Abstract The current papers discuss the impact of separating the upstream and downstream forcing function contributions in an embedded compressor rotor. Previously, the authors have discussed separating the influence at a torsional mode crossing, and this paper expands this study to a higher-order mode crossing. This crossing occurs at a slower speed, and higher-order crossings are difficult to predict due to their low response for this configuration. The paper focuses on the multi-row interaction at multiple operating points and the mistuned blade response prediction. The computational model for this analysis uses the time transformation method within the commercial code CFX to help simplify the computational domain. The configuration chosen for this study is that the number of rotor blades and downstream stator blades have a common multiple, which allows the usage of a small number of passages in these rows. However, the upstream stator count does not satisfy this criterion. Hence, the time tilting method helps account for this difference in pitch and simplifies the computational domain. Both the upstream and the downstream rows excite the embedded rotor at different crossings. Additionally, the effect of wave reflection from a downstream or upstream row is considered. The responses are characterized at multiple operating points, i.e., nominal loading and high loading. The second section of this paper describes the details of the in-house mistuning code utilized to predict the blade responses of the individual rotor blades in the system. One of the inputs to this code is the modal force obtained using unsteady CFD. The in-house code is based on the FMM, which couples both aerodynamic and structural dynamics to predict the blade responses. The key conclusions from this study were: 1) Physical wave reflections from the downstream stator were constructive at both the operating points. 2) The analysis at the 88EO crossing helped quantify the impact of physical wave reflections from a downstream rotating row. The influence was found to be destructive at both operating points, 3) The 3-row case with the downstream rotor in the computational model yielded the most accurate prediction of the blade response supporting earlier conclusions by the same authors that a downstream row was essential to predict the forcing function accurately.