Forcing superposition and decomposition of an embedded compressor rotor
This paper investigates superposition and decomposition methods when determining the forcing function on an embedded rotor in a multistage axial compressor. Under investigation is a 3-row stator/rotor/stator configuration where two stators, having the same vane count, can be circumferentially clocked to change the rotor excitation. The combined forcing function on the rotor in any arbitrary clocking arrangement is assumed to be a linear combination of two known clocking positions. In this study, the forcing function from these known positions are first approximated by two 2-row CFD solutions, namely stator/rotor and rotor/stator. It is found that theses approximations are poor, since they do not include the significant effect of stator-stator interaction. Next, these two forcing components are obtained via a linear decomposition using two 3-row CFD solutions at two clocking positions. A linear superposition is performed based on these decomposed results to estimate the combined rotor forcing for any arbitrary clocking position. It is found that although the trend of rotor forcing with respect to clocking is qualitatively captured, considerable quantitative discrepancies exist that refute the proposed linearity assumption. This assumption oversimplifies the multi-row aerodynamic interactions: a change in the stator-stator clocking position does not only alter the relative phase between two stator-induced forcing functions to the rotor, but also these two components themselves.