Spatial Decomposition of Aerodynamic Forcing and Interference Diagram: Theory and Aeromechanic Validation on an Industrial Axial Compressor

This paper proposes a new strategy to decompose the overall unsteady aerodynamic forcing, and an improved use of the Interference Diagram capable of detecting additional possible resonances in the operating range of turbomachines. The identification of potential resonance conditions, or crossings, in the Interference Diagram is based on the assumptions of a cyclic symmetry tuned structure and a forcing function having an aliased engine order driven only by the airfoil count of the rotor/stator interactions of interest. Despite this, it is not rare that unexpected resonances appear during compressor validation tests, often calling for painful and time-consuming redesigns. This is witnessed by the unsteady analysis of an 11-stage axial compressor in which the spatial content of engine order forcing is influenced also by further rotor/stator interactions that cause the onset of additional crossing conditions. The theory based on spatial decomposition is validated with full 3D unsteady forced response simulations of the entire multistage compressor. Numerical results obtained by a modal work approach are compared with experimental data focusing on two resonances, the first of which is detected by the classical interference diagram, while the second one is justified only by the improved use discussed here. In both cases, the predicted blade responses are in good agreement with measurements. The theory associated with the generation mechanism of the additional nodal diameters is further explained using the results of dedicated numerical experiments conducted by changing the relative stator and rotor airfoil count on a reduced compressor domain.