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This article contains an investigation of the unsteady acoustic forcing on a centrifugal impeller due to coupled blade row interactions. Selected results from an aeromechanical test campaign on a GE Oil and Gas centrifugal compressor stage with a vaneless diffuser are presented. The most commonly encountered sources of impeller excitation due to upstream wake interaction were identified and observed in the testing campaign. A 30/rev excitation corresponding to the sum of upstream and downstream vane counts caused significant trailing edge vibratory stress amplitudes. Due to the large spacing between the impeller and the return channel vanes, this 30/rev excitation was suspected to be caused by an aero-acoustic excitation rather than a potential disturbance. The origin of this aero-acoustic excitation was deduced from an acoustic analysis of the unsteady compressor flow derived from CFD. The analysis revealed a complex excitation mechanism caused by impeller interaction with the upstream vane row wakes and subsequent acoustic wave reflection from the downstream return channel vanes. The findings show it is important to account for aero-acoustic forcing in the aeromechanical design of low pressure ratio centrifugal compressor stages.

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This paper presents an integrated numerical procedure to predict the generation, in-duct propagation, and radiation of multiple pure tone (MPT) noise of aircraft engines. Reynoldsaveraged Navier-Stokes (RANS) computational fluid dynamics (CFD) simulations using part-annulus grids have been performed to resolve the non-uniform shock signature just upstream of the fan blades. A linear superposition method is then used to reconstruct the full-annulus pressure field using the part-annulus CFD results and the as-manufactured blade stagger angles measured by a coordinate-measuring machine. In order to account for the nonlinear propagation of the shock waves inside the nacelle, a one-dimensional model has been employed to simulate the propagation of MPT from upstream of the fan leading edge to the nacelle lip. For far-field propagation, a commercially available software ACTRAN/TM is used to linearly propagate the acoustic modes to far-field microphone locations. The entire analysis process has been applied to predict MPT noise of a typical high bypass ratio engine at operating conditions when the relative tip Mach number of the fan is transonic. Comparisons against static engine test measurement are made for in-duct, near-field, and far-field sound pressure levels. Good agreement has been observed between predictions and measured data.