AEROACOUSTIC EFFECTS ON THE FORCING OF FAN AND COMPRESSOR BLADES DUE TO DISTORTION

A distorted air stream entering an aeroengine fan or compressor leads to harmonic forces on the rotating blades. These aerodynamically induced forces are well known causes for blade vibration and associated fatigue problems. Significant levels of distortion can arise from different sources like side wind and high angles of attack that normally occur at specific operating conditions. For aircraft with the engine closely integrated with the fuselage the engine will be exposed to distortion during the entire flight cycle. This increases the risk of operating in conditions where blade resonances are excited. The sensitivity to the forcing is also accentuated in this case as the exposure over time gives a large number of fatigue cycles. Unsteady response to a specific distortion can be modeled using CFD methods to a high degree of fidelity. With a focus on understanding the aeroacoustic interaction the analyses used here considers generic harmonics of the distortion. Harmonic responses are calculated from low to transonic speeds for a range of cases. Major phenomena and driving parameters affecting the forcing strength and pressure amplitudes in the blade passage are identified from the analyses. It is demonstrated that the forcing strength is strongly affected by the cut-on/cut-off conditions upstream and downstream of the blades. Also, depending on design parameters of the blade, the aeroacoustics of the blade passage is important for the resulting forcing. The analyses used are made in 2D over a wide range of flow conditions as well as geometric variations. The results of the study provides an increased understanding of the harmonic forcing of blades. A simple model is proposed that can identify condition where increased pressure amplitudes in the blade passage may be expected. The sensitivities to parameters may also give some guidance in how design and operation can be adapted to reduce the aerodynamic forcing.