Application of planar beamforming method to identification of spinning acoustic modes

Geometrical and mechanical parameters of multi-layer panels used as sound-absorbing liners in the ducts of aircraft engines are determined by acoustic impedance requirements of the liners. The requirements are stated based on the analysis of sound modes propagation in ducts and in particular in the inlet. Therefore, knowledge of the sound field modal structure in an intake duct is essential for the choice of geometrical and mechanical parameters (depth of layers, dimensions and forms of honeycomb chambers, percent open area, material of construction and core, type of glue and etc.) of sound-absorbing liners. Experimental determination of the modal structure can be made with a microphone array mounted inside or outside the duct. In the present work, a planar microphone array placed outside the inlet duct has been used for measuring spinning acoustic modes, and the obtained data was processed with the beamforming method. Spinning modes were produced by a special generator based on the inlet of PS-90 turbofan engine. Sound was generated by 34 acoustic drivers JBL 2451H placed around the circumference under the test rig. Tests were carried out without flow. Experimental investigations were performed in the brand new anechoic chamber of Perm National Research Polytechnic University. The measurement results show that planar beamforming method locates a spinning mode at a point, which position depends on the mode number (this phenomenon is similar to the results obtained with planar beamforming method for propeller or open rotor noise). The conclusion has been made that measurements with planar beamforming method of spinning modes radiated from an inlet allow, in principle, determining noise modal structure in the duct but practical implementation of the method for real aircraft engines requires special research. Localization of spinning modes at a point has to be taken into account in the analysis of aircraft engine noise sources measured with beamforming method.