NOISE SOURCE IDENTIFICATION IN A PROPFAN MODEL BY MEANS OF ACOUSTICALNEAR-FIELD MEASUREMENTS

For the exploration of the dominant aerodynamic noise sources, the pre ssure fluctuations in the exit plane (near field) of the propfan mode, CRISP (Counter Rotating Integrated Shrouded Propfan) were measured wi th conventional 1/4 inch microphones. The pressure field of the tone c omponents was resolved into a distribution of duct modes. Knowledge of the dominant modes allows conclusions about the dominant noise genera tion mechanisms, because the different noise sources inside the propfa n create different sets of modes. The CRISP concept is developed by Mo toren- und Turbinen-Union Munchen (MTU). The experimental propfan has two counter-rotating rotors of 0.4 m diameter and equal speed. The shr oud is supported by seven struts located downstream of the second roto r. Measurements were made with equal (B-1 = B-2 = 10) as well as unequ al (B-1/B-2 = 10/12) blade numbers and under different operational con ditions. The highest overall harmonic levels were found for the config uration with equal blade numbers. In this case, the blade passing freq uency component is generated mainly by the rotor 2/struts interaction, and the higher blade tone harmonics, which dominate the overall tone noise level, are produced by the interaction of the two rotors. In the case of unequal blade numbers, all even harmonics of the shaft freque ncy (H = 2, 4, 6, ...) can be generated by the rotor 1/rotor 2 interac tion. The harmonics below H = 22, however, are excited as non-propagat ional modes only and were found to have small amplitudes in the exit p lane. The rotor 1/rotor 2 interaction is the main noise generation mec hanism for the configuration with a short axial distance between the r otors. When the rotor distance is enlarged, the rotor/rotor interactio n noise is reduced and, as a consequence, the contributions from the r otor/struts interactions become important. In addition to the experime nts, a theoretical method is described for the prediction of the frequ encies and azimuthal modes generated by two rotors with arbitrary spee ds, directions of rotation and blade numbers. This method is helpful f or the design of low noise rotor systems. (C) 1997 Academic Press Limi ted.