Experimental Study on Sound Propagation Through a Duct Section With Large Pylons

Kurzfassung

The pressure distribution of an acoustic mode emitted into the bypass duct of an aeroengine is significantly altered by the pylons' boundary conditions. The presented experimental study aims to analyze the associated propagation effects in detail. For this purpose, large pylons are installed in an annular duct section and subjected to individual modes of controlled propagation angles. The modes are generated by a spinning mode generator consisting of two loudspeaker rings, which is operated in such a way that all modes of other azimuthal and radial orders are effectively suppressed. The transmitted, scattered and reflected fractions of the incident mode are measured using mode detection microphone arrays in the upstream annular duct and in the pylon section. To account for the impact of the pylons' shape on the mode propagation in the inter-pylon duct the radial mode decomposition technique is extended accordingly. The results demonstrate that the incident mode perceives the propagation conditions of the inter-pylon section in a distinctly different manner depending on the angle of incidence. This leads to diverse transmission behavior and manifests itself in the form of various mode orders being generated in the inter-pylon ducts. At the trailing edge of the pylon section, the resulting sound pressure distribution is dominated either by the order of the incident mode, its inverse order, or by a scattering mode.