Experimental Exploration on the Influences of Inflow Distortions on the Broadband Noise Generation of Three Subsonic Fans

Kurzfassung

Inlet flow distortions in aircraft engines and ventilation systems can significantly increase broadband fan noise by amplifying existing sound sources and generating new ones. This experimental study aims to understand the main noise excitation mechanisms and how they depend on fan geometry and inflow characteristics. The focus of the investigations is on subsonic axial fans, with two application-oriented scenarios examined in detail: an Urban Air Mobility propulsor positioned above the wing (ATW), and a configuration representing boundary layer ingestion (BLI) for embedded engines. A subsonic fan test rig equipped with flush-mounted microphones was used to perform broadband mode analysis and quantify sound power spectra. To assess individual noise contributions, a semi-empirical model based on Hanson's theory was calibrated and used to approximate the noise resulting from the interaction between the duct boundary layer and the rotor in the case without inflow distortion. The study systematically varied parameters including the number of rotor blades, the number of stator vanes, the flow rate and the rotational speed.The results show that interactions of the duct boundary layer with the rotor create broad spectral peaks around and the blade passage frequency, which remain relatively constant across different flow rates. Conversely, noise induced by inflow distortions increases with the flow rate, demonstrating a more significant impact on fans with fewer rotor blades. A slight dependence on the stator vane number was observed, indicating that distortion noise is partly influenced by the modulation of rotor wakes. Furthermore, the residual sound power, after subtracting boundary-layer-rotor interaction and distortion components, exhibits characteristic rotor-stator interaction trends, scaling approximately with the product of the vane number and chord length.