Assessment of In-Duct Fan Broadband Noise Measurements in a Modern Low-Speed Test Rig

A comprehensive experimental study is conducted at a new aeroacoustic fan test rig with the objective to improve the understanding of the dominant fan noise source mechanisms. For this purpose, the aerodynamic properties are resolved with hotwire probes and total pressure rakes and the acoustic characteristics are resolved with arrays of wall-flushed microphones in the inlet section. The interpretation of the results is supported by a physics-based, analytical fan model. The presented study focuses on the rotor-incoherent noise components. They are obtained by a cyclostationary analysis, which ideally removes tonal components from the sound pressure spectrum. The remaining spectrum consists of broadband noise and narrowband spectral components. The narrowband components are located around the blade passing frequencies and are associated with elongated turbulent structures, which create partly correlated noise sources when interacting with the rotor. The sound power of the broadband and narrowband components is determined with help of a radial mode decomposition. Correlations to varying mean flow speed and loading of the rotor blades are found. By means of an experimental variation, i.e. the installation of a honeycomb in the fan inlet, the dependence of the narrowband noise components on the lateral turbulence and the integral length scale is demonstrated. Comparisons with the analytical fan noise model give insight into the underlying dependencies. The model calculates the steady and unsteady flow field based on simplified state-of-the-art aerodynamic modelling. From this the strengths of the acoustic sources that are distributed radially along the individual blade spans are derived. The fast calculations allow extensive parameter studies, that help to trace back acoustic phenomena to their physical noise source mechanisms. The paper concludes with a comparison to results obtained for another test rig with similar dimensions. Both rigs show slightly different trends due to different blade designs.