Direct and inverse approaches for modal detection using an external microphone array in the presence of a background flow

• Identification of acoustic modes caused by rotor-stator interaction using external measurements • Consideration of a realistic bellmouth-shaped intake geometry • Calculation of modal amplitudes while assuming a background mean flow • Validation of the novel approach on a fan-OGV test rig Accurately characterizing the acoustic modal content generated by rotor-stator interaction in ducted fans is critical for noise mitigation. However, conventional in-duct measurement techniques are hampered by significant drawbacks, including limited physical space, intrusive sensor placement, and susceptibility to turbulent flow, all of which can compromise data accuracy. In this paper, we introduce a methodology that determines the in-duct modal amplitudes from non-intrusive, external measurements. Our approach utilizes a hemispherical microphone array located in the far-field to capture the radiated acoustic pressure. This data, combining both experimental measurements and numerical simulations, is then processed using an iterative Bayesian Inverse Approach. This technique was specifically chosen to reconstruct the source modal content while effectively reducing the artifacts associated with array sidelobes. The results demonstrate that the modal amplitudes determined by our external measurements yield the same results as those from conventional in-duct approaches and even provide a higher degree of accuracy. This successful implementation, validated through a second test case with a modified outlet-guide-vane configuration, confirms that our method overcomes the core limitations of internal measurements. It offers a more robust and flexible approach for aeroacoustic analysis that is less sensitive to turbulent conditions and spatial constraints.