Flow diagnostic-free methodology for identifying propeller recirculation onset in anechoic chambers

Acoustic testing of propellers in enclosed environments presents a fundamental challenge in reproducing static free-flight conditions due to the onset of flow recirculation, as reported in previous studies. Building on the concept of a facility time-scale parameter, this paper proposes and demonstrates a methodology for identifying the onset of flow recirculation without the use of advanced flow-measurement instrumentation. An experimental campaign was conducted in an anechoic chamber across a range of test points, encompassing multiple low-Reynolds-number propeller sizes and rotational speeds. The onset of recirculation was inferred through facility-scale considerations and analysis of the resulting acoustic response, rather than through direct flow diagnostics. The extent and severity of acoustic contamination were quantified using sound pressure level measurements obtained from a far-field microphone array. The results elucidate clear dependencies between the recirculation onset and key parameters governing the facility time scale, including propeller diameter and angular speed. The results confirm that flow recirculation measurably alters the propeller tonal acoustic signature. The strongest effect was observed in the 2nd to 9th BPF harmonics, where differences of up to 7 dB were recorded between time segments, while the 10th–15th harmonics showed smaller variations. The findings contribute toward the development of more robust and standardised methodologies for acoustic testing of propellers and unmanned aerial vehicles.