RESUMO
An experimental campaign was undertaken to identify the effects posed by flow recirculation on the dynamic performance and acoustic emissions of an isolated rotor, operating in hover in a sealed anechoic chamber. It is shown that flow recirculation is an artifact of testing rotary-wing systems in an enclosed environment, and results in a significant amplification of tonal and broadband noise components. The acoustic emissions produced while the recirculated flow is ingested into the rotor disk vary from those emitted by similar rotors operating in clean inflow conditions. A variety of turbulence suppression screens were employed to impede the propagation of the rotor wake, in an effort to delay the onset of recirculation. This mitigation strategy proved to be effective for one treatment configuration and significantly extended the duration available to acquire clean measurements. The presence of this treatment is thought to alter the coherent structures comprising the recirculated flow and thus the noise emissions following the onset of flow recirculation.
RESUMO
An experimental campaign was undertaken to identify the effects of flow recirculation on an isolated rotor's acoustic emissions in a closed anechoic chamber. It is shown that flow recirculation results in a significant increase in higher harmonic noise, with an increase of more than 15 dB in some harmonics. This increase in noise is due to the nature of testing in a closed facility and does not represent the acoustic emissions of a similar rotor in hovering flight outdoors. Future measurements of rotors and full unmanned aerial vehicles in closed anechoic chambers must acknowledge recirculation effects and attempt to mitigate the effect on reported measurements.