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.

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.

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This paper describes the development of a new dual rotor test stand for the Open Jet Flow-Through Anechoic Chamber at the Penn State University. The Coaxial Acoustic Test System (CATS) can accommodate the testing of isolated, tandem, and coaxial rotor configurations operating in hover, climb, cruise, and descent flight conditions. The stand is equipped with a traversable microphone array that enables acoustic measurements to be acquired at a variety of azimuthal directivity angles. This paper highlights the design process as well as some of the encountered issues and their solutions. Sample results for two representative coaxial co-rotating rotor configurations are also presented. The first configuration consists of two vertically separated “stacked” rotors mounted on a single shaft. The rotors are mounted on separate shafts in the second configuration, so that the azimuthal position of the blades varies throughout the measurement. Close agreement was achieved in the average thrust and torque measurements for the two rotor configurations. The separated configuration had higher tonal noise levels than the stacked configuration, although broadband noise levels were similar.

Flow recirculation is known to develop inside a closed anechoic chamber when testing unmanned aerial system (UAS) rotor components and vehicles. This flow recirculation modifies the inflow through the vehicle’s rotors, which results in significant impacts to the measured acoustic signature. A measurement campaign was undertaken at NASA Langley Research Center in which a UAS rotor was tested inside a small anechoic wind tunnel. Acoustic signatures were obtained with the downwash exhausting down the wind tunnel, and with the tunnel exhaust plugged forcing flow recirculation. Several methods to mitigate the acoustic impacts of flow recirculation were then employed with the wind tunnel in the plugged configuration. The effectiveness of the mitigation strategies are discussed, along with implications for future testing and standards development.