Mitigating the pitfalls of testing unmanned aerial system vehicles and components in anechoic chambers

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.