Acoustic Characterization of the NASA Langley 14- by 22-Foot Subsonic Tunnel using a Phased Array

A characterization test was conducted in the NASA Langley Research Center 14- by 22-Foot Subsonic Tunnel to assess recent acoustic modifications in the facility. Test section reflectivity, background noise and shear layer effects were evaluated by taking measurements of known noise sources. Both conventional speakers and a high-pressure air source were employed as noise generation mechanisms. These were mounted to a pole or fairing at a consistent location in the test section. The source waveforms were used to represent the expected frequency content of scaled Urban Air Mobility vehicles. Acoustic measurements were taken using two unique microphone arrays placed outside the core flow, one of which was a 54-element streamwise-traversing phased array. The other was an 11-element linear tower array. The applicability of phased array acoustic beamforming techniques to mitigate shear layer effects and reject background noise is presented. Additionally, beamforming results are compared to results from single-microphone processing techniques. Flow speeds investigated ranged from Mach 0.00 (static) to Mach 0.12. The tunnel has been partially acoustically treated but still exhibits flow noise, reducing the signal-to-noise ratio of the source under investigation. Conventional beamforming was able to capture high frequency content passing through the shear layer better than single microphone measurements. CLEAN deconvolution improves source mapping at low frequencies.