Wind Tunnel Continuous-Scan Acoustic Analysis of Ducted Fan Tones Interacting With a Shielding Wall

Abstract High-bypass ratio ducted fans emit tonal and broadband noise in both forward and aft directions. While produced by common mechanisms, the noise radiating in each direction is typically treated as its own directional source. Proper modeling of forward- and aft-emitted fan noise enables more efficient noise suppression strategies. In this article, wind tunnel continuous-scan (CS) acoustic measurements of a fan simulator are analyzed in two configurations: with and without a barrier wall structure. The barrier wall is used to separate the forward-emitted fan noise from the aft-emitted fan noise through shielding. As acoustic shielding is also an important noise reduction strategy for next-generation aircraft, the barrier wall is an effective surrogate to study the effect of propulsion airframe aeroacoustics (PAA), where the wall creates shielding and/or scattering similar to a wing or airframe surface. The goal of this article is to use previously demonstrated high-resolution, CS acoustic analysis methods to (1) decompose isolated fan noise measured in the NASA Glenn Research Center (GRC) 9- by 15-foot (9 × 15) low-speed wind tunnel (LSWT) into respective aft- and forward-emitted source models and (2) predict the fan noise interaction with a barrier wall. Acoustic signals from the scanning microphone are combined with a once-per-revolution tachometer signal to separate each shaft harmonic tone's amplitude and phase in the time domain. The complex pressure envelope obtained from this decomposition is next fit to an axially distributed ring source model by solving an inverse problem. The source model is verified against the isolated tunnel measurement data and then again projected with a line-of-sight barrier wall shielding effect. The barrier wall measurements provide a comparison dataset for the analytical shielding prediction developed from the isolated dataset. Results suggest that the source model localizes the fan sources well enough to perform analytical source separation, offering the potential to eliminate the barrier wall from testing, which would provide a substantial time or cost savings. The findings also suggest that combining high-resolution acoustic measurements with analytical methods could realize an integrated test and analysis paradigm for efficient PAA design.