Removal of Spurious Noise From Permeable FWH Data Surfaces

The use of permeable Ffowcs Williams and Hawkings data surfaces in conjunction with scale-resolving computations has become the preferred approach for predicting the far-field noise signature of complex aircraft configurations for which shielding, scattering, and other propagation effects are important. However, this approach can render inaccurate levels that may be unrecognized when only single-microphone data are available. Specifically, the passage of a strong, turbulent wake through the downstream segment (endcap) of a permeable data surface may generate spurious sources that can affect the noise signature significantly, even when standard endcap averaging is used. The development of methods to reduce the impact of these fictitious sources on single-microphone spectra is presented in this paper. The methods are based on beamforming the signals for each endcap at a given single observer location. Studies are first conducted using synthetic white noise signals simulating multiple endcaps. Sample results from CFD simulations of the NASA generic low boom concept are then used to illustrate the presence of spurious noise sources at the endcaps and the potential of the proposed methods to eliminate or significantly reduce the impact of these sources on single-microphone far-field spectral data. Advantages and limitations of three proposed methods are discussed in detail, including the impact of the number of endcaps, the amount of uncorrelated noise present in the signals, and the length of the data available to compute the required cross-spectral matrices.