Guidelines for accurate sound source quantification in closed-section wind tunnels

Abstract

Experimental aeroacoustic measurements conducted in wind tunnels are crucial for informing the design process of various devices, including aircraft components or wind turbine blades. Whereas closed-section wind tunnels typically offer better aerodynamic conditions compared to their open-jet counterparts, they often introduce challenges related to noisier test environments and the optimal placement of acoustic sensors, such as microphone arrays, within the test section. As advancements in noise reduction measures lead to quieter test models and our knowledge regarding the location of their main noise sources improves, accurately quantifying the sound sources within these models becomes increasingly important. The present manuscript offers valuable guidelines aimed at enhancing the precision of sound source quantification. It provides practical recommendations regarding microphone placement and the utilization of post-processing techniques, such as manipulations of the cross-spectral matrix. The experimental setup consists of an array of 16 microphones in two different configurations (flush-mounted and recessed in cavities). To assess the quantification accuracy, a speaker playing broadband noise at different sound pressure levels outside of the flow serves as a known reference sound source. A wide range of signal-to-noise ratios (SNRs) are achieved by employing different flow velocities and speaker settings. The results indicate that relatively accurate sound source quantification can be achieved with SNRs down to −10 dB. Lastly, a scaling law for the expected quantification error is proposed in terms of the number of microphones within the array and the SNR. In this manner, the experimental setup can be adapted accordingly to obtain the required level of accuracy.