The far-field acoustic emissions of an isolated scale-model propeller ingesting turbulent inflow are experimentally investigated in wind-tunnel measurements. Phase-averaging and phase-shifting signal processing techniques are used to separate the deterministic (tonal) and random (broadband) components of the recorded sound signals. It is reported that a combination of the two techniques prevents tones unrelated to the shaft rotational frequency from contaminating the estimate of the broadband part of the signal. Scaling and directivity analyses of the obtained broadband component highlight the presence of two noise generation regimes depending on the considered frequency range. In particular, a quasi-omnidirectional directivity pattern is observed for frequencies for which the Helmholtz number is much lower than unity when turbulence is ingested. On the other hand, a dipole-like pattern with minima close to the propeller’s rotational plane gradually appears for higher frequencies. A time- and frequency-domain analysis through the Continuous Wavelet Transform (CWT) method shows how the increase in broadband noise is due to a large number of short-duration pulses linked to the ingestion of turbulence.

Psychoacoustic listening experiments were conducted to investigate the human response to propeller noise under turbulent inflow conditions. The sound stimuli considered were recorded in aeroacoustic wind tunnel experiments featuring an isolated six-bladed propeller. The propeller operated under different isotropic inflow turbulence conditions and collective blade pitch angles. Higher inflow turbulence intensity levels caused a growth in the broadband noise emissions of the propeller, increasing the overall loudness levels. On the other hand, sharpness and tonality consistently decreased due to the increase in low-frequency noise and the tone-masking effect by the aforementioned broadband noise, respectively. The listening experiments aimed to elucidate the effect of these contradicting trends on sound perception. It was observed that lower noise annoyance ratings were reported for higher inflow turbulence intensity levels and lower collective blade pitch angles. Overall, only the tonality metric provided a statistically significant correlation with the annoyance ratings, indicating the importance of this perceptual aspect in propeller noise, in combination with loudness and sharpness. Other energy-based sound metrics, like the effective perceived noise level, failed to correctly describe the results of the listening experiment. In general, this analysis is valuable for the perception-influenced design of devices equipped with propellers.

The far-field acoustic emissions of a six-bladed propeller were investigated in aeroacoustic experiments in an openjet wind tunnel. The propeller was operating in different isotropic inflow turbulence conditions generated by turbulence grids placed upstream of the exit plane of the wind tunnel nozzle. In addition, the collective pitch angle of the propeller blades was also varied throughout the measurements. A preliminary directivity analysis of different acoustic and psychoacoustic metrics was performed to investigate the influence of the inflow turbulence intensity and collective pitch angle on the noise emissions and sound perception. In general, increasing the inflow turbulence levels did not modify the conventional metrics recorded, e.g. equivalent sound pressure level. Nevertheless, it considerably increased the broadband noise emissions of the propeller, the loudness, and the overall psychoacoustic annoyance metrics. However, notable reductions in tonality (due to partial tone masking because of the higher levels of broadband noise) and sharpness were reported for increasing turbulence intensities. Overall, this analysis is valuable for the perception-influenced design of devices equipped with propellers, such as drones or urban air mobility vehicles, to account for installation effects.

The application of an acoustically absorbent material (melamine foam) is investigated for the treatment of turbulence grids in an anechoic open-jet wind tunnel facility featuring an axisymmetric contraction. A comparative study of both the generated turbulence and the grids’ self-noise is performed. It is found that the application of melamine foam on the downstream side of the grids marginally affects the produced turbulence, while providing an efficient suppression of tonal peaks in the grids’ self-noise spectrum. Broadband noise levels instead show opposing trends depending on the frequency range considered. On the one hand, a general decrease due to the acoustic treatment is observed for Strouhal numbers lower than unity. On the other, an increase, in the form of broad peaks, is seen to occur over certain higher frequency ranges.

The issue of acoustic reflections caused by an axisymmetric nozzle is addressed both numerically and experimentally. The contraction (with an exit diameter of 420 mm and a contraction ratio of 2) is representative of typical open-jet wind tunnel exits. Such a category of wind tunnels are extensively used in aeroacoustic studies due to the possibility of acoustic treatment of the test chamber (plenum) walls and the placement of acoustic sensors outside of the flow. The distance between the wind tunnel’s nozzle exit and the model being tested is usually limited to around one hydraulic diameter in order for the model to be fully contained within the exit jet’s core. Partly for this reason, the wind tunnel nozzle normally protrudes inside the anechoic plenum to distance the tested model from the (acoustically-treated) walls. This effectively creates a cavity, which is in communication with the test section. Acoustic simulations through a commercial finite-element code (COMSOL Multiphysics) show that the presence of the nozzle leads to interference patterns within the test section and a substantial modification of a source’s measured directivity pattern. Experimental measurements in a fully anechoic chamber confirm these results. Melamine foam inserts on both the exit flange and part of the inner walls of the contraction are shown to mitigate the issue partly.

An experimental aeroacoustic study on the influence of the collective blade pitch angle in the noise emissions by an isolated propeller under different turbulent inflow conditions is presented. Acoustic and aerodynamic measurements are conducted in an anechoic, open-jet wind tunnel facility. Different inflow turbulence characteristics are achieved by employing square-mesh, square-bar turbulence grids positioned ahead of an additional 2:1 contraction at the wind tunnel's exit. It is found that the ingestion of grid-generated turbulence does not significantly impact the thrust produced by the propeller for any of the tested collective blade pitch angles. On the other hand, turbulence ingestion greatly increases noise production in both broadband and tonal components. The grouping of broadband noise around the Blade Passing Frequency (BPF) and its harmonics ("haystacking'') does not prove to be a phenomenon of particular relevance in grid-generated turbulence ingestion. A directivity analysis shows that an increase in inflow turbulence intensity is responsible for increased noise emissions downstream of the propeller.