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M. Alì
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This study investigates the aeroacoustic behavior of a low-Reynolds-number propeller in forward flight subjected to large-scale inflow disturbances. The incoming flow is modeled as single-frequency sinusoidal vortical gusts, enabling a systematic assessment of the effects of gust frequency, initial phase, and direction on aerodynamic performance and noise generation. The numerical setup is first validated against experimental data under steady inflow conditions. The results show that the loading fluctuations caused by the incoming gust result in discrete tonal components in the acoustic spectrum at frequencies determined by the combination of the gust frequency and multiples of the rotational frequency. These components arise from a double modulation mechanism, and their amplitude is further shaped by inter-blade interference effects. The phase of the gust with respect to the rotor primarily affects the phase of the blade response, thereby modifying the noise directivity, particularly at low frequencies. When the gust is inclined relative to the mean flow, the interaction becomes more complex, leading to a richer tonal spectrum with high intensity tones extending up to the 10th harmonic of the blade passing frequency. Overall, the results provide a physical interpretation of the coupling between rotating blades and large-scale inflow disturbances, supporting the development of improved models for unsteady tonal noise prediction.
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This study investigates the aeroacoustic behavior of a low-Reynolds-number propeller in forward flight subjected to large-scale inflow disturbances. The incoming flow is modeled as single-frequency sinusoidal vortical gusts, enabling a systematic assessment of the effects of gust frequency, initial phase, and direction on aerodynamic performance and noise generation. The numerical setup is first validated against experimental data under steady inflow conditions. The results show that the loading fluctuations caused by the incoming gust result in discrete tonal components in the acoustic spectrum at frequencies determined by the combination of the gust frequency and multiples of the rotational frequency. These components arise from a double modulation mechanism, and their amplitude is further shaped by inter-blade interference effects. The phase of the gust with respect to the rotor primarily affects the phase of the blade response, thereby modifying the noise directivity, particularly at low frequencies. When the gust is inclined relative to the mean flow, the interaction becomes more complex, leading to a richer tonal spectrum with high intensity tones extending up to the 10th harmonic of the blade passing frequency. Overall, the results provide a physical interpretation of the coupling between rotating blades and large-scale inflow disturbances, supporting the development of improved models for unsteady tonal noise prediction.
Journal article
(2026)
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Mario Alì, Andrea Piccolo, Riccardo Zamponi, Daniele Ragni, Francesco Avallone
This work investigates the effect of grid-generated turbulence ingestion on noise generation in a propeller operating at a low Reynolds number using high-fidelity, scale-resolved simulations. The numerical setup reproduces experiments carried out at Delft University of Technology, where inflow turbulence is generated by a grid placed within a duct. It is found that, upstream of the propeller, the longitudinal correlation length of the streamwise velocity component increases with respect to the case without the propeller. The opposite happens for the transversal one. The turbulent inflow impinging on the propeller blades does not alter the mean flow characteristics over the propeller blades, e.g., the mean static pressure coefficient. However, it increases the root mean square of the pressure fluctuations up to the turbulent reattachment point of the laminar separation bubble, while leaving the downstream region mostly unaffected. This causes a broadband increase in the radiated noise in the low-to-mid frequency range, as confirmed by applying Amiet’s noise-prediction model with input data sampled near the propeller blades’ leading edge. The far-field noise spectra are characterized not only by an increase in the broadband noise with respect to the clean inflow case, but also by tonal components at multiples of the blade-passing frequency. It is found that these tones are caused by the footprint of the turbulence grid that introduces flow inhomogeneities at the propeller location for this specific configuration. It is recommended, when performing experiments and simulations, to verify if any footprint of the turbulence grid is present, not only by performing single-point measurements but also by measuring the time-averaged flow field before installing the propeller.
...
This work investigates the effect of grid-generated turbulence ingestion on noise generation in a propeller operating at a low Reynolds number using high-fidelity, scale-resolved simulations. The numerical setup reproduces experiments carried out at Delft University of Technology, where inflow turbulence is generated by a grid placed within a duct. It is found that, upstream of the propeller, the longitudinal correlation length of the streamwise velocity component increases with respect to the case without the propeller. The opposite happens for the transversal one. The turbulent inflow impinging on the propeller blades does not alter the mean flow characteristics over the propeller blades, e.g., the mean static pressure coefficient. However, it increases the root mean square of the pressure fluctuations up to the turbulent reattachment point of the laminar separation bubble, while leaving the downstream region mostly unaffected. This causes a broadband increase in the radiated noise in the low-to-mid frequency range, as confirmed by applying Amiet’s noise-prediction model with input data sampled near the propeller blades’ leading edge. The far-field noise spectra are characterized not only by an increase in the broadband noise with respect to the clean inflow case, but also by tonal components at multiples of the blade-passing frequency. It is found that these tones are caused by the footprint of the turbulence grid that introduces flow inhomogeneities at the propeller location for this specific configuration. It is recommended, when performing experiments and simulations, to verify if any footprint of the turbulence grid is present, not only by performing single-point measurements but also by measuring the time-averaged flow field before installing the propeller.