Tiziano Pagliaroli

This study investigates the aeroacoustic effects of phase synchronization between co-rotating propellers in side-by-side configurations. Time-domain Proper Orthogonal Decomposition (POD) is employed on pressure fluctuation data to separate tonal and broadband components, enabling a detailed analysis of acoustic interference mechanisms. Experimental results demonstrate that synchronization significantly influences the tonal component, highlighting distinct regimes of constructive and destructive interference depending on the rotor phase delay. Specifically, the analysis reveals a notable reduction in noise amplitude at specific phase shifts, particularly affecting the second harmonic. An explanation of this scientific outcome is provided and illustrated schematically. Additionally, probability density functions indicate pronounced modifications in the statistical characteristics of pressure fluctuations related to phase synchronization, further supporting the effectiveness of the decomposition approach. he results demonstrate the effectiveness of rotor synchronization as a useful active noise control strategy in order to achieve a significant noise mitigation. A non-linear analysis is applied to the tonal component, revealing a path to chaos via intermittency during the transition between constructive and destructive interference. More in detail, a local birth and death of periodic oscillations via Hopf-like bifurcation is observed. This aspect highlights the intricate and fascinating chaotic nature of the interference transition. The chaotic behavior of the tonal component is related to the macro time scale of the pressure fluctuation, and has been incorporated into the mathematical model.

This study investigates noise generation from co-rotating rotors arranged in a side-by-side configuration. The analysis examines the effects of different phase delays and separation distances. A simple mathematical model is developed to provide insight into constructive and destructive noise interference. An experimental campaign was carried out to validate the proposed analytical model. Furthermore, the study introduces a space–time proper orthogonal decomposition technique to separate broadband and tonal components. Subsequently, wavelet analysis is applied to the tonal component, revealing a transition to chaos via intermittency, characterised by the local birth and decay of periodic oscillations. This phenomenon highlights the intricate and fascinating chaotic nature of interference transitions. The chaotic behaviour of the tonal component is related to the macro time scale of pressure fluctuations, and has been incorporated into the mathematical model. This model has several applications, including its potential use in the development of active control systems and the design of quieter distributed propulsion systems.

The main goal of the present study is to explore the noise mitigation potential using an active control strategy based on rotor phase synchronization. This work is focused on the effects of the inflow velocity on the noise interference effect. The inflow velocity does not affect the phase at which the interference phenomenon is observed, as expected. On the other hand, the intensity of the pressure fluctuations is influenced by the inflow velocity for all of the rotor phase shift conditions investigated. Specifically, as the inflow velocity increases, maintaining a constant rotational speed, in the Overall Sound Pressure Level graphs, a reduction of approximately 10 dB is observed. This effect also applies to cases of destructive interference, highlighting the remarkable versatility of this noise reduction technique.

An investigation of twin corotating rotors’ interaction effects was performed by load (thrust and torque) measurements, flow field dynamics through Time-Resolved Particle Image Velocimetry, and acoustic emissions using a microphone array. Two rotors, each with a diameter of D = 393.7 mm and equipped with three blades, were investigated in a side-by-side configuration, to simulate a multirotor propulsion system. The mutual distance between the propellers is 1.02 D, and four different rotating speeds, i.e., 2620, 3500, 4360, and 5200 RPM, were explored. In such a configuration, thrust and torque undergo a reduction compared to that found for a single propeller configuration. The level of aerodynamic load fluctuations increases as well. The interaction of the wakes produces a recirculation region at the external periphery of the shear layers. An innovative approach involving the coupling of Proper Orthogonal Decomposition (POD) and Wavelet Transform has been employed to investigate the dominant structures within the flow and their mutual influence. The results reveal that the interacting wakes are dominated by a wave-like motion pulsating at Harmonics of the Blade Passing Frequency (HBPF) of 1/3. Higher orders of POD modes capture coherent vortical structures, including tip vortices pulsating at HBPF = 1. The aeroacoustic investigation shows that the noise level, in terms of the Over All Sound Pressure Level, presents a remarkable increment concerning that generated by the single propeller.

In the last decade, the drone market has grown rapidly for both civil and military purposes. Due to their versatility, the demand for drones is constantly increasing, with several industrial players joining the venture to transfer urban mobility to the air. This has exacerbated the problem of noise pollution, mainly due to the relatively lower altitude of these vehicles and the proximity of their routes to extremely densely populated areas. In particular, both the aerodynamic and aeroacoustic optimization of the propulsive system and of its interaction with the airframe are key aspects of unmanned aerial vehicle design that can signify the success or the failure of their mission. The industrial challenge involves finding the best performance in terms of loading, efficiency and weight, and, at the same time, the most silent configuration. For these reasons, research has focused on an initial localization of the noise sources and, on further analysis, of the noise generation mechanism, focusing particularly on directivity and scattering. The aim of the present study is to review the noise source mechanisms and the state-of-the-art control strategies, available in the literature, for its suppression, focusing especially on the fluid-dynamic aspects of low Reynolds numbers of the propulsive system and on the interaction of the propulsive system flow with the airframe.

The aim of the present manuscript is to investigate the noise footprint of an isolated propeller in different flight configurations for the propulsion of a hybrid-electric aircraft. Experimental tests were performed at the Low-Turbulence Tunnel located at Delft University of Technology with a powered propeller model and flush-mounted microphones in the tunnel floor. The propeller was investigated at different advance ratios in order to study the noise impact in propulsive and energy harvesting configurations. For brevity, this work only reports the results at the conditions of maximum efficiency in both propulsive and energy harvesting regimes, for a fixed blade pitch setting. Comparing these two configurations, a frequency-domain analysis reveals a significant modification in the nature of the noise source. In the propulsive configuration, most of the energy is related to the tonal noise component, as expected for an isolated propeller; however, in energy harvesting configuration, the broadband noise component increases significantly compared to the propulsive mode. A more detailed analysis requires separation of the two noise components and, for this purpose, an innovative decomposition strategy based on proper orthogonal decomposition (POD) has been defined. This novel technique shows promising results; both in the time and in the Fourier domains the two reconstructed components perfectly describe the original signal and no phase delays or other mathematical artifices are introduced. In this sense, it can represent a very powerful tool to identify noise sources and, at the same time, to define a proper control strategy aimed at noise mitigation.

This paper studies the spontaneous radiant energy emitted by a trapped vortex flame, focusing on the intermittent occurrence of the flame blowout during thermoacoustic oscillation. We tackle this issue by a wavelet-based auto-conditioning technique, that on the grounds of an energy criterion, performs an objective detection and selection of those events whose energy content exceeds a proper threshold. The result is the time history of a signal containing extinction-reignition events only. As further outcomes, the detected intermittent events are statistically characterized in terms of shape, amplitude, time lag, and timescales. Moreover, intermittent events are found to be in phase with the tonal component of the radiant energy signal, whereas they occur randomly in time. Although the wavelet auto-conditioning technique is a well-established method in fluid dynamics and aeroacoustics, we apply it in the combustion field, where alternative methods, based on wavelet transforms, have already been successfully applied. Additional information about intermittency in the flame is inferred from the chaotic analysis: recurrence plot (RP) of radiant energy shows that the time of boundary crisis is much larger than the characteristic timescale of extinction-reignition event. Furthermore, cross-conditioning of RPs with intermittent events found by wavelet transform results in a characteristic “pike-like” pattern, which identifies an intermittent state different from the most common intermittency types already described in the literature.