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"Aeroacoustics"
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Tonal-Noise Assessment of Quadrotor-Type UAV Using Source-Mode Expansions
The present work deals with the modeling of the aerodynamic sound generated by the propellers of small-size drones, taking into account the effects of horizontal forward flight with negative pitch and of installation on supporting struts. Analytical aeroacoustic formulations are used, dedicated to the loading noise. The fluctuating lift forces on the blades are expanded as circular distributions of acoustic dipoles, the radiated field of which is calculated by using the free-space Green’s function. This provides descriptions of the sound field, valid in the entire space. The stationary mean-flow distortions responsible for the lift fluctuations and at the origin of the sound are estimated from existing numerical flow simulations and from ad hoc models. Installation and forward-flight effects are found to generate much more sound than the steady loading on the blades associated with thrust. Therefore, the models are believed reliable fast-running tools that could be used for preliminary low-noise design through repeated parametric calculations, or for noise-impact estimates corresponding to prescribed urban traffic.
Journal Article
Aerodynamic and Aeroacoustic Interactional Effects for a µUAV
An experimental campaign has been conducted at the Italian Aerospace Research Center to quantify the interactional effects caused by rotor-rotor interaction on a commercially available µ UAV platform. Aerodynamic and aeroacoustic measurements have been carried out for different configurations: data obtained for the isolated rotor, i.e. the baseline configuration, has been compared to the quadcopter configuration. Additionally, the quadcopter with a single operating rotor has been tested to isolate the effects of the quadcopter’s supporting arms. As expected, the aerodynamic performance of the quadcopter configuration is worse than the baseline configuration, although by a small margin, with the quadcopter exhibiting a 5% reduction in the overall thrust coefficient. The presence of the quadcopter’s support arms also degraded the acoustic field, causing the single rotor configuration to be more annoying to listeners across all azimuth angles.
Journal Article
Wavepacket models for supersonic jet noise
Gudmundsson and Colonius (J. Fluid Mech., vol. 689, 2011, pp. 97–128) have recently shown that the average evolution of low-frequency, low-azimuthal modal large-scale structures in the near field of subsonic jets are remarkably well predicted as linear instability waves of the turbulent mean flow using parabolized stability equations. In this work, we extend this modelling technique to an isothermal and a moderately heated Mach 1.5 jet for which the mean flow fields are obtained from a high-fidelity large-eddy simulation database. The latter affords a rigourous and extensive validation of the model, which had only been pursued earlier with more limited experimental data. A filter based on proper orthogonal decomposition is applied to the data to extract the most energetic coherent components. These components display a distinct wavepacket character, and agree fairly well with the parabolized stability equations model predictions in terms of near-field pressure and flow velocity. We next apply a Kirchhoff surface acoustic propagation technique to the near-field pressure model and obtain an encouraging match for far-field noise levels in the peak aft direction. The results suggest that linear wavepackets in the turbulence are responsible for the loudest portion of the supersonic jet acoustic field.
Journal Article
The generation of screech tones by shock leakage
The mechanism underpinning the generation of screech tones has remained an open question for many years. In this paper, direct experimental observations of the shock-leakage mechanism first proposed by Manning & Lele (AIAA Paper 1998, p. 282) are presented. Ultra-high-speed schlieren images are filtered to preserve only upstream-propagating components, with the upstream motion of the shock tip and subsequent emission of an acoustic wave visible for a number of operating conditions. The flow visualizations are supported by the ray-tracing model for shock leakage of Shariff & Manning (Phys. Fluids., vol. 25, issue 7, 2013, 076103), applied to velocity fields corresponding to a reconstructed screech cycle. The predictions of the model, when applied to real data, are in close agreement with the phenomena observed in the flow visualizations. It is demonstrated that shock leakage does not necessarily occur either at the point of maximum wave amplitude or maximum vorticity fluctuation. While the first point of shock leakage is shown to vary between cases, sound emission from multiple sources is observed for most cases considered. Finally, it is shown that variations in vortex strength captured in the velocity data are sufficient to explain variation in shock-leakage location observed in the flow visualization data.
Journal Article
Axisymmetric superdirectivity in subsonic jets
We present experimental results for the acoustic field of jets with Mach numbers between 0.35 and 0.6. An azimuthal ring array of six microphones, whose polar angle,
$\\theta $
, was progressively varied, allows the decomposition of the acoustic pressure into azimuthal Fourier modes. In agreement with past observations, the sound field for low polar angles (measured with respect to the jet axis) is found to be dominated by the axisymmetric mode, particularly at the peak Strouhal number. The axisymmetric mode of the acoustic field can be clearly associated with an axially non-compact source, in the form of a wavepacket: the sound pressure level for peak frequencies is found be superdirective for all Mach numbers considered, with exponential decay as a function of
$ \\mathop{ (1\\ensuremath{-} {M}_{c} \\cos \\theta )}\\nolimits ^{2} $
, where
${M}_{c} $
is the Mach number based on the phase velocity
${U}_{c} $
of the convected wave. While the mode
$m= 1$
spectrum scales with Strouhal number, suggesting that its energy content is associated with turbulence scales, the axisymmetric mode scales with Helmholtz number – the ratio between source length scale and acoustic wavelength. The axisymmetric radiation has a stronger velocity dependence than the higher-order azimuthal modes, again in agreement with predictions of wavepacket models. We estimate the axial extent of the source of the axisymmetric component of the sound field to be of the order of six to eight jet diameters. This estimate is obtained in two different ways, using, respectively, the directivity shape and the velocity exponent of the sound radiation. The analysis furthermore shows that compressibility plays a significant role in the wavepacket dynamics, even at this low Mach number. Velocity fluctuations on the jet centreline are reduced as the Mach number is increased, an effect that must be accounted for in order to obtain a correct estimation of the velocity dependence of sound radiation. Finally, the higher-order azimuthal modes of the sound field are considered, and a model for the low-angle sound radiation by helical wavepackets is developed. The measured sound for azimuthal modes 1 and 2 at low Strouhal numbers is seen to correspond closely to the predicted directivity shapes.
Journal Article
Modelling of jet noise: a perspective from large-eddy simulations
In the last decade, many research groups have reported predictions of jet noise using high-fidelity large-eddy simulations (LES) of the turbulent jet flow and these methods are beginning to be used more broadly. A brief overview of the publications since the review by Bodony & Lele (2008, AIAA J. 56 , 346–380) is undertaken to assess the progress and overall contributions of LES towards a better understanding of jet noise. In particular, we stress the meshing, numerical and modelling advances which enable detailed geometric representation of nozzle shape variations intended to impact the noise radiation, and sufficiently accurate capturing of the turbulent boundary layer at the nozzle exit. Examples of how LES is currently being used to complement experiments for challenging conditions (such as highly heated pressure-mismatched jets with afterburners) and guide jet modelling efforts are highlighted. Some of the physical insights gained from these numerical studies are discussed, in particular on crackle, screech and shock-associated noise, impingement tones, acoustic analogy models, wavepackets dynamics and resonant acoustic waves within the jet core. We close with some perspectives on the remaining challenges and upcoming opportunities for future applications. This article is part of the theme issue ‘Frontiers of aeroacoustics research: theory, computation and experiment’.
Journal Article
Recurrence Analysis Applied to Ultrasonic Absorptive Coating
Metasurfaces are, at the actual state-of-the-art, a constant topic for scientific community as they find widespread applications in several fields, including acoustics and aeroacoustics. In this paper, the potential of recurrence analysis was evaluated by applying it to some metasurfaces. These devices effectiveness has been previously demonstrated with a reflection coefficients analysis in the Fourier and Wavelet domains. A novel strategy based on the Recurrence Analysis, which is a no man’s land applied on the metasurfaces, was performed. The evaluation of the recurrence plot and phase space attractors led to interesting results. The results show an introduction of non linearities after the forcing wave interacts with the metasurfaces. In light of that, there is the evidence that the chaotic analysis principles may be a powerful tool to characterize the performance of metasurfaces in particular through the calculation of the chaotic indexes. This method could be integrated with previous ones for the evaluation of more complex and multiscale geometries that nowadays show huge potential in too many fields of application.
Journal Article
Coherent structure and sound production in the helical mode of a screeching axisymmetric jet
The structure of a screeching axisymmetric jet in the helical C mode at a nozzle pressure ratio of 3.4 issuing from a convergent nozzle is studied using high-resolution particle image velocimetry. Proper orthogonal decomposition (POD) is used to extract the dominant coherent structures within the jet. The first two modes produced by the POD are used to reconstruct a phase-averaged data sequence. A triple decomposition into mean, coherent and random velocity components is performed. The embedded shock structures within the jet are shown to strongly modulate the coherent axial stresses within the shear layer and to weakly modulate the random axial stresses. Analysis of the third and fourth moments of the velocity probability density function is used as an indicator of possible regions of shock–vortex interaction and thus screech tone generation. Peaks of kurtosis (flatness) occur at the second, third and fourth shock–boundary intersection points, with the radial position shifting towards the centreline with increasing downstream distance. Analysis of the coherent component of vorticity shows that the largest fluctuations in coherent vorticity occur at the high-speed side of the shear layer in an area extending from the second to the fourth shock cell. With reference to prior literature, the argument is made that it is this increased magnitude of coherent vorticity fluctuation that is the primary factor in the determination of which shock cells act as dominant screech sources.
Journal Article
Enhancement of a Free-wake Model by using PIV Measurements for the Study of a Small-scale Propeller with Dissimilar Blade Geometries
This research seeks to improve a free-wake model for the study of the hover performance of a small, two-bladed propeller using the Boundary Element Methodology (BEM), implemented in the medium-fidelity aerodynamic solver RAMSYS, coupled with the tool ACO-FWH for acoustic validation. It emphasises the necessity of accounting for subtle geometric variations, which significantly affect aeroacoustic predictions, specifically sub-harmonic SPL spectra. An optimisation process, employing a Genetic Algorithm, determines optimal parameters for the Bhagwat-Leishman vortex core model, enhancing acoustic signature accuracy beyond geometric correction alone. The findings confirm the effectiveness of a carefully calibrated medium-fidelity approach for capturing the details of the aeroacoustic behaviour of small-scale propellers, contingent on accurate geometric representation and optimised wake modelling parameters.
Journal Article