Effects of flow recirculation on unmanned aircraft system (UAS) acoustic measurements in closed anechoic chambers

Stephenson, James H.; Weitsman, Daniel; Zawodny, Nikolas S.

doi:10.1121/1.5092213

Cited by 54 publications

(14 citation statements)

References 3 publications

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“…9-14 However, the BPF higher harmonics are less pronounced than in some of the other data. Stephenson and coworkers 33,42 suggested that flow recirculation in a closed chamber can increase the magnitude of BPF harmonics (in line with other studies 53 ). With our test set-up comprising a rotor-wall clearance of at least ∼ 8D p to all sides (except for the ground plane at ∼ 4D p clearance), this flow recirculation effect may be minor with, as a consequency, a lower-magnitude higher harmonic noise content (e.g.…”

Section: Acoustic Field Characteristicssupporting

confidence: 62%

“…Fig. 4 of Stephenson et al 42 ) in comparison to other studies conducted with more confined setups. This explanation is speculative for the time-being, as no flow measurements were performed.…”

Section: Acoustic Field Characteristicsmentioning

confidence: 74%

“…The induced flow direction was downward to prioritize clean inflow conditions to the rotor. The rotor was located in the center of the anechoic test environment, to ensure that any flow recirculation in the anechoic chamber-possibly affecting higher harmonic noise 42 -would be symmetric. The rotor disk was located four rotor diameters above the suspended floor to minimize ground effects.…”

Section: A Experimental Setup and Acquisitionmentioning

confidence: 99%

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Quantifying modulation in the acoustic field of a small-scale rotor using bispectral analysis

Baars

Bullard

Mohamed

2021

AIAA Scitech 2021 Forum

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This paper describes a methodology to quantify inter-frequency modulation in the acoustic field of a small-scale rotor. How the blade passing frequency modulates the intensity of the higher-frequency (broadband) noise content is of specific interest, as this modulation is a major factor in the human perception of rotor noise from advanced air mobility vehicles and drones. A proposed modulation-parameter is based on post-processing steps that are applicable to a single acoustic time series. First, an auto-bispectral analysis assesses the dominant nonlinear, quadratic inter-frequency coupling between the blade passing frequency and the higher-frequency noise content. Secondly, the degree of modulation is determined using a robust parameter: a correlation parameter between the (low-frequency) modulating BPF signal and an envelope of the (higher-frequency) carrier signal. Provided that a single parameter is obtained for a given acoustic time series, the directivity pattern of the modulation strength can be inferred from data available from standard acoustic measurement campaigns. For illustration, an 11 inch diameter single-rotor in hover is considered, with acoustic data taken at 420 microphone positions within a plane perpendicular to the rotor disk. It is revealed that modulation is confined to a sector θ ≈ (10 • , −45 • ), where θ = 0 • is the rotor plane and negative angles are in the direction of the rotor-induced flow. The strongest modulation appears around θ ≈ −15 • . This work aids in quantifying the phenomenological description of modulation, namely that it results from the periodic advance and retreat of certain rotor blade's noise sources, relative to a stationary observer.

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Section: Acoustic Field Characteristicssupporting

confidence: 62%

Section: Acoustic Field Characteristicsmentioning

confidence: 74%

Section: A Experimental Setup and Acquisitionmentioning

confidence: 99%

See 1 more Smart Citation

Quantifying modulation in the acoustic field of a small-scale rotor using bispectral analysis

Baars

Bullard

Mohamed

2021

AIAA Scitech 2021 Forum

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“…Turbulent inflow noise can be predicted using a computational method such as that described in [21] or analytical methods such as that described by [22]. Note that tests conducted in anechoic chambers or confined spaces will produce a recirculating flow, which can produce high levels of turbulent inflow noise [23,24]; •…”

mentioning

confidence: 99%

Quantification of the Psychoacoustic Effect of Noise from Small Unmanned Aerial Vehicles

Hui

Kingan

Hioka

et al. 2021

IJERPH

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This paper presents the results of a study evaluating the human perception of the noise produced by four different small quadcopter unmanned aerial vehicles (UAVs). This study utilised measurements and recordings of the noise produced by the quadcopter UAVs in hover and in constant-speed flight at a fixed altitude. Measurements made using a ½″ microphone were used to calculate a range of different noise metrics for each noise event. Noise recordings were also made using a spherical microphone array (an Eigenmike system). The recordings were reproduced using a 3D sound reproduction system installed in a large anechoic chamber located at The University of Auckland. Thirty-seven participants were subjected to the recordings and asked to rate their levels of annoyance in response to the noise, and asked to perform a simple cognitive task in order to assess the level of distraction caused by the noise. This study discusses the noise levels measured during the test and how the various noise metrics relate to the annoyance ratings. It was found that annoyance strongly correlates with the sound pressure level and loudness metrics, and that there is a very strong correlation between the annoyance caused by a UAV in hover and in flyby at the same height. While some significant differences between the distraction caused by the UAV noise for different cases were observed in the cognitive distraction test, the results were inconclusive. This was likely due to a ceiling effect observed in the participants’ test scores.

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“…Experimentalists studying the noise of unmanned aerial systems (UAS) take caution to ensure that noise due to recirculation of their own turbulent wake does not dominate noise measurements conducted in enclosed areas. 155–157 That research, along with other recent research, 158,159 suggest that ingestion noise can contribute significantly to UAS noise. Thus this is an important area for future research, which could also have some relevance for eVTOL aircraft.…”

Section: Broadband Noisementioning

confidence: 71%

Challenges and opportunities for low noise electric aircraft

Greenwood

Brentner

Rau

et al. 2022

International Journal of Aeroacoustics

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A new class of electric aircraft is being developed to transport people and goods as a part of the urban and regional transportation infrastructure. To gain public acceptance of these operations, these aircraft need to be much quieter than conventional airplanes and helicopters. This article seeks to review and summarize the aeroacoustic research relevant to this new category of aircraft. First, a brief review of the history of electric aircraft is provided, with an emphasis on how these aircraft differ from conventional aircraft. Next, the physics of rotor noise generation are reviewed, and the noise sources most likely to be of concern for electric aircraft are highlighted. These are divided into deterministic and nondeterministic sources of noise. Deterministic noise is expected to be dominated by the unsteady loading noise caused by the aerodynamic interactions between components. Nondeterministic noise will be generated by the interaction of the rotor or propeller blades with turbulence from ingested wakes, the atmosphere, and self-generated in the boundary layer. The literature for these noise sources is reviewed with a focus on applicability to electric aircraft. Challenges faced by the aeroacoustician in understanding the noise generation of electric aircraft are then identified, as well as the new opportunities for the prediction and reduction of electric aircraft noise that may be enabled by advances in computational aeroacoustics, flight simulation, and autonomy.

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Effects of flow recirculation on unmanned aircraft system (UAS) acoustic measurements in closed anechoic chambers

Cited by 54 publications

References 3 publications

Quantifying modulation in the acoustic field of a small-scale rotor using bispectral analysis

Quantifying modulation in the acoustic field of a small-scale rotor using bispectral analysis

Quantification of the Psychoacoustic Effect of Noise from Small Unmanned Aerial Vehicles

Challenges and opportunities for low noise electric aircraft

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