# An Experimental Study of the Aeroacoustic Properties of a Propeller in Energy Harvesting Configuration

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## Abstract

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## 1. Introduction

## 2. Materials and Methods

#### 2.1. Experimental Setup

#### 2.2. Proper Orthogonal Decomposition

#### 2.3. Wavelet Transform

## 3. Results

## 4. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Abbreviations

DEP | Distributed electric propulsion |

EP | Electric propulsion |

FT | Fourier transform |

HBPF | Harmonics of the blade passing frequency |

HEP | Hybrid-electric propulsion |

POD | Proper orthogonal decomposition |

SPSL | Sound pressure spectrum level |

WT | Wavelet transform |

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**Figure 2.**Photos of the experimental setup employed for the measurement campaign at Delft University of Technology’s Low-Turbulence Tunnel.

**Figure 4.**Repeatability test: comparison between noise spectra. (

**a**) test performed at advance ratio J = 0.7. (

**b**) test performed at advance ratio J = 1.2.

**Figure 5.**Application of the POD–based decomposition strategy to a synthetic signal: a noisy sine wave. (

**a**) Signals, total and reconstructed, in the time domain; (

**b**) the total and reconstructed tonal signals in the frequency domain; (

**c**) the total and reconstructed broadband signals in the frequency domain.

**Figure 7.**Contour map of the sound pressure spectrum level of the near-field pressure signals along the axial distance for each operational condition. (

**a**) Propulsive configuration. (

**b**) Transitional configuration. (

**c**) Regenerative condition.

**Figure 8.**Probability density function of the near field pressure signals for each operational condition. The PDFs are reported in both linear (

**a**) and logarithmic scales (

**b**).

**Figure 9.**Application of the POD–based decomposition strategy to the test cases under analysis. (

**a**) Time history of the raw pressure signal and of its tonal and broadband components for the propulsive regime. (

**b**) Nondimensional spectra for the propulsive regime. (

**c**) Energy percentage associated with the POD modes for the propulsive regime. (

**d**) Time history of the raw pressure signal and of its tonal and broadband components for the transition regime. (

**e**) Nondimensional spectra for the transitional regime. (

**f**) Energy percentage associated with the POD modes for the transitional regime. (

**g**) Time history of the raw pressure signal and of its tonal and broadband components for the regenerative configuration. (

**h**) Nondimensional spectra for the regenerative configuration. (

**i**) Energy percentage associated with the POD modes for the regenerative regime.

**Figure 10.**Convergence analysis of the eigenvalues for the POD decomposition for each operational condition. (

**a**) First eigenvalue ${\lambda}_{1}$; (

**b**) second eigenvalue ${\lambda}_{2}$; (

**c**) third eigenvalue ${\lambda}_{3}$.

**Figure 11.**Probability density function of the tonal component of pressure signals for each operational condition. The PDFs are reported in both linear (

**a**) and logarithmic scales (

**b**).

**Figure 12.**Probability density function of the broadband component of pressure signals for each operational condition. The PDFs are reported in both linear (

**a**) and logarithmic scales (

**b**).

**Figure 13.**Wavelet intensity of the near-field pressure signal for the raw signal and the tonal and broadband components for each operational condition. (

**a**) $w\left(p\right)$ for the propulsive regime. (

**b**) $w\left({p}_{T}\right)$ for the propulsive regime. (

**c**) $w\left({p}_{BB}\right)$ for the propulsive regime. (

**d**) $w\left(p\right)$ for the transitional regime. (

**e**) $w\left({p}_{T}\right)$ for the transitional regime. (

**f**) $w\left({p}_{BB}\right)$ for the transitional regime. (

**g**) $w\left(p\right)$ for the regenerative condition. (

**h**) $w\left({p}_{T}\right)$ for the regenerative condition. (

**i**) $w\left({p}_{BB}\right)$ for the regenerative condition.

**Table 1.**Pitch angle $\beta $, rotational speed n and advance ratio J values for each operational condition investigated.

Test Case | $\mathit{\beta},\phantom{\rule{0.166667em}{0ex}}\mathit{deg}$ | $\mathit{n},\phantom{\rule{0.166667em}{0ex}}\mathbf{Hz}$ | $\mathit{J}\phantom{\rule{0.166667em}{0ex}}[-]$ |
---|---|---|---|

Propulsive | ${15}^{\circ}$ | $134.0$ | $0.60$ |

Transitional | ${15}^{\circ}$ | $98.5$ | $0.75$ |

Energy harvesting | ${15}^{\circ}$ | $67.0$ | $1.10$ |

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**MDPI and ACS Style**

Candeloro, P.; Martellini, E.; Nederlof, R.; Sinnige, T.; Pagliaroli, T.
An Experimental Study of the Aeroacoustic Properties of a Propeller in Energy Harvesting Configuration. *Fluids* **2022**, *7*, 217.
https://doi.org/10.3390/fluids7070217

**AMA Style**

Candeloro P, Martellini E, Nederlof R, Sinnige T, Pagliaroli T.
An Experimental Study of the Aeroacoustic Properties of a Propeller in Energy Harvesting Configuration. *Fluids*. 2022; 7(7):217.
https://doi.org/10.3390/fluids7070217

**Chicago/Turabian Style**

Candeloro, Paolo, Edoardo Martellini, Robert Nederlof, Tomas Sinnige, and Tiziano Pagliaroli.
2022. "An Experimental Study of the Aeroacoustic Properties of a Propeller in Energy Harvesting Configuration" *Fluids* 7, no. 7: 217.
https://doi.org/10.3390/fluids7070217