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Development of computational and experimental noise assessment and suppression methodologies for the next generation of silent distributed propulsion configurations

Periodic Reporting for period 3 - SilentProp (Development of computational and experimental noise assessment and suppression methodologies for the next generation of silent distributed propulsion configurations)

Reporting period: 2022-10-01 to 2023-09-30

Distributed Electric Propulsion (DEP) is particularly attractive from a noise reduction perspective since the induced high frequency noise is associated with: i) Increased atmospheric absorption benefiting citizens and ii) Improved structural shielding benefiting passengers as well as citizens.

SilentProp has achieved the following objectives:

-SilentProp achieved significant milestones in the experimental study of noise emitted from Distributed Electric Propulsion (DEP) configurations. The team successfully designed and implemented an experimental DEP setup for comprehensive noise measurements, paving the way for an in-depth analysis of noise characteristics of DEP systems. This work provided valuable insights into the physics of noise generation in multi-propeller systems, with extensive evaluations of the aeroacoustics of both propeller-propeller and propeller-wing interactions in leading-edge mounted DEP setups. Specifically, results were delivered for at least 50 different cases focusing on these interactions.

-A distributed source model has been developed and implemented into DEA which enables the study of the influence of correlated sources, such as phase-locked propellers, on the vibrational behaviour of structures. In addition, the influence of the turbulent boundary layer represented by standard models (such as the Corcos model) has been investigated and implemented into DEA software. Combined, these models reproduce a detailed picture of the vibration sources impacting aircraft during flight using DEA, and can be applied to estimate the sound level distribution across the aircraft.

-Related to noise suppression and shielding technologies, SilentProp confirmed the efficacy of various noise reduction strategies through experimental methods. The project focused on passive, adaptive, and serration treatments for DEP systems. Results were provided for a minimum of 6 different technologies, specifically aimed at noise mitigation. These efforts contributed significantly to the development of more efficient and quieter DEP technologies, aligning with the broader objectives of enhancing environmental sustainability and operational performance in aviation. SilentProp explored directional noise control techniques, with an emphasis on propeller phase locking effects, delivering findings from at least 40 distinct cases and achieving directional noise reduction by about 15-25 dB under forward flow conditions (J>0).
WP1 - Propellers’ parameters were selected, and propellers are being designed and manufactured at Mejzlik
- Aeroacoustic analyses were conducted under both static (no inflow) and forward flow conditions, with varying inflow velocities between 9 to 14 m/s, and a constant propeller rotation of 5000 rpm. Two identical 5-bladed propellers with a diameter of = 9” and Pitch to Diameter ratio / = 1 were mounted on a NACA 0018 wing. The results highlighted the significant impact of relative phase to minimize the propeller noise, particularly at the first Blade-Passing Frequency (BPF) for all directivity angles. Experimental results revealed that:

- Phase synchronization effectively reduces the sound pressure level in the low-frequency band, especially at the first blade-passing frequency.
- A relative phase difference of 90° between the propellers resulted in distinct acoustic characteristics, characterized by reduced BPF amplitudes across all directivity angles.
- The tip-to-tip separation distance between the rotors had no significant impact on noise reduction. This highlighted the dominant influence of relative phase and forward flow conditions in determining noise levels.

WP2 - Developed and validated the method to numerically compute the near-field and far-field acoustic characteristics of propellers.
- The capability and use of a new acoustic model, called “sponge layer”, are assessed for achieving robust non-reflective boundary conditions.
- Machine Learning (ML) and Radon-Cumulative-Distributed Transform (RCDT) with Proper Orthogonal Decomposition (POD) based techniques are investigated to compute Reduced Order Models (ROM) of DEP propeller aerodynamics and acoustics.
- Established the potential and limitations of the ROM in reproducing aerodynamic and acoustic outputs.
- The effects of flow and geometric conditions on DEP noise were discussed based on the numerical results, concerning the advance ratio, propeller pitch, and the leading-edge installation effects.
- Study several cases to analyse the effects of multi-propeller interactions on DEP noise.

WP 3 - Noise shielding and suppression technologies development
- Investigative efforts encompass a range of techniques including acoustic shielding, implementation of porous materials, and control of the propeller phase.
- five main suppression technologies have been investigated experimentally: acoustic shielding of the propeller, porous material interaction with the turbulence interaction noise, trailing edge serrations, phase synchronization of propellers.
- CFD simulations using Lattice Boltzmann Method (LBM) implemented in ProLB software were performed to assess porous treatment efficiency for noise reduction.
- A TMM solver for curved structure has been implemented in AlphaCell and validated against literature work.
- The far field noise analysis shows that porous treatment of the leading edge helps reducing the noise at low frequency but incurs a greater penalty at high frequency.

WP4 - Project Management control and dissemination & outreach.
The major technology breakthrough made by SilentProp currently lies in the area of noise reduction for Distributed Electric Propulsion (DEP) systems, which play a crucial role in the development of environmentally friendlier and more efficient aviation technologies. SilentProp's contribution is particularly significant in providing essential data for noise assessment, a critical challenge in integrating DEP systems into aviation. Data is readily available online, and this database enables the updating of existing CFD (Computational Fluid Dynamics) and CAA (Computational Aeroacoustics) codes. These updates improve the prediction of aerodynamic and aeroacoustic interactions occurring between the wing and the propellers' slipstream.

By focusing on the noise interaction between multiple propeller systems, SilentProp targets a specific and technically challenging aspect of DEP systems that previous research has not extensively explored. This approach indicates that SilentProp is pioneering in its efforts to understand and reduce the acoustic impacts of DEP systems. The major innovation SilentProp introduces is the mitigation of noise in DEP systems through the use of electronically phase-synchronized propellers, with significant implications for advancing cleaner, quieter, and more efficient aviation. This innovation is expected to offer valuable insights and data that support the CleanSky 2 program's objectives, as well as broader environmental and operational goals in the aviation industry.

The next stage would consist of combining these noise reduction solutions to address both structure vibrations and airborne sound sources. In this regard, morphological adaptative design with energy harvesting capabilities seems to be a promising route, provided the accurate numerical tools as well as appropriate manufacturing gears are deployed.
Example Propeller 2
Aeroacoustic Facility with Test Rig
Example Propeller 1
5 Bladed Propeller installed on Rig