Aeroacoustics of Propeller Boundary Layer Ingestion

Abstract

The environmental impact of aviation has become a prominent challenge, prompting
a focus on sustainable innovations aimed at reducing emissions, noise, and energy
consumption. Key initiatives such as the UK’s Jet Zero strategy and the European
Commission’s Flightpath 2050 seek to achieve net-zero emissions in the aviation sector
by 2050. This has spurred the exploration of a wide range of novel and complex aircraft
configurations that promise improved efficiency over conventional designs. Boundary
layer ingestion (BLI) is a popular arrangement which involves the propulsion system
ingesting the airframe boundary layer or wake. This has been shown to have significant
aerodynamic benefits, however, it can also result in increased noise emissions, adversely
affecting public health and social acceptance, especially in urban environments. The
understanding of the noise generation and propagation mechanisms present in BLI
configurations remains limited, with also a notable absence of empirical data to assist in
BLI noise model validation. To address this gap, the current study experimentally investigates the aeroacoustics of a propeller ingesting turbulent boundary layers, with a focus
on exploring parameter sensitivity, noise source identification, and flow-field behaviour.
Two experimental setups were built and tested in the Aeroacoustic Facility at the University of Bristol: (1) a zero-pressure-gradient (ZPG) boundary layer configuration, achieved
by positioning the propeller adjacent to a flat plate, and (2) an adverse-pressure-gradient
(APG) boundary layer configuration, representative of “buried” BLI concepts, achieved by
placing the propeller adjacent to a curved plate. Far-field microphone arrays, near-field
surface pressure sensors, propeller force, and hot-wire anemometry are utilised to obtain
detailed knowledge of the far-field acoustics and near-field hydrodynamics. An angular
encoder also allowed phase-averaged noise and velocity analyses. A comprehensive test
matrix was carried out, resulting in a large empirical dataset. Key findings showed that
thicker, more turbulent boundary layers significantly increase broadband turbulence
ingestion noise (TIN), and enhances tonal harmonics. The TIN and tonal emissions are
observed more strongly downstream, directed along the blade normals. The influence
of the inflow reduces at very high thrust, as blade self-noise dominates. APG flow ingestion further increases the emitted noise and exhibits strong haystacking along with
additional tonal harmonics, for the same thrust, compared to ZPG inflows. The current
study has advanced the empirical understanding of BLI aeroacoustics by providing a
detailed dataset and analysis of the noise and flow characteristics across several design
and operational parameters. The findings reveal insights that can be considered during
the preliminary design of quieter and more efficient aircraft employing BLI.

Date of Award	17 Jun 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Mahdi Azarpeyvand (Supervisor) & Nick Zang (Supervisor)