Acoustic Streaming in Air for Flow Manipulation and Control

Abstract

This thesis investigates bulk-driven acoustic streaming in air, focussing on its potential
for flow manipulation and control. The research is motivated by recent developments in
high-powered ultrasonic sources for haptics and levitation applications, as well as the
growing interest in novel methods for aeroacoustic treatments. Understanding and characterising
bulk-driven acoustic streaming flows in air is becoming increasingly crucial, and the potential
applications of acoustic streaming warrant exploration. The study employs two types of ultrasonic
transducers, namely Langevin horns and focused arrays of open-type flexural transducers. The
research is structured around four main objectives: characterising the transducers’ behaviour,
measuring and predicting second-order streaming velocity fields, investigating acoustic streaming interactions with turbulent boundary layers, and exploring methods to enhance acoustic
streaming flows. These objectives are designed to address critical knowledge gaps in the field and
pave the way for innovative applications of acoustic streaming in air.
Initially, the thesis presents a comprehensive characterisation of the ultrasonic transducers,
focussing on their non-linear behavior and establishing calibration procedures. This phase is
crucial for ensuring consistent and predictable operation in subsequent experiments. Numerical
models developed for both transducers show excellent agreement with experimental measurements, demonstrating peak sound pressure levels of 164.5 and 166.6 dB for the Langevin horn and
focussed array, respectively. The distinct acoustic field shapes of each transducer are examined,
along with an analysis of acoustic non-linearities observed due to the high acoustic pressures
produced.
The second phase employs particle image velocimetry (PIV) to characterise airborne acoustic
streaming flows induced by both transducers. Measurements reveal streaming velocities up
to 0.15 m/s for the Langevin horn and 0.2 m/s for the focused array, with distinct flow field
shapes for each transducer due to their different first-order pressure field shapes. A numerical
model is developed to predict second-order streaming fields, showing reasonable agreement with
experimental results and validating the weak non-linearity assumption for the tested cases.
This characterisation is essential for understanding the behaviour of acoustic streaming flows in
various applications, including haptics and levitation, where such flows can impact performance.
The third part introduces a novel method of boundary layer manipulation using bulk-driven
acoustic streaming. Experiments in a wind tunnel demonstrate that acoustic streaming flows
increase mean velocity within the turbulent boundary layer while reducing turbulence intensity
in its lower half. The Langevin horn, in particular, shows significant effects, reducing turbulent
energy content by up to 3 dB across a wide frequency range in the lower boundary layer. This
method of flow manipulation is likened to flow injection treatments, demonstrating the potential
for utilising acoustic streaming in practical aeroacoustic applications, such as trailing-edge noise
reduction.
Finally, the thesis explores the phenomenon of enhanced acoustic streaming flows induced by porous materials in high-amplitude acoustic fields. This discovery significantly broadens the
potential applications for acoustic streaming. Certain porous materials, such as Kevlar fabric
and specific filter papers, are shown to increase streaming velocities by an order of magnitude.
A working hypothesis suggests that materials must possess sufficient acoustic attenuation to
generate streaming forces without excessive flow resistance, supported by numerical modelling
that couples acoustic attenuation in a porous domain to streaming flow equations. The chapter
presents a detailed characterisation of the materials’ acoustic properties and flow resistances,
revealing correlations with observed streaming enhancement levels. A proof-of-concept transducer
design is proposed, suggesting that when combined with a streaming-enhancing material, a
second iteration of the wind tunnel experiment could yield substantially more significant results
in terms of boundary layer manipulation.
The findings presented here contribute to growing topics within the field of ultrasonics such
as haptics and levitation, while primarily focussing on developing novel methods for aeroacoustic
treatments. The discovery of streaming enhancement through porous materials opens new
avenues for research and practical applications in various areas, including boundary layer
manipulation for trailing-edge noise reduction. By providing an in-depth investigation of airborne
streaming and demonstrating novel methods for its application and enhancement, this thesis
advances the understanding of acoustic streaming in air and explores its potential uses.

Date of Award	4 Feb 2025
Original language	English
Awarding Institution	University of Bristol
Sponsors	Dyson Technology Ltd.
Supervisor	Bruce W Drinkwater (Supervisor), Anthony J Croxford (Supervisor) & Mahdi Azarpeyvand (Supervisor)