The reduction of sound from aircraft engines
Submitting InstitutionKeele University
Unit of AssessmentMathematical Sciences
Summary Impact TypeTechnological
Research Subject Area(s)
Physical Sciences: Astronomical and Space Sciences
Engineering: Mechanical Engineering, Interdisciplinary Engineering
Summary of the impact
In response to many EU directives (e.g. 89/629/EEC, 2002/30/EC), and to
the threat of financial penalties, the aircraft industry has long
considered it a matter of the utmost importance to develop tools for the
reduction of aircraft noise. Chapman's ray theory of aeroengine noise,
created and developed in 1994-2000, provided such a tool. The impact of
this work has extended through aircraft industry giants such as
Rolls-Royce to consumers and the general public worldwide, because of its
influence on the design of quieter aircraft.
Following application of the same theory to broadband underwater
acoustics, the impact now extends to the government's plans for the next
generation of nuclear submarines. This is a £25 billion project to design
and build the Successor class, to replace the Vanguard class of Trident
submarines. Chapman's ray theory has been used in the current Assessment
Phase leading to Main Gate in 2016, when the Government will decide on
The impetus to create a ray theory of aeroengine noise arose from two key
aspects of the state of aircraft noise research in the early 1990's:
- The prevailing theory of fan-generated sound in aeroengine ducts
contained a fundamental error, in that sound rays were mistakenly
believed to lie on smooth helices, rather like the stripes on a barber's
pole, winding around a cylinder of fixed radius. In fact, the sound
field has a very different structure.
- Advances in theoretical understanding were urgently needed, for
development and certification of new engines, to account for the
high-frequency, short wavelength sound field produced by a rapidly
rotating fan with many blades. Such fans were by then the norm in large
high-bypass aeroengines, as used in all large commercial aircraft
throughout the world.
The error (1) was catastrophic for prediction methods, because `barber's
pole' rays (if they really existed) would send net energy in the duct
direction only, with no provision for the enormous sideways propagation of
sound energy which actually occurs from the front face of an aeroengine.
Although barber's-pole type rays do not exist, this did not stop sketches
of them appearing in some research papers.
No real progress with research program (2) was possible until Chapman
demonstrated that the sound rays are not smooth helices at all, but are
piecewise-linear helices, consisting of a sequence of straight-line
segments joined up at sharp corners. In consequence, the rays all have a
sideways, i.e. radial, component in their direction. This fact has an
enormous impact on the sound produced by an aircraft engine, because it
implies that in an aeroengine duct the sound rays bounce repeatedly from
the duct wall, to emerge from the front face of the duct at definite
sideways angles, which Chapman calculated explicitly as a function of the
parameters specifying the modes in the duct.
An immediate development was that within two years Chapman obtained a
complete theory of aeroengine fan noise, which not only modelled the
source of the noise on the fan, but also tracked the energy flow all the
way through the duct and out into the far field, where the energy is
perceived as noise. Within a few more years, Chapman elucidated various
intricate patterns of focusing in the sound field, and his research
student Hocter calculated many complete directivity patterns of aeroengine
sound fields, hence determining the sound received in all directions from
The programme of research on aircraft engine noise occupied a major part
of Chapman's research time as a Lecturer and then Reader at Keele
University in the period 1994-2000, and resulted in his promotion to a
Professorship. As part of this programme, Chapman supervised a PhD
student, S. T. Hocter from 1996-1999, with funding provided by an EPSRC
Doctoral Grant, to work on the sound radiation from the front face of the
aeroengine duct. Also as part of the programme, Chapman supervised C. J.
Powles, who obtained a Nuffield bursary in 2000 to work on energy paths
within the duct. The following year, Powles began an EPSRC-funded PhD at
Keele under Chapman's supervision, on the generation of sound rays by the
leading edges of the fan blades in the duct, completing his PhD in 2004.
In the period 2010-2012, Chapman extended the theory to broadband noise
in an MoD research project, with Thales Underwater Systems Ltd and other
universities, on the next generation of nuclear submarines, the Successor
class. The theory determines the ray directions of underwater sound from
References to the research
JFM: Journal of Fluid Mechanics; JSV: Journal of Sound and Vibration;
PRSLA: Proceedings of the Royal Society of London A.
C. J. Chapman 1994 Sound radiation from a cylindrical duct. Part
I. Ray structure of the duct modes and of the external field. JFM
C. J. Chapman 1996 Sound radiation from a cylindrical duct. Part
II. Source modelling, nil-shielding directions, and the open-to-ducted
transfer function. JFM 313, 367-380.
C. J. Chapman 1999 Caustics in cylindrical ducts. PRSLA 455,
S. T. Hocter 1999 Sound radiation from a cylindrical duct. PhD
thesis, University of Keele.
S. T. Hocter 1999 Exact and approximate directivity patterns of
the sound radiated from a cylindrical duct. JSV 227,
C. J. Chapman 2000 Similarity variables for sound radiation in a
uniform flow. JSV 233, 157-164.
S. T. Hocter 2000 Sound radiated from a cylindrical duct with
Keller's geometrical theory. JSV 231, 1243-1256.
S. T. Hocter 2000 Sound reflection into a cylindrical duct. PRSLA
C. J. Powles 2002 Energy paths in sound fields. PRSLA 458,
C. J. Powles 2004 Supersonic leading-edge noise. PhD thesis,
University of Keele.
Details of the impact
The impact of this work has taken place via Rolls-Royce, the second
largest maker of aircraft engines in the world, and a world leader in gas
turbine technology, and via the MoD and Thales Underwater Systems. A large
number of research contracts between Rolls-Royce and university research
groups have exploited Chapman's ray theory of aeroengine noise; this
research has concerned the design of new aeroengines, and has greatly
reduced the risk that certain types of aircraft might be banned. The work
has also been exploited in EU projects arising from EU directives. The
exploitation has taken place outside of Keele, most notably through the
long-standing connections of Rolls-Royce with the Department of Applied
Mathematics and Theoretical Physics, Cambridge University, and the
Institute of Sound and Vibration, Southampton University. The exploitation
in underwater acoustics took place via consultancy for the Successor class
nuclear submarine project, initiated by DSTL, and led to the report
Broadband Hydroacoustic Research (ref. 9).
Two recent articles co-authored by senior Rolls-Royce engineers are
explicit about the impact of Chapman's work on noise reduction. The first,
co-authored by Dr A. B. Parry, is the survey article `Modern Challenges
Facing Turbomachinery Aeroacoustics', published in 2012 in the Annual
Review of Fluid Mechanics (ref. 1). The second article, co-authored by Dr
A. J. Kempton and published in 2010, is explicit about the way in which
Chapman's work is needed for prediction of the far-field directivity of
broadband noise using measurements made in the duct (ref. 2).
A world-class group which has exploited Chapman's ray theory is that led
at Cambridge University by Professor N. Peake, working jointly with
Rolls-Royce over a period of many years. The impact trail from Chapman's
work to industrial practice in Rolls-Royce lies in a series of EPSRC CASE
awards between Rolls-Royce and Cambridge University in which Chapman's ray
theory explicitly plays an underpinning role. A long-lasting contact at
Rolls-Royce is Dr A. B. Parry, an engineering specialist in aerothermal
methods, aeroacoustics, and aerodynamics.
Full details of four of the above-mentioned research awards are given in
refs. 2-6, in which the published papers noted refer to the collaboration
with Rolls-Royce and explain the relation of the results to Chapman's
work. These details provide the impact trail which leads from Chapman's
work to Rolls-Royce. The young researchers who worked with N. Peake
include E. J. Brambley, A. J. Cooper, C. J. Heaton, G. M. Keith, and B.
Veitch. The areas investigated in these research projects were
- Resonant phenomena in gas turbines,
- Aeroacoustic models of fan noise,
- Wave propagation and resonance in aeroengines,
- Turbomachinery broadband noise.
The research awards cover the period 1998-2009. Given that Chapman's ray
theory provides basic underpinning science, and aeroengine development is
a long-term process, this continuity of use over an extended period is an
essential part of the impact which the work has to the present day. In
detail, the above research awards have led, via Chapman's ray theory, to
advances at Rolls-Royce relating to the acoustic effects of
- the rotor, stator, and guide vanes in the aeroengine duct;
- the precise shape of the duct, including for example the non-circular
cross-section, the curvature of the centre-line, the variation in
duct-liner properties, and angling of the front face of the duct; and
- the interaction of the effects in (1) and (2), which occurs because of
scattering and diffraction.
Another world-class group which has disseminated Chapman's work to
Rolls-Royce and the worldwide aircraft industry is the Rolls-Royce
University Technology Centre in Gas Turbine Noise at Southampton
University. This is housed in the Institute of Sound and Vibration
Research (ISVR). The research workers there have made frequent use of
Chapman's ray theory, often making use of Hocter's papers noted in Section
3, on sound radiation and reflection from the front face of the aeroengine
duct. This contribution to Rolls-Royce's research is indicated in ref. 2.
A further dissemination route of Chapman's work has been directly to the
European aircraft industry, via EU projects in Frameworks 5 and 6. The EU
directives 2002/30/EC and 2002/49/EC were to reduce aircraft noise, and
included the setting of noise standards. Two such projects were
SILENCE(R), referring to Significantly Lower Community Exposure to
Aircraft noise, and MESSIAEN, referring to Methods for Efficient
Simulation of Aircraft Engine Noise. Refs. 7-8 give published papers
arising from these projects which refer to Chapman's ray theory and
provide a link to the European aircraft industry.
Sources to corroborate the impact
ARFM: Annual Review of Fluid Mechanics; JFM: Journal of Fluid
Mechanics; JSV: Journal of Sound and Vibration
N. Peake & A. B. Parry 2012 Modern challenges facing
turbomachinery aeroacoustics. ARFM 44, 227-248.
C. R. Lowis, P. F. Joseph & A. J. Kempton 2010 Estimation
of far-field directivity of broadband aeroengine fan noise using an
in-duct axial microphone array. JSV 329, 3940-3957.
- Rolls-Royce/EPSRC CASE award GR/L80317, 1998-2001. A mathematical
investigation of resonant phenomena in gas turbines. A. J. Cooper
& N. Peake 2001, 2005 JFM 445, 207-234, 523,
219-250; N. Peake & A. J. Cooper 2001 JSV 243,
- Rolls-Royce/EPSRC CASE award GR/M21638/01, 1999-2002. Development and
validation of aeroacoustic models for fan noise. G. M. Keith &
N. Peake 2002 JSV 255, 129-146, 147-160; C. J. Heaton
& N. Peake 2005 JFM 540, 189-220.
- Rolls-Royce/EPSRC CASE award RG/42494, University Gas Turbine Research
Partnership Programme, 2004-2007. Wave propagation and resonance in
aeroengines. E. J. Brambley & N. Peake 2008 JFM 596,
- Rolls-Royce/EPSRC CASE award EP/D035031/1, 2006-2009. Mathematical
modelling and computational engineering prediction of turbomachinery
broadband noise. B. Veitch & N. Peake 2008 JFM 613,
- Rolls-Royce/EU-FP5 SILENCE(R) project, 2001-2005. Significantly lower
community exposure to aircraft noise. A. McAlpine, R. J. Astley, A.
J. Kempton et. al. 2006 JSV 294, 780-806.
- Rolls-Royce/EU-FP6 MESSIAEN project 502938, 2003-2007. Methods for
efficient simulation of aircraft engine noise. G. G. Vilenskii &
S. W. Rienstra 2007 JFM 583, 45-70.
- Thales Underwater Systems, project DSTlx-10062650/BHAR, 2010-2012.
Broadband Hydroacoustic Research (Phase II). D. Yumashev, I. D.
Abrahams, C. J. Chapman et al. 2012, 1-47.
Source to corroborate the impact of the work on Rolls-Royce and the
aircraft industry: Engineering Specialist — Aerodynamics, Rolls-Royce plc.
Source to corroborate the spin-off impact on the submarine industry
(defence work): Naval Systems Department, DSTL.
Source to corroborate the impact of the work on the European aircraft
industry: Institute of Sound and Vibration Research, University of
Source to corroborate the impact of the work on the aircraft industry:
Centre for Mathematical Sciences, Cambridge University.