Flow Acoustic Effects on a Commercial Automotive Air Intake Silencer

142 Pages·2017·19.41 MB·English

Flow Acoustic Effects on a Commercial
Automotive Air Intake Silencer
A Numerical Study using Computational Fluid Dynamics
Master’s Thesis in Applied Mechanics
LINUS ZACKRISSON
Department of Applied Mechanics
CHALMERS UNIVERSITY OF TECHNOLOGY
Gothenburg, Sweden 2017

Master’s thesis 2017:76
Flow Acoustic Effects on a Commercial
Automotive Air Intake Silencer
A Numerical Study using Computational Fluid Dynamics
LINUS ZACKRISSON
Department of Applied Mechanics
Division of Fluid Dynamics
Chalmers University of Technology
Gothenburg, Sweden 2017
Flow Acoustic Effects on a Commercial Automotive Air Intake Silencer
A Numerical Study using Computational Fluid Dynamics
LINUS ZACKRISSON
© LINUS ZACKRISSON, 2017.
Supervisor: Magnus Knutsson, Volvo Car Group
Lars Davidsson, Department of Applied Mechanics
Examiner: Lars Davidsson, Department of Applied Mechanics
Master’s Thesis 2017:76
Department of Applied Mechanics
Division of Fluid Dynamics
Chalmers University of Technology
SE-412 96 Gothenburg
Telephone [phone]
Cover: Plane acoustic waves seen as difference in acoustic pressure fluctuations
propagating in the air intake duct system.
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Printed by Reproservice/Department of Applied Mechanics
Gothenburg, Sweden 2017
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Flow Acoustic Effects on a Commercial Automotive Air Intake Silencer
A Numerical Study using Computational Fluid Dynamics
LINUS ZACKRISSON
Department of Applied Mechanics
Chalmers University of Technology
Abstract
Noise generated by turbo-compressors in combustion engine air intake systems is
often mitigated by broad-band high-frequency duct silencers. The acoustic per-
formance of a developed silencer design is generally obtained numerically without
influence of duct mean flow, using acoustic computer aided engineering (CAE) tools.
Discrepanciesarehowevercreatedbetweentheacousticsoftherealairintakesystem
and numerical model of the system since mean duct flow is always present in the
real system, due to the aspiration of the engine. This Master’s thesis project aims
to capture flow effect on silencer acoustic performance numerically using computa-
tional fluid dynamics (CFD). In cooperation with Volvo Car Group, a commercially
existing part of an air intake system, including a silencer composed of two Helmholtz
resonators, is studied. An already established academic CFD methodology is ex-
plored, used, expanded and streamlined to investigate flow effects on acoustic prop-
erties of the complex silencer-duct system. The acoustic properties from the CFD
simulation are then compared to experimental data and acoustic CAE.
The established CFD methodology is integrated and applied using the commercial
CFD software Star-CCM+, studying the silencer acoustic behaviour with several
mean inlet flow speeds. Using Star-CCM+, mean flow and acoustic wave propaga-
tion is simulated simultaneously in the defined computational domain of the given
silencer-duct system. Acoustic waves in a frequency band of interest related to
the eigenfrequency of the silencer, are inserted through a time-varying inlet bound-
ary condition. The numerical setup mimics an acoustic experimental measurement
setup where the propagating acoustic waves are captured as pressure signals in vir-
tual microphones, to calculate the acoustic silencing property of transmission loss
(TL). Turbulence is modelled using the unsteady Reynolds-averaged Navier-Stokes
equations (URANS). Numerical parameters in the CFD setup are studied in regards
to numerical accuracy and computational efficiency, to find the most optimal model
in describing the flow effect on silencer acoustic performance. The most optimal
resulting CFD methodology was able to capture transmission loss characteristics
with reasonable accuracy, under predicting the resulting eigenfrequency shift with
roughly 8.5 % difference for all flow speeds; as well as a 11.4 % transmission loss
peak difference, both in comparison to experimental data.
Different duct geometrical changes were then studied using the optimally developed
CFD setup, in order to improve silencer acoustic performance under flow condition.
Through a numerical process, six different geometrical changes were developed with
varyingstrategies. Thebestdesignresultedina15%increaseinfirstorderresonance
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peak transmission loss and removal of eigenfrequency shift as well as only increasing
pressure drop by 1.1 %, in comparison to a reference simulation.
Keywords: Computational fluid dynamics (CFD), Aeroacoustics, Air intake si-
lencer, Helmholtz resonator, Flow effect, Transmission loss, Noise reduction, Star-
CCM+, Unsteady Reynolds averaged Navier-Stokes equations (URANS), geometri-
cal change.
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Acknowledgements
The presented Master’s thesis project has been conducted in cooperation with the
NVH & Driving Dynamics Centre at Volvo Car Group, spring 2017. I would like
to express my deepest gratitude towards my supervisor Magnus Knutsson as well
as co-worker Simone Vizzini for their continuous encouragement, excellent support
and feedback throughout the project. Further, I give thanks to Axel Kierkegaard at
Siemens PLM for showing great interest in my project, providing me with valuable
information and support regarding Star-CCM+ and CFD. I also want to thank
my professor, supervisor and examiner Lars Davidsson at Chalmers University of
Technology’s Division of Fluid Dynamics for the interesting discussions regarding
the project and valuable feedback.
Finally, I want to show my appreciation by thanking all co-workers at the NVH &
Driving Dynamics Centre as well as surrounding project groups of interest at Volvo
Car Group, it has been a pleasure to work at the office everyday beside all wonderful
and friendly people.
Linus Zackrisson, Gothenburg, June 2017
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