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Predicting the sound field from aeroacoustic sources on moving vehicles: Towards an improved urban environment
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design. (Aerodynamics)ORCID iD: 0000-0002-7006-067X
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In a society where environmental noise is becoming a major health and economical concern, sound emissions are an increasingly critical design factor for vehicle manufacturers. With about a quarter of the European population living close to roads with heavy traffic, traffic noise in urban landscapes has to be addressed first. The current introduction of electric vehicles on the market and the need for sound systems to alert their presence is causing a shift in mentalities requiring engineering methods that will have to treat noise management problems from a broader perspective. That in which noise emissions need not only be considered as a by-product of the design but as an integrated part of it. Developing more sustainable ground transportation will require a better understanding of the sound field emitted in various realistic operating conditions, beyond the current requirements set by the standard pass-by test, which is performed in a free-field. A key aspect to improve this understanding is the development of efficient numerical tools to predict the generation and propagation of sound from moving vehicles.

In the present thesis, a methodology is proposed aimed at evaluating the pass-by sound field generated by vehicle acoustic sources in a simplified urban environment, with a focus on flow sound sources. Although it can be argued that the aerodynamic noise is still a minor component of the total emitted noise in urban driving conditions, this share will certainly increase in the near future with the introduction of quiet electric engines and more noise-efficient tyres on the market.

This work presents a complete modelling of the problem from sound generation to sound propagation and pass-by analysis in three steps. Firstly, computation of the flow around the geometry of interest; secondly, extraction of the sound sources generated by the flow, and thirdly, propagation of the sound generated by the moving sources to observers including reflections and scattering by nearby surfaces. In the first step, the flow is solved using compressible detached-eddy simulations. The identification of the sound sources in the second step is performed using direct numerical beamforming with linear programming deconvolution, with the phased array pressure data being extracted from the flow simulations. The outcome of this step is a set of uncorrelated monopole sources. Step three uses this set as input to a propagation method based on a point-to-point moving source Green's function and a modified Kirchhoff integral under the Kirchhoff approximation to compute reflections on built surfaces. The methodology is demonstrated on the example of the aeroacoustic noise generated by a NACA air inlet moving in a simplified urban setting. Using this methodology gives insights on the sound generating mechanisms, on the source characteristics and on the sound field generated by the sources when moving in a simplified urban environment.

Abstract [sv]

I ett samhälle där buller håller på att bli ett stort hälsoproblem och en ekonomisk belastning, är ljudutsläpp en allt viktigare aspekt för fordonstillverkare. Då ungefär en fjärdedel av den europeiska befolkningen bor nära vägar med tung trafik, är åtgärder för minskat trafikbuller i stadsmiljö en hög prioritet. Introduktionen av elfordon på marknaden och behovet av ljudsystem för att varna omgivningen kräver också ett nytt synsätt och tekniska angreppssätt som behandlar bullerproblemen ur ett bredare perspektiv. Buller bör inte längre betraktas som en biprodukt av konstruktionen, utan som en integrerad del av den. Att utveckla mer hållbara marktransporter kommer att kräva en bättre förståelse av det utstrålade ljudfältet vid olika realistiska driftsförhållanden, utöver de nuvarande standardiserade kraven för förbifartstest som utförs i ett fritt fält. En viktig aspekt för att förbättra denna förståelse är utvecklingen av effektiva numeriska verktyg för att beräkna ljudalstring och ljudutbredning från fordon i rörelse.

I denna avhandling föreslås en metodik som syftar till att utvärdera förbifartsljud som alstras av fordons akustiska källor i en förenklad stadsmiljö, här med fokus på strömningsgenererat ljud. Även om det aerodynamiska bullret är fortfarande en liten del av de totala bullret från vägfordon i urbana miljöer, kommer denna andel säkerligen att öka inom en snar framtid med införandet av tysta elektriska motorer och de bullerreducerande däck som introduceras på marknaden.

I detta arbete presenteras en komplett modellering av problemet från ljudalstring till ljudutbredning och förbifartsanalys i tre steg. Utgångspunkten är beräkningar av strömningen kring geometrin av intresse; det andra steget är identifiering av ljudkällorna som genereras av strömningen, och det tredje steget rör ljudutbredning från rörliga källor till observatörer, inklusive effekten av reflektioner och spridning från närliggande ytor. I det första steget löses flödet genom detached-eddy simulation (DES) för kompressibel strömning. Identifiering av ljudkällor i det andra steget görs med direkt numerisk lobformning med avfaltning med hjälp av linjärprogrammering, där källdata extraheras från flödessimuleringarna. Resultatet av detta steg är en uppsättning av okorrelerade akustiska monopolkällor. Steg tre utnyttjar dessa källor som indata till en ljudutbredningsmodel baserad på beräkningar punkt-till-punkt med Greensfunktioner för rörliga källor, och med en modifierad Kirchhoff-integral under Kirchhoffapproximationen för att beräkna reflektioner mot byggda ytor. Metodiken demonstreras med exemplet med det aeroakustiska ljud som genereras av ett NACA-luftintag som rör sig i en förenklad urban miljö. Med hjälp av denna metod kan man få insikter om ljudalstringsmekanismer, om källegenskaper och om ljudfältet som genereras av källor när de rör sig i en förenklad stadsmiljö.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. , p. 70
Series
TRITA-AVE, ISSN 1651-7660 ; 2017:25
Keywords [en]
pass-by simulations, aeroacoustic sources, urban sound propagation, direct numerical beamforming, linear programming deconvolution, Kirchhoff approximation, moving sources, detached-eddy simulation, Ffowcs Williams-Hawkings, vehicle design
Keywords [sv]
förbifartssimuleringar, aeroakustiska källor, urban ljudutbredning, direkt numerisk lobformning, avfaltning med hjälp av linjärprogrammering, Kirchhoffs approximation, rörliga källor, detached-eddy simulation, Ffowcs Williams-Hawkings, fordonsdesign
National Category
Engineering and Technology Fluid Mechanics and Acoustics Vehicle Engineering
Research subject
Vehicle and Maritime Engineering
Identifiers
URN: urn:nbn:se:kth:diva-205791ISBN: 978-91-7729-364-4 (print)OAI: oai:DiVA.org:kth-205791DiVA, id: diva2:1090371
Public defence
2017-05-18, F3, Lindstedtsvägen 26, Sing-Sing, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20170425

Available from: 2017-04-25 Created: 2017-04-24 Last updated: 2017-04-25Bibliographically approved
List of papers
1. Aerodynamic and aeroacoustic analyses of a submerged air inlet in a low-Mach-number flow
Open this publication in new window or tab >>Aerodynamic and aeroacoustic analyses of a submerged air inlet in a low-Mach-number flow
2016 (English)In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 133, p. 15-31Article in journal (Refereed) Published
Abstract [en]

Computational aerodynamic and aeroacoustic analyses of a submerged air inlet are performed at a low Mach number. A hybrid method is used, in which the flow in the vicinity of the inlet is solved through detached eddy simulation (DES) and the acoustic pressure in the far-field is computed through the use of a Ffowcs Williams and Hawkings integral. Several surfaces of integration are used, both solid and permeable. The inlet design is based on an experimental inlet developed by the National Advisory Committee for Aeronautics (NACA). The flow is solved first through steady-state RANS simulation, then time-dependent DES is run from the converged results. The results from both RANS simulations and DES show good agreement with experimental data from NACA, both in terms of integral quantities and surface pressure coefficients. Pressure fluctuations are observed on both sides of the lip of the inlet, and are greater at low velocity ratios, with the velocity ratio defined as the ratio between the flow velocity at the duct entrance and in the free stream. A transition is observed between a quasi-laminar flow at a velocity ratio of 0.8 and a turbulent flow at velocity ratios of 0.6 and 0.4. This turbulent behaviour at low velocity ratios is associated with much higher acoustic levels in the far-field. At low velocity ratios, the acoustic spectra in the far-field exhibit a broadband character with maximum levels distributed around a characteristic frequency given by the ratio between the flow velocity at the duct entrance and the duct entrance depth. At high velocity ratios, the spectra show tonal characteristics with peaks at around 90 percent of this characteristic frequency and at the corresponding harmonics. A comparison between the spectra from solid and permeable surfaces reveals that volume sound sources are negligible at this low Mach number. A visualization of the integrands in the Ffowcs Williams and Hawkings integral show that sound sources are located on both sides of the lip of the inlet, at the position of impact of the vortices, and along the vortex wakes. Some observations regarding the use of solid and permeable surfaces of integration are made.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
submerged air inlet, NACA duct, NACA inlet, detached eddy simulation, DES, Ffowcs Williams and Hawkings
National Category
Fluid Mechanics and Acoustics
Research subject
Vehicle and Maritime Engineering; Aerospace Engineering; Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-174180 (URN)10.1016/j.compfluid.2016.04.010 (DOI)000377740000002 ()2-s2.0-84963936303 (Scopus ID)
Note

QC 20160523

Available from: 2015-10-01 Created: 2015-10-01 Last updated: 2017-12-01Bibliographically approved
2. Identifying equivalent sound sources from aeroacoustic simulations using a numerical phased array
Open this publication in new window or tab >>Identifying equivalent sound sources from aeroacoustic simulations using a numerical phased array
2017 (English)In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 394C, p. 203-219Article in journal (Refereed) Published
Abstract [en]

An application of phased array methods to numerical data is presented, aimed at identifying equivalent flow sound sources from aeroacoustic simulations. Based on phased array data extracted from compressible flow simulations, sound source strengths are computed on a set of points in the source region using phased array techniques assuming monopole propagation. Two phased array techniques are used to compute the source strengths: an approach using a Moore-Penrose pseudo-inverse and a beamforming approach using dual linear programming (dual-LP) deconvolution. The first approach gives a model of correlated sources for the acoustic field generated from the flow expressed in a matrix of cross- and auto-power spectral values, whereas the second approach results in a model of uncorrelated sources expressed in a vector of auto-power spectral values. The accuracy of the equivalent source model is estimated by computing the acoustic spectrum at a far-field observer. The approach is tested first on an analytical case with known point sources. It is then applied to the example of the flow around a submerged air inlet. The far-field spectra obtained from the source models for two different flow conditions are in good agreement with the spectra obtained with a Ffowcs Williams-Hawkings integral, showing the accuracy of the source model from the observer's standpoint. Various configurations for the phased array and for the sources are used. The dual-LP beamforming approach shows better robustness to changes in the number of probes and sources than the pseudo-inverse approach. The good results obtained with this simulation case demonstrate the potential of the phased array approach as a modelling tool for aeroacoustic simulations.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
sound sources, modelling, phased array, beamforming, simulations, dual-LP beamforming
National Category
Fluid Mechanics and Acoustics
Research subject
Vehicle and Maritime Engineering
Identifiers
urn:nbn:se:kth:diva-192164 (URN)10.1016/j.jsv.2017.01.051 (DOI)2-s2.0-85011589602 (Scopus ID)
Note

QC 20160921

Available from: 2016-09-06 Created: 2016-09-06 Last updated: 2017-11-21Bibliographically approved
3. A Kirchhoff approximation-based numerical method to compute multiple acoustic scattering of a moving source
Open this publication in new window or tab >>A Kirchhoff approximation-based numerical method to compute multiple acoustic scattering of a moving source
2015 (English)In: Applied Acoustics, ISSN 0003-682x, Vol. 96, p. 108-117Article in journal (Refereed) Published
Abstract [en]

Within the scope of a study of external noise propagation from moving ground vehicles, a numerical method is developed to compute the acoustic field emitted by a moving source in the presence of scattering objects such as roads, buildings or noise-shields. This method is developed with the purpose of being used in a vehicle design process and therefore it must have a low computational cost, which requires a certain number of approximations. The case of a fixed point source is studied first then the effect of a movement of the source is taken into account through the introduction of a retarded time. The acoustic source is assumed to be represented by one or many harmonic monopoles of possibly different frequency moving with a constant speed in a quiescent flow field. Scattering from nearby perfectly reflecting objects is computed through a Kirchhoff–Helmholtz integral equation applying the Kirchhoff approximation. A ray-surface intersection algorithm to compute shadow areas is proposed. The method is validated against analytical solutions and experimental results for a fixed source, and against a higher-order finite difference time-domain method for the multiple scattering of a moving source. Results are good and show that this method can potentially be used to predict urban noise.

Place, publisher, year, edition, pages
Elsevier, 2015
Keywords
Multiple scattering, Moving source, Kirchhoff–Helmholtz integral, Kirchhoff approximation, Urban noise
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-163047 (URN)10.1016/j.apacoust.2015.03.016 (DOI)000355033900013 ()2-s2.0-84926306952 (Scopus ID)
Note

QC 20150424

Available from: 2015-03-26 Created: 2015-03-26 Last updated: 2017-12-04Bibliographically approved
4. Pass-by noise signature of aerodynamic sound sources in urban environment: A numerical approach
Open this publication in new window or tab >>Pass-by noise signature of aerodynamic sound sources in urban environment: A numerical approach
2017 (English)In: 23rd AIAA/CEAS Aeroacoustics Conference, American Institute of Aeronautics and Astronautics, 2017Conference paper, Published paper (Other academic)
Abstract [en]

A numerical methodology is presented to simulate the sound field generated by moving aerodynamic sound sources in a simplified urban environment. The framework is described step by step from source identification to pass-by simulations. The applicability of the methodology is demonstrated on the example of a NACA inlet, a type of submerged air inlet typical placed on cars or trains. The methodology is based on three steps that use previous research published by the authors. In a first step, the flow around the NACA inlet is solved using compressible detached-eddy simulation. In a second step, identification of the sources is performed using direct numerical beamforming with linear programming deconvolution, based on array pressure data extracted from the flow simulation. In a third step, the sources are propagated in a urban environment by means of a simplified boundary element method based on a moving point source Green’s function and a Doppler-corrected Kirchhoff integral using the Kirchhoff approximation. The ground is accounted for through the image source method. The results are analysed in terms of pass-by time-frequency descriptions and sound pressure level. Due to its fully numerical framework, this methodology can be used to evaluate the contribution of aerodynamic sound sources to the pass-by signal early in the design process of ground vehicles. It should also be possible to include other sound sources such as the engine or the exhaust in the propagation step to get a more complete pass-by description and evaluate the contribution of the individual sources to the total emitted noise, providing that they can be modelled as monopoles and have a statistically stationary frequency content.

Place, publisher, year, edition, pages
American Institute of Aeronautics and Astronautics, 2017
Keywords
pass-by noise, aeroacoustic sources, aerodynamic sound, urban, simulation, beamforming, kirchhoff
National Category
Fluid Mechanics and Acoustics Vehicle Engineering
Research subject
Vehicle and Maritime Engineering
Identifiers
urn:nbn:se:kth:diva-197892 (URN)10.2514/6.2017-3697 (DOI)2-s2.0-85023632217 (Scopus ID)
Conference
23rd AIAA/CEAS Aeroacoustics Conference, 2017, Denver, United States, 5 June 2017 through 9 June 2017
Note

QC 20161208

Available from: 2016-12-08 Created: 2016-12-08 Last updated: 2018-01-09Bibliographically approved

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