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Journal of Sound and Vibration

Journal of Sound and Vibration 340 (2015) 39–60

http://d0022-46

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journal homepage: www.elsevier.com/locate/jsvi

Aeroacoustics research in Europe: The CEAS-ASC reporton 2013 highlights

G.J. Bennett a,n, J. Kennedy a, C. Meskell a, M. Carley b, P. Jordan c, H. Rice a

a Department of Mechanical and Manufacturing Engineering, Parsons Building, School of Engineering, Trinity College Dublin,Dublin 2, Irelandb Department of Mechanical Engineering, University of Bath, Bath BA2 7AY, Englandc Département Fluides, Thermique, Combustion, Institut PPRIME, CNRS – Université de Poitiers – ENSMA, UPR 3346, 86036 Poitiers, France

a r t i c l e i n f o

Article history:Received 25 July 2014Accepted 1 December 2014

Handling Editor: P. Joseph

community on an European scale and European aeronautics activities internationally.In this context, “aeroacoustics” encompasses all aerospace acoustics and related areas. Each

Available online 5 January 2015

x.doi.org/10.1016/j.jsv.2014.12.0050X/& 2014 Elsevier Ltd. All rights reserved.

esponding author. Tel.: þ353 1 896 1383.ail address: gareth.bennett@tcd.ie (G.J. Benn

a b s t r a c t

The Council of European Aerospace Societies (CEAS) Aeroacoustics Specialists Committee(ASC) supports and promotes the interests of the scientific and industrial aeroacoustics

year the committee highlights some of the research and development projects in Europe.This paper is a report on highlights of aeroacoustics research in Europe in 2013,

compiled from information provided to the ASC of the CEAS.During 2013, a number of research programmes involving aeroacoustics were funded by

the European Commission. Some of the highlights from these programmes are summarisedin this paper, as well as highlights from other programmes funded by national programmesor by industry. Furthermore, a concise summary of the CEAS-ASC workshop “Atmosphericand Ground Effects on Aircraft Noise” held in Seville, Spain in September 2013 is included inthis report.

Enquiries concerning all contributions should be addressed to the authors who are givenat the end of each subsection.

This issue of the “highlights” paper is dedicated to the memory of Prof. John A.Fitzpatrick, Professor of Mechanical Engineering, Trinity College Dublin, and a valuedmember of the Aeroacoustics Specialists Committee. John passed away in September 2012and is fondly missed across the globe by the friends he made in the AeroacousticsCommunity. This paper is edited by PhD graduates and colleagues of John's who conductresearch in aeroacoustics, inspired by his thirst for knowledge.

& 2014 Elsevier Ltd. All rights reserved.

1. CEAS-ASC workshop

The 17th CEAS-ASC workshop was held in the Higher Technical School of Engineering of Seville, Spain, on September 24–25,2013. Its topic was “Atmospheric and Ground Effects on Aircraft Noise” and it was organised by Nico van Oosten (AnotecConsulting) together with Emilio Campos (Association of Aeronautical Engineers of Spain, AIAE). The chairman of thescientific committee was Emilio Campos, who will act as a guest editor of a special issue of the International Journal ofAeroacoustics dedicated to the workshop.

ett).

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–6040

The atmosphere, ground, and topography characteristics cause effects on the propagation of sound that can becomeimportant and since the aircraft noise propagation phenomena are relevant with a view to a proper evaluation of the noiseimpact around airports, and particularly to the computation of noise contours, those effects must be taken into account inthe aircraft noise calculated as received on the ground. The interest in this subject was demonstrated by the 57 attendantsfrom 20 countries (EU states, Norway, Ukraine, Russia, USA, and Japan) and the 20 papers presented over 2 days. There werethree keynote lectures, the first one being simulation of sound propagation in the atmosphere: effect of turbulence andnonlinearities, given by professor Daniel Juvé (École Centrale de Lyon, France), that explained the role of random turbulentfluctuations on the propagation of sound waves, presenting a review of experimental work to simulate these effects at thelaboratory scale, and discussing numerical simulations, based on a Monte-Carlo approach, both in the frequency and timedomains. The second lecture, Efficient outdoor sound propagation modelling in time-domain, was presented by professorTimothy van Renterghem (University of Ghent, Belgium), who discussed the finite-difference time-domain (FDTD) methodas a reference solution for complex sound propagation problems dealing particularly with some relevant numericaldiscretisation schemes, and presenting alternative approaches for long-distance sound propagation, including movingcalculation frames and hybrid modelling. Dr. Joyce E. Rosenbaum (Computer Sciences Corporation, Cambridge, MA, USA),delivered the third lecture, enhanced propagation of aviation noise in complex environments: a hybrid approach, aimed atdescribing the hybrid propagation model, an approach that is a composite of three methods, the generalised terrainparabolic equation (GTPE), fast field program (FFP), and straight ray models, and analysing cases using different sourceheights and including uneven terrain, refractive atmosphere, and ground type transitions.

There were 17 contributed papers, presented in five sessions entitled: Meteorological effects – Effects of turbulence –

Modelling of atmospheric and ground effects on aircraft noise – Long range sound propagation studies – Assessment ofatmospheric and ground effects in methodologies related to airport noise. Some papers were devoted to studies ofmeteorological effects on outdoor sound propagation aimed at developing an aircraft noise prediction model with a view tooptimizing noise abatement operational procedures. The role of turbulence on sound propagation was investigated in somecontributions including the case of sound level increasing in shadow zones caused by hills, screens, or atmosphericrefraction. A number of papers discussed the application of ray tracing codes to aircraft noise propagation in both cases ofnear airport and en-route, including in some of them in-flight noise measurements. Long-range sound propagation in aninhomogeneous atmosphere was dealt with in other paper, by using a time-domain approach for moving sources employingthe linearised Euler equations. The influence of ground surface and topography, including artificial and terrain barriers, onairport noise was discussed in other contributions. The effect of atmospheric temperature gradients and ground impedanceon sound was another subject, attacked by solving exactly the acoustic wave equation in a polytrophic atmosphere over aflat impedance ground. Lastly, the use of CFD/CAA methods to investigate atmospheric and ground effects in aircraft noiseprediction was the subject of another paper, in an attempt to explain observed differences between predictions and flighttest results.

Written by E. Campos: emilio.campos@coiae.com, Association of Aeronautical Engineers of Spain (AIAE), Madrid, Spain.

2. Airframe noise

2.1. VALIANT: towards improved accuracy of airframe noise simulation techniques

VALIANT was a Collaborative Project addressing the external noise challenges raised by the development of greenaircrafts, which ran for 45 months over the period 2009–2013. The consortium included the following partners: VKI(Belgium, Coordinator), KIAM (Russia), ECL (France), TUB (Germany), ONERA (France), TsAGI (Russia), NLR (Netherlands),DLR (Germany), CIMNE (Spain), NTS (Russia), NUMECA (Belgium) and LMS (Belgium) [1]. The project focused on broadbandairframe noise (AFN) by tackling both landing gears and high lift devices, which are the two main contributors to AFN of anaircraft at approach. Among various other important aspects, the fulfillment of the ACARE objectives involves theimprovement of accurate and fast prediction techniques to enable virtual prototyping and shorten development cycles.While previous research programs dealing with AFN, such as RAIN, SILENCE(R), AWIATOR, TIMPAN, CLEANSKY andOPENAIR, studied realistic rather than generic configurations, VALIANT was rather aiming at validating/improving noiseprediction tools tested on generic but representative configurations, representing a very complementary contributiontowards the ACARE goals. Having generic configurations permits to establish very complete and accurate experimentaldatabases that can be used for the unambiguous validation of the prediction tools, which is an essential step towards theirimprovement.

Four specific flow configurations revealing the basic mechanisms of AFN generated by the most relevant elements of areal aircraft were selected in VALIANT for a thorough investigation aimed at validating and improving the broadband AFNpredictive tools: gap turbulence interaction, a flapþwing configuration, a slatþwing configuration,and a two-strutsconfiguration (see Fig. 1). In terms of nondimensional numbers, the Reynolds numbers of the generic cases were about oneorder of magnitude smaller than for their full-scale counterpart, but the Mach numbers were roughly respected.

The first half of the project has concentrated on thorough experimental campaigns, especially to obtain the necessarymeasured data for the simulations, and on the first-pass application of state-of-the-art simulation techniques. Numericalsimulations (especially LES and DES) being quite sensitive to boundary conditions, some measurements providing theseinputs had to be conducted in the early stage of the project to permit a timely start of the simulation work. The second half

Fig. 1. Four generic configurations investigated in VALIANT (top-left: wing-slat, top-right: gap, bottom-right: wing-flap, bottom-left: two-struts),illustrated by their respective test rigs (wing-slat and wing-flap: in ECL anechoic room; gap: in TsAGI anechoic room; two-struts: in NLR anechoic room)and sample numerical results (wing-slat: ONERA RANS optimisation; gap: KIAM DES; wing-flap: NTS zonal RANS-LES; two-struts: KIAM DES). The variousapproaches for flow and acoustic simulation that were developed in the project are also listed.

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–60 41

of the project was focusing on the improvement and application of the existing tools based on the experiences gainedthrough the first pass simulations. The last part of the project finalised all the developments and demonstrated theireffectiveness for noise prediction.

The main outcomes of the project are the following. Firstly, even for configurations geometrically rather simplified, thevalidation of the simulation approaches remains a difficult task. A number of installation effects, for example wind tunnelfree jet deflection or shear layer acoustic refraction, need to be properly matched between simulations and experiments topermit a meaningful comparison. Secondly, the simulation of the various cases for realistic Mach numbers and Reynoldsnumbers about one order of magnitude smaller than for their full-scale application are still very demanding computation-ally, and can hardly be yet incorporated in optimisation loops. For these reasons, the development of hybrid methods in ageneral sense (i.e. based on the acoustic analogy, using zonal RANS-LES/DES decomposition), and of semi-analytical (e.g.linearised airfoil theory) or stochastic approaches (SNGR/RPM), remains a worthwhile effort to be pursued in the future.To conclude, the VALIANT work has permitted gathering a lot of physical insight on the sound generation mechanisms,which will undoubtedly support the efforts towards the design of quieter air transport systems.

Written by C. Schram: schram@vki.ac.be, L. Koloszar, von Karman Institute for Fluid Dynamics, Belgium

2.2. Lattice-Boltzmann aeroacoustic study of LAGOON landing gear configuration

An aeroacoustic validation study of the Lattice-Boltzmann/FW-H hybrid approach was conducted by [2] using theLAGOON nose landing-gear database [3]. Grid convergence and solution accuracy was extensively demonstrated usingsteady and unsteady pressure measurements on the surface of the gear, steady and unsteady velocity measurement in thewake of the gear, far-field noise measurements and predicted maximum value of the PNL along a nominal approachcertification trajectory. One- and two-point cross-spectra of the velocity components were also predicted and compared tomeasurements. The noise spectra at microphones located along two circular arcs away from the model were in goodagreement with the measurements. Discrepancies were observed at angles away from the overhead direction that may beattributed to experimental uncertainties related to the wind tunnel installation (jet shear layer refraction and nozzlediffraction) or to a lack of statistical convergence in the simulation. Finally, a tonal noise generation mechanism involvingthe coupling between Rossiter-type modes taking place in the rim cavities and a pure acoustic wheel-to-wheel mode wasdiscovered and related to the occurrence of tonal peaks in the gear sideline direction and to generation of strong dynamicloads in the side direction [4].

Written by D. Casalino: damiano@exa.com, Exa GmbH, Stuttgart, Germany.

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–6042

2.3. Shear layer driven acoustic modes in a cylindrical cavity

The interior acoustic pressure of a cylindrical cavity driven by a shear layer has been investigated. Existing cavity flowliterature is generally limited to rectangular cavities, where the resonance is either longitudinal or the result of exciteddepth modes inside the cavity. This study focuses on a partially covered cylindrical cavity and in particular on the excitationof higher order, azimuthal duct modes. This geometry is frequently found in the transportation industry, e.g. windowbuffeting in cars/trains and the landing gear bays of aircraft and it is demonstrated that the excitation of higher order modes,which are commonly overlooked, can not only dominate the pressure field but can also combine with depth modes atrealistic free-stream velocities. Experiments performed show how lock-on between the Helmholtz resonance as well aslongitudinal and azimuthal acoustic modes with the shear layer hydrodynamic modes can be generated [5], see Fig. 2.Numerical simulations, experimental acoustic mode decomposition and PIV, provide significant insight into the interactionbetween the shear layer and the cylinder and the application of the vortex sound theory of Powell, modified by Howe for itsapplication to wall bounded flows, allowed the acoustic energy generated in the shear layer to be measured and the acousticsources to be localised in space [6,7], see Fig. 3. The effects of the cavity opening size and location were studied and it wasfound that the judicious placement of the cavity can allow high amplitude modes to be suppressed. The study highlightshow the thorough understanding of these neglected resonance phenomena is important for designing systems to mitigateagainst undesirable resonance.

Written by Gareth J. Bennett: gareth.bennett@tcd.ie, Trinity College Dublin, Ireland

2.4. Wall pressure fluctuations induced by supersonic turbulent boundary layers

A numerical database has been analysed to investigate cross-statistics of wall-pressure fluctuations induced bysupersonic turbulent boundary layers developing over a rigid smooth wall without a pressure gradient. The Mach numberspans from 2 to 4 and a relatively large range of Reynolds numbers is considered. Results from the analysis [8] show for thefirst time that the cross-statistics of wall-pressure fluctuations at such high Mach numbers is very similar to that usuallyobserved in incompressible flow conditions. Furthermore, it is demonstrated that theoretical predictions provided by thewell-known Corcos and Efimtsov models apply well when the adjustable coefficients are set properly.

Written by Alessandro Di Marco Alessandro.dimarco@uniroma3.it, R. Camussi, Department of Engineering, University RomaTRE, Rome, Italy. & M. Bernardini and S. Pirozzoli, Department of Mechanical and Aerospace Engineering, University La Sapienza,Rome, Italy.

2.5. The reduction of aerofoil-turbulence interaction noise by using wavy leading edges

A high-order accurate aeroacoustic simulation has been performed at the University of Southampton to study thereduction of aerofoil–gust interaction (AGI) noise by using wavy leading edges [9]. It was found that the ratio of the wavyleading-edge peak-to-peak amplitude (LEA) to the longitudinal wavelength of the incident gust (λg) is the key parameter tocharacterise the reduction of the AGI noise. It was observed that the reduction of the AGI noise increases with LEA=λg andsignificantly for ðLEA=λgÞ40:3. Also, a similarity was found between the directivity patterns of the noise reduction for thesame value of LEA=λg . The existing work that was mainly based on ideal sinusoidal gust functions is followed by an extendedwork (under the support of EPSRC – EP/J007633/1) with inclusion of realistic incident turbulence to study the broadbandresponses from various wavy leading-edge geometries.

Written by J.W. Kim: j.w.kim@soton.ac.uk, University of Southampton, United Kingdom

0 10 20 30 40 500

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Fig. 2. Pressure spectral density inside the cavity subjected to grazing flow. The orifice is located off centre. There is a dramatic jump from a depth mode(D1) to an azimuthal/depth combination mode (AZ1D1) at higher velocities excited by the second shear layer hydrodynamic mode.

Fig. 3. Distribution of the net acoustic energy generated per acoustic cycle on the orifice. (a) 46.3 m/s, SL1-D1. (b) 48.4 m/s, SL2-AZ1D1. (c) 51.5 m/s, SL2-D3.

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–60 43

3. Fan and jet noise

3.1. ORINOCO: jet noise reduction with plasma actuators and investigation of instability waves physics

After several years of effort in jet noise reduction, acoustic benefits have been obtained due to the increase of the bypassratio of the double stream nozzle engine (even if this choice was not motivated by aeroacoustics considerations) and due topassive devices such as chevrons. Technology breakthroughs are now necessary if further reductions are to be made anexample of which might be active devices which could be switched off after take-off. ORINOCO is the cooperation betweenEurope and Russia for advanced engine noise control based on plasma actuators. It is one of the first projects co-funded bythe European Commission and the Ministry of Industry and Trade of Russian Federation. Consequently, the project was ledby two coordinators: Onera (F) and TsAGI (RU) and 13 partners participated in the project: Aviadvigatel (RU), CIAM (RU),CIRA (It), CNRS Pprime (Fr), Erdyn Consultant (Fr), GPI-RAS (RU), JIHT-RAS (RU), LMFA/ECL (Fr), NLR (NL), TRINITI (RU) andthe University of Roma Tre (It). ORINOCO started in August 2010 and was completed in January 2014, in accordance with theinitial schedule. External feedback of the results was provided by an industrial expert panel which was created with threemembers from both Europe and the Russian Federation: Airbus, Alenia-Aermacchi, Beriev, Snecma, Sukhoi and Tupolev.

Plasma technologies which had initially been developed for flow control were confronted with technical aspects far fromacoustics when applied to jet noise reduction. This use of plasma actuators is a novel concept that required fundamentalapproaches to understand the interaction mechanisms with the main jet shear layer and the resulting radiated sound.During the 41 months of the project, three main points were investigated: the development of plasma actuators, evidence ofthe existence of an instability wave (also known as wave packets) and the evaluation of the capability of plasma actuators toreduce this instability wave.

Several plasma techniques were improved and developed: the Dielectric Barrier Discharge, the Barrier Corona Discharge,the Magnetically Driven Arc Discharge, the Gliding Surface Discharge and Plasma Synthetic Jets. Their effect on the main jetis the generation of ionic wind parallel or normal to the jet axis or micro-jet impingement on the main jet. All thesetechniques have proven their efficiency in generating coherent structures associated with instability waves, see Fig. 4.

Theoretical investigation of the physics of the instability wave concept was carried out to determine instability waveparameters and for the formulation of active control strategies. Experimentally, an Artificial Instability Wave (AIW) was

Fig. 5. Noise reduction (0.7 dB) in the far field with high frequency dielectric barrier discharge (jet velocity 100 ms�1– red: actuator off; blue: actuator on –

TsAGI/JIHT RAS). (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)

Fig. 4. Vortex generation by plasma synthetic jet (jet velocity 200 ms�1 – PIV measurement of radial velocity – Onera).

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–6044

generated in a single stream jet with acoustic excitation from a loudspeaker. After having demonstrated the feasibility of thesuppression of the AIW with external acoustic excitations, the same results were obtained with plasma actuators: thesuppression of the AIW was proven using PIV measurements [10].

Coupling mechanisms between the azimuthal modes of the jet were also investigated by theoretical analysis and anexperimental campaign based on a corrugated nozzle [11]. The same effect was then obtained with high frequency dielectricbarrier discharge actuators which succeeded in reducing the noise in the far field, see Fig. 5.

Finally, the analysis of near field pressure [12] and PIV measurements allowed ORINOCO to demonstrate that instabilitywaves can be a mechanism for noise generation in a jet. Deeper investigation pointed out that linear stability analysis is notable to explain the noise radiated in the far field rather it is the Jittering behavior of the instability waves which involvesnonlinear dynamics [13].

Acknowledgements: The research leading to these results received funding from the European Union Seventh Frame-work Programme (FP7 2007-2013) under Grant agreement no. 266103 and the Ministry of Industry and Trade of RussianFederation (Project ORINOCO).

Written by F. Cléro (franck.clero@onera.fr), Onera, France and V.F. Kopiev (vkopiev@mktsagi.ru), TsAGI, Russian Federation.

3.2. Jet-noise reduction by fluidic active control

Developing solutions to control and reduce jet noise generated by aircrafts is a major concern as underlined in ACARErecommendations. The Large-Eddy Simulation of a high Reynolds isothermal single jet (Re¼1,000,000, M¼0.9) controlledby fluidic crossed actuators has been realised under PRACE 5th call with the elsA software on a 2 billion point mesh. Theconservative filtered Navier–Stokes governing equations are solved with a finite volume formulation based on a sixth-ordercompact-scheme [14] and a DRP six-step low-storage Runge–Kutta for time advancement [15]. Radiative and nonreflectiveboundary conditions are also used to avoid spurious noise reflections from boundaries.

Direct noise computation of the controlled jet is performed using eight pairs of crossed microjets imposed via NSCBCboundary conditions inside the nozzle leap. The resulting near-field velocity of the main jet and the microjets action isdepicted in Fig. 6. The microjets influence on the main jet is local and does not induce significant modifications on the meanflow field downstream the jet potential core. However, a decrease of the sound pressure level in the low-frequency range is

Fig. 7. RANS mean-flow for nozzle configurations with lobe mixer, top: baseline nozzle; bottom: nozzle with nozzle guide elements.

Fig. 6. Isosurface of streamwise velocity U ¼ 0:5Uj , coloured with density. (For interpretation of the references to color in this figure caption, the reader isreferred to the web version of this paper.)

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–60 45

observed. In agreement with experimental results [16], the local increase of the turbulence level in the impact zone iscorrelated to a significant reduction of the radiated overall sound pressure level.

Written by G. Daviller: daviller@cerfacs.fr, CERFACS, France. J.-F. Boussuge CERFACS, France. M. Montagnac, CERFACS, France.H. Deniau, ONERA, France. M. Gazaix, ONERA, France. C. Bogey, CNRS-LMFA, France.

3.3. Efficient hybrid RANS-CAA method for prediction of dual- and single-stream jet mixing noise

In the German BMWi funded joint research project OPTITHECK, jet noise reduction means have been investigated andoptimised. The principal research subject was the development and the validation of an efficient jet noise predictionconcept for realistic passive noise reduction means. Numerical nozzle design tools for industry require short computationaltime on medium size hardware architectures and have to provide sufficiently high sensitivity to predict differences of noiselevel for even small modifications of nozzle geometry. According to these requirements, a broadband jet noise predictionmethodology has been successfully tested and validated. The hybrid approach comprises the following building blocks:firstly, a solution of RANS equation for the steady part of the jet-flow together with turbulence statistics. Secondly,fluctuating modal sound sources derived from the Tam and Auriault cross-correlation model [17], efficiently obtained from2-D stochastic realisations [18] using the random particle method (RPM) [19]. Finally, a sound propagation step withaxisymmetric LEE plus porous FW-H far-field extrapolation was performed for each azimuthal mode [20]. Results for dual-stream jets [21–23] have shown that this approach meets the industrial requirements as stated above. Latest resultsdemonstrate the applicability to lobe-mixer configurations with and without nozzle guide elements. In Fig. 7 the x-velocitydistribution from SST-k-omega RANS simulation with DLR solver TAU is shown. Fig. 8 presents relative noise levels and noiselevel differences using the DLR CAA solver PIANO. The general trends agree well with those found in measurements.

Written by A. Neifeld: andrej.neifeld@dlr.de, C. Appel, R. Ewert, DLR, Germany, M. Steger, M. Rose, Rolls-Royce Deutschland,Germany.

3.4. Evidence for wavepackets in jets

Breakey et al. [24] have used time resolved PIV with a sample frequency of 9 kHz and near field microphones toinvestigate a low Mach number subsonic jet (M¼0.25). It has been found that the PIV data at high frequencies are uncertaindue to inherent limitations of PIV: temporal aliasing and location precision. This does not affect the time-domain

Fig. 8. Comparison between CAA computed spectra of different nozzle geometries (solid lines) and corresponding experimental data (symbols)configurations of dual-stream jets (D¼0.157 m) in forward flight (Ma¼0.25) with mean nozzle exit velocity Ma¼0.87, evaluated for observer positionat θ¼ 901 and jet axis distance R¼1.0 m. (b) Relative sound pressure level differences of three modified configurations (colored lines and symbols) withrespect to reference nozzle configuration (baseline/baseline) for CAA computations and experimental data. (For interpretation of the references to color inthis figure caption, the reader is referred to the web version of this paper.)

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–6046

correlations, but does impact on the estimates of turbulent quantities (e.g. coherence) in the frequency domain.Nonetheless, analysis of the correlation signature gives evidence that large-scale, convecting, wave-like structures areassociated with sound production, a result consistent with observations by many recent investigators. Furthermore analysisusing proper orthogonal decomposition of near-field microphone measurements in a high speed subsonic jet in CEATPoitiers supports the concept of the so-called wavepackets being present in the flow beyond the end of the potential coreand the far-field data indicates that these wavepacket signatures propagate to the far-field [25]. This analysis gave rise to aquestion of how to assess whether one signal (e.g. the reconstructed far-field estimate) compares well with another (e.g. themeasured far-field pressure). The structural similarity metric (SSIM) used by Hines and Harte [26] to determine speechintelligibility has been applied by Breakey and Meskell [27] to aeroacoustic data subject to common distortions. It has beenfound that the SSIM is a more reliable indicator of similarity than traditional metrics such as mean error or RMS.

Written by Craig Meskell: cmeskell@tcd.ie and David E.H. Breakey, Trinity College Dublin, Ireland.

3.5. On an acoustical oscillation energy for shear flows

The paper gives the exact solution of the acoustic-vortical wave equation for a unidirectional shear flow over animpedance wall with an hyperbolic tangent shear velocity profile [28]. It specifies the complex reflection and transmissioncoefficients (amplitude and phase) and also the pressure field in the boundary layer without restriction on wave frequency.An acoustic Hamiltonean or oscillation energy is introduced that describes the energy exchange between sound and themean flow. It is shown that the oscillation energy has an extremum at the critical layer, that is a maximum for propagationupstream and a minimum for propagation downstream. The critical layer where the Doppler shifted frequency vanishescorresponds (i) zero pressure, (ii) the transformation between acoustic and vortical modes, (iii) leads to the appearance of acontinuous spectrum.

Written by Luis M.B.C. Campos luis.campos@tecnico.ulisboa.pt, M.H. Kobayashi, Univ. Hawai, USA.

4. Helicopter noise

4.1. TEENI project outputs

TEENI (Turboshaft Engine Exhaust Noise Identification) was a level 1 project which began on April 1, 2008, and hasfinished since March 31, 2013. The project consortium consisted of 11 partners: Turbomeca (France, coordinator), ANOTEC(Spain), AVIO (Italy), Bruel & Kjaër (Denmark), COMOTI (Romania), DLR (Germany), EPFL (Switzerland), INASCO (Greece),Microflown (The Netherlands), ONERA (France), and TCD (Ireland), with an overall budget of €3.3 M.

The main project objective was to decompose the broadband noise radiated from the exhaust of a Turbofan engine intoits constituent parts and to identify with which internal engine module the component was associated. Broadband corenoise is the second most dominant noise source of a Turboshaft engine, but probably the first in flight, due to installationeffects. The project was concerned with only the engine and thus the propeller was not included within the scope of the

Fig. 9. Instrumented Ardiden engine at Uzein open air test bed.

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–60 47

work. Turboshaft exhaust noise is assumed to be a mix of combustion and turbine noise, with very little jet noise. It is alsorepresentative of what is generally called core noise in aircraft engines.

The TEENI work programme is divided in three inter-dependent work packages (WP):

�

WP1: Innovative sensor development to provide reference measurements of fluctuating quantities within the enginegiven its harsh conditions. Acoustic velocity and acoustic intensity probes were judged insufficiently mature to meet theengine test requirements, but two innovative cooled unsteady pressure and temperature probes were successfullydeveloped.

�

WP2: Noise source breakdown technique (NSBT) development, to determine the dominant emission location fromexternal measurements. Several techniques were evaluated, considering internal and external measurements on smallscale tests, as well as various algorithms and techniques. Sound propagation through the turbine(s) and tools to help theconsideration of individual engine noise sources was also developed.

�

WP3: An Ardiden Turboshaft engine full-scale test was conducted. This test included and tested the novel sensors andapplied the noise source breakdown techniques to provide a first example of noise decomposition per module of aTurboshaft engine (see Fig. 9).

TEENIs main outcomes were:

�

Pressure and temperature probes for measuring unsteady quantities, adequate for full-scale engine testingð650 1CoTo1400 1CÞ, with a (more than) 4 kHz bandwidth for pressure probes and (more than) 1 kHz bandwidthfor thermocouples.

�

A comprehensive full-scale engine test database, the first of its kind, with extensive internal measurements and far-fieldinstrumentation.

�

A noise breakdown realised out of a panel of original signal processing methods which have been developed during theproject, using internal measurements to understand the origin of noise measured in the far-field.

�

Elements of understanding of noise generation, propagation and radiation through the exhaust, from experimental andtheoretical points of view.

Direct combustion noise was identified at low frequencies and its relative importance is shown to be increased after theHP turbine, and maintained throughout the downstream measurements. The increased noise levels at higher frequencies

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–6048

(but still 1 kHz) after the HP turbine may be related to indirect combustion noise. The influence of the power turbine, verysignificant at high frequencies, was more difficult to stress in the TEENI frequency band of interest (0 4 kHz). However,broadband noise origin for frequencies above 2.5 kHz was not fully explained.

TEENI project, coordinated, by E. Bouty, TurbomecaWritten by E. Bouty: eric.bouty@turbomeca.fr, Turbomeca, France.

5. Aircraft interior noise

5.1. In-flight measurements of the of coherence length varying with altitude and speed using flight test data

Surface pressure fluctuations from several flight attitudes were evaluated in order to find the influence of altitude andspeed on the coherence lengths of turbulent structures in the boundary layer. Flight tests were performed using an A320airplane as a test carrier. Three consecutive window banks in the vicinity of the wing were used to install a pressuretransducer array. The array consisted of three aluminum windows equipped with a total of 30 pinhole-mounted pressuretransducers [29–31]. Assuming a homogenous and ergodic flow the spatial differences between each transducer pairingwere considered and the narrow-band coherence was calculated for each pairing. A two-dimensional exponential functionwas fitted through this coherence, taking the local frequency-dependent flow direction into account [32]. The decaycoefficient representative for a remaining coherence of 36 percent was plotted over the frequency and compared with theEfimtsov prediction [33] in Fig. 10. The prediction was found to considerably underestimate the coherence lengths valuesbelow 1 kHz. The prediction parameters were adapted to fit the measurements and the resulting curves were comparedbetween different flight altitudes at constant flow velocity and different flow velocities at constant altitude. In Fig. 11 thecoherence length at the lowest speed and altitude is shown in black. Based on this reference, variation of speed is shown in

Fig. 10. Comparison of measured coherence length at FL250, TAS¼210 m/s and the predicted values from the Efimtsov model.

Fig. 11. Fitted coherence length using the Efimtsov ansatz function. Black: reference red variation of speed at constant altitude; blue: variation of altitude atconstant speed. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–60 49

red and variation of altitude is shown in blue, respectively. Marginal variation of coherence length is observed in the fittedcurves although the change of viscosity with increasing altitude was believed to have such an effect.

Written by Stefan Haxter: stefan.haxter@dlr.de, C. Spehr, DLR, Germany.

5.2. Numerical approximations on the predictive responses of plates under stochastic and convective loads

The quality of the predictive structural (and structural-acoustic) response of an elastic structural domain, under arandom and convective load, is analyzed through a discussion of the main steps of a numerical procedure [34]. Thetransformation of the pressure distribution into discrete locations is one of the most critical steps in fact, this depends on (i)the model assumed for wall pressure fluctuation description (the assumed turbulent boundary layer model), (ii) theintegration scheme and (iii) the frequency range. These three aspects are the main aims of this work. In particular, therobustness of the numerical results is evaluated with regard to the adopted numerical approximations. The transformationof the pressure distribution into discrete locations can be computationally expensive for the required level of the numericalaccuracy. This work reports a dimensionless metric for the plate response, which is a measure of the average vibrationenergy of a plate, versus a dimensionless frequency, which is a function of structural mesh size and convective flow speed.The use of a consistent formulation, in the finite element scheme, can enhance the quality of the results but it can besometimes unfeasible because the TBL model must be easily integrable and the (structural element) integration domainmust have a regular geometry. The increase of the mesh size leads to an insignificant improvement (Fig. 12). Hence, somenumerical approaches, dealing with the high frequency bands, are analyzed and some enhancements are proposed aimingat a reduction of the computational cost. They use the uncorrelated pressure field characteristics of turbulent boundary layeras frequency increases and try to compensate the load aliasing due to the discretisation mesh. Finally, the solution is carriedout in a geometrical scaled domain. The computational time, through these successive approximations, reduces of fourorders of magnitude.

Fig. 12. Comparison among analytical and numerical results vs. dimensionless frequency improvements in the load discretisation and mesh size.

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–6050

Written by Francesco Franco, Universit di Napoli “Federico II”, Napoli, Italy, Department of Industrial Engineering – AerospaceDivision – francesco.franco@unina.it. Sergio De Rosa, Universit di Napoli “Federico II”, Napoli, Italy, Department of IndustrialEngineering – Aerospace Division – sergio.derosa@unina.it. Elena Ciappi, CNR-INSEAN, Via di Vallerano 139, Roma, Italy, elena.ciappi@cnr.it

6. Propeller noise

6.1. Low frequency sound absorption of resonators with flexible tubes

Locally reacting liners, as those used in aeronautical engine nacelles, are generally sandwich resonators with a perforatedplate linked to an honeycomb material above a rigid plate (SDOF). Their absorption behavior can be described approximatelywith the principle of an Helmholtz resonator. The frequency range of absorption is so essentially controlled by the thicknessof the honeycomb cavity. The impedance can depend non linearly on the incident particle velocity level (or sound pressurelevel). So, acoustic vortices of particle velocity can occur at the resonator surface thus modifying the impedance. In order toenlarge the frequency range of absorption, different types of SDOF liners can be piled up to constitute 2DOF or 3FOF liners.Nevertheless, their physical law is not suited to an absorption to the lowest frequencies, as needed for future ultra highbypass ratio (UHBR) engines with shorter and thinner nacelles (frequencies around 500 Hz). To drastically improve theircapabilities, a possible approach is to carry out a perforated panel resonator with flexible tube bundles to generate asignificant shift of the frequency range of absorption towards lower frequencies. This concept, simulated and tested byOnera, through French projects (MANSART and CORALIE), allows a significant change in the acoustic impedance due to thelarge thickness of the resistive and reactive material and the coupling with the surrounding cavity [35]. Applied for anaeronautical liner (Figs. 13 and 14), the resonance frequency decreases considerably compared to an initial resonator (factorof about 1/5). These first results allow us to consider that these resonators, whose behaviour is linear according the soundpressure level, could be an alternative to classical resonator, on the assumption of an automatic manufacturing process(requirements of robustness and cleaning theoretically satisfied as for classical resonator) and an appropriate behaviourwith grazing flow.

Written by F. Simon: Frank.Simon@onera.fr, ONERA, France.

6.2. Feasibility of front-rotor trailing-edge-blowing for the reduction of CROR interaction noise

A possible way to decrease interaction tone noise of a contra-rotating open rotor (CROR) is through the application oftrailing edge blowing (TEB), by reducing the momentum deficit of the front rotor wake and therefore its interaction with the

Fig. 13. Perforated plate with tubes manufactured by ATECA (project MANSART).

Fig. 14. Measured absorption coefficient of a liner without and with hollow tubes (20 mm) SPL 135 dB.

Fig. 15. Comparison of aft rotor unsteady thrust loading component, for (a) baseline and (b) TEB cases.

Fig. 16. Overall sound pressure level (OASPL) in streamwise direction (x/D) for baseline (blue) and blowing (green) configuration, with inset displaying A-weighted OASPL. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–60 51

aft rotor. Although similar techniques have been successfully applied to rotor–stator interaction noise in turbomachinery/fans some 20 years ago [36], never has it been investigated for CRORs. In this contribution, we present an assessment offront rotor TEB for the reduction of CROR interaction noise [37,38]. For this purpose, a DLR-designed generic CROR [39] hasbeen modified to include TEB on the front rotor blades. With the DLR CFD-code TAU, uRANS simulations have been run forthe baseline and the TEB configuration. Subsequently, an aeroacoustic analysis has been performed with the Ffowcs-Williams/Hawkings tool APSIMþ for both configurations. The results show a negligible differences of the aerodynamicperformance, with significantly lower unsteady loading of the aft rotor when the TEB technique is active [cf. Fig. 15(a) and(b)]. The aeroacoustic results illustrate that indeed the interaction tones are significantly ameliorated, with a reduction overalmost the complete polar angle range seen when TEB is active. With respect to the A-weighted overall sound pressure level(OASPL), a reduction of approximately 2.5 dBA in the maximum level for the TEB configuration is achieved for the hereconsidered geometry (see Fig. 16). The decreased noise radiation with TEB mainly stems from a noise reduction in the firsttwo interaction tones.

Written by R.A.D. Akkermans: rinie.akkermans@dlr.de, A. Stuermer, and J.W. Delfs, German Aerospace Center (DLR), Instituteof Aerodynamics and Flow Technology, Germany.

6.3. WENEMOR – wind tunnel tests for the evaluation of the installation effects of noise emissions of an open rotor advancedregional aircraft

The WENEMOR project was developed in response to the requirements of the European Clean Sky Joint TechnologyInitiative in order to assess the aero-acoustic noise emissions for an advanced regional counter rotating open rotor (CROR)aircraft configuration [40]. The main objective of the project was the establishment of an experimental database for CROR

Fig. 17. Installed CROR simulators on an advanced regional aircraft design featuring a U-tail.

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–6052

noise emission. This database is currently being utilised for the validation of numerical codes and simulations beingdeveloped in the larger Clean Sky Green Regional Aircraft program.

The project consortium consisted of seven partners namely two universities (Trinity College Dublin (Coordinator) andUniversita Politecnica delle Marche), a large European wind tunnel facility (Pininfarina) and several SMEs (Eurotech,Teknosud, MicrodB and Paragon S.A.) with specific competencies in design, manufacture, noise measurement and dataanalysis. The project utilised a proposed design for an advanced regional aircraft configuration, developed within the CleanSky Green Regional Aircraft project, for the aircraft model geometries. The aircraft model was manufactured at 1:7.5 scaleand was designed to be parametric following a modular approach. The aircraft model was configurable with interchangeabletail pieces, variable fuselage length, engine pylon rotation and elongation and was controllable for angle of attack. Thepropulsion system simulators were based on an advanced CROR design and featured realistic modern blade profilesproduced in agreement with GRA partners and suggested by SNECMA. The installed CROR simulators on the aircraft modelare shown in Fig. 17.

The aircraft model was instrumented with flush mounted pressure sensors in the aircraft fuselage. Near field noisemeasurements were made using a microphone array on a traversing arm. Far-field sound measurements were acquired on3 microphone beam forming arrays and also on a far-field linear array of microphones. One engine simulator wasinstrumented with 22 kulites sensors on both the front and rear blade rows to measure blade surface pressure profilessimultaneously with the far-field sensors. The instrumented engine simulator was also investigated in an uninstalledconfiguration on a single pylon for a variety of angles of attack, flow speeds and operating conditions. In total over 250 nearand far-field measurement sensors were deployed for the test campaign. The data produced by this extensive measurementcampaign have been used to generate the most comprehensive database of installed and isolated CROR noise emissioncurrently available.

The CROR noise emission was investigated as a function of aircraft geometry [41]. The effects of tail geometry, enginepylon length, engine pylon rotation and wing to engine distance were investigated. It was found that the pusher engineconfigurations were between 5 and 7 dBA quieter than the geometrically equivalent tractor configurations. For the pusherengine configuration the lower noise emissions were achieved through the use of longer engine pylons and higher angles.The best performing configuration overall featured a U-tail design with the pusher engine configuration [42]. This designproduced a further reduction in far-field sound of up to 5 dBA over the baseline pusher configuration.

EU FP7. Cleansky. Coordinator – Gareth J. Bennett, Trinity College DublinWritten by J. Kennedy: kennedj@tcd.ie, Gareth J. Bennett, Trinity College Dublin, Ireland.

7. Techniques and methods in aeroacoustics

7.1. Panel methods for leading edge airfoil noise prediction

Leading edge airfoil noise, which can be generated either by the airfoil interaction with turbulence or the wake of anupstream aerodynamic component, is a concern for many applications such as contra-rotating open rotors, aeroenginerotor–stator interaction and high-density wind-farms. The conceptual design of components for these applications demandsfast and accurate methodologies for aeroacoustics predictions. A panel method coupled with Curles analogy has beendeveloped at KU Leuven and its results have been verified for the turbulence–airfoil interaction noise case, showingexcellent agreement with analytical results [43]. This methodology is shown capable to extend analytical methodologiesconsidering complex airfoil geometries and incoming flows with addition of small computational cost. Recently, the results

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–60 53

of this numerical methodology have been compared with experiments executed at the von Karman Institute for FluidDynamics [44], and are seen to accurately predict the airfoil wall pressure jump (Fig. 18) and tonal noise generation based onmeasured upstream flow quantities (Fig. 19).

Written by L.D. Santana leandro.desantanadantas@mech.kuleuven.be, C. Schramn and W. Desmet, Katholieke UniversiteitLeuven, Belgium; nvon Karman Institute, Sint-Genesius-Rode, Belgium.

7.2. A stochastic particle based method for broadband noise prediction of confined flow applications

In confined flow systems, e.g. automotive intake- and exhaust systems or HVAC systems broadband flow generated noisehas an important contribution to overall noise level. For the numerical prediction of the broadband noise, unsteady flowsimulations are usually required. A promising alternative is the stochastic reconstruction of an unsteady turbulent velocityfield. Since confined flows can have a very nonuniform mean flow, a particle based method, which relies on the localfiltering of white noise is most suited [45]. KU Leuven has developed a hybrid numerical methodology [46]. The sound

Fig. 19. Comparison between the experimental far-field noise (continuous line) and the panel method predicted tones (crosses). Airfoil profile DINNO, flowvelocity 19 m/s and angle of attack 01.

Fig. 18. Comparison of the measured wall pressure fluctuation (continuous line) with the tones predicted numerically (crosses) for the airfoil chord-wiseposition x/c¼0.3.

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–6054

propagation is computed by a Runge–Kutta discontinuous Galerkin method which solves variants of the linearised Eulerequations [47]. The generation of the source terms is performed by the stochastic reconstruction method using a Gaussianenergy spectrum and a Langevin model for evolution of turbulence. The hybrid method has been verified using an analyticalreference solution for a synthetic source in uniform mean flow conditions [18] (Fig. 20).

Written by B. Vanelderen: bart.vanelderen@mech.kuleuven.be, W. De Roeck and W. Desmet, KU Leuven, Belgium.

7.3. Optimisation of an advanced aeroacoustics hybrid approach, and application to a landing gear noise problem

Aircraft noise has become a societal issue, whose reduction is a technical challenge. In its effort towards the developmentof advanced computational method for aircraft noise prediction, ONERA has promoted the so-called hybrid approach [48],which consists in weak-coupling separate methods and solvers, so as to simulate the entire noise production chain(generation, propagation, and far-field radiation). Within this framework, an existing hybrid approach procedure [48] hasbeen recently optimised, which relies on the weak-coupling of unsteady computational fluid dynamics (CFD) andcomputational aeroacoustics (CAA) methods. Several innovative techniques were proposed, among which (i) a moreflexible weak-coupling procedure (the so-called non-reflecting interface [1]); (ii) a more robust interpolation method (theso-called interpolation by parts [49]) and (iii) a more accurate finite differences (FD) derivative scheme (the so-calledintrinsically optimised FD [49]). All these outcomes allowed optimizing the overall CFD–CAA weak-coupling procedure sothat it can cope with all stringent constraints that are dictated by real-life problems without being jeopardised by someof their unavoidable side effects (such as the signal degradation to which CFD unsteady data may be subjected to,when sampled and/or interpolated for being exploited in a CAA sense [50,49]). In addition, the relevance of such optimisedCFD–CAA weak-coupling procedure was assessed through its application to a landing gear noise emission problem (LAGooN

Fig. 21. Noise emission by a simplified nose landing gear (LAGooN program), via a CFD-CAA coupled computation (CFD computation performed by Dr. BenKhelil with Oneras elsA solver, CAA calculation achieved with Oneras sAbrinA.v0 code). Near-field CFD results (instantaneous pressure fluctuation field, andQ-criterion of iso-surfaces coloured by the streamwise velocity component). (For interpretation of the references to color in this figure caption, the readeris referred to the web version of this paper.)

Fig. 20. Spectrum observed from a synthetic source in a uniform mean flow. The numerical and the analytical solution are compared for different flowspeeds and time scales of turbulence.

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–60 55

program, supported by Airbus) thus, a weakly-coupled CFD–CAA calculation was performed [51] (see Fig. 21), being thenfavourably compared to both experimental data and numerical outputs (by an acoustic analogy). All this allowed validatingfurther the present CFD–CAA hybrid approach, as well as illustrating better its potential regarding an effective application toreal-life problems.

Written by Stéphane Redonnet (stephane.redonnet@onera.fr) and G. Cunha, Onera.

7.4. Microphone-array measurements on a scaled model at real-flight Reynolds numbers

Aerodynamic measurements are often performed in cryogenic and/or pressurised wind tunnels which are capable ofincreased (up to real-flight) Reynolds number flows – since conventional wind tunnels cannot generally achieve real-flightReynolds numbers. Based on previous research [52–54], at the DLR Institute of Aerodynamics and Flow Technology themicrophone array measurement technique has now been further developed to acquire aeroacoustic data in an environmentwhich is both pressurised and fully cryogenic and thus at real-flight Reynolds numbers [55]. The measurements wereconducted at the facility of the European Transonic Windtunnel GmbH which can provide real-flight Reynolds numbers byvirtue of both decreased temperature and increased pressure. Measurements were carried out for a wide range ofoperational parameters and different Reynolds numbers up to real-flight Reynolds numbers using a scaled Airbus K3DY half-model. The results showed a significant Reynolds number dependency for various sources: sources on the slat with a strongtonal character, which disappear with a rise of the Reynolds number and various peaks with combined Strouhal- andReynolds number dependencies. Of particular note are various dominant sources appearing on the flap at real-flightReynolds numbers, giving a major contribution to the overall level (see Fig. 22). To our knowledge, this is the first time thatairframe noise data of a small-scale model have been acquired at real-flight Reynolds numbers. The ability of measuringairframe noise at real-flight Reynolds numbers gives now the possibility of separating the effect of the Reynolds numberfrom the effects of model-fidelity and Mach number on aeroacoustic behavior.

Written by T. Ahlefeldt: thomas.ahlefeldt@dlr.de, DLR, Germany.

7.5. An unsteady aerodynamic formulation for efficient rotor tonal noise prediction

An aerodynamic/aeroacoustic solution methodology for prediction of tonal noise emitted by helicopter rotors andpropellers has been presented in [56]. It is particularly suited for configurations dominated by localised, high-frequencyinflow velocity fields as those generated by blade–vortex interactions. This approach consists of a three-step procedure: (i)evaluation of blade downwash given by the combination of the blade motion with the velocity field induced by wakes andinteraction effects present in the aerodynamic environment, (ii) semi-analytical evaluation of blade loads by the sectional,frequency-domain Küssner–Schwarz theory (speed-up component of the process), (iii) radiation of sound by blade motionand blade loads sources through the Ffowcs Williams and Hawkings Equation (FWHE). Downwash effects fromwake inflow-velocity are provided by a three-dimensional (3D), unsteady, panel-method formulation suited for the analysis of rotorsoperating in complex aerodynamic environments [57]. Numerical results in terms of blade loads, noise signatures and soundpressure level contours show that the proposed analytical-numerical approach yields a computationally efficient solutionprocedure that may be used in preliminary design/multidisciplinary optimisation applications. Specifically, comparisons

Fig. 22. Comparison of the CLEAN-SC source maps (dB) at M¼0.203 and an angle of attack of 3 but different Reynolds numbers. The corresponding third-octave Strouhal number for both plots is St¼180. The left plot shows the map at a Reynolds number of 1.42�106, the right plot the map at the real-flightReynolds number of 20�106.

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–6056

with aeroacoustic predictions obtained by the FWHE where airloads are directly evaluated by the BEM formulation [57]highlight a good level of agreement in terms of both noise field (magnitude and directivity) and blade airloads, in particularfor operating conditions governed by strong localised, high-frequency blade-wake impact phenomena. A key-point of theaerodynamic formulation herein discussed is that computational costs required to evaluate blade airloads are reduced up toseven times with respect to those needed by the aerodynamic BEM approach in Ref. [57] hence it represents a good trade-offbetween accuracy of predictions and computational costs.

Written by M. Gennaretti, University ROMATRE, (Italy) m.gennaretti@uniroma3.it, C. Testa, CNR-INSEAN (Italy) claudio.testa@cnr.it, G. Bernardini University ROMATRE, (Italy) g.bernardini@uniroma3.it

7.6. Use of two-port acoustic data to investigate turbo-compressor surge

A novel method for investigation of turbocharger flow instabilities (surge) has been recently proposed [58]. The methodis based on the experimental determination of the full acoustic two-port data. The active part of this data describes thereflection free sound generation and the passive part, the scattering of sound. The scattering data (2�2 matrix) containsinformation about possible flow–acoustic interaction and the resulting amplification of incident sound [59]. The existence ofsuch amplification has been investigated for a compressor operating close to low-end mass flow limit called surge whereflow instabilities are expected [58]. As the amplification of incident sound depends on the amplitude and phase relationbetween the inlet and outlet branch, all the possible combinations have to be considered. The active data can also be used tocompute the coherence between the sound emitted up- and downstream. It is found that at flow instabilities such asrotating stall peaks occur in the source coherence corresponding to the existence of a correlated source structure. Themethod has been tested on automotive turbo-compressor in the unique test facility for two-port testing of turbo-compressors at KTH in Stockholm [60].

Written by R. Kabral: kabral@kth.se, M. Åbom and H, Bodén, KTH, CCGEx, Sweden.

7.7. A weak-scattering model for turbine-tone haystacking

Turbine tones radiated from the exhaust nozzle of a turbofan engine propagate through turbulent jet shear layers whichcauses scattering of sound. In the far-field, measurements of the tones may exhibit spectral broadening, known colloquiallyas haystacking, where owing to scattering, the tones are no longer narrow band peaks in the spectrum. A new theoreticalmodel to predict spectral broadening for a tone radiated through a circular jet is detailed in Refs. [61,62]. The recent work inRef. [62] extends the analysis found in Ref. [61] which considered the prediction of spectral broadening at far-field observerlocations outside the cone of silence. The new analysis extends the theoretical model to predict spectral broadening at anypolar angle, including inside the cone of silence. The modelling uses high-frequency asymptotic methods and a weak-scattering assumption. A realistic shear layer velocity profile and turbulence characteristics are included in the model. Anillustrative example is shown in Fig. 23. The spectral broadening of a tone is shown for a selection of varying turbulenceconvection velocities. The application of this work, in addition to turbine-tone haystacking, also could include theassessment of noise measurements in open jet wind tunnels. Measurements of tones, obtained by microphones locatedoutside the jet, may suffer from haystacking because the sound waves propagate through the jet shear layer before reachingthe microphones.

The work was funded by the European 6th framework project TURNEXWritten by A. McAlpine: am@isvr.soton.ac.uk, Institute of Sound and Vibration Research, University of Southampton, UK.

Fig. 23. Effect of varying the turbulence convection velocity on the scattered acoustic field at polar angle equal to 90 degrees. Predictions of the normalisedfar-field spectral density. (Taken from Ref. [2]).

G.J. Bennett et al. / Journal of Sound and Vibration 340 (2015) 39–60 57

7.8. Closed form integrals of Pridmore–Brown modes

A new class of integrals involving solutions of the Pridmore–Brown equations (sometimes denoted as Pridmore–Brownmodes) have been found [63]. The Pridmore–Brown equations describe modal acoustic waves in straight lined ducts withradially nonuniform mean flow and mean temperature. The integrals can be considered as generalisations of the classicintegrals of products of Bessel functions. The new integrals have been successfully applied to efficient and accurate(compared to the classical approach) mode matching.

Written by S.W. Rienstra, s.w.rienstra@tue.nl, Technical University Eindhoven TUE.

8. Miscellaneous topics

8.1. Aqueous foam to reduce the blast wave phenomenon

Current studies on the environment of launchers at lift-off aim at controlling the blast wave generated at the ignition ofsolid rocket motors. The blast wave generated on the launch pad is characterised by two components: the ignition over-pressure (IOP) impinging on the bottom of the trench and radiating from the opening of the trench entrance from aperturesat the entry of the duct and the duct over-pressure (DOP) propagating from the downstream exit of the duct. In order toinvestigate research studies and reduction devices, a system able to simulate the blast wave phenomenon was recentlydeveloped in the MARTEL facility, installed in CEAT of Poitiers University. The principle of the blast wave generator isdetailed in [51]. It is produced by the deflagration of methane–air mixture ignited inside a spherical volume, under pressureup to 100 bars at a temperature of about 2500 K. A campaign in the Martel facility, funded by CNES, has been performed tostudy the influence of the trench geometry on the blast wave. No significant modifications of the blast wave radiation occurwhen varying the shape of the trench, except when removing its upper side cover where a stronger blast wave can beobserved. Additional tests when filling up the trench with foams have been realised (Fig. 24). The foam is generated by themixture of compressed air with a foaming solution. A simple device has been implemented to control the expansion ratio ofthe foam to about 20. Preliminary results show that the presence of foam at the trench entrance reduces the DOP but not theIOP. When the foam fills the throughout trench, a strong attenuation on both the IOP and DOP is observed (Fig. 25). In thislatter configuration, measurements inside the trench show that the DOP is both reduced and delayed by the foam, comparedto the configuration without foam . The authors acknowledge CNES for fruitful discussions and for supporting this study.

Written by C. Bresson: claude.bresson@onera.fr, P. Malbqui, ONERA, France.

8.2. Verification of the inverse cut-off effect in a turbomachinery stage

Conventional cut-off design is a standard way to reduce airfoil interaction noise in low pressure turbines (LPT). However,with respect to the last rotor and turbine exhaust case (TEC) interaction, a cut-off effect can normally not be achieved due tothe large discrepancy of airfoil counts. As a means of circumventing these restrictions, an inversely cut-off TEC has beendesigned by means of analytical and numerical acoustic tools. The corresponding design has then been tested at a 1 stagecold flow LPT rig at the Graz University of Technology back-to-back to a standard TEC design (compare with reference [64]).The results in terms of the acoustic power level reduction (PWL) at the design condition approach show a significant effectboth on the target interaction (�7 dB PWL) and the turbine stage PWL (�10 dB PWL). The latter can be explained by a

Fig. 24. Setting-up of the trench filled with foam in Martel facility.

Fig. 25. Blast wave without and with the foam in the trench.

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modified scattering effect at the inversely cut-off TEC. Compared to the numerical predictions (LEE, ref. [65]), the measuredeffect is only limited by the noise floor within the test and analysis data. In addition, the design robustness has been provenby an rpm variation of the approach condition.

Written by Dominik Broszat, MTU Aero Engines AG, Munich, Germany, dominik.broszat@mtu.de Additional authors: ThorstenSelic and Andreas Marn, Graz University of Technology, Graz, Austria.

8.3. Whistling of corrugated pipe segments

Rudenko and Nakibolu [66,67] have complemented a systematic study of the whistling induced by a flow through acorrugated pipe. Following the PhD work of Nakiboglu [68] on the whistling of single and double cavities, the focus has beenon the design of silent pipe systems with corrugated pipe segments. A model has been developed to predict the effect ofacoustic boundary conditions on the whistling of a corrugated segment embedded in a smooth pipe. A surprising result isthat even with anechoic terminations a sufficiently long corrugated pipe segment can whistle [66]. An engineering tool hasbeen developed to predict the influence of the corrugation geometry [67]. Continuing the study on the interaction betweentwo successive cavities, a silent corrugated pipe has been designed and presented at the da Vinci completion of ERCOFTAC[69]. The work was awarded a shared first price.

Written by A. Hirschberg: A.Hirschberg@tue.nl, Technische Universiteit Eindhoven, Applied Physics, NT/MTP, Room CC.3.01b,TU/e, Postbus 513, 5600 MB Eindhoven, The Netherlands, O. Rudenko, G. Nakiboglu.

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