[american institute of aeronautics and astronautics 49th aiaa aerospace sciences meeting including...

American Institute of Aeronautics and Astronautics

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Hybrid liner concept with integrated active devices for active noise reduction in engine intakes and exhausts

Michael Bauer* and Conny Krauspe† EADS Innovation Works, 81663 Munich, Germany

Noise reduction for aero-engine inlets is usually achieved passively by absorbing acoustical treatment or liner. Further reduction or cancellation of annoying tonal noise components is possible by active means. The solution presented in this paper is a combination of active devices which are integrated into the passive lining. A prototype liner section was manufactured and tested in a large scale fan rig under realistic flow conditions.

Nomenclature BPF = blade passage frequency ANC = active noise control R = number actuator carrying rings N = total number of actuators SAA = special acoustic absorber α = absorption coefficient SPL = sound pressure level rpm = revolutions per minute d = diameter l = length UHBR = ultra-high bypass ratio

I. Introduction OISE reduction e.g. in turbofan inlets is usually done passively by an acoustical treatment1. This treatment applies very effective for broadband noise in a certain frequency range and the flow conditions for which the

liner was designed. There is a variety of different acoustic liners, optimized for the fan/stator configuration. But in some cases they do not sufficiently reduce the tonal noise components. This may be problematic, especially for take-off and approach. Take-off is remarkable due to the high noise levels and approach can be critical due to the related flight path giving a long exposure time for observers in the community on the ground. A further reduction of the major tonal fan noise components BPF, 2xBPF, etc. is necessary and active concepts have already been developed, e.g. in publicly funded projects2. The problem that always has to be overcome is the loss of liner performance by using additional active devices. Those devices often reduce the effective liner surface or change the impedances on the surface inside the intake in such way that absorption loss is coming along with ANC systems. This is the description of a hybrid liner for an aero-engine intake duct, consisting of a combination of active devices (actuators), integrated into the passive lining.

* Research Team Leader, Dept. Structures Engineering, Production & Aeromechanics, AIAA Senior Member. † Undergraduate (formerly), Dept. Structures Engineering, Production & Aeromechanics.

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49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition4 - 7 January 2011, Orlando, Florida

AIAA 2011-842

Copyright © 2011 by EADS Innovation Works. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.


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II. Hybrid Liner Design and Functionality Tests

A. Principle of the Hybrid Liner Design The advanced integration of an active device inside the passive lining will ensure both: Preservation of the

liner’s performance and application of active measures for additional noise reduction at selected tonal components by means of active noise control (ANC). The actuators will be classically arranged in a number of R rings around

the perimeter of the duct/intake. Each ring carries N actuators. This is a common ANC setup, where selected propagable modes of the noise field are targeted to be reduced. The number of actuators and actuator rings depends on the noise field, it’s characteristics, mainly on the propagable modes which have to be reduced.

The new approach described here is the integration of the active device into the acoustic liner and the complete covering of the active devices by a permeable sheet, such as so-called felt metal. The integration of the active device into the liner material (usually honeycomb material) will be demonstrated by the prototype and is described later. There will be no surface inhomogeneity allowed over

the hybrid duct area. The active devices can be driven electromechanically, electrodynamically, by hydraulics or pneumatically or by using the energy of the air flow itself (i.e. the actuators as aeroacoustic noise sources). Here, a piezoelectric mechanism was considered to be used for the test hardware in an earlier stage. But all those devices may have in common, that a special conical structure, designed for the dimensions of the liner will enhance the acoustic properties of the active device, for optimal sound propagation towards the permeable sheet and into the duct section. Those miniaturized acoustic horns are indicated in Fig. 1. Their use can reduce the necessary electrical power for driving the actuators.

B. Prototype Design & Manufacturing A prototype hybrid liner was designed and

assembled according to the principle given in Fig.1. A passive acoustic liner was established with a 35 mm thick layer of honeycomb material (Hexcel Composites, type CRIII-F40-5052-.0019-3.1N) in combination of so-called special acoustic absorbers, known as SAA4. The SAA had 25 mm diameter, 35 mm height, and were integrated at 560 per m2 into the honeycomb matrix by using core splice foam. A felt metal layer (Technetics Corporation, type D12-S3/1813) was glued upon to provide a homogeneous surface at the inner side of duct ring, leaving 0.8 m of free diameter for the later use in the large scale fan test rig5. This combination of honeycomb and SAA is characterized by a rather broadband absorption in

Figure 1. Principle design of the hybrid liner3. Sectional drawing of anactuator (schematic), embedded in a conventional acoustic liner.

Figure 2. Duct ring during liner integration. This duct ringleaves a free diameter of 800 mm and a length of 620 mm. Due to the large flanges for installation in the large scale test rig, theusable length for applying the hybrid liner is 550 mm.


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the desired frequency range of interest4. Figure 2 shows the duct section during it’s manufacturing. The flanges for the installation in the test rig’s test section are already mounted. The pattern of the later SAA and actuator positions is visible inside the duct. Details of the liner surface are shown in Fig. 3. The positions of the actuators and the SAA

can be well identified due to colour patterns from the manufacturing process. Additional N = 63 actuators were positioned in R = 3 rings along the perimeter of the duct section, staggered by a rotational angle between the rings of 5.7°. The actuators were integrated into the liner in the same way that is shown in Fig.1, providing a perfect homogeneous liner surface. Figure 4a shows the loudspeakers, which finally were selected for the rig tests and already prepared for integration. Preliminary tests with special piezoelectric driven actuators showed their capability to achieve the desired SPL for the ANC application. The actuators’ dimensions (140 mm length, 100 Gramm weight) were optimized regarding SPL, size, and weight. But regarding the costs for piezoelectrics, power amplifiers, and their integration into the duct section, heavier but cheaper commercial HF drivers “Eminence APT-50”6 were selected, since weight did not matter for the large scale tests. The

loudspeakers were combined with aluminum cones of 25 mm opening diameter, which corresponds to the SAA’s dimension. These miniature horns were made of a simple conical shape.

C. Functionality Test Before starting with the rig tests, the basic acoustical characteristics of the hybrid liner had to be assessed. The

achievable SPL for the integrated active part was of main interest, because up to now only free field measurements of the free and undisturbed loudspeaker were available. But inside the liner the noise propagation will be different and it was not known if the targeted 120 dB could be reached after integration. To evaluate the SPL inside the duct section, one loudspeaker was fully integrated and microphones were arranged as indicated in Fig. 5.

Figure 3. Detail view of the hybrid liner surface inside the assembled duct section.

a)b)

a)b)

Figure 4. Commercial electrodynamic HF drivers equipped with aluminum horns and measured SPL. The SPL were measured with tonal excitation at 0.5 m distance in an anechoic room. The dependency on the electricalpower consumption of the loudspeaker is evident. Remarkable is the result that a loud speaker at 17 Watt powerproduces almost the same or lower SPL than the actuator at 10 Watt power, but equipped with the miniaturehorn.


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The HF drivers were capable to generate a sufficiently high

tonal SPL of above 120 dB inside the duct section in the desired frequency range from 1,500 to 2,700 Hz. Those laboratory tests also showed that a HF driver of the selected type - here also called loudspeaker or speaker - without this conical miniature horn, was consuming 17 Watt of electrical power but produced the same or even lower SPL than a speaker at 10 Watt electrical power but equipped with a horn (Fig. 4b).

The absorption characteristics for the passive part of the

hybrid liner was evaluated in a simplified test, where the hybrid liner duct was exposed in the laboratory to an external white noise sound field, generated by a loudspeaker installed outside of the duct section. The following measurements were performed for two principle conditions:

The liner surface was covered by a reflective foil to simulate an acoustic hard wall, exposed to the exterior white noise source.

The pure liner was exposed to the exterior white noise source.

The SPL for the two different situations was measured

inside the duct section at different microphone positions which were distributed over the cross section, but also close to the duct wall (Fig.5). The direct comparison of the measured SPL yields an estimated absorption coefficient α ~ 0.9 for the wall region, which will be responsible for the suppression of the propagable modes.

The functionality tests were carried out by using simplifications, such as the lack of flow, which again will

change the properties compared to the static situation. But the derived numbers gave a first rough idea about the hardware’s quality before entering the large scale test.

III. Testing a Hybrid Liner Prototype

A. Test Setup For the large scale tests, the hybrid liner ring was integrated into the duct system of a multistage two shaft

compressor test facility at the DLR Institute of Propulsion Technology at Cologne, Germany. Different operation conditions7

were tested at selected fan speeds with the rpm 2570 min-1, 3150 min-1, and 3225 min-1. The corresponding frequencies 1885 Hz, 2310 Hz, and 2365 Hz for those rpm were the targets during the ANC experiments, where a frequency of about 2300 Hz is related to 2xBPF for a modern aero-engine at approach. This was the main design point of the hybrid liner. In the ANC experiments only modes were selected for cancellation which were able to

propagate inside the rig duct and thus were able to contribute significantly to the radiation into the free field. The free field was detected inside the acoustically treated settling chamber (d = 8 m, l = 18 m) of the test facility8 with a movable microphone antenna7,8, consisting of 40 microphones and providing a noise map of a hemisphere of 6 m in diameter around the inlet area. When rotating the antenna in steps of e.g. 10°, a total number of 760 microphone positions on this hemisphere was covered.

Figure 6 shows the principle of the overall test setup, which is roughly sketched in Ref. 9. The SPL were measured inside the intake duct of the fan by flush mounted microphones as well as in the far field in front of the air intake. The ANC test setup was already described in detail, e.g. in Ref. 7 and 8, where control algorithm and

Table 1. Design parameters of the used UHBR Fan 8.

rotor blade number 22 stator blade number 38 hub-to-tip ratio at the rotor 0.275 shaft speed at design point 7846 rpm blade tip speed 330 m/s relative Mach number at blade 1.05

Figure 5. Simplified setup for testing the absorption coefficient of the liner wall. Twoconfigurations were tested, one with the hybridliner surface, an other with a reflective foil, covering the liner. The duct’s interior wasexposed to white noise from a source, installed outside of the duct section.


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analysis method were in the focus of investigation. In Fig. 7 the hybrid liner is already installed in the test rig duct system.

Figure 7. Suction duct in test rig with installed habrid absorber duch section. The hybrid absorberduct was installed directly in front of the fan rotor. The visible details on the liner surface are related tothe positions of the actuator systems and the special acoustic absorbers and are only created by opticaleffects from the manufacturing process.

Figure 6. Principle test setup of the hybrid liner with ANC system, integrated into the suction duct of the fan test rig. The error microphones for the ANC tests were positioned in a separate microphone section. The far field ofthe radiated fan noise was mapped on a hemispherical surface, using a microphone antenna inside the settlingchamber.


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B. Test Results The SPL inside the suction duct was recorded at the error microphone positions during all operated liner and fan

conditions, also for the baseline test where the hybrid liner was replaced by a hard walled duct section. The flow velocity was at about 85 m/s. The electric power consumption of each actuator was limited to 2 Watt by the ANC controller since 63 actuators not necessarily needed to be driven at full power. Examples for the interior SPL are shown in the diagrams of Fig. 8, whereas the far field noise is mapped in Fig. 9 and Fig. 10.

Figure 8. Test results for the tonal component at 2xBPF, measured inside the suction duct. Figure 8a shows the 2xBPF tone at 1885 Hz, purely influenced by the liner’s passive effect (blue curvewith liner, red curve hard wall duct without liner). Figure 8b is the 2xBPF tone near 2310 Hz (greencurve with ANC, blue curve without ANC, both in the presence of the liner).

Figure 9. Test results7,8 from far field measurements in a fan test rig, fan at 3150 rpm.. Figure 10a shows the far field noise image, measured on a hemispherical surface over the fan intake and representing the primary noise field using the hard walled duct section without any acoustic treatment. Figure 10b is a far field noise map for the same hemisphere in the presence of the liner, but with deactivated ANC.


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Regarding the pure passive performance of the hybrid liner section compared to a hard walled baseline duct, the

interior broadband noise inside the duct was reduced by 3 - 5 dB. For the 2xBPF tonal component a reduction of about 9.5 dB was observed (Fig. 8a). Enabling the active part of the liner during the ANC experiment enhances the tonal reduction by additional 5 - 6 dB (Fig. 8b).

The influence of the hybrid liner configuration on the far field noise, which was radiated from the fan intake into the settling chamber of the test facility, is evident from the noise maps. The overall noise reduction was estimated from the available microphone antenna data. Compared to the baseline duct with fully reflective hard walls, the passive hybrid liner reduces the broadband sound power of the radiated noise approximately 5 dB while the active devices are still switched off. The activation of the ANC system additionally reduces the overall sound power by 3 - 4 dB. Those results demonstrate the functionality of the described hybrid liner concept.

IV. Summary and Conclusions A hybrid liner solution for noise reduction at aero-engine intakes was presented in this paper. This solution

consists of a combination of active devices, integrated into the passive lining of the intake. With an appropriate design a prototype hybrid liner was assembled and tested in a large scale fan rig. The acoustic effectiveness for the principle design of this liner type was experimentally demonstrated under realistic flow conditions for a model fan. The results confirm the performance of the passive acoustic liner by a reasonable broadband reduction in the relevant frequency range. Additional application of ANC devices further reduces the major tonal components. Finally it has to be mentioned that the application of the described hybrid liner configuration is not limited to intakes of aero-engines.

Figure 10. Test results7,8 from far field measurements in a fan test rig, fan at 3150 rpm. Figure 10a shows the far field noise image, measured on a hemispherical surface over the fan intake andrepresenting the primary noise field using the passive effect of the hybrid liner duct, i.e. in the presence of the liner but deactivated the ANC system (identical to Fig. 9b). Figure 10b is an example for a farfield noise map on the same hemisphere but with the liner duct active, using ANC for the here selectedradial mode (6,0). The structured pattern in Fig. 6b results from effects generated by the appliedmodal control7.


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Acknowledgments The partial financial support of the project NASGeT9 by the German Government within the national aeronautic

research program “LuFo” is acknowledged (funding reference number 20T0310A). The far field microphone data have been processed and provided by the German Aerospace Center (DLR).

References 1Smith, M.J.T., “Aircraft Noise”, Cambridge University Press, Chapter 4, 1989. 2Lissek, H., “Shunt loudspeaker technique for use as acoustic liner”, Internoise , Ottawa, Canada, 2009. 3Bauer, M., Krauspe, C., „Schallabsorber für den Strömungskanal einer Gasturbine“, German Patent and Trade Mark Office,

10 2009 005 163.5-13, applied 15 Jan 2009. 4Uhlig, R., Borchers, I.U., Drobietz, R., Möser, M., ”Analytical Modelling of Special Acoustic Absorbers”, AIAA-2004-

3012, 10th AIAA/CEAS Aeroacoustics Conference, Manchester, UK, May 2004. 5Deutsches Zentrum für Luft- und Raumfahrt e.V., „Fan für künftige Hochbypasstriebwerke“, Forschungsbilanz und

wirtschaftliche Entwicklung 2007/2008, Köln, December, 2008. 6Eminence Speaker LLC, Eminence APT-50 Super Tweeter, Technical Data, http://www.parts-express.com/pdf/290-530.pdf,

[cited 16 April 2010]. 7Zillmann, J., Tapken, U., “Tonal Noise Radiation from UHBR Fan - Active Control of Radiation Characteristic”, AIAA-

2009-3226, 15th AIAA/CEAS Aeroacoustics Conference, Miami, USA, May 2009.

8Tapken, U., Raitor, T., Enghardt, L., “Tonal Noise Radiation from UHBR Fan - Optimized In-duct Radial Mode Analysis”, AIAA-2009-3288, 15th

AIAA/CEAS Aeroacoustics Conference, Miami, USA, May 2009. 9Saueressig, G., M. Bauer, M., Holste, F., Neise, W., Haag, K., “Quiet Traffic – Working Group on Aircraft Noise and its

Focal Point Quiet Commercial Aircraft”, Euronoise, Tampere, Finland, 2006.

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