NLR-TP-2001-132

Mode detection with an optimised arrayMode detection with an optimised arrayMode detection with an optimised arrayMode detection with an optimised arrayin a model turbofan engine intakein a model turbofan engine intakein a model turbofan engine intakein a model turbofan engine intakeat varying shaft speedsat varying shaft speedsat varying shaft speedsat varying shaft speeds

E.R. Rademaker, P. Sijtsma and B.J. Tester

NationaalNationaalNationaalNationaal Lucht- en Ruimtevaartlaboratorium Lucht- en Ruimtevaartlaboratorium Lucht- en Ruimtevaartlaboratorium Lucht- en RuimtevaartlaboratoriumNational Aerospace Laboratory NLR

NLR-TP-2001-132

Mode detection with an optimised arrayMode detection with an optimised arrayMode detection with an optimised arrayMode detection with an optimised arrayin a model turbofan engine intakein a model turbofan engine intakein a model turbofan engine intakein a model turbofan engine intakeat varying shaft speedsat varying shaft speedsat varying shaft speedsat varying shaft speeds

E.R. Rademaker, P. Sijtsma and B.J. Tester*

* Rolls-Royce plc, United Kingdom

This investigation has been carried out partly under a contract awarded by the EuropeanCommission, contract number BPPR-CT-0473, and partly as part of NLR’s basic researchprogramme, Working Plan Number A.1.D.2.

This report is based on a presentation held at the 7th AIAA/CEAS AeroacousticsConference, 28-30 May 2001, Maastricht, The Netherlands.

The contents of this report may be cited on condition that full credit is given to NLR andthe authors.

Division: Fluid DynamicsIssued: 22 March 2001Classification of title: Unclassified

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Contents

I. Introduction 3

II. Theory 3Optimisation of the sparse array 3Spectral analysis on data acquired at sweep conditions 4

III. Experimental Set-up 5Test facility 5Pressure transducers of the intake array 5Data acquisition system 5Test programme 6

IV. Results 6Subsonic fan 6Supersonic fan 6Modal spectra at 1 BPF 6

V. Conclusions 6

Acknowledgement 7

References 7

11 Figures

(12 pages in total)

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AIAA 2001-2181

MODE DETECTION WITH AN OPTIMISED ARRAYIN A MODEL TURBOFAN ENGINE INTAKE AT VARYING SHAFT SPEEDS

Edward R. Rademaker*, Pieter Sijtsma+

National Aerospace Laboratory NLR, 8300 AD Emmeloord, The Netherlands

and Brian J. Tester±

Rolls-Royce plc, DE24 8BJ Derby, United Kingdom

Modal measurement techniques in engine intakes have been used previously to analyse thegenerated fan noise. A proven method is to use a wall-mounted array of Kulite transducers andoperate the (model) turbofan under constant shaft speeds. A drawback of this method is the largenumber of (expensive) microphones and acquisition channels needed to obtain complete m-modespectra at high engine orders. Furthermore, to get a full scan of the m-mode spectra as a function ofshaft speed, many measurements are required. The issue of the large number of microphones wasaddressed by using a sparse array instead of an equidistant array. An array optimisation technique,similar to a technique used for the design of phased microphone arrays for sound sourcelocalisation, was used to define such a sparse intake array. This array consists of 100 Kulites and isable to determine without aliasing the modal spectrum from m = −−−−79 to m = ++++79, which isappropriate to determine the modal content up to 3 BPF of a modern turbofan. This array wastested in a Rolls-Royce model fan rig at Ansty as a part of the RESOUND project. A new digitaldata-acquisition system made it possible to simultaneously and continuously record the Kulitepressure data as the engine speed was varied continuously from idle to maximum speed or viceversa, with each acceleration/deceleration lasting for a period of 9 minutes. Time histories of theKulites were processed giving power spectra of the engine orders, which revealed the rotor lockedtonal components. For each rotor revolution, a Discrete Fourier Transform was applied and, afteraveraging over a number of revolutions, the m-mode spectra were determined. In this way, a fullmodal scan with respect to shaft speed in a very limited testing time was obtained.

I. Introduction

The principal aircraft and engine manufacturers inEurope are facing increasing pressure to reduceaircraft noise levels. This arises both from thecommunity expectations of improved quality of lifeand from the need to compensate for the expectedgrowth in air traffic. Several research programmes arecarried out, one of which is the RESOUNDprogramme (Reduction of Engine Source Noisethrough Understanding and Novel Design led byRolls-Royce), which addresses the challenge ofreducing the turbomachinery noise at source. For thereduction of fan noise, the potential of severaltechnologies such as reduced fan tip speed, swept andactive stators was investigated.

*Research Engineer, Aeroacoustics Department, P.O. Box 153,e-mail: radema@nlr.nl+Research Engineer, Aeroacoustics Department, P.O. Box 153,e-mail: sijtsma@nlr.nl±Senior Research Scientist, Rolls-Royce plc, P.O. Box 31, e-mail: brian.tester@rolls-royce.comCopyright 2001 by the National Aerospace Laboratory NLR.Published by the American Institute of Aeronautics andAstronautics, Inc. with permission.

Tests were carried out in the Rolls-Royce model fanrig at Ansty. In addition to acoustic measurements inthe far field, modal measurements upstream of theintake liner with an optimised wall-mounted sparsearray of 100 Kulites were performed to quantify thenoise benefits. Due to the availability of a new, high-performance data acquisition system1 at NLR, modalmeasurements could be continuously carried outduring accelerations and decelerations of the fan.Within a data acquisition period of about ten minutes,a full scan of the modal content in the engine intakeas a function of shaft speed could be obtained. Theoptimisation of the sparse array, the modalmeasurement technique and some preliminary resultsare described in this paper.

II. Theory

The optimisation technique of the sparse array andthe spectral analysis technique for data acquiredduring engine acceleration and deceleration or‘sweep’ are separately described hereafter.

Optimisation of the sparse arrayThe modal content in the intake at a certain frequencycan be determined with a circumferential array of

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microphones. The circumferential pressure field isexpressed by

( ) imm

mp A e θθ

∞−

=−∞

= ∑ . (1)

Microphone locations are , 1,..,k k Nθ = . Modalamplitudes can be calculated according to

1

1 ( ) kN

imm k

kA p e

Nθθ

=

= ∑ . (2)

If only a single mode is present, in other words, if( ) ip e µθθ −= , then the array output is

( )

1

1k

Ni m

mk

A eN

µ θ−

=

= ∑ , (3)

which gives for m µ= the amplitude 1. For m µ≠a residue will be found depending on the array.

The traditional, equidistant array 2k k Nθ π=(for 1,..,k N= ) yields

2 ( ) /

1

1 Ni m k N

mk

A eN

π µ−

=

= ∑ , (4)

• ZllN,µmmA ∈+== for1 (aliasing),• ZllN,µmmA ∈+≠= for0 (no side lobes).

A practical solution to avoid aliasing is to take thenumber of microphones at least equal to two timesthe mode number, for which the first radial mode iscut-off. However, at higher engine orders, whenmany circumferential modes are propagating, thiswould require a large number of microphones.

As a spin-off from the development of phasedmicrophone arrays for sound source location2, adifferent approach was taken. By the use of a non-equidistant array, one can increase the modal rangefrom ( 1) 2 ( 1) 2M m M− − ≤ ≤ − with M N>and with the number of microphones N unchanged.This will be at the expense of the occurrence of side-lobes, which one can try to minimise. One couldeither minimise Am, equation (3), for

M m Mµ µ− < < + and m µ≠ or minimise thesummation J, which is applied here.

22

2, 1 1

2| | k

M M Nim

mm M m m k

J A eN

µθ

µ µ

+

= − ≠ = =

= =∑ ∑ ∑ (5)

This expression is independent of µ and can beminimised as a function of the parameters kθ . Sincethe derivatives of J can be evaluated analytically, thisminimisation could be done relatively quickly usingthe Conjugate Gradient Method.

The optimised array for 100N = and 159M =( 79 79m− ≤ ≤ ) is given in figure 1. The residualresponse in dB shows the presence of side lobes (Fig.2). Note that equation (2) is used to determine the

modal spectra, which implies that no additionalweighting functions are used (e.g. depending on thearc segment covered by the microphone). Themaximum side lobe value, which is equal to thedynamic range, is 0.1229 (-18.2 dB). So, the modalrange with 100 microphones has been increased at theexpense of the occurrence of side lobes. The dynamicrange may decrease somewhat further, when severaldominant modes are present. With four dominantmodes the value has decreased to 14.7 dB (Fig. 3).

In theory, it is possible to enlarge the signal/noiseratio by subtracting the side lobe spectrum of adominant mode from the total spectrum. This will beinvestigated further. Note that a precise knowledge ofthe Kulite sensitivities is then required.

Spectral analysis on data acquired at sweepconditionsFor the determination of fan noise, it is commonpractice to perform two types of acquisitions duringengine sweeps:

• A data acquisition with the sampling processbeing coupled to rotation of the shaft (alsodenoted as external clock data).

• A data acquisition with a fixed samplingfrequency governed by the internal clock of theacquisition system. (internal clock data).

In previous experiments, the so-called phase-lockedtime domain averaging technique was applied on theexternal clock data. The cross-power matrix wasestablished for the internal clock data. The latter datasuffer from the mismatch between the fixed timewindow of a FFT processing and the varying periodof one revolution of the shaft leading to spectralleakage. The amount of leakage is strongly dependenton shaft speed. Modal spectra can be determinedfrom both internal and external frequency spectra.

In the present experiment, data acquired with theinternal clock are used to both obtain the auto-powersand the simulated phase-locked time domainaveraged spectra (though also external clock datawere acquired for backup and for benchmarking thenew data acquisition system). The 1-p signal (onepulse per revolution of the shaft) is used to determinethe varying number of samples acquired perrevolution. The stepwise determination of the spectrais as follows:

Step 1The trigger times tj are determined (Fig. 4). Thesetrigger times are defined as the moment during oneperiod (revolution), when the (positive or negative)derivative of the 1-p signal reaches it maximum. Theperiod lengths are updated using the found values fortj.

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Step 2A Fourier transform for each channel and timeinterval is performed:

1

1,

1 1

2 ( ) exp 2j

j

tjk k

j nj j j jt

t tp s t in dt

t t t tπ

−

−

− −

−= − − −

∫ , (6)

where sk(t) is the time signal in channel nr. k. Sincethe number of samples in an interval 1j jt t −− is ingeneral not a power of 2, an FFT is not possible.Therefor, the discrete Fourier transform is simplyevaluated as:

−−

−=−

−

=∑

1

1

1max, 2exp)(2 max

jj

jv

v

v

kknj tt

tinsp ντ

πτν

, (7)

where max1 1 2 ....j jt tντ τ τ− ≤ < < < < .

Step 3:The spectra are averaged in two different ways, witha specified number of averages S (50 for sweeps)

"Complex average": ,1

1 Sk kn j n

jC p

S == ∑ (7)

and "Auto-power average": 21

,21

1 Sk kn j n

jA p

S == ∑ . (8)

By complex averaging, the non-periodic noise(broadband noise) will be filtered out.

Step 4:The modal amplitudes are calculated from thecomplex averaged spectra of N Kulites:

,1

1k

Nimk

m n nk

B C eN

θ

=

= ∑ , (9)

where θk is the angular position of the k-th transducer.

III. Experimental Set-up

Test facilityTests were carried out at the ANCTF (Ansty Noiseand Compressor Test Facility, Fig. 5) of Rolls-Royce.The large anechoic room has dimensions of46x37x12 m3. A model turbofan of 0.86 m diameterin the present test is mounted in a wall of theanechoic room at a height of about 4 m. The modelturbofan operated in intake configuration consists of aone stage compressor, with 26 wide chord fan blades,58 ESS vanes (Engine Section Stator) in the core ductand 58 swept OGV vanes (Outlet Guide Vanes) in theby-pass duct (Fig. 6). A linear SDOF (Single DegreeOf Freedom) segmented liner with 8 splices of 2 cmwidth is mounted upstream of the fan. Treatment Lover D ratio is 0.35. The optimised array of 100Kulites is located upstream of the liner. Air is drawninto the intake through an intake flare and aTurbulence Control Screen. The fan is driven by anelectric motor of 10 MW. Fan rotational speed varies

from 4000 rpm (idle) to 10,191 rpm (fan design speed= 100%).

Pressure transducers of the intake arrayModified Kulite XT-190 piezo resistive pressuretransducers of differential type were used in the array,which are the same as previously used for flight testson a Fokker 1003. Nominal range is 5 PSI. Thedimensions of the transducers are a body length of 40mm and a diameter of 3.86 mm. Temperaturecompensation is specified for a thermal sensitivityshift of less than 1% in the temperature range from –35 °C to 20 °C. The resonance frequency is above 80kHz, ensuring a flat amplitude characteristics andnegligible phase differences between the transducersin the 0 to 10 kHz frequency range. Combined non-linearity and hysteresis is within 1% of full-scale.

The differential transducers were used in “sealedgage mode”, the volume beneath the diaphragm wasclosed. The transducers were calibrated in an endplate of an impedance tube of 100 mm diameter usingpure tone sound of 1000 Hz with a B&K 4135 ¼ inchmicrophone as reference.

Data acquisition systemThe data are acquired with a new modular dataacquisition system1, so-called VIPER, made to thespecifications of DNW-LLF and NLR by GBMmBH. Maximum number of channels per front-end is48. The modular system can be extended to acomplete system of 960 channels with a sampling rate200 kHz per channel. DNW-LLF and NLR at presenteach have a system of 128 channels, consisting of twoVIPER units of 48 channels and 1 unit of 32channels. For the present test, a system with 176(3x48 and 1x32) channels was used. In addition to thesignals from the 100 Kulite array in the intake, OGVmounted Kulites were recorded. Total number ofmeasurement channels was 165. General features ofthe system are:

• Simultaneous acquisition on all input channels.• High speed over-sampling A/D converters.• 16 bits resolution.• Integrated transducers conditioning units and

amplifiers.• Maximum continuous sampling rate of 200 kHz

per channel with two hard-disks installed in eachfront-end. Maximum acquisition time onlylimited by total hard-disk capacity.

• Signal to noise ratio better than 70 dB at a gainof 1000x.

• Capability to on-line resample and store the dataon basis of a reference signal (tracking ratiotuner functionality included).

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During engine sweep operations with a maximumelapsed time of 9 minutes, a number of 165 channelswere recorded with an internal sampling frequency of44 kHz. Additionally, for benchmarking the newsystem, phase-locked acquisitions were done(external sampling) with 256 samples per revolution.Note that the results presented in this paper are basedon the internal clock data.

Test programmeModal measurements in the intake were taken atconstant and varying nominal shaft speeds accordingto table 1 (Tref = 288.15 °C). The fan was operated attwo aerodynamic conditions denoted by low and highworking lines. Maximum shaft speed is determinedby the maximum allowable bearing vibration levelsand dependent on ambient temperature.

Table 1: Overview of the shaft speed, 2 working linesConstant shaft speed Idle, 50, 60, 70, 75, 80,

85, 90, 95 and 100%Engine sweeps (660rpm/minute)

Decels max. to 40%Accels 40% to 90%

IV. Results

The results shown here are to illustrate the modedetection technique under decelerations of the fan.Benefits from the new noise reduction technologiesinvestigated in the RESOUND project are notdiscussed in this paper. Note that all results shownhere are measured at “high working line” conditionswith highly loaded wide chord fan blades andmeasured with the optimised circumferential Kulitearray upstream of the liner.

Subsonic fanAuto-power spectra of mikes 1, 2 and 3 at 60%reduced engine speed, acquired under steady andsweep conditions, are compared in figure 7. Thesound field in the intake of a subsonically rotating fanis dominated by the blade passing frequency andharmonics. The differences in broadband levelsbetween both acquisitions for individual microphonesare within the range of ±1 dB. The differences insound pressure levels at blade passing frequency andharmonics are larger, e.g. 3 dB for mike 1 at 1 BPFand 7 dB for mike 2 at 2 BPF. However, thesedifferences are of the same order of magnitude as thedifferences between the microphones 1 to 3 at steadyor sweep conditions. A comparison between themodal spectra at 1 BPF acquired from bothacquisitions at 60% engine speed gives a moreconsistent result (Fig. 8). Note that only a range of 20dB is taken, which corresponds to the dynamic rangeof the optimised array. The differences in levelsbetween a number of dominant modes is less than 1

dB (m = -2, 2, 10 and 11). For other modes thedifferences are somewhat larger (4 dB for m=-14 and2 dB for m = -6).

It is concluded that at subsonic conditions themeasurements under sweep conditions reveal similarauto-power and modal spectra at 1 BPF as thoseacquired under steady conditions.

Supersonic fanAuto-power spectra of Kulites 1, 2 and 3 at 80%reduced engine speed, acquired under steady andsweep conditions, are compared in figure 9. Thepresence of buzz saw noise is noted (for frequencieslower than engine order 10 and for frequencies near 2and 3 BPF). The spectra of all microphones are verysimilar. The modal spectra at 1 BPF show a goodagreement (Fig. 10). The differences between thedominant mode levels are less than 1 dB.

The new modal measurement technique undersupersonic conditions even reveal better results thanunder subsonic conditions. This is due to thedominant rotor alone field, where at each frequencyonly a few modes prevail.

Modal spectra at 1 BPFThe advantages of a measurement technique undersweep conditions can be further exploited whenmodal spectra at fixed frequency are determined as afunction of engine speed. This has been done for thedominant modes at blade passing frequency (Fig. 11).Note that besides dominant mode levels also the 100microphones averaged SPL at 1 BPF (denoted bySPL in the legend) and the summation level of thedominant modes (denoted by Total) are given. Forshaft speeds above 80%, the sound field at 1 BPF inthe intake is fully dominated by the rotor alone mode(m = 26). However at transonic and subsonicconditions (i.e. for engine speeds between about 50%and 75%), the sound field is fully dominated byscattered modes, which are generated, essentially, bythe interaction of the rotor-alone mode with thespliced liner. The mode spacing number is 8, whichequals the number of splices. Strong modal scatteringby splices and other intake inhomogenities was alsopreviously found from flight tests3. In the cause of theRESOUND project, an improved liner has been used.

V. Conclusions

As a spin-off from the development of microphonearrays for source location, an array optimisationtechnique was used to define a sparse intake array of100 Kulites capable to determine the modal contentup to 3 BPF of a modern turbofan. This arraymounted upstream of the intake liner was tested in aRolls-Royce model fan rig at Ansty as a part of the

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RESOUND project. The circumferential modalspectrum in the experiment ranges from mode –79 to+79 without aliasing. A new digital data-acquisitionsystem made it possible to simultaneously andcontinuously record the Kulite pressure data as theengine speed was varied continuously from idle tomaximum shaft speed or vice versa, with eachacceleration/deceleration lasting for a period of 9minutes.

Time histories of the Kulites were processed givingthe standard power spectra at the engine orders,which revealed the rotor locked tonal components.Instead of re-sampling, the Digital Fourier Transformwas used and m-mode spectra were determined. Inthis way, a full modal scan with respect to shaft speedin a very limited testing time was obtained, whichgave valuable data for the assessment of fan noisereduction means.

Further work on the development of an optimisedacoustic array is being considered. The modalmeasurement range can be enlarged by increasing thenumber of microphones and/or by selecting adifferent ratio between the modal range and thenumber of microphones (1.6 in the present

experiment). Furthermore, the suppression of sidelobes by improved post-processing software shouldbe further investigated.

Acknowledgement

This work was carried out within the Brite/Euramproject RESOUND.

References

1. Holthusen, H., Smit, H., “A new data acquisitionsystem for microphone array measurements inwind tunnels”, AIAA-paper 2001-2169, 7thAIAA/CEAS Aeroacoustics conference May2001, Maastricht, The Netherlands.

2. Sijtsma, P., “Optimum arrangements in a planarmicrophone array”, presented at the First CEAS-ASC Workshop: Wind Tunnel Testing inAeroacoustics, DNW, 5/6 November 1997.

3. Sarin, S.L., Rademaker, E.R., “In-flight acousticmode measurements in the turbofan engine inletof Fokker 100 aircraft”, AIAA-paper 93-4414

Fig. 1 The optimised circumferential array, 100 microphones, modal range –79 ≤≤≤≤ m ≤≤≤≤ +79

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m − µ

Fig. 2 Residual modal response function of the sparse array, dynamic range is 18.2 dB

m

Fig. 3 Modal response function with four dominant modes, dynamic range is 14.7 dB

jt

Fig. 4 Sketch of 1-p signal used to determine the period of one revolution of the shaft

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Fig. 5 The Rolls-Royce Ansty Noise and Combustion Test Facility (ANCTF)

100 Kulite Ring

Fig. 6 The 0.86 m diameter model fan rig

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0 26 52 78 104 130

Engine order

SPL

mike 1 steadymike 2 steadymike 3 steadymike 1 sweepmike 2 sweepmike 3 sweep

5 dB

Fig. 7 Comparison between measured auto-power spectra of microphones 1, 2 and 3 of the sparsearray, steady and sweep conditions, 60% reduced engine speed, high working line

-80 -60 -40 -20 0 20 40 60 80

Mode

Mod

e Le

vel

1 BPF steady1 BPF sweep

5 dB

Fig. 8 Comparison between measured circumferential modal spectra at 1 BPF, steady and sweepconditions, 60% reduced engine speed, high working line

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0 26 52 78 104 130

Engine order

SPL

mike 1 steadymike 2 steadymike 3 steadymike 1 sweepmike 2 sweepmike 3 sweep

5 dB

Fig. 9 Comparison between measured auto-power spectra of microphones 1, 2 and 3 of the sparsearray, steady and sweep conditions, 80% reduced engine speed, high working line

-80 -60 -40 -20 0 20 40 60 80

Mode

Mod

e Le

vel

1 BPF steady1 BPF sweep

5 dB

Fig. 10 Comparison between measured circumferential modal spectra at 1 BPF, steady and sweepconditions, 80% reduced engine speed, high working line

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4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 9000 9500

RPM

Mod

e Le

vel

SPLTotalm= 26m= 18m= 10m= 2m= -6m=-14

10 dB

Fig. 11 Dominant m-mode spectra at 1 BPF as a function of shaft speed