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AbstractNowadays different novel aviation engine

architectures are being investigated in order toprovide further fuel consumption benefit, noisereduction and emission decreasesimultaneously. The most popular are thesolutions taking into account further increase ofengine bypass ratio and Open rotorconfiguration design.

In 2000 leading aero engine companiesreturned to investigation of Open rotor schemeon a new level on base of 3D designmethodology. Experimental data received instatic conditions during tests of engineprototype JE-36 with Open rotor showed thatthe aircraft powered by the Open rotor enginecould comply with the ICAO Chapter 4requirements. Though some papers devoted tothe Open rotor noise issues, demonstratedoptimistic views on progress in terms of noisereduction, noise issue is still the main challengein practical realization of OR design.

CIAM has C-3A test facility equipped bythe counter rotating drive system for CR fanmodel experimental investigations and ananechoic chamber for taking acousticmeasurements in forward and rear hemispheressimultaneously.

Results of acoustic measurements ofunducted pulled type propfan and ducted pulledtype propfan of identical rotor blade designreceived at C-3A test facility with the anechoicchamber are represented. It is shown that in alloperating conditions under study the Open rotornoise spectrum contains only discrete

components at blade passing frequency of firstand second rotor and discrete components atsummary frequency BPF1+BPF2.

Comparison of ducted and unductedcounter rotating fan models noise spectradiscovered significant difference, especially interms of broadband noise components. Thus atapproach mode the ducted counter rotating fanmodel broadband noise component is on 8-10 dB lower that the same of the Open rotor.However the most intensive tonal components ofboth fan models (BPF1+BPF2) turned out to bepractically identical.

NomenclatureCRF – counter rotating fan; (биротативныйвинтовентилятор),CROR – unducted CRF (Open Rotor);GTF – geared turbofan;BPF1 – blade passing frequency of first rotor;BPF2– blade passing frequency of second rotor.

1 IntroductionNowadays different novel aviation engine

architectures are being investigated in order toprovide further fuel consumption benefit, noisereduction and emission decreasesimultaneously. The most popular are thesolutions providing further increase of bypassratio, such as Geared Turbofan engine (GTF),Counter Rotating Fan engine (CRF) andCounter Rotating Open Rotor (CROR).

COMPARISON OF DUCTED AND UNDUCTEDCOUNTER ROTATING FAN MODEL NOISE

Yuri Khaletskiy, Victor Mileshin**CIAM, 2, Aviamotornaya str. 111116, Moscow, Russia

Keywords: noise, counter rotating fan, open rotor, combination frequency

KHALETSKIY Y., MILESHIN V.

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Previously, the CRF models performanceswere investigated within the scope of CRISPprogram (Counter Rotating Integrated ShroudedPropfan) [1, 2]. The fan of 400 mm diameter hasbeen driven from the refrigerating turbine. Bymeans of the differential reduction gearbox itsrotors were counter rotated with identicalcircumferential speeds of about 227 m/s.However this program has been closed due toexcessively high sound pressure levels (SPL)generated during operation.

Schemes of turbofan engine with counterrotating fan (CRF) have been attracted attentionof Russian aircraft engine designers for a longtime. So, CRF engine NK-93 had beendeveloped and passed bench tests in designbureau named after N.D. Kuznetsov (Samara,Russia). Its bypass ratio was equal to 16.However in those years the realization of thisengine has not been finished.

The subsequent development of enginedesign technique has caused a renewed interestto CRF architecture. Within the frame ofEuropean program VITAL the experimentalresearch of acoustic performances of three CRfan models has been carried out. The mainresults of this project have been stated in [3-5].

Particularly it was demonstrated that thetypical CRF noise spectrum differedsignificantly compared with the typical singlerotor fan (SRF) noise spectrum by presence oflarge quantity of tonal components at the bladepassing frequencies and their harmonics of bothfan rotors and rotor of the booster stage. Also,the CRF noise spectrum includes combinationfrequencies due to the interaction of rotatingrows. At the same time these discrete noisecomponents at combination frequencies are themost energy carrying. Hovewer whencomparing noise levels of an airplane poweredby the conventional turbofan and CRF engines itwas revealed, that CRF engines provideddecrease in cumulative noise level on 7 EPNdB.

Fig. 1. Unducted Counter Rotating Fan modelCRTF2А

Usually tests of CROR models are carriedout in wind tunnels, where the fan modeloperates in conditions of incident flow with thespeed corresponds to the real flight conditions[3-4]. It is not possible to conduct such tests inconditions of C-3A test facility. Moreover, thefan model was designed only for a nacelleconfiguration, i.e. angles of attack were far fromthe optimum. Therefore the results of the givenstudy have a preliminary character, interestingin terms of detecting correlations between noiselevels of the CROR and CRF main tonal noisesources.

2 Basic features of fan acousticperformances

2.1 CROR model noiseThe CROR tonal noise includes three

groups of components: blade passing frequencyof the first rotor, blade passing frequency of thesecond rotor and rotors interaction noise. Theblade passing frequencies of rotors is defined byequations:

F1m = mZ1f1 и F2n = nZ2f2,where Z1 and Z2 – blades count, f1 and f2 -

rotors rotational speeds, m and n - arbitraryintegers. In its turn, the noise spectrum of rotorsinteraction is defined by the relationship:

Fmn = mZ1f1 + nZ2f2

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COMPARISON OF DUCTED AND UNDUCTED COUNTERROTATING FAN MODEL NOISE

where m and n – arbitrary integers. Note,that in the last equation one of the integers maybe equal to zero. In this regard, BPF noise andinteraction noise separation generally representsnot a trivial task. This challenge, however, isgreatly facilitated by the fact that due todifferent physical nature the noise of rotationand the interaction noise have differentdirectivity diagrams. Rotation noise directivitydiagrams have lobed shape, while interactionnoise directivity diagrams may have severalpeaks, some of which can be directed at a smallangle to the CROR axis.

The relative contribution of variouscomponents to CROR noise depends on theoperating mode. It is known, that rotorsinteraction noise dominates at modern CROR atTake-off. Even at frequencies which aremultiples of BPF the noise radiation may havethe directivity diagram typical for theinteraction noise. On the contrary, at Cruisemode rotation noise dominates, the basiccontribution is given by the buzz-saw noisegenerated on blades operating at transonicspeed.

The important feature of CRF is thepossibility of interaction noise generation atfrequencies lower, than rotor BPF frequencies.In the case, if rotation speeds of both rotorshave common multiple period the lower limitfrequency at which the fan radiates maycorrespond to this period in theory (in realitythis frequency usually is usually slightly higheras it is simple to demonstrate on a specificexample). If there is no common multipleperiod, then in some cases the spectrum maycontain harmonics of indefinitely lowfrequencies. Low frequencies radiation couldlead to the design complexity of the aircraftnoise reduction system. So, there is a commonpoint of view according to which rotationalspeeds of rotors should be identical.

In this case the lowest frequency is equal tothe rotational speed multiplied by number ofspatial periods of the fan keeping within 2π. Butit is necessary to note, that generally [3.2] theinteraction noise at low frequencies for whichone of the numbers “m” or “n” is negative(according to the equation above), should be

essentially more quietly than the harmonicsnoise for which both numbers are positive.

In current experimental campaign the fanmodel rotational speed changed from 24% up to100% in regard to the nominal mode.

As examples Fig. 2 and 3 present thenarrow-band spectra of the CROR model atoperating modes corresponding to Flyover andApproach in terms of thrust in the direction ofmaximum radiation in the rear hemisphere. It isclearly seen, that only discrete components atfrequencies BPF1+BPF2; BPF1 and BPF2 arereally significant in the noise spectrum.

Frequency, Hz

Fig. 2. CROR model narrowband noisespectrum at Flyover in direction 110°

Frequency, Hz

Fig. 3. CROR model narrowband noisespectrum at Approach in direction 110°Figures 4-6 show directivity diagrams of

the most intensive tonal noise components forthe CROR model at 0.84 N (maximum mode),0.75 N (Flyover) and 0.54 N (Approach).Unlike the ducted CRF having two peaks infront and rear hemispheres, the CRORdirectivity diagram has only one peak at 100-110 degrees. The level of the maximum in therear hemisphere is on 5 dB or more higher than

SPL,

dB

SPL,

dB

KHALETSKIY Y., MILESHIN V.

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the levels in the front hemisphere (10…70 deg)for all three considered modes.

Fig. 4. Directivity diagrams of the mostintensive fan tonal components of the CROR

model at maximum mode 0.84 N

Fig. 5. Directivity diagrams of the mostintensive fan tonal components of the CROR

model at 0.75 N (Flyover)

Fig. 6. Directivity diagrams of the mostintensive fan tonal components of the CROR

model at mode 0. 45 N (Approach)

2.2. Ducted fan model noisePreviously, the ducted CRF model

acoustic performances have been investigatedexperimentally and theoretically [5-10]. It has

been established, that the main CRF modelnoise was generated by its two rotor wheels (R1and R2), rotating in opposite directions, and onerotor (R3) of the booster stage rotating with R2turns. Levels of tonal noise components withfrequencies mf1 and nf2 (m> 0 and n> 0) andtheir harmonics turned out to be lower incomparison with combination components withfrequencies f = mf1 + nf2 (m> 0 and n> 0).

The CRF noise spectrum is characterisedby presence of big quantity of discrete noisecomponents with blade passing frequencies ofthree rotors and their harmonics and also withcombinational frequencies including itsinteraction with basic tonal noise sources.Exactly these discrete noise components withcombination frequencies determine the CRFacoustic response within the whole frequencyrange.

2.3. CRF and CROR noise spectrumcomparison

Since we compare noise spectrum ofdifferent types fan the impulse of the exhaustjet, proportional to the engine thrust, wasselected as the criterion of operating modesequivalence. Figure 7 represents measuredvalues of the exhaust jet impulse versus shaftrotational speed.

Fig. 7. Dependance of jet impulse versusoperating mode (N)

From the plot on figure 7 it is clearly seenthat operating modes similar in terms of jets

Impu

lse,

H

SPL,

dB

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COMPARISON OF DUCTED AND UNDUCTED COUNTERROTATING FAN MODEL NOISE

impulse lie within the range of 4-7 kN, whichcorresponds to Approach and Flyover modes.

Figures 8-11 represent ducted (CRTF2A)and unducted (CROR) models noise spectra inApproach and Flyover. As it was mentionedabove, the tonal components at BPF frequenciesof R1 and R2 and at sum frequency dominate inthe spectra of the CROR model at bothoperatimg modes. For the ducted CRF modeltonal components at BPF frequencies BPF1 andBPF2 are essentially lower as compared withthe sum frequency BPF1+BPF2, though at thesame time exceeding on 10 dB the broadbandnoise. Dominant tonal components of the ductedCRF model noise at sum frequencyBPF1+BPF2 also have quite significantharmonics, mostly the first, i.e. at the frequency(2BPF1+2BPF2). But in Flyover mode in therear hemisphere there are significant tonalcomponents at second harmonic(3BPF1+3BPF2) and at combination frequency(3BPF1+2BPF2) (Fig. 11). The broadband noiseof the unducted CROR model turned out to bemuch higher, on average by 15 dB, as comparedwith the ducted CRF model. This difference inbroadband noise component of the consideredfan models can be attributed to the lack ofincident airflowing of the unducted CRORmodel simulating the real flight conditions

Frequency, HzFig. 8. CRF and CROR noise spectrum atApproach in front hemisphere

Frequency, HzFig. 9. CRF and CROR noise spectrum atApproach in rear hemisphere

Frequency, HzFig. 10. CRF and CROR noise spectrum atFlyover in front hemisphere

Frequency, HzFig. 11. CRF and CROR noise spectrum atFlyover in rear hemisphere

ConclusionsComparison of ducted and unducted

counter rotating fan models noise spectrademonstrated that both models generate themost powerful tonal noise at combinationfrequency BPF1+BPF2 and its harmonics. Inaddition, spectra include tonal components atblade passing frequency of first and secondrotor.

SPL,

dB

SPL,

dB

SPL,

dB

SPL,

dB

KHALETSKIY Y., MILESHIN V.

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Unlike the CRF model, the CROR modelnoise directivity diagrams have showedsignificant exceeding of noise levels in the rearhemisphere relative to the forward hemisphereon 5-8 dB.

Apparently considerable exceeding of thebroadband noise component of the CRF modelas compared to the CROR model is due to themethodological measurement conditions in theanechoic chamber without incident airflow.

References[1] Holste F., Neise W. Experimental determination of

the main noise sources in a propfan model byanalysis of the acoustic spinning modes in the exitplane. 14th A1AA Aeroacoustics Conference, Aachen,Germany, Paper DGLR/AIAA 92-02-138, 1992

[2] Holste F., Neise W. Acoustical near fieldmeasurement on a propfan model for noise sourceidentification. 16th AIAA Aeroacoustics Conference,Munich, Germany, Paper CEAS/AIAA-95-178,1995.

[3] Woodward R. Noise of a Model High SpeedCounterrotation Propeller at SimulatedTakeoff/Approach Conditions (F7/A7). 11th AIAAAeroacoustics Conference, Sunnyvale, California,AIAA-87-2657, October 19-21, 1987.

[4] Khalid S.A., Wojno J.P., Breeze-Stringfellow A. andothers. Open Rotor Designs For Low Noise AndHigh Efficiency. Proceedings of ASME Turbo Expo2013, June 3-7, 2013, San Antonio, Texas, USA,GT2013-94736

[5] Khaletskiy Y., Mileshin V. and Shipov R. Acoustictest facility for aero engine fans. Acoustics 2008Paris.

[6] Khaletskiy Y. Results of C-3A Test FacilityDevelopment and CRTF1 Acoustic Features. Book ofAbstract. VITAL Final Workshop, Budapest, 9-10March 2009, p. 60

[7] Khaletskiy Y., Mileshin V., Shipov R. Study ofcounter rotating fan noise at anechoic chamber.Proceeding of the 8th European Conference on NoiseControl «EuroNoise», 2009, Edinburgh, Paper 0268

[8] Khaletskiy Y, Shipov R, Mileshin V and Povarkov V.“Experimental Study of the Counter Rotating ModelFan Noise”, Ecological Problems of Aviation,Proceedings of Central Institute of Aviation Motors#1347, Moscow, 2010, pp 76-83.

[9] Khaletskiy Y, Mileshin V, Talbotec J, Nicke E. Studyon Noise of Counter Rotating Fan Models at CIAMAnechoic Chamber. Proceeding of the ICASConference, Paper 897, 2012, Brisbane, Australia.

[10]H. Brouwer, “Analytic description of the noiseradiation from single- and contra-rotating propellers”,ICAS2010-5.2.3, 27-the International Congress of the

Aeronautical Sciences, 19 - 24 September 2010,Nice, France.

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