American Institute of Aeronautics and Astronautics
NOISE AND FLOW STUDIES OF COAXIAL JETS AT INCIDENCECraig Mead
Owen KenningQinetiQ Ltd, Farnborough, Hampshire, Great Britain
ABSTRACT
When an aircraft takes off and climbs away from theairfield, it flies with a significant angle of attackrelative to its direction of motion, so that the jetexhaust from the engine will experience a cross-flow.It is known that a jet in a cross flow will producenoise above that of a jet aligned with the flow. Ifprediction accuracy is to be improved, the magnitudeof the noise changes associated with coaxial jets in across flow must be quantified and understood.
Accordingly, a series of model-scale tests have beenperformed to establish noise effects of such crossflow on coaxial jets. A nozzle with bypass diameter116 mm and a geometry representative of modernhigh bypass ratio aeroengines was placed at 8°incidence to a cross flow. Noise measurements weremade in several azimuthal planes at flow conditionsrepresentative of modern aeroengines.
The results of these tests are presented: Significantnoise increases occurred as a result of the jet being atincidence to the flight-stream. These were observedto vary both with the polar emission angle, andazimuthally around the jet, and also with jetcondition. The increases are significantly larger thancan be explained by considering the reducedcomponent of the flight velocity in the direction ofthe jet axis.
To progress the understanding of these noise changes,steady-state Computational Fluid Dynamics studieshave been carried out to investigate the developmentof the jet flow and the changes to shear layerturbulence levels as a result of the cross flow.Significant increases in turbulent kinetic energydownstream of the jet nozzle exit were observed inthe presence of a cross flow. The results of thesestudies are presented and discussed in the light of theacoustic test results.
INTRODUCTION
BACKGROUND
Model-scale noise tests and static engine trials areusually performed in isolation from the aircraftstructure. However, installation of the engine on theaircraft can substantially effect the character of theradiated noise. Such ‘installation effects’ were firstidentified at the NGTE (National Gas Turbine
Establishment, now QinetiQ) in the mid 1970’s1,2,3,and these can seriously reduce the accuracy ofpredictions based on isolated models or engines.
A number of different mechanisms contribute to theoverall installation effect. The magnitudes of theseare dependent on variables such as the jet flowconditions and the engine and wing geometry. Forexhaust noise in an engine-under-wing configuration,reflection of the radiated noise by the wing andinteraction of the exhaust flow with the wing aregenerally considered to be the major contributors4,5.However, this assessment is largely based uponmodel-scale tests where the jet flow is aligned withthe direction of flight. This is potentially misleadingsince during takeoff, the aircraft, and hence exhaustflow, is generally at incidence to the direction offlight. Studies6 have shown that considerable noiseincreases can occur when jets are placed at incidencesof this order to a cross flow. While it is expected thatthe downwash from the wing could reduce the ‘real’incidence seen by the jet and go some way towardalleviating the problem, the magnitude of this is alsocurrently an unknown.
JETS IN CROSS FLOWS
It is known that a jet in a cross flow will producenoise above that of a jet aligned with the flow.Previous work6 has demonstrated that for a 300 m/s,800 K single-stream jet at 10° incidence to a 90 m/scross flow, noise increases over 3 dB OASPL (OverAll Sound Pressure Level) can occur. However, littleinformation is currently available with regard to thenoise produced by coaxial jets in cross flows.
The noise produced by a single-stream jet can becharacterised using relatively few parameters; forexample, the velocity, temperature and jet diameter.Consequently, correlation of a database is areasonable solution for predicting single-stream jetnoise. Indeed, prediction methods based on suchcorrelations are still more accurate than the analyticalsolutions currently available.
The situation is considerably more complicated forthe case of coaxial jets, because there are a largernumber of variables that need to be considered; forexample, the velocity, temperature and area of eachstream are all important parameters. This aloneconsiderably increases the complexity of the databaseand correlation required for noise prediction.
9th AIAA/CEAS Aeroacoustics Conference and Exhibit12-14 May 2003, Hilton Head, South Carolina
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Nevertheless, attempts have been made to developsuch correlations [e.g. Section 7 of Reference 7, andReference 8], with limited success. In addition, forreal engine geometries there are further, secondary,parameters, such as the distance the core nozzleprotrudes from (or sits within) the fan nozzle, and thesize of the centre plug, that must be considered.
Predicting the noise from coaxial jets at incidence bycorrelation requires the addition of two furtherparameters, the cross-flow velocity and angle ofincidence. An attempt to do so has been made inSection 7 of Reference 7, in which a noise increase isdefined which is added only to the low-frequency,mixed-jet component of the coaxial jet noise model,as follows:
(Eqn. 1)
Where: α is the angle of incidence in degrees,θ is the angle of the observer from the jetaxis in degrees, andMa is the flight Mach number.
However, this was derived from data from jets withlower bypass ratios and higher velocities than thosetypical of today’s aeroengines. It’s accuracy isdifficult to check on the basis of aircraft noisemeasurements, since the relevant noise changes aredifficult to separate from the flight effect and otherinstallation effects.
Nevertheless, if prediction accuracy is to beimproved, the problem must be revisited, and themagnitude of the noise changes associated withcoaxial jets in a cross flow must be quantified andunderstood. Accordingly, the main objective of theinvestigation described in this report was to establishthe magnitude of the noise increases arising from theincidence to the direction of motion of a jet from anozzle typical of current aeroengines.
The approach taken has been to examine the noisebehaviour of isolated coaxial jets in a cross flowexperimentally, together with a CFD investigation ofthe flow development.
ACOUSTIC EXPERIMENTS
TEST PHILOSOPHY
The nozzle chosen for this exercise wasrepresentative of a typical large aeroengine (BypassRatio ~6) with a bypass nozzle diameter of 116 mm.It is a separate exhausts configuration, with a bypass-to-core area ratio of approximately 3½. In order tosimplify data interpretation, the engine support
pylons were removed, so the nozzle was completelyaxisymmetric. A photo of the nozzle is presented asFigure 1.
Figure 1: Photo of nozzle used for acoustic tests.
The model was mounted with incidences 0° and 8° tothe flight-stream axis, the latter being an angle ofincidence typical of an aircraft during takeoff. It wasalso rotated about the flight stream axis so that noisemeasurements could be made in several azimuthalplanes. The angles of rotation, φ, used were 0°, 30°,90°, and can be thought of as an elevation angle, i.e.if the nozzle were mounted on an aircraft, φ = 0°would correspond to measurements made in thehorizontal plane containing the nozzle, φ = 30° tomeasurements made at ground level with the aircraft30° above the horizon (typical of the sidelinecertification point), and φ = 90° to measurementswith the aircraft flying directly overhead (typical ofthe flyover certification point).
The two jet flow conditions, shown in Table 1 (at ISAsea-level temperature), were chosen to berepresentative of the full-power takeoff and cutbackcertification conditions of a modern aeroengine.Three flight-stream speeds were chosen; 0 m/s (i.e.static), 70 m/s and 100 m/s.
Condition Vcore(m/s)
TTcore(K)
Vbypass(m/s)
TTbypass(K)
Takeoff 492 821 341 354Cutback 300 717 256 326
Table 1: Flow conditions
EXPERIMENTAL LIMITATIONS
The NTF flight stream diameter is 0.5 m, and the airsupply to the nozzle is situated at the centre of theflight stream. In order to achieve an angle ofincidence the air supply needed to be turned at somepoint upstream of the nozzle. Ideally, this would bewell upstream, to allow the flow to settle after beingturned and ensure a symmetric velocity profile acrossthe nozzle. Unfortunately, the small flight-streamdiameter requires that the turn be as close to thenozzle exit as possible. It is therefore likely that theflow from the nozzle will not be completelyaxisymmetric.
2
6.0180
8.15.0
−
=
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The source region of a jet extends some ten nozzlediameters downstream from the nozzle. However, inthe current set-up, the jet reaches the outside of theflight-stream after approximately 5 nozzle diameters,as shown in Figure 2. Since the characteristicfrequency of the noise produced at a point in a jet is afunction of distance from the nozzle, this effectivelylimits the lowest frequency of the creditable noisedata that can be obtained. For a static single-streamjet this lower limit would be approximately 3 kHz(equivalent to 120 Hz full scale), and this has beenused as the lower frequency limit in the subsequentanalysis. However, it must be recognised that for acoaxial jet in flight the characteristic frequency at 5nozzle diameters is somewhat higher, possibly ashigh as 10 kHz (250 Hz full scale) in this case.However, the problem will be alleviated somewhat bythe turning of the jet by the cross flow. Reducing themodel scale factor would decrease this lowerfrequency limit, but the upper frequency limit was setby the analyser at 40 kHz. This is equivalent to only1.6 kHz at full scale, and reducing the model scalewould decrease this still further.
Figure 2: Impact of the limited extent of the flight stream
TEST DETAILS
The tests were carried out in the Noise Test Facility(NTF) at QinetiQ Farnborough. The NTF is a verylarge anechoic chamber in which noise tests can beperformed on model-scale jet nozzles. Two silencedcoaxial streams of air are supplied to the model torepresent the core and bypass streams of the engine.The flows are heated, using LPG on the core and anelectric heater on the bypass, to represent the exactexhaust temperatures. A third stream is used tosimulate the effects of the aircraft’s motion.
During the tests, aerodynamic data, in the form ofmultiple pressure and temperature measurements,were acquired continually. These were immediatelycomputed to provide the jet conditions of the modeland are continually updated during the test.Calibrated orifice plates upstream of the modelenabled calculation of the mass flows. The accuracy
of the pressure measurements was maintained byregular calibration of the pressure transducers.
The noise measurements were made using amicrophone array with ½ inch Bruel and Kjaer(B&K) type 4133 microphones and type 2645 pre-amplifiers. These were mounted on masts at the sameheight as the model at a nominal distance of 12 m (40ft), at angles between 30° and 120° to the jet axis at10° intervals. The microphones were powered byB&K 2807 power supplies, with their output fed to abank of conditioning amplifiers under computercontrol. Each channel was then multiplexed into aNorwegian 830/831 analyser, which measures levelsin 1/3rd-octaves up to 40 kHz.
ANALYSIS DETAILS
The measured 1/3rd-octave sound pressure levelshave been corrected to a polar distance of 6 metresand lossless atmospheric conditions9. The data takenunder static conditions, (i.e. zero flight speed) werecorrected for the chamber background noise, whichincluded the electrical background noise of the dataacquisition systems. The noise levels andmeasurement angles for data taken under simulatedflight conditions were corrected for shear-layerrefraction10 and for the background noise of the flightstream. This correction assumes that the noise sourceslie on the wind-tunnel axis, which is not correct inthis case. However, the errors so introduced are likelyto be small compared to those arising from the moresignificant experimental limitations discussed above.An additional angle correction was applied to the datawhere necessary to correct for the angle between theflight-stream axis and the jet axis. Finally, by addinga Doppler frequency shift to the flight data, the resultsachieved are those that would be obtained if the jetwere actually in flight.
ACOUSTIC RESULTS
EFFECT OF TURNING THE FLOW
As previously discussed, the turning of the flow toachieve the required incidence occurred at a pointundesirably close to the nozzle exit. As a result, theflow from the nozzle is not expected to be completelyaxisymmetric. This is likely to cause a change in thenoise produced by the jet, which will varyazimuthally around the jet. To quantify this, the staticnoise data at the cutback jet condition have beenplotted in Figure 3 for each elevation angle φ. Eachplot shows 1/3-octave sound pressure levels (SPL)against frequency for each φ at a single emissionangle. Four representative emission angles (θj,defined relative to the jet axis) have been chosen; 50°,70°, 90° and 110°. Where data were not available atthese angles, the data at the closest available angleshave been interpolated on a linear basis.
~ 5 nozzle diameters
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Figure 3:Comparison of static test data for the cutback jet condition
To reduce inconsistencies in the data, an upperfrequency limit of 20 kHz has been applied.
It can be seen that noise changes up to 2 dB do indeedoccur. However, at this cutback condition thedifferences are generally small, and should not have asignificant effect on the subsequent analysis.
Figure 4:Comparison of static test data for the takeoff condition
The static data for the takeoff condition have beenplotted in a similar manner in Figure 4.
At this condition there are much more significantnoise changes of up to 4½ dB in the lower angles.This condition has a lower bypass-to-core velocityratio than the cutback condition, and this, togetherwith the fact that the greatest noise changes are seen
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at angles closest to the jet axis, suggest that theproblem lies with the hot core flow. The subsequentanalysis will mostly consider noise differencesassociated with increasing flight velocity, and, whilethe datum static noise levels will change for eachconfiguration, the noise differences should be lessaffected. It is likely that some small error will beintroduced however, particularly at angles near to thejet axis.
FLIGHT EFFECT AT ZERO INCIDENCE
Example results for the tests with the jet alignedalong the flight-stream axis are presented for thecutback condition, in the form of 1/3-octave SPLsagainst frequency, in Figure 5 (for just tworepresentative angles to the jet axis).
Figure 5:Effect of flight at zero incidence, cutback jet condition
It can be seen that the effect of flight is to reduce thenoise produced by the jet, as a result of the reducedshear between the jet and the ambient medium. Theeffect is largest at angles near to the jet axis.
The slight irregularities in the data at the highestflight stream velocity (Va = 100 m/s) are consideredto arise because the signal is close to the noise floor(i.e. the noise produced by the flight stream iscorrupting the signal from the jet). The noise
produced by the flight stream is greatest at lowfrequencies, and this is the reason that much of thelow-frequency data have been lost in simulated flight.
The results for the takeoff condition are similarlypresented in Figure 6.
Figure 6:Effect of flight at zero incidence, takeoff jet condition
Again, similar behaviour is observed here, with thenoise reductions in flight greatest at low angles to thejet axis. In this case however, it is also apparent thatthe flight effect increases with frequency. This isunlike the single-stream jet case, in which allfrequencies reduce by a consistent amount, and canbe understood by considering the aerodynamicstructure of a jet. At high frequencies, the noise isproduced close to the nozzle, in the bypass-to-ambient shear layer. The high-frequency noise is afunction of the relatively low speed bypass flow. Thelow-frequency noise, however, is produced welldownstream, where the bypass flow has mixed withthe hot fast core flow. The velocity of this flowregion is therefore greater than that of the bypassstream. The noise reduction in flight arises from thereduced shear between the jet and ambient fluid, andis a function of the relative velocity between the jetand the ambient medium. As the flight velocityincreases, the change in relative velocity is greater
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when the jet velocity is lower. The high frequencynoise produced by the low-speed bypass flowtherefore undergoes a greater noise reduction in flightthan the low frequency noise produced by the highervelocity mixed flow.
FLIGHT EFFECT AT 8° INCIDENCE, φ = 0°
Figure 7 shows the effect of flight on the noise for thecutback condition, when the jet is at 8° incidence tothe flight stream. The measurements are made here inthe horizontal plane, and the jet is exhausting directlydownward, i.e. φ is 0°.
Figure 7: Effect of flight on the jet at the cutback jet condition,α = 8°, φ = 0°
As for the zero incidence case, it is apparent that thejet noise is reduced in flight. However, the reductionobserved here is less than that for the zero incidencecase, by around 2½ dB.
Figure 8 shows the effect of flight on the noise for thetakeoff condition, again with the jet at 8° incidence tothe flight stream and φ equal to zero.
Effects are seen of similar magnitude to thoseobserved at the cutback jet condition.
Figure 8: Effect of flight on the jet at the takeoff jet condition,α = 8°, φ = 0°
AZIMUTHAL VARIATION OF THE FLIGHT EFFECT AT8° INCIDENCE
In order to reduce the data to a form that can be moreeasily assessed, Overall Sound Pressure Levels(OASPL) have been calculated over the frequencyrange 3 kHz to 20 kHz (equivalent to 120Hz to800 Hz at full scale). These have then been used tocalculate a noise increase associated with the crossflow at each nozzle elevation angle φ and eachemission angle. The results have been plotted inFigure 9.
It can be seen that for all emission angles except 90°,the maximum noise increase, of up to 6 dB, wasmeasured at an elevation angle of 30°. Of course, dueto the sparsity of the data, all that can be establishedis that the maximum noise increase occurs in theregion 0° < φ < 90°. This is significant, as the sidelinecertification measurement is generally made atelevation angles around 30° to 40°. Why an exceptionto the general trend should occur at θj = 90° is notclear; neither real effects nor inaccurate data havebeen ruled out.
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Figure 9: Azimuthal variation of the noise increase arisingfrom the cross flow
It is also apparent that the largest noise increasesoccur at the cutback condition. Whether this is aconsequence of the lower jet velocity, or the
increased bypass-to-core velocity ratio, cannot bereadily determined from these data.
These noise increases are considerably larger than the0.1 dB suggested by Equation 1. Nor can they beexplained by the reduced component of the flightvelocity in the direction of the jet axis, which canonly reasonably account for 0.3 dB. It is recognisedthat in a real aircraft situation, the downwash fromthe wing will have some alleviating effect, but itseems unlikely that this can account for all of theremaining noise increases. Unfortunately, it isimpossible to reproduce the downwash in the testsituation without introducing other installation effectsassociated with the presence of the wing.
It is possible that a turbulent wake is being shed fromthe nozzle due to its incidence to the flow. Previouswork11,12 has shown that turbulence shed near to a jetcan significantly increase the noise it produces.However, the situation here is not dissimilar to anengine nacelle at incidence to the flow, and so maynot be unrepresentative of the aircraft configuration.
One outstanding concern is that the measured noiseincreases might arise from the interaction of the jetwith the flight-stream shear layer, rather than theincidence effect being studied. It is difficult to rulethis out, but shear layer interaction noise might beexpected to be greatest when the jet is pointedtowards the microphones, i.e. at an elevation angle of90°, and this is not the case here.
It is worth observing at this point that the azimuthangle of the measurement point varies with emissionangle. Azimuth angle, θazi, is defined as the angle tothe microphone in the plane perpendicular to the jetaxis, as shown in Figure 10. The microphonearrangement used for the tests described here wasrepresentative of a level flypast in a flight-testsituation, where the aircraft flies past a microphonearray at constant altitude, but with a finite pitch angle.Because the jet axis is not in the same plane as themeasurement array, the azimuth angle will differbetween microphones. It is important that this isrecognised, because the effect being studied varieswith azimuth angle.
Figure 10: Definition of azimuth angle
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FLIGHT EXPONENTS
The reduction in jet noise from static to flightconditions can be written as follows13,7:
(Eqn. 2)
Where; Vj is the jet velocity,Va is the aircraft or flight-stream velocity,Ma is the flight Mach number,m(θj) is the flight exponent, and is a functionof emission angle θj.
When deriving the flight exponent from experimentaldata, significant differences have been observedbetween data from pure-jet rig tests and aircraftmeasurements14. The flight exponents calculated fromaircraft data are significantly lower, indicating thatthe noise reduction actually seen by an aircraft inflight is less than that expected from wind-tunneltests. This has been attributed to the fact that theinstallation effects associated with real aircraft havenot been properly separated from the data.
It was considered a worthwhile exercise therefore, toestablish how the incidence effects being consideredhere would reduce the flight exponent. Accordingly,the (1 + Ma cosθj) term in Equation 2 has beensubtracted from the calculated OASPL noisereductions. The result has then been plotted againstthe component of the jet velocity along the jet axis,and a straight line passing through the origin has beenfitted to the data. The gradient of this line then givesthe flight exponent. An example set of plots is given,for the zero incidence case, in Figure 11. Thecalculated flight exponents are listed in Table 2.
Inci
denc
e
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nent
at θ
j=50
°
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0°
0° - 7.8 6.1 4.9 4.48° 0 6.7 5.0 3.7 3.1
30 4.6 4.0 3.2 2.490 5.1 4.0 2.7 3.0
Table 2: Calculated flight exponents
The exponents for the zero incidence jet are inreasonable agreement with those previously obtainedfor a single stream jet in a wind tunnel13. Asexpected, the exponents at 8° incidence are lowerthan those obtained for zero incidence, but are not aslow as those obtained from aircraft measurements.The variation of the exponents with elevation anglenaturally reflects the variation of the noise increasediscussed previously.
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Figure 11: Calculation of flight exponents
CFD STUDIES
In order to better understand the noise changes, it isdesirable to know how the cross flow affects the jetdevelopment. This could be achieved by directmeasurement, but CFD techniques offer analternative (and cheaper) way of acquiring theinformation.
( )( )
+
−
=∆ ja
m
aj
j MVV
VLogOASPL
j
θθ
cos110 10
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Unfortunately, the available CFDstudy was aimed at a slightlydifferent aircraft/engine combination,however, the nozzle configurationwas similar to that used for theacoustic tests. Both are nozzles forbypass ratio 6 aeroengines, andalthough there are slight geometricdifferences, these should notsignificantly change the way that thecross flow interacts with the jet.There is also a 1º incidencedifference, the CFD study being for anozzle at 7º to the cross flow. Again,this should not significantly effectthe jet development, at least withinthe accuracy of this study.
COMPUTATIONAL APPROACH
Computational analyses were carriedout using the commercially availableunstructured Fluent v.5 CFD flowsolver15. This cell-centred finitevolume code was run steady-state tosolve the compressible RANS(Reynolds Averaged Navier-Stokes)equations. Turbulence closure wasachieved by using a k-ε model with arealizable constraint and wallfunctions were employed to resolvethe near wall flow behaviour.
Two CFD grids were constructed – one for the 0ºincidence case and the other for the 7º incidence case.Due to the 3 dimensional nature of flows at incidence,both CFD grids were constructed covering 180º in thejet azimuthal direction with a symmetry planeboundary condition. The grids were constructedusing flow aligned hexahedral cells designed tocapture the jet shear layer development. Thecomputational domains extended from x/DS=-1 tox/DS=50 axially and y/DS=25 to y/DS=35 radially.The datum co-ordinate position of (x/DS=0, y/DS=0)was set for the 0º incidence case where the secondarynozzle exit plane crossed the nozzle centreline; forthe jet at incidence case, the nozzle was rotated by 7ºabout this point i.e. the z-axis. Both grids containedapproximately 1.8 million nodes. Interestingly, it wasfound that convergence was much more difficult toachieve with the 0º incidence case than the 7ºincidence case.
RESULTS
The following results are for a single flow conditioncorresponding to a high-power takeoff for a modernaeroengine. Figure 12 shows the axial jetdevelopment of the jets for 0° and 7° incidence interms of axial development of velocity and turbulent
kinetic energy. These results show that the cross flowintroduces an asymmetry into the jet developmentthat most apparent in the turbulent kinetic energy.The blockage of the jet causes the ambient flow onthe underside of the jet to be accelerated slightly. Inthe initial region of the jet, closest to the nozzle, thisresults in a reduction in shear between the bypass andambient flows, leading to a reduction in turbulentkinetic energy (and by inference, reduced noiseproduction in this region of the jet).
However, this reduced shear also results in areduction in the spreading rate of the jet, so that,beyond the end of the potential core, the velocitygradient across the jet radius is higher than it is forthe zero incidence case. This leads to an increase ofapproximately 10% in turbulent kinetic energy in thisportion of the jet. Since this region corresponds to thenatural region of peak turbulent kinetic energy (andby inference, peak noise production), the net effectmay be expected to be a noise increase – as, indeed,was observed in the acoustic tests.
From Lighthill16 we can deduce that the acousticpressure is proportional to the u2, where u is thefluctuating velocity (assuming isotropic turbulence).Since turbulent kinetic energy is also proportional tou2, a simple calculation would indicate that the soundpressure level should increase by ~1.6 dB for a 10%
Velocity
(m/s)
Ds : 0 2 4 6 8 10 12
Turbulent Kinetic Energy
(m2/s2)
Ds : 0 2 4 6 8 10 12
Figure 12: Axial flow development
10
American Institute of Aeronautics and Astronautics
increase in jet turbulence.This is, perhaps, lower thanexpected given the acousticresults.
Figure 13 shows slicesthrough the jet at variousaxial stations, takenperpendicular to the mean jetflow direction. It is apparentthat the jet is considerablydistorted by the cross flow,developing a horseshoe shapein the downstream regions.Close analysis of the CFDresults has established theformation of a pair of largecounter-rotating vortices onthe upper side of the jet.
This deformation of the flowwould be expected to alterthe radiation of the noisefrom the jet, particularly atangles close to the jet axis.This may explain some of thenoise changes seen in theacoustic test results.Quantification of the effect ofthis asymmetry on the noiseis not really possible usingsimple techniques. Accurateprediction would require the use of computationalmethods such as Linearised Euler solvers.
CONCLUDING REMARKS
A series of model-scale tests have been performed toestablish noise effects of cross flow on coaxial jets. Arepresentative bypass ratio 6 nozzle was used at flowconditions representative of modern aeroengines. Anincidence to the cross flow of 8°, typical of an aircrafttakeoff, was used, and measurements were made inseveral azimuthal planes.
Due to the limited extent of the flight stream, severalexperimental limitations have had to be accepted, butnevertheless the data are considered to be ofacceptable quality for the purpose of thisinvestigation, which was primarily to establish thelikely magnitude of these effects.
Noise increases of up to 6 dB were observed as aresult of the jet being at incidence to the flight-stream, which were observed to vary both with thepolar emission angle, and azimuthally around the jet.The increases were generally greatest at low emissionangles and at an azimuthal plane equivalent to anaircraft flying 30° above the horizon than for the
aircraft either in the same horizontal plane as, orflying directly over, the observer.
The noise increases varied with jet condition, beinglarger at cutback than takeoff, but whether this is aconsequence of the lower jet velocity, or theincreased bypass-to-core velocity ratio, cannot bereadily determined from these data.
The noise increases are considerably larger than canbe explained by either a consideration of the reducedcomponent of the flight velocity in the direction ofthe jet axis, or by the ‘angle of attack factor’ given inReference 7. It is recognised that the downwash fromthe wing will have some alleviating effect on thenoise increases, but it is considered unlikely that thiswould fully account for the difference.
CFD studies have been performed which indicate thatthe noise changes are likely to be associated bothwith an increase in turbulent kinetic energy in thelower part of the jet just beyond the end of thepotential core, and also with changes to thepropagation of the noise from the jet as a result ofdeformation of the jet flow by the cross flow.
(m/s0 Ds 2 Ds 4 Ds 6 Ds 8 Ds 10 Ds 12 Ds
(m2/s20 Ds 2 Ds 4 Ds 6 Ds 8 Ds 10 Ds 12 Ds
Velocity
Turbulent Kinetic Energy
Figure 13: Flow development(viewed perpendicular to jet flow direction)
11
American Institute of Aeronautics and Astronautics
RECOMMENDATIONS
This investigation was intended to establish themagnitude of the cross-flow effect, and theexperimental limitations imposed by the flight-streamsize were considered acceptable in this context.However, the work has established that considerablenoise increases may arise as a result of cross flow,and these must be understood in greater depth ifaircraft noise predictions are to be improved. Thiswould require, as a starting point, further tests in alarger wind-tunnel, over a wider range of flowconditions, incidence angles, and azimuth angles.Recent refurbishments to the QinetiQ noise testfacility, including a larger (1.8 m) flight-simulationstream, should allow such tests to be undertaken.Better model design, to eliminate turbulent wakes,should also be considered.
Tests aimed at quantifying installation effects for arealistic aircraft configuration, including incidence,have recently been performed in Europe17. The scopeof the tests was limited to a single angle of incidence,nevertheless, these data represent a valuable resourcefrom which to begin assessing the impact of aircraftflight incidence on jet exhaust noise.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the UKDepartment of Trade and Industry for their support ofthis work under the ‘CARAD’ aeronautics researchprogramme. Thanks are also due to NoiseEngineering group at Rolls-Royce in Derby for theirsupport, both technical and financial.
REFERENCES
1 W D Bryce and R A Pinker, ‘Concerning engineinstallation effects on aircraft noise’, NGTE NoteNo. NT. 1015, March 1976.
2. D J Way and B A Turner, ‘Model testsdemonstrating under-wing installation effects onengine exhaust noise’, AIAA 80-1048, June 1980.
3. R C K Stevens, W D Bryce and V M Szewczyk,‘Model and full-scale studies of the exhaust noisefrom a bypass engine in flight’, AIAA 83-0751,April 1983.
4 B N Shivashankara and A M Blackner, ‘Installedjet noise’, AIAA 97-1601, May 1997.
5 C J Mead and P J R Strange, ‘Under-winginstallation effects on jet noise at sideline’,AIAA 98-2207, Proceedings of the 4thAIAA/CEAS Aeroacoustics Conference,June 1998.
6 R C K Stevens, ‘Noise measurements from hotjets at incidence to a flight stream’,Memorandum 78068, National Gas TurbineEstablishment, December 1978.
7 ‘Gas Turbine Jet Exhaust Noise Prediction’, SAEARP876 Revision D, Society of AutomotiveEngineers, Issued March 1978, Revised January1994.
8 ‘Computer-based estimation procedure forcoaxial jet noise’, ESDU Engineering Data Item86019, ESDU International plc, May 1991.
9 E N Bazley, ‘Sound Absorption in air atfrequencies up to 100kHz’, NPL Acoustics ReportAC74, February 1976.
10 J R Jacques, ‘The noise from moving aircraft;some relevant models’, PhD Thesis, CambridgeUniversity Engineering Department, August 1975.
11 D J Way and R A Pinker, ‘Preliminary testsmodelling acoustic installation effects on the jetprovost’, Paper No. 12 of NGTE MemorandumM78060, National Gas Turbine Establishment,December 1978.
12 D J Way, ‘Some acoustic tests demonstrating theinstallation effects arising from a flat platepositioned upstream of a jet exhaust’, Paper No.14 of NGTE Memorandum M78060, NationalGas Turbine Establishment, December 1978.
13 B J Cocking and W D Bryce, ‘Subsonic jet noisein flight based on some wind-tunnel tests’,AIAA-75-462, Proceedings of the 2nd AIAAAeroacoustics Conference, March 1975.
14 K W Bushell, ‘Measurement and prediction of jetnoise in flight’, AIAA-75-461, Proceedings of the2nd AIAA Aeroacoustics Conference, March1975.
15 ‘Fluent 5 User Guide’, Fluent Inc, 200116 M J Lighthill, ‘On sound generated
aerodynamically I, General theory’, Proceedingsof Royal Society London, 565-587, 1952.
17 L C Chow, P Lempereur and K Mau ‘Reductionof Airframe and Installation Noise’, Proceedingsof Inter-Noise, pp. 189-194, December 1999.
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