1American Institute of Aeronautics and Astronautics
A COMPUTATIONAL AND EXPERIMENTAL INVESTIGATION OFSERRATED COAXIAL NOZZLES
Gary J. Page*, James J. McGuirk#, Mamdud Hossain‡, Nicola J. Hughes§ , Miles T. Trumper‡‡
Department of Aeronautical and Automotive Engineering, Loughborough University,
Loughborough, Leicestershire. LE11 3TU
United Kingdom
AbstractSerrations on the nozzles of high bypass ratio turbo-fan engines are a promising technique to reduce jetnoise. A study is carried out using both experimentallaser measurements and computational Reynoldsaveraged predictions to help understand the fluidmechanics which may lead to a reduction in jet noise.Serrations are present on both primary (core) and sec-ondary (bypass) nozzles and these are set at a zeroangle to the aerodynamic flow lines. Results are pre-sented for mean and fluctuating axial velocities up toseven primary nozzle diameters downstream. Experi-ments indicate that serrations give a reduction in peakturbulence intensity in the bypass/freestream shearlayer which may be a mechanism to reduce jet noise.CFD gives good agreement with experiment for meanvelocity, but is poor for fluctuating velocities. Initialtests with a hexahedral mesh indicate that these givebetter agreement with experiment as compared to thetetrahedral meshes used in the current work.
IntroductionJet noise is an important component of the noise emis-sion of civil turbofan aircraft. Even for high bypassratio engines, jet noise is the most prominent source atthe full power take off condition.
Nozzle designs are actively being sought whichmay result in significant jet noise reductions for highbypass ratio, separate jet exhaust configurations. Anovel approach by which this might be achieved is tomodify the complex coaxial jet flow developmentdownstream of the nozzles by means of trailing edge
serrations (or ‘chevrons’). Model-scale exhaust testshave measured substantial noise reductions underboth static and flight-simulation conditions1. Whilstdesigns for these serrated nozzles have been flighttested2, it is not clear as to the physical mechanismsby which noise reduction is achieved.
The aim of this work is to undertake a fundamen-tal study, initially using high quality laser dopplervelocimetry measurements and Reynolds AveragedNavier-Stokes CFD predictions, to explore the fluiddynamic changes introduced by the serrations. Thisknowledge will provide a basis for extending existingsemi-empirical noise predictions3,4 to reflect correctlythe influence of geometric features such as serrations.
For low to medium bypass ratio engines, internalmixing of the core and bypass streams using a forcedmixer offers significant jet noise reductions. Forced
Figure 1. Boeing 777 Rolls-Royce Trent 800full scale flight test
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lobed mixers have been extensively studied bothexperimentally5 and computationally6. The lobesdirect hot core flow outwards into the bypass, andcool bypass flow downwards into the core, strong vor-tical structures are created which enhances mixing inthe initial portion before reaching the jet exit plane. Athigh bypass ratios, however, the noise benefit of inter-nal mixing is small due to both the high flow arearatio and the high velocity ratio. In addition, a longbypass cowl has significant weight and drag penalty.
In the context of single stream jets, small tabs onthe nozzle lip, perpendicular to the flow, have beenused to give enhanced mixing of the initial shearlayer7. However, the protrusion of the tab is likely togive undesirable thrust loss and an increase in highfrequency noise.
Nozzle serrations are a more subtle approachwith low protrusion into the flow and consequentlythrust losses are kept to an acceptable level. Neverthe-less, a significant distortion of the shear layer is intro-duced and their impact on noise emission from acoaxial jet can be dramatic.
Kenzakowski et al.8 showed CFD predictions ofthe experimental configurations used in the noisetests1. Both chevron and delta tabs were considered,but only on the core nozzle since the experimentaltests had concluded that bypass serrations were lesseffective at reducing noise. The chevron geometrywas relatively severe, with alternating chevronsdeflected up and down by 3o from the nominal corestreamlines - this gave a strong distortion of the core/bypass shear layer with lobe like structures that ‘pinchoff’ downstream. However, the authors comment thatthe fan/freestream mixing layer is still the dominantturbulent region (due to the small velocity differencebetween core and bypass) and the overall fan/freestream shear layer growth is not significantlyaltered by the presence of the chevrons (on the core).
Thomas et al.9 also included the influence of thenacelle pylon in their predictions, using the samechevron geometry as Kenzakowski et al. Orientationof the chevrons relative to the pylon was found tohave a significant effect on the lobe development cre-ated by the chevrons near the pylon.
Both these computational studies show how theturbulence kinetic energy field is affected by the pres-ence of chevrons, but no experimental turbulence fielddata was available for comparison. It is also assumedthat noise reduction is purely a result of the injectedvorticity from the chevrons increasing core andbypass flow mixing.
Page et al.10 showed how a standard Reynoldsaveraged CFD method is capable of giving goodagreement with experimental meanand fluctuatingdata for a ‘clean’ coaxial jet configuration11. If thiscapability is also true for serrated nozzles then such atechnique would be a useful tool in improving noiseprediction models.
The present work uses non-intrusive laser dopplervelocimetry to obtain both mean and fluctuating datafor clean and serrated coaxial configurations and com-pares these to RANS CFD predictions using a k-ε tur-bulence model on an unstructured tetrahedral mesh.
MethodologyExperimentalExperimental measurements were carried out in awater-tunnel facility designed specifically for near-field jet-mixing problems7. The tunnel is of recirculat-ing design, has a working section 1.125m long, 0.37mwide and 0.3m high, which was made of Perspex toallow ample optical access. For measurements on co-axial nozzles, the primary and secondary jet nozzlesare fed from separately pumped flow circuits withflow rates monitored by rotameters. It is also possibleto provide a third co-flowing parallel stream outsideof the jet nozzles to simulate a flight stream. Turbu-lence management units are provided in the supplyducts feeding both jet flows in order to provide as uni-form an exit profile as possible, and low turbulencelevels at the nozzle exit. The tertiary co-flow is alsoused to create a low velocity stream (typically around5% of the primary jet velocity) in order to stabilise theflow in the furthest downstream region of the watertunnel; measurements with and without co-flowshowed this had little effect on the jet development.Figure 2 shows a schematic of the water tunnel.
A Dantec LDA system was used to provide meanvelocity and turbulence data. The system used a 5Wargon-ion laser, with the light separated into blue andgreen wavelengths (488nm and 514nm) within atransmitter box. A Bragg cell was used to provide a40MHz shift to the beams to eliminate fringe bias andremove directional ambiguity. The beams were trans-mitted to a two dimensional probe via optical fibres.The multi-mode fibres were also capable of transmit-ting the reflected light scattered from seeding particlesin the flow to photo multipliers, since the probe alsoincorporated its own receiving optics. Each photomultiplier contained a narrow band filter for its ownspecific colour. Mean and turbulence intensity of twovelocity components could be obtained by processingeach colour independently using standard Dantec
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BSA processors. Cross coupling of the signals withinthe acquisition software enabled the evaluation of theReynolds shear stress. The use of a large beamexpander increased the size of the transmitted beams,but decreased the beam ‘waist’, thereby enabling areduction in the size of the interference pattern con-tained within the measurement volume.
For flow visualisation purposes, a Laser InducedFluorescence (LIF) technique was used. Fluoresceindye was added in dilute quantity to either the primaryor secondary jet stream. The dye was injected farupstream, so that it was fully mixed at nozzle exit. Asecond 5W argon-ion laser mounted on a moveabletable was used as a light source. The beam was passedthrough a Dantec 80x20 light sheet probe containing acylindrical lens to create a light sheet (thickness2.5mm); this was shone through the lower surface ofthe water tunnel. The light sheet could be orientedeither parallel or perpendicular to the jet axis. Whendye-containing-water flowed through the light sheet,it fluoresced a green colour and the instantaneouscross section of the jet could be clearly seen. Thisallowed the effect of the serrations to be easily visual-ised and confirmed they had a major influence on thenozzle shear layers, with significant distortions of thenominally circular jet shape. The flow visualisationpictures were also used to guide the locations of lon-gitudinal and transverse measurement planes for thedetailed LDV measurements. Figure 3 shows the LIFset-up and Fig. 4 an example of a typical flow visuali-sation. The test conditions for this picture were avelocity ratio (primary/secondary) of 0.7, with plaininner and serrated outer nozzles. The jet cross-sectionwas captured at a downstream distance of 1 primarynozzle diameter, with dye added just to the outer sec-
ondary flow. The individual inward and outwardmovements of the jet shear layer caused by thestreamwise vortex emanating from the serrations isclearly visible.
ComputationalThe CFD methodology solves the compressible Rey-nolds Averaged Navier Stokes equations and uses ak-ε model coupled with wall-functions for turbulenceclosure. The algorithm uses an edge based data struc-ture and can handle combinations of tetrahedral, hexa-hedral, prism and pyramid elements.
A median dual finite volume formulation isadopted. Spatial discretisation is based upon centraldifferencing of the flux vector with a smoothing fluxbased upon one dimensional characteristic variables.This is effectively an upwind difference scheme basedupon the characteristic variables of the system and is
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formally second order accurate. Viscous fluxes arecomputed by interpolating nodal gradients to edges,with a modification so that the components of the gra-dient aligned with the edge are replaced with a simpledifference along the edge.
An explicit Runge-Kutta scheme is used toadvance the solution in time towards the steady statewith preconditioning and multigrid for convergenceacceleration. In addition a low speed preconditioner isemployed to allow this density-based method to com-pute very low speed flows. Further details may befound in Moinier and Giles12 and Moinier13, and itsapplication to a lobed mixer geometry in Salman etal.14
The nozzle geometry was created parametricallywithin the SolidEdge CAD package, and this wasimported into the ICEM Tetra grid generation system.Although previous work has discovered potentialerrors in turbulent mixing predictions due to the adop-tion of tetrahedral meshes14 (as compared to hexahe-dral meshes), it was felt that the geometricalflexibility and ease of mesh adaption of tetrahedralmeshes would be of great benefit to this type of flowproblem.
ResultsGeometryThe geometry used is representative of a separate flownozzle with an external plug, and an area ratio (sec-ondary to primary) of five. Clean and serrated primaryand secondary nozzles were available allowing fourdifferent configurations to be studied. This paperreports the clean primary and secondary nozzles(datum configuration) and serrated primary and sec-ondary nozzle (serrated configuration) only. The ser-rations are geometrically somewhat different fromchevrons previously reported in the literature. Effec-tively, the triangular tip of the chevron has been‘squared off’ as has the triangular cut -out (seeFig. 5).
Figure 6 shows the serrated nozzle geometry andthe coordinate system adopted. The origin is placedalong the nozzle centreline at the axial position of the
primary nozzle lip. Results will be presented with dis-tances non-dimensionalised by the primary diameter(Dp) and velocities non-dimensionalised by the sec-ondary exit velocity (Vs). Results shown here are forthe ratio of secondary to primary velocity (λ) equal to0.7, Table 1 shows the primary and secondary dimen-sions and flow conditions.
Primary (p) 58 2.15 10% of Vp 0.1 x primary inlet channel height
Secondary (s) 116 1.5 10% of Vs 0.1 x secondary inlet channel height
Tertiary (t) - 0.1 10% of Vt set to give low turbulent viscosity
Table 1: Flow conditions for velocity ratio λ=0.7
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The size of the nozzle in the experimental facilitymeant that the furthest downstream experimentallocation was at 12 primary diameters, whilst the CFDdomain was continued until 31 primary diameters.
Longitudinal LDA traverses were taken in thex-yplane and axial slices were taken in they-z plane atvarious downstream stations.
The CFD meshes contained triangular prisms toresolve the boundary layers on the nozzle walls withtetrahedra in the remainder of the domain. The meshwas designed to cluster a large number of cells in theregions of the jet shear layers. The smallest volumeswere approximately 1mm in size. Due to symmetry,only a 30o sector was modelled. Figure 7 shows themesh for the serrated configuration and consists of215 000 tetrahedra and 45 000 prisms which resultedin 412 000 nodes and 2.7 million edges.
Mean VelocityComputed stream ribbons are shown in Fig. 8; theseare placed just inside the primary and secondary noz-zle lips. The primary flow deflects outwards as it trav-els though the cut-out region of the serration. Thisresults in a distorted shear layer, but little evidence ofa strong vortical flow nature. The flow near the cut-outof the secondary serration is less distorted and in factthe predictions show that the outer co-flow is penetrat-ing inwards. As will be seen later, this phenomena isnot found in the experiments.
Figure 9 shows axial velocity contours for thedatum nozzle. It is noticeable that the wake region dueto the bullet is stronger in the CFD predictions. It isbelieved that this is due to a poor geometric definitionof the bullet in the solid model used for grid genera-tion. A qualitative comparison of axial velocity is
shown in Fig. 10, apart from the bullet wake region, ingeneral good agreement is found.
Comparing the datum results with the serratednozzle axial velocities (Fig. 11 and Fig. 12) showsthat the bullet wake region is stronger in both experi-ment and CFD. The experiment also indicates that theaxial velocity near the exit of the primary nozzle issignificantly reduced (this effect is present, but isweaker in the CFD). The consequence of these twoeffects is that the potential core velocity is reduced.Qualitative comparison in Fig. 12 also shows reasona-ble agreement but the reduction (in comparison todatum) of jet velocity downstream is underpredicted.
Turbulent FluctuationsOf interest to noise prediction models is the turbu-lence intensity in the shear layers. Results are shownfor the RMS of axial turbulent fluctuations for thedatum (Fig. 13 and Fig. 14) and serrated (Fig. 15 andFig. 16) nozzles, in the plane through the centre of thecut-out. The most important observation is that theexperiments indicate that downstream in the second-ary/co-flow shear layer, the turbulent fluctuations aresignificantly reduced. Since this shear layer is consid-ered to be the dominant noise producing source forthis type of coaxial jet, this reduction in turbulenceintensity could be the mechanism by which noisereduction is observed. As a consequence of the flowgradients around the serrations, turbulent fluctuationsare slightly increased in the early development of theprimary/secondary shear layer. As the turbulenteddies are small in this region, this is likely toincrease high frequency noise.
The CFD results clearly under predict the turbu-lent axial fluctuations. Previous work showed that thistype of RANSk-ε approach is capable of reasonable
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predictions of normal stresses in coaxial jets10. How-ever, in the current work unstructured tetrahedralmeshes have been adopted (as compared to structuredhexahedral meshes used previously); there is someevidence that discretization error on tetrahedralmeshes can give underprediction of normal stresses6.Preliminary calculations have been carried out usinga structured hexahedral mesh and these results areshown as a dashed line in Fig. 14 and Fig. 16. Clearlythe fluctuating axial velocity is in much better agree-ment with experiment, and importantly is able to pre-dict the reduction in turbulence fluctuationsdownstream caused by the presence of the serration
Finally, an axial slice just downstream of the pri-mary nozzle exit is shown in Fig. 17. Whilst the shapeof the mean velocity contours looks similar, carefulinspection of the secondary/co-flow shear layer con-firms that the experiment has flow in the secondarycut-out moving radially outwards, whilst in the CFD itis moving inwards. It is believed that the horizontalwiggles apparent in both experimental plots is an arti-fact of the experimental facility. Whilst many testswere carried out to determine the cause of this anom-aly this has proved elusive.
ConclusionsResults have been shown for experimental measure-ments and computational predictions for both cleanand serrated coaxial nozzle configurations relevant tohigh bypass ratio turbofan engines. The serrationswere present on both primary (core) and secondary(bypass) flows and were set at zero angle to the aero-dynamic flow lines.
The main observations are:
1. The serrations distorted the jet shear layers, butdid not cause a strong vortical type flow.
2. The presence of serrations reduced the axial veloc-ity in the primary potential core.
3. In the experiments, the serrations significantlyreduced the peak turbulent fluctuations in the sec-ondary / co-flow shear layer. It is believed that thismay be a mechanism to reduce jet noise.
4. The CFD were in good agreement with experi-ment for the mean axial velocity, but was poor forthe turbulent fluctuating axial velocity. The latterappeared to be linked to numerical issues associ-ated with tetrahedral meshes, and improvementwas found by the use of hexahedral meshes.
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Further work will concentrate on increasing thefidelity of the CFD predictions by using hexahedralmeshes and more refined tetrahedral meshes. Thisshould allow RANS CFD methods to be coupled tosemi-empirical noise prediction techniques in orderthat the effect of serrations can be taken into account.
AcknowledgementsThe computational portion of this work was fundedthrough EPSRC grant GR/M84985 in conjunctionwith Rolls-Royce Plc and QinetiQ. The experimentalwork on serrated nozzles has been separately fundedby Rolls-Royce Plc and QinietiQ. The authors are par-ticularly grateful for the guidance given by PaulStrange (RR) and Craig Mead (QinetiQ).
“Acoustics and thrust of separate-flow exhaustnozzles with mixing devices for high-bypass-ratioengines”, AIAA Paper 2000-1961, AIAA/CEAS,21st AIAA Aeroacoustics Conference) Lahaina,HI, 12-14 June 2000.
2 Nesbitt, E., Elkoby, T.R., Bhat, T.R.S., Strange,P.J.R. and Mead, C., “Correlating model-scale andfull-scale test results of dual flow nozzle jets,”AIAA Paper no AIAA-2002-2487, 8th AIAA/CEAS Aeroacoustics Conference, Breckinridge,CO, 17-19 June 2002.
3 Fisher, M.J., Preston, G.A. and Bryce, W.D., “AModelling of the Noise from Simple Coaxial Jets,Part I: with Unheated Primary Flow,”Journal ofSound and Vibration, Vol. 209, 1998, pp385-403.
4 Fisher, M.J., Preston, G.A. and Mead, C.J., “AModelling of the Noise from Simple Coaxial Jets,Part I: with Heated Primary Flow,”Journal ofSound and Vibration, Vol. 209, 1998, pp405-417
5 McCormick, D.C. and Bennett J.C. Jr., “Vorticaland Turbulent Structure of a Lobed Mixer FreeShear Layers,”AIAA Journal, Vol. 32, no. 9,pp1852-1859, 1994.
6 Salman, H., McGuirk, J.J. and Page G.J., “ANumerical Study of Vortex Interactions in LobedMixer Flow Fields,” AIAA Paper 99-3409, 30thAIAA Fluid Dynamics Conference, Norfolk, VA,July 1999.
7 Behrouzi, P. and McGuirk, J.J., “ExperimentalStudies of Tab Geometry Effects on MixingEnhancement of an Axisymmetric Jet,”JSME
International Journal, Series B, Vol. 41, 1998,pp908-917.
8 Kenzakowski, D.C., Shipman, J., Dash, S.M.,Bridges, J.E. and Saiyed, N., “Turbulence modelstudy of laboratory jets with mixing enhancementsfor noise reduction,” AIAA Paper 2000-0219,AIAA 38th Aerospace Sciences Meeting andExhibit, Reno, NV, 10-12 June, 2000.
9 Thomas, R.H., Kinzie, K.W. and Pao, S.P., “Com-putational analysis of a pylon-chevron core nozzleinteraction,” AIAA Paper 2001-2185, 7th AIAA/CEAS Aeroacoustics Conference, Maastricht, TheNetherlands, 28-30 May 2001.
10 G.J. Page, J.J. McGuirk, P. Behrouzi, M. Hossain,M.J. Fisher, “A CFD Coupled Acoustics Approachfor the Prediction of Coaxial Jet Noise,’ NATORTO-AVT Symposium on Ageing Mechanismsand Control, Part A - Developments in Aero- andHydro-Acoustics, Manchester, UK, 8-11 Oct.2001
11 Ko, N.W.M and Kwan, A.S.H, “The Initial Regionof Subsonic Co-axial Jets,”Journal of FluidMechanics, Vol. 73, 1976.
12 Moinier, P. and Giles, M.B., “PreconditionedEuler and Navier-Stokes Calculations on Unstruc-tured Grids,” 6th ICFD Conference on NumericalMethods for Fluid Dynamics,Oxford, UK, 1998
13 Moinier, P. “Algorithm developments for anunstructured viscous flow solver,” PhD thesis,University of Oxford, UK, Trinity Term 1999.
14 H.Salman, J.J.McGuirk, G.J.Page and P.Moinier,“The Influence of Unstructured Mesh Type on thePrediction of Convoluted Shear Layers,” ECCO-MAS 2000, Barcelona, 11-14 September 2000
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