Mixing Process in a Lobed Jet Flow

AIAA JOURNAL Vol.40,No.7, July 2002

Mixing Process in aLobedJet Flow

† ‡ § Hui Hu,¤ TetsuoSaga, ToshioKobayashi, andNobuyuki Taniguchi Universityof Tokyo,T okyo153-8505, Japan

Ahigh-resolutionstereoscopic particleimage velocimetry (PIV) systemwas used to conduct three-dimensional measurementof anair jet exhaustedfrom a lobednozzle. The characteristics ofthe mixingprocess inthe lobed jet oware revealedclearly and quantitatively from the stereoscopic PIV measurementresults. The instantaneous velocityand streamwise vorticitydistributions revealed that the large-scalestreamwise vortices generatedby the lobednozzle breakinto smaller but not weaker streamwise vortices asthey travel downstream. This is the proposedreason why a lobednozzle wouldenhance both large-scale mixing and small-scale mixing reported by otherresearchers. The overalleffect ofthe lobednozzle onthe mixingprocess wasevaluated by analyzing the ensemble-averagedstreamwise vorticitydistributions. The strengthof the ensemble-averagedstreamwise vortices wasfound to decay very rapidly over the rst twodiameters (rst sixlobe heights ),thento decay at amoremoderate rate farther downstream.The averagedturbulent kinetic energy pro le alsoindicated that most of the intensive mixingbetween the core jet andambient owoccurred withinthe rst twodiameters. These results indicatethat the maximumeffectiveness regionfor the lobednozzle toenhance mixing is about the rst twodiameters of the lobednozzle (rst sixlobe heights ).

Nomenclature beenapplied widely in turbofan engine exhausts and ejectors to D =diameterof thelobed nozzle, 40 mm reducetakeoff jet noise and speci c fuelconsumption. 1;2 More re- H =lobeheight, 15 mm cently,lobed mixers/ nozzleshave also emerged as anattractive ap- K =turbulentkinetic energy, .1=2U 2/.u 2 v 2 w 2/ proachfor enhancing mixing between fuel and air in combustion 0 0 C 0 C 0 K .z/ =averaged-turbulentkinetic enegy, chambersto improve the ef ciency of combustionand toreduce the N formationof pollutants. 3 ½W .x; y; z/K .x; y; z/ dx dy Besidesthe continuous efforts to optimize the geometry of lobed mixers/nozzlesfor better mixing enhancement performance and ½W .x; y; z/ dx dy R widenthe applications of lobed mixers/ nozzles,extensive studies Re =Reynoldsnumber R aboutthe mechanism by which the lobed mixers/ nozzlessubstan- U; V; W =velocitythree components tiallyenhance mixing have alsobeen conducted in past years. Based U0 =meanvelocity of the air jet at the inlet of the onpressure,temperature, and velocity measurements of the ow- lobednozzle, 20.0 m/ s elddownstream of alobednozzle, Paterson 4 revealedthe existence u0; v0; w0 =rmsvalues of velocity uctuations oflarge-scalestreamwise vortices in lobed mixing owsinduced by x; y; z =coordinates thespecial geometry of lobed nozzles. The large-scale streamwise µin =inwardpenetration angle of the vorticeswere suggested to be responsible for the enhanced mixing. lobednozzle, 22 deg Werle et al.5 andEckerle et al. 6 foundthat the streamwise vortices µout =outwardpenetration angle of the inlobed mixing owsfollow a three-stepprocess by which the lobedsnozzle, 14 deg streamwisevortices form, intensify, and then break down and sug- ¹ =viscosityof air gestedthat the high turbulence resulting from the vortex breakdown ½ =densityof air improvedthe overall mixing process. Elliott et al. 7 suggestedthat $z =streamwisevorticity, .D=U0/.@v=@x @u=@y/ thereare three primary contributors to the mixing processes in lobed ¡ mixing ows.The rstis the spanwise vortices, which occur in any freeshear layers due to the Kelvin –Helmholtzinstability. The sec- ondis the increased interfacial contact area due to the convoluted Introduction trailingedge of the lobed mixer. The last element is the stream- OBED mixers/nozzles,which are essentially splitter plates wisevortices produced by thespecialgeometry of thelobed mixer. L withcorrugated trailing edges, are uidmechanic devices used Basedon pulsed laser sheet owvisualization with smoke and three- toaugmentmixing in avarietyof applications.Such devices have dimensionalvelocity measurements with a hot-lm anemometer, McCormickand Bennett 8 suggestedthat it is theinteraction of the spanwiseKelvin –Helmholtzvortices with the streamwise vortices Received 27February 2001; revision received 7November2001; accepted thatproduce the high levels of mixing. forpublication 18 December 2001.Copyright c 2002by theauthors. Pub- lishedby theAmerican Instituteof Aeronautics °and Astronautics, Inc., with Althoughthe existence of unsteadyvortices and turbulent struc- permission.Copies of thispaper may bemade forpersonal or internaluse, turesin lobed mixing owshas been revealed in those previous onconditionthat the copier pay the $10.00 per-copy fee tothe Copyright studiesby qualitative owvisualization, the quantitative, instanta- Clearance Center,Inc., 222 Rosewood Drive, Danvers, MA 01923;include neous,whole- eld velocity and vorticity distributions in lobedmix- thecode 0001-1452/ 02$10.00in correspondencewith the CCC. ing owshave never been obtained until recent work of thepresent ¤ JapanSociety for the Promotion of Science Research Fellow,Institute authors.9 InRef.9, both planar laser induced uorescence (PLIF) ofIndustrial Science, 4-6-1Komaba, Meguro-Ku; currently Postdoctoral andconventional two-dimensional particle image velocimetry (PIV) Research Associate, TurbulentMixing and Unsteady Aerodynamics Lab- techniqueswere used to study lobed jet mixing ows.Based on the oratory,A22, Research ComplexEngineering, Michigan State University, East Lansing,MI 48824;[email protected]. Member AIAA. directlyperceived PLIF owvisualization images and quantitative †Research Associate, Instituteof Industrial Science, 4-6-1Komaba, PIV velocity,vorticity, and turbulence intensity distributions, the Meguro-Ku. evolutionand interaction of variousvortical and turbulent structures ‡Professor,Institute of Industrial Science, 4-6-1Komaba, Meguro-Ku. inthelobed jet owswere discussed. §Associate Professor,Institute of Industrial Science, 4-6-1Komaba, Theconventional two-dimensional PIV systemused in theearlier Meguro-Ku. workof theauthors 9 isonly capable of obtaining two components

1339 1340 HU ET AL. ofvelocity vectors in the planes of illuminating laser sheets. The weredivided into two streams; one is used to seed the core jet ow out-of-planevelocity component is lost,whereas the in-plane com- andthe otherfor ambient air seeding. ponentsmay be affectedby an unrecoverable error due to perspec- Figure2 showsthe schematic of thestereoscopic PIV systemused. tivetransformation. 10 Forhighly three-dimensional owssuch as Thelobed jet owwas illuminated by a double-pulsedNd:Y AG lobedjet mixing ows,conventional PIV measurementresults may laser set (NewWave;50-mJ/ pulse ) withthe laser sheet thickness of notbe able to reveal their three-dimensional features successfully. about2.0 mm. Thedouble-pulsed Nd:Y AG laserset can supplythe Ahigh-resolutionstereoscopic PIV system,which can provide all pulsedlaser (pulsedillumination duration 6 ns ) atafrequencyof threecomponents of velocity vectors in a measurementplane si- 15Hz.T wohigh-resolution (1000 1000)cross-correlationcharge- multaneously,is used in the present study to measure an airjet ow coupleddevice (CCD) cameras (TSI£ PIVCAM10-30 ) were used to exhaustedfrom a lobednozzle. Based on thestereoscopic PIV mea- performstereoscopic PIV imagerecording. The two CCD cameras surementresults, some characteristics of themixing process in the werearranged in an angular displacement con guration to get a lobedjet oware discussed. largeoverlapped view. With the installation of tilt-axismounts, the lensesand camera bodies were adjusted to satisfythe Scheimp ug ExperimentalSetup andStereoscopic PIV System condition (see Ref. 11).Inthe present study, the distance between Figure1a showsthe lobed nozzle used in thepresent study. It has theilluminating laser sheet and image recording planes of theCCD sixlobe structures at thenozzletrailing edge. The width of each lobe camerasis about 650 mm, andthe angle between the view axis of the is6mm, andthe lobe height is 15 mm (H 15 mm).Theinward and twocameras is about 50 deg. For such an arrangement, the size of D 2 outwardpenetration angles of the lobed structures are µin 22 deg thestereoscopic PIV measurementwindow is about 80 80 mm . D £ and µout 14deg,respectively. The diameter of thelobednozzle is TheCCD camerasand double-pulsed Nd:Y AG laserswere con- 40 mm (DD 40 mm). nectedto a workstation (hostcomputer ) viaa synchronizer (TSI Figure1b Dshowsthe air jet experimental rig used in the present LaserPulsesynchronizer ),whichcontrolled the timing of thelaser study.A centrifugalcompressor was used to supplyair jet ows.A sheetillumination and the CCD cameradata acquisition. In the cylindricalplenum chamber with honeycomb structures was used to presentstudy, the time interval between the two pulsed illumina- settlethe air ow .Througha convergentconnection (convergentratio tions is 30 ¹s. 50:1),theair ow is exhaustedfrom the lobed nozzle. The velocity Ageneralin situ calibration procedure was conducted to obtain rangeof theair jet out of theconvergent connection (attheinlet of themapping functions between the image planes and object planes. 12 thetest nozzle )couldbe variedfrom 5 to35m/s.A meanspeed of the Atargetplate (100 100 mm) with 100-¹m-diamdots spaced at £ airjet owat theinlet of thelobed nozzle of U0 20:0m/swasused, intervalsof 2.5mm wasused for the in situ calibration. The front whichcorresponds to aReynoldsnumber Re D5:517 105 (based surfaceof thetarget plate was aligned with the center of thelaser onthenozzle diameter D 40 mm).Theair jet D owwas £seeded with sheet,and then calibration images were captured at threelocations 1 5-¹mdi-2-ethylhexyl-sebacD t (DEHS) dropletsgenerated by a acrossthe depth of thelaser sheets. The space interval between these seeding» generator. The DEHS dropletsout of the seeding generator locationsis 0.5mm forthe present study.

a) Lobednozzle

b) Experimentalrig Fig.1 Lobednozzle andair jet experimentalrig. HU ET AL. 1341

Themapping function used was taken to be a multidimensional PIV (HR-PIV) softwaredeveloped in-house. The HR-PIV software polynomial,which is fourth order for the directions (X and Y di- isbasedon HRprocessesof a conventionalspatial correlation op- rections) parallelto the laser sheet plane and second order for the erationwith offsetting of thedisplacement estimated by the former direction (Z direction) normalto the laser sheet plane. The coef- iterationstep and hierarchical reduction of theinterrogation window cientsof the multidimensional polynomial were determined from sizeand search distance in the next iteration step. 13 Comparedwith thecalibration images by using a least-squaremethod. conventionalcross-correlation-ba sedPIV imageprocessing meth- Thetwo-dimensional particle image displacements in each im- ods,the HR-PIV methodhas advantages in spuriousvector suppres- ageplanewas calculated separately by usinga hierarchicalrecursive sionand spatial resolution improvement of thePIV result.Finally,

Fig.2 Stereoscopic PIV system.

a) Instantaneousvelocity eld c) Instantaneousstreamwise vorticitydistribution

b) Ensemble-averagedvelocity eld d) Ensemble-avergedstreamwise vorticitydistribution Fig.3 Stereoscopic PIV measurementresults inthe Z/D = 0:25 (Z/H = 0:67) cross plane. 1342 HU ET AL. whenthe mapping functions obtained by thein situcalibration and distributions,ensemble-averaged velocity, and streamwise vortic- thetwo-dimensional displacements in thetwo image planes are used, ity elds.The ensemble-averaged velocity and streamwise vorticity allthree components of thevelocity vectors in the illuminating laser eldsgiven in Figs. 3 –5werecalculated based on 250 frames of sheetplane were reconstructed. instantaneousstereoscopic PIV measurementresults. Because 32 32pixelinterrogation windows with 50% overlap In the Z=D 0:25 (Z=H 0:67) cross plane (almostat the exit wereused for £the nalstep of therecursive correlation operation, ofthe lobed nozzle D ),thehigh-speed D core jet owwas found to thespatial resolution of thepresent stereoscopic PIV measurement havethe same geometry as thelobed nozzle. The signature of the isexpectedto beabout2.5 2.5 2.0 mm3.Toevaluatethe mea- lobednozzle in the form of a six-lobestructure can be seenclearly surementaccuracy of thepresent £ £stereoscopic PIV system,a laser fromboth the instantaneous and ensemble-averaged velocity elds dopplervelocimetry (LDV) systemwas used to conduct simulta- (Fig. 3).Theexistence of very strong secondary streams in thelobed neousmeasurement of the lobed jet ow.Thestereoscopic PIV jet owis revealed very clearly in the velocity vector plots. The core measurementresults were compared with the LDV results.It was jet owejects radially outward in the lobe peaks, and ambient ows foundthat the velocity differences between the measurement results injectinward in the lobe troughs. Both the ejection of thecore jet ofthestereoscopic PIV systemand the LDV dataare less than 2.0% owand the injection of the ambient owsgenerally are following atthecompared points. Therefore, the accuracy level of thevelocity theoutward and inwardcontours of thelobed nozzle, which results vectorsobtained by the stereoscopic PIV systemis expected to be inthegeneration of sixpairs of counterrotating streamwise vortices about2.0% and that of therms of thevelocity uctuations, u0; v0, inthelobed jet ow.The maximum radial ejection velocity of the and w0,andturbulent kinetics energy K areabout 5.0%. An adap- corejet owin the ensemble-averaged velocity eldis foundto be tive scheme14 wasused in thepresent study to calculate streamwise about5.0 m/ s,which is almostequal to the value of U sin.µ /. A 0 ¢ out vorticity $z distributionsfrom the velocity eldsobtained by the largehigh-speed region can also be seenclearly from the ensemble- stereoscopicPIV measurement.The accuracy level of the stream- averagedvelocity eld,which represents the high-speed core jet wisevorticity data in the present study is expected to be within owin the center of thelobednozzle. 10.0%.Further details about the in situ calibration, reconstruction Theexistence of the six pairs of large-scale streamwise vor- proceduresof the stereoscopic PIV systemand thecomparison of the ticesdue to the special geometry of thelobed nozzle can be seen stereoscopicPIV andLDV measurementsmay be foundin Ref. 15. moreclearly and quantitatively from the streamwise vorticity dis- tributionsshown in Figs. 3c and 3d. The size of theselarge-scale ExperimentalResults andDiscussion streamwisevortices is foundto be ontheorder of thelobe height. Figures 3–5showsthe stereoscopic PIV measurementresults in Comparedwith those in theinstantaneous streamwise vorticity eld threetypical cross planes of the lobed jet ow,whichinclude typi- (Fig. 3c),thecontours of thelarge-scale streamwise vortices in the calinstantaneous velocity elds,simultaneous streamwise vorticity ensemble-averagedstreamwise vorticity eld (Fig. 3d) were found

a) Instantaneousvelocity eld c) Instantaneousstreamwise vorticitydistribution

b) Ensemble-averagedvelocity eld d) Ensemble-avergedstreamwise vorticitydistribution Fig.4 Stereoscopic PIV measurementresults inthe Z/D = 1:5 (Z/H = 4:0) cross plane. HU ET AL. 1343

a) Instantaneousvelocity eld c) Instantaneousstreamwise vorticitydistribution

b) Ensemble-averagedvelocity eld d) Ensemble-avergedstreamwise vorticitydistribution Fig.5 Stereoscopic PIV measurementresults inthe Z/D = 3:0 (Z/H = 8:0) cross plane. tobemuchsmoother. However, they have almost the same distri- scalestreamwise vortices observed at theexit of thelobed nozzle butionpattern and magnitude as their instantaneous counterparts. havebroken down into many smaller streamwise vortices. However, Thesimilarities between the instantaneous and ensemble-averaged notethat the maximum vorticity value of the instantaneous small- streamwisevortices might suggest that the generation of large- scalestreamwise vortices is found to be almost at the same level as scalestreamwise vortices at theexit of thelobed nozzle to be quite thoseat theexit of the lobed nozzle. steady. Fromthe ensemble-averaged streamwise vorticity distribution in Thelobed jet owwas found to become much more turbulent thiscross plane (Fig. 4d),notethat the strength of thelarge-scale in the Z=D 1:5 .Z=H 6:0/ crossplane compared to that at the ensemble-averagedstreamwise vortices in the lobed jet owhas exitof thelobed D nozzle. DInstead of generallyfollowing the inward dissipated.The maximum value of the ensemble-averaged stream- andoutward contours of the lobed trailing edge at the exit of the wisevorticity is only about one- fth of that at the exit of the lobed lobednozzle, the secondary streams revealed in theinstantaneous nozzle.Because the present lobed jet owis afreejet,the centers of velocity eld (Fig. 4a) becomemuch more random and unsteady. thelarge-scale streamwise vortices were found to have spread out- Thesignature of the lobed nozzle in the form of asix-lobestructure wardas they travel downstream, which is alsorevealed very clearly isnearly indistinguishable from the instantaneous velocity eld. intheensemble-averaged velocity vector plot given in Fig. 4b. The Althoughthe high-speed region in the center of the lobed nozzle samephenomena were also found in the LDV measurementresults stillcan be discerned from the ensemble-averaged velocity eld, ofBelovichand Samimy. 16 thesize of the high-speed region was found to decrease substan- Asthe downstream distance increases to Z=D 3:0 .Z=H tiallydue to intensive mixing between the core jet owand ambient 8:0/,thelobed jet mixing owbecame so turbulent D that the sig- D ow.Theensemble-averaged velocity vector plot also shows that natureof the lobed nozzle can no longer be identi ed from the theensemble-averaged secondary streams in the lobed jet owhave instantaneousvelocity eld (Fig. 5a).The oweld is completely becomemuch weaker. The maximum radial ejection velocity of the lledwith many small-scale vortices and turbulent structures. The corejet owhas decreased to about 2.0 m/ s,which is onlyabout ensemble-averagedvelocity eldin this cross plane shows that the 40%ofthatat theexit of thelobed nozzle. distincthigh-speed region in the center of the lobed jet owhas Thesix pairs of large-scalestreamwise vortices generated by the dissipatedso seriously that isovelocity contours of thehigh-speed lobednozzle could no longer be identied from the instantaneous corejet owhave become small concentric circles. The ensemble- streamwisevorticity distribution in the Z=D 1:5 .Z=H 4:0/ averagedsecondary streams in thiscross plane become so weak (the crossplane shown in Fig. 4c. Instead of large-scale D streamwise D maximumsecondary stream velocity of lessthan 0.8 m/ s ) that they vortices,many smaller streamwise vortices were found in the in- cannot be identi ed easily from the ensemble-averaged velocity stantaneousstreamwise vorticity eld.This indicates that the large- vectorplot. 1344 HU ET AL.

Fromthe instantaneous streamwise vorticity distribution in the Thepreceding stereoscopic PIV measurementresults have shown Z=D 3:0 .Z=H 8:0/ crossplane given in Fig. 5c,note that there thatthe size of theinstantaneous streamwise vortices in the lobed areso Dmany small-scale D streamwise vortices in thelobed jet mixing jet owdecreases as thedownstream distance increases, that is, the owthat they almost lledthe measurement window .However,the large-scalestreamwise vortices generated by thelobed nozzle broke maximumvorticity value of theseinstantaneous small-scale stream- intosmaller vortices as they travel downstream. However, the maxi- wisevortices was found to be stillat thesame level of thosein the mumvorticity values of the smaller vortices were found to be almost upstreamcross planes. Because of serious dissipation caused by atthesame level as theirparent streamwise vortices. These results theintensive mixing between the core jet owand ambient ow, suggestedthat the dissipation of thelarge-scale streamwise vortices almostno apparentstreamwise vortices can be identied fromthe generatedby the corrugated trailing edge of the lobed nozzle did ensemble-averagedstreamwise vorticty distribution (Fig. 5d) in the nothappen abruptly but rather appeared to be a gradualprocess. Z=D 3:0 .Z=H 6:0/ crossplane. Thelarge streamwise vortices were found to break down into many BasedD on thestereoscopic D PIV measurementresults at 12 cross smallerbut not weaker streamwise vortices as thedownstream dis- planes,the ensemble-averaged three-dimensional oweld in the tanceincreases. Thus, besides the large-scalemixing enhancement, neardownstream region of the lobed jet owwas reconstructed. mixingat a nerscale could also be achievedin thelobed jet mix- Figure6a showsthe three-dimensional ensemble-averaged velocity ing ow.The result, therefore, agrees well with those obtained by vectors.The velocity isosurfaces of the reconstructed three- Belovichet al. 17 dimensional oweld are given in Fig. 6b. The velocity magnitudes Thelobed nozzle might be thought to actas auidstirrer in the ofthese isosurfaces are 4.0, 8.0, 12.0, and 16.0 m/ s,respectively. lobedjet owto mix the core jet owwith ambient ow.The stirring Notethat the high-speed core jet owhas the same geometry as effectof thelobed nozzle on themixing processes can be evaluated thelobed nozzle, that is, six-lobe structure, at the exit of thelobed fromthe evolution of theensemble-averaged streamwise vortices. nozzle.Because of the stirring effect of the large-scale streamwise Figure7 showsthe decay pro le oftheensemble-averaged stream- vorticesgenerated by thelobed nozzle, the six-lobe structure of the wisevorticity in the lobed jet ow.Note that, over the rsttwo corejet owwas rounded up rapidly.At Z=D 3:0 .Z=H 8:0/ diametersdownstream of thelobed nozzle (rstsix lobe heights ), downstream,the isosurfaces were found to become D concentric Dcylin- theensemble-averaged streamwise vorticity decayed very rapidly ders,which are very similar to those in a circularjet ow. andthen the decay rate slowed farther downstream. This may indicatethat the stirring effect of the lobed nozzle to enhance the mixingbetween the core jet owand ambient owis concentratedprimarily in the rsttwo diameters (rstsix lobe heights ). In thecross plane of Z=D 3:0 .Z=H 8:0/,theensemble-averaged streamwisevorticity dissipated D so seriouslyD that the strength of the 1 ensemble-averagedstreamwise vortices became only about 10 th of thatat theexit of thelobed nozzle. This means that the stirring effectof thelobed nozzle has almost disappeared at thisdownstream location. Thecharacteristics of themixing process between the core jet ow andambient owin thelobed jet owmay be revealedmore quanti- tativelyfrom the averaged-turbulent kinetic energy pro le given in Fig.8. Inthe present paper, the averaged-turbulent kinetic energy

a) Velocityvectors

Fig.7 Decayof the ensemble-averagedstreamwise vorticity.

b) Velocityisosurfaces Fig.6 Reconstructed three-dimensional oweld. Fig.8 Averagedturbulent kinetic energy pro le. HU ET AL. 1345 ofthelobed jet owat eachmeasured cross plane was calculated thecore jet owand ambient owis concentratedprimarily in the byusingthe following equation: rsttwo diameters of thelobed nozzle (rstsix lobe heights ). The averagedturbulent kinetic energy pro le also indicated that most ½W.x; y; z/K .x; y; z/ dx dy oftheintensive mixing between the core jet owand ambient ow K .z/ N D ½W .x; y; z/ dx dy occurredover the rsttwo diameters of thelobed jet ow ( rst six R lobeheights ).Theisovelocity contours of thelobed jet owin the Itwas found that the averaged-turbulent R kinetic energy grew very fartherdownstream regions (Z=D > 3:0, Z=H > 12:0) are found to rapidlywithin the rstdiameter of the lobed nozzle and then de- beconcentriccircles, which are quite similar as those in acircularjet creasedto amoremoderate rate farther downstream. The averaged- ow.Theseresults indicate that the mixing enhancement caused by turbulentkinetic energy pro le reached its peak value at about thespecial geometry of thelobed nozzle concentrates primarily in Z=D 2:0 (Z=H 5:33) andthen began to decrease. This may the rsttwo diameters downstream of thelobed nozzle (rstsix lobe beexplained D as follows. D Within the rstone diameter of the test heights).Themixing between the core jet owand ambient owin nozzle,due to thestirring effect of thelarge-scale streamwise vor- thefarther downstream region (Z=D > 3:0, Z=H > 12:0) will occur ticesgenerated by thelobed nozzle revealed in thegiven streamwise bythesame gradient-type mechanism as thatfor a circularjet ow. vorticitydistributions, the core jet owand ambient owmixed in- tensively.In the downstream region of Z=D > 1:0 (Z=H > 2:67), References thelarge-scale streamwise vortices generated by the lobed nozzle 1Kuchar,A. 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This is 1997,pp. 838 –84l. proposedas thereason that a lobednozzle would enhance both large- 17Belovich,V .M.,Samimy,M., andReeder, M.F.,“ DualStream Axisym- scalemixing and small-scale mixing reported by otherresearchers. metric Mixingin thePresence ofAxial V orticity,”Journalof Propulsionand Theoverall effect of the lobed nozzle on the mixing process in Power,Vol.12, No. 1, 1996,pp. 178 –185. thelobed jet owwas evaluated by analyzingthe evolution of the 18Glauser, M.,Ukeiley,L., andWick, D., “ Investigationof Turbulent ensemble-averagedstreamwise vorticity distributions. The strength FlowsVia PseudoFlow Visualization, Part 2: The Lobed Mixer Flow Field,” oftheensemble-averaged streamwise vortices was found to decay ExperimentalThermal and Fluid Science ,Vol.13, No. 2, 1996,pp. 167 –177. veryrapidly in the rsttwo diameters of thelobed nozzle, then turn toamoderatedecay rate farther downstream. This may indicate that J. P. Gore thestirring effect of the lobed nozzle to enhance mixing between AssociateEditor