Time Evolution And Mixing Characteristics Of Hydrogen And Ethylene .

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PHYSICS OF FLUIDS 18, 026101 共2006兲Time evolution and mixing characteristics of hydrogen and ethylenetransverse jets in supersonic crossflowsA. Ben-Yakara兲Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712M. G. Mungal and R. K. HansonDepartment of Mechanical Engineering, Stanford University, Stanford, California 94305共Received 8 February 2005; accepted 28 July 2005; published online 13 February 2006兲We report an experimental investigation that reveals significant differences in the near-flowfieldproperties of hydrogen and ethylene jets injected into a supersonic crossflow at a similarjet-to-freestream momentum flux ratio. Previously, the momentum flux ratio was found to be themain controlling parameter of the jet’s penetration. Current experiments, however, demonstrate thatthe transverse penetration of the ethylene jet was altered, penetrating deeper into the freestream thanthe hydrogen jet even for similar jet-to-freestream momentum flux ratios. Increased penetrationdepths of ethylene jets were attributed to the significant differences in the development oflarge-scale coherent structures present in the jet shear layer. In the hydrogen case, the periodicallyformed eddies persist long distances downstream, while for ethylene injection, these eddies losetheir coherence as the jet bends downstream. The large velocity difference between the ethylene jetand the freestream induces enhanced mixing at the jet shear layer as a result of the velocity inducedstretching-tilting-tearing mechanism. These new observations became possible by the realization ofhigh velocity and high temperature freestream conditions which could not be achieved inconventional facilities as have been widely used in previous studies. The freestream flow replicatesa realistic supersonic combustor environment associated with a hypersonic airbreathing engineflying at Mach 10. The temporal evolution, the penetration, and the convection characteristics ofboth jets were observed using a fast-framing-rate 共up to 100 MHz兲 camera acquiring eightconsecutive schlieren images, while OH planar laser-induced fluorescence was performed to verifythe molecular mixing. 2006 American Institute of Physics. 关DOI: 10.1063/1.2139684兴I. INTRODUCTIONA useful scramjet combustor requires enhanced mixingof fuel and air. Because of the high velocities associated withsupersonic/hypersonic flight speeds, mixing is slow compared to the residence time of the flow. Efficient performanceof very high-speed combustor systems requires fuel and airmixing at the molecular level in the near field of the fuelinjection.One of the simplest injection configurations to enhancenear-field mixing is the transverse 共normal兲 injection of fuelfrom a wall orifice. As the fuel jet, sonic at the exit, interactswith the supersonic crossflow, an interesting but rather complicated flowfield is generated. Figure 1 illustrates the general flow features of an underexpanded transverse jet injectedinto a supersonic crossflow. As the crossflow is displaced bythe fuel jet a three-dimensional 共3D兲 bow shock is produceddue to the blockage of the flow. The bow shock causes theupstream wall boundary layer to separate, providing a regionwhere the boundary layer and jet fluids mix subsonicallyupstream of the jet exit. This region, confined by the separation shock wave formed in front of it, is important in transverse injection flowfields owing to its flame-holding capabila兲Author to whom correspondence should be addressed. Electronic 2兲/026101/16/ 23.00ity in combusting situations, as has been shown in previouspublications.1,2Mixing properties of normal injection into supersonicflows are controlled by the jet vortical structure which can bepartially extrapolated from studies of jets in subsonic flows.The experimental studies performed by Fric and Roshko3provide some insight into the vortical structure of a transverse jet injected into a low-speed crossflow. Their photographs, obtained using the smoke-wire visualization technique, illustrate four types of coherent structures: 共1兲 thenear-field jet-shear layer vortices; 共2兲 the far-field counterrotating vortex pair 共CVP兲; 共3兲 the horseshoe vortex whichwraps around the jet column; and 共4兲 the downstream wakevortices originating from the horseshoe vortex. Figure 1shows the presumed vortical structures for the jet in supersonic crossflow 共which are known to exist in subsonic jet-incrossflow兲 as they were partially observed by numerousstudies.2,4,5The origins of the jet vortical structures were studied byseveral researchers.3,6,7 Among those studies, Yuan et al.7performed a large-eddy simulation of transverse jets in subsonic crossflows. Their results revealed that the majority ofthe jet vortical structures arose from the Kelvin-Helmholtz共K-H兲 instability of the jet-shear layer in the near field. Interestingly, they do not observe the formation of vortex ringsaround the periphery of the jet as was assumed in previous18, 026101-1 2006 American Institute of PhysicsDownloaded 29 Apr 2006 to 171.64.10.189. Redistribution subject to AIP license or copyright, see http://pof.aip.org/pof/copyright.jsp

026101-2Phys. Fluids 18, 026101 共2006兲Ben-Yakar, Mungal, and Hansontransverse jet injection into supersonic crossflows, determined that in the far field the eddies tend to travel withvelocities that are closer to the freestream velocity. This indicates that in high speed freestream conditions, these largecoherent structures, where the fuel and air are mixed by slowmolecular diffusion, will also travel at high speeds. Consequently the combustion process will be mixing controlled.High mixing efficiency, however, must be achieved inthe near field of the fuel injection for the success of hypersonic propulsion systems. Therefore, it is important to understand how these structures and their growth rates evolve asflow and jet conditions are changed. Two types of fuel arebeing considered for use in supersonic combustion: 共1兲 hydrogen and 共2兲 hydrocarbon fuels. The large differences inthe molecular weights of these fuels result in a wide variation in injection velocities that might lead to a substantialvariation in the jet shear layer growth rate and the mixingproperties. However, none of the previous jet penetrationstudies9–14 found any dominant differences between jets withdifferent molecular weights. Penetration was shown to bedependent primarily on the jet-to-freestream momentum flux,J, expressed byJ FIG. 1. Schematic of an underexpanded transverse injection into a supersonic crossflow. 共a兲 Instantaneous side view at the centerline axis of the jet;共b兲 3D perspective of the averaged features of the flowfield 共Ref. 14兲.studies. Instead they find two kinds of vortices originatingfrom the jet exit boundary layer: 共1兲 regularly formed spanwise rollers on the upstream and downstream edges 共largescale jet shear layer vortices兲, 共2兲 quasisteady vortices, theso-called “hanging vortices” that form in the skewed mixinglayers 共mixing layers formed from nonparallel streams兲 oneach lateral edge of the jet leading to the formation of theCVP.The near-field mixing of transverse jets is dominated bythe so-called “entrainment-stretching-mixing process” drivenby large scale jet-shear layer vortices. In the region near theinjector exit, the injectant fluid moves with a higher velocitytangent to the interface than the freestream fluid. As a result,large vortices are periodically formed engulfing large quantities of freestream fluid and drawing it into the jet-shearlayer 共macromixing兲. These large scale vortices also stretchthe interface between the unmixed fluids. Stretching increases the interfacial area and simultaneously steepens thelocal concentration gradients along the entire surface whileenhancing the diffusive micromixing.Preliminary examinations5,8 of the convection characteristics of these large-scale structures, developed in sonic共 u2兲 j 共 pM 2兲 j ,共 u2兲 共 pM 2兲 共1兲where the subscript j corresponds to the jet exit conditionsand corresponds to freestream conditions ahead of a bowshock.One exception to this is the work of Auvity et al.15where low momentum slot jets of helium and nitrogen areinjected into hypersonic boundary layers. These authors notea significant difference in the nature of the boundary layerdue to gas composition which might serve as a precursor tothe types of results to be presented below.Most transverse jet-in-crossflow studies were, however,carried out in cold supersonic flows 共namely low velocities兲generated in blow-down wind tunnels. The freestream temperatures and velocities in these facilities were usually lowerthan that expected in a real supersonic combustor environment. Comprehensive studies still need to be performed todetermine the mixing properties of different types of fuels ina relatively realistic supersonic combustor environment.These observations gave rise to the following question: “Isthere any other mechanism or controlling parameter otherthan jet-to-freestream momentum flux, which might alter thelarge eddy characteristics of the jet shear layer and thereforeaffect its near field mixing in realistic conditions?”Thus, we were challenged to study the flow features ofhydrogen and ethylene transverse jets exposed to high-speedsupersonic freestreams at realistic conditions leading to highlevels of shear. Such an effort requires the use of an impulsefacility to achieve high speed flows with high temperatures.The application of nonintrusive flow diagnostic techniques athigh repetition rates provides information on the temporalevolution of fast flow structures. The freestream conditions,generated using an expansion tube facility, simulate a realistic supersonic combustor environment for a Mach 10 flightspeed.Downloaded 29 Apr 2006 to 171.64.10.189. Redistribution subject to AIP license or copyright, see http://pof.aip.org/pof/copyright.jsp

026101-3Phys. Fluids 18, 026101 共2006兲Transverse jets in supersonic crossflowsTABLE I. Supersonic crossflow 共nitrogen兲 conditions.PropertySymbolUnitValueMach numberVelocityStatic temperatureStatic pressureM U T p m/sKkPa3.38 0.042360 25129032.4DensityStagnation enthalpy Htot, ᐉatmkg/ m30.320.0846MJ/ kg s3.9 0.1270 10mmmm500.75Steady flow timeDistance of injection port from the flat plate leading edgeBoundary layer thickness at the injection portReynolds number at the injection portThe outstanding questions that we are investigating are:How do the jet shear layer vortices develop and which parameters control their stability and coherence? What is thecontribution of the jet shear layer vortices to the near-fieldmixing? Does the penetration mechanism only depend onjet-to-crossflow momentum ratio as has been proposed forthe last 40 years or is there any other mechanism leading tohigher penetration and better mixing properties? In the following sections, our observations will be discussed and willprovide new insights into the above questions.II. EXPERIMENTAL APPROACHA. Expansion tubeWe use an expansion tube to provide a relatively accurate simulation of the true flight conditions at the entrance ofa typical supersonic combustor in the Mach 10 flight range.Due to the large total enthalpies 共greater than 3 MJ/ kg兲 associated with high flight speeds beyond Mach 8, only impulse facilities are capable of providing the conditions forground testing, typically with short test times 共 0.2– 2 ms兲.Table I summarizes the freestream supersonic flow conditions used in the current experiments. A detailed characterization of the flow properties are discussed elsewhere.16The expansion tube facility with its dedicated lasers andoptical arrangement is schematically illustrated in Fig. 2. Theflow facility is 12 m in length, has an inner diameter of 89mm, and includes three main sections: driver, driven, andexpansion sections. The operation cycle of the expansiontube is initiated by bursting the double diaphragms, whichcauses a shock wave to propagate into the test gas and produce a flow of intermediate velocity with an increased pressure and temperature. The shocked test gas 共in the drivensection兲 then accelerates through the expansion section andemerges from the end of the tube.Downstream of the exit of the expansion section asquare viewing chamber of 27 27 cm cross section ismounted. This test chamber is equipped with an opposed pairof square 共13 13 cm兲 quartz windows for observation and afused silica window on top of the chamber for admission ofthe vertical laser sheet for laser-based diagnostics such asOH-PLIF. Re U ᐉ / 2.2 105B. Injection system and its calibrationThe injection system is positioned right at the exit of theexpansion tube inside the test section 共Fig. 3兲. The systemconsists of a flat plate with an attached high speed solenoidvalve 共less than 1 ms response time, General Valve Series 9,Iota One controller兲 which allows near-constant injectionflow rates during the expansion tube test time period. For theresults presented here, an underexpanded transverse jet ofhydrogen with a d j 2 mm port diameter has been used. Thejet port is located at a distance 30 mm downstream of thetube exit and about ᐉ 50 mm downstream of the flat plateleading edge. At the injection location, the freestream boundary layer thickness, developing on the flat plate, is approximately 0.75 mm. Table II summarizes the jet flow properties at the exit of the sonic orifice.Calibration of the injection system was performed to determine the stagnation pressure losses through it. This wasaccomplished by comparing the Mach disk height of an underexpanded jet into still air with a well-known empiricalcorrelation. Schlieren flow visualization was used to measurethe Mach disk height for different pressure ratios.17 The ex-FIG. 2. Expansion tube facility 共12 m in length and 89 mm inner diameter兲and imaging system.Downloaded 29 Apr 2006 to 171.64.10.189. Redistribution subject to AIP license or copyright, see http://pof.aip.org/pof/copyright.jsp

026101-4Phys. Fluids 18, 026101 共2006兲Ben-Yakar, Mungal, and Hansonimaging was used to observe the temporal development ofthe jet. This combined with the traces obtained using a fastresponse pressure transducer located at the jet exit, allowedthe determination of the optimum valve actuation time共 1.5 ms before start of test time兲.C. Flow diagnosticsFIG. 3. Schematic of the injection system.pected jet Mach disk position, based on the correlation suggested by Ashkenaz and Sherman18 as a function of jetstagnation pressure 共ptot,j兲 to effective back pressure 共peb兲ratio, is冉 冊ptot,jy1 0.67 ·djpeb1/2共2兲,where y 1 is the height of the Mach disk. On the basis of thiscorrelation, measurements indicated a stagnation pressureloss of 48% for hydrogen injection and 41% for ethyleneinjection during valve operation 共note that the fuels weresupplied from flow lines of different length兲.In addition, the valve actuation time and the tube firinghave to be synchronized such that the jet is fully developedby the time the steady test flow conditions are obtained.Within that constraint, the time interval between the valveactuation and the test gas arrival should be short enough toavoid significant changes in the expansion section initialpressure. To determine the jet development time, schlierenTABLE II. Jet exit flow properties at the sonic orifice.Jet exit conditionsHydrogenEthyleneMjU j 共m/s兲T j 共K兲p j 共MPa兲 j 共kg/ m3兲JM w,j 共g / gmol兲 jd j 共mm兲 j 共m2 / s兲Red j U jd j / j112052460.490.481.4 0.121.4220.16 10 6150 00013152630.557.021.4 0.1281.2721.32 10 6477 000We utilize a MHz repetition rate imaging system to acquire a sequence of schlieren images of the supersonic flow,since tracking the structural evolution of high-speed flowsrequires acquisition of images at fast repetition rates. Detailed descriptions of the ultrahigh-speed schlieren systemand its synchronization with the expansion tube operation areprovided elsewhere.8 Although the schlieren technique hassome limitations, since it integrates the effects of densitygradients along the beam propagation path, it can still beused to identify and track structures along the edge of the jet.Features internal to the jet can be discerned only with a planar light-sheet technique such as planar laser induced fluorescence 共PLIF兲. We therefore included in our study PLIF ofOH radicals to gain further information on the molecularmixing.2 OH-PLIF maps the regions of ignition where thefuel and the crossflow 共air or oxygen兲 are mixed and burn atthe molecular level.The ultrafast-framing schlieren system includes threecomponents: 共1兲 a high-speed framing camera 共Imacon 468,manufactured by Hadland Photonics兲, 共2兲 a long durationlight source 共xenon flashlamp兲, and 共3兲 mirrors and knifeedge in a standard Z arrangement. The IMACON 468 consists of eight independent intensified CCD cameras for highspeed framing that can capture eight consecutive imageswith variable exposure and interframing times down to 10ns. The single optical input is divided uniformly by a specialbeamsplitter and directed onto eight different intensifiedCCD modules, each with a 576 384 array of 22 22 msize pixels. The light source is a high intensity xenon flashdischarge unit 共Hadland Photonics model 20–50 flash systemwith an extension to 200 s duration兲. The unit has threeranges providing 20 s , 50 s, and 200 s durations, withdischarge energies of 125 J, 375 J, and 700 J per pulse,respectively.In the optical setup, two f / 10 共f number is defined as theratio of the focal length to the radius of the mirrors兲, 200 cmfocal length concave mirrors are used to collimate the lightthrough the test section and then refocus it onto a knife edge共razor blade兲. This knife edge 共KE兲 at the focal point of thesecond schlieren mirror is used to partially cut off the deflected rays for observing the schlieren effect 共visualizationof density gradients兲. The test object is then imaged with asingle 共constant focal length兲 lens onto the intensified CCDcameras. Two different focal length lenses 共an f / 12.5, 100cm focal length lens and an f / 6, 49 cm focal length lens兲were used to image different sizes of the field of interest. The100 cm focal length lens imaged a field of view of 28 18 mm onto the 12.7 8.5 mm CCD array, demagnifyingthe object by 0.44. However, with a 49 cm focal length lensa larger field of view of 50 30 mm could be imaged. Theexposure time of the intensified cameras was adjusted to re-Downloaded 29 Apr 2006 to 171.64.10.189. Redistribution subject to AIP license or copyright, see http://pof.aip.org/pof/copyright.jsp

026101-5Transverse jets in supersonic crossflowsPhys. Fluids 18, 026101 共2006兲solve the turbulent flow features. A detailed discussion ofresolution considerations can be found in Ben-Yakar andHanson.8III. RESULTS AND DISCUSSIONWe have studied the flowfield properties of both hydrogen and ethylene transverse jets using nonintrusive diagnostic techniques such as ultrafast-framing schlieren and OHPLIF. The jet-to-freestream momentum flux ratio 共J兲 ischosen to be identical 共J 1.4兲 for both cases. On the otherhand, note that the exit velocities of both jets are quite different due to the substantial difference in their molecularweights 共see Table II兲.In the following sections, we will first present the general flowfield properties of transverse injection into a supersonic crossflow. Then we will discuss the characteristics ofthe large scale eddies, their convection and mixing propertiesand the jet penetration as observed using time-correlatedschlieren images and finally the OH-PLIF results. Althoughthe results presented here are only for one value of J, it isworth noting that experiments with different values of J provided similar results.A. General flowfield featuresSchlieren imaging provides a visual observation of bothinstantaneous and average characteristics of the flowfield depending on the exposure time of the image. While a shortduration schlieren image 共100-200 ns exposure time兲 revealssome of the instantaneous vortex and shock structure of theflowfield, a long duration schlieren image 共3 s exposuretime兲 provides information on the average and more steadyproperties.Figure 4 shows two instantaneous schlieren images ofhydrogen and ethylene jets injected into a supersonic crossflow of nitrogen. Note that the x axis is normalized with thejet diameter d j 2 mm. In jet-in-crossflow studies, it is common to present the jet trajectories in x / d j冑J space.19 Since Jis identical in both cases presented here a comparison can bemade between their penetration features in x / d j space.Freestream fluid flows from left to right, and the fuel jetsenter from the bottom at x / d j 0.Several interesting features, such as the large-scale structures at the jet periphery and the bow shock are very apparent in these images. The large-scale eddies are periodicallygenerated in the early stages of the jet/freestream interaction.While those eddies exist in both cases, they demonstrate significant differences in their development as they convectdownstream. In the hydrogen case, these structures preservetheir coherence with distance while in the ethylene case theydisappear beyond about 12 jet diameters downstream. Thisresult that is consistent in all visualizations obtained, is not aschlieren contrast issue, rather it is most likely related to theenhanced mixing characteristics of the flowfield. As will bediscussed in the following section, the schlieren contrast forethylene injection is expected to be 10 times larger thanthe hydrogen case in the absence of mixing 共hot nitrogen vscold ethylene兲. The schlieren contrast will diminish when thehot freestream fluid begins to mix with the cold ethylene jetFIG. 4. Examples of hydrogen 共a兲 and ethylene 共b兲 injections into a supersonic crossflow 共nitrogen兲. Exposure time of each image was 200 ns. The xaxis is normalized by the jet diameter d j 2 mm.while creating a region of reduced density gradient. The ethylene structures are larger and penetrate deeper into thecrossflow. Besides the bow shock, additional weak shockwaves are formed around the ethylene eddies indicating theirsubsonic motion relative to the freestream. A detailed examination of these large scale structures is performed using highspeed schlieren movies and will be discussed in the following sections.Figure 4 also shows that the bow shock is almost mergedwith the jet close to the injection location with a very smallstandoff distance and curves sharply downstream. Its localshape appears to depend strongly on the large scale shearlayer structures, especially close to the jet exit where thefreestream behind the steep bow shock is subsonic. As aresult, the bow shock reveals local fluctuations in position,which are small in the hydrogen case but significant in theethylene case.In Fig. 4, there appears to be more “speckling” in theimages of the ethylene jet than appears to be in the case forthe hydrogen jet. The difference in the level of “speckling”might be due to any or all of the following reasons: 共1兲higher Reynolds number of the ethylene jet, 共2兲 slower ethylene jet flow that smears less the turbulent structures duringthe camera exposure time, and 共3兲 higher sensitivity of theethylene jet to the schlieren effect.Figure 5 shows an example for the hydrogen flowfield,visualized with a longer exposure time 共3 s兲. Additionalfeatures are emphasized and become visually observable:such as the upstream separation shock wave and the down-Downloaded 29 Apr 2006 to 171.64.10.189. Redistribution subject to AIP license or copyright, see http://pof.aip.org/pof/copyright.jsp

026101-6Ben-Yakar, Mungal, and HansonPhys. Fluids 18, 026101 共2006兲FIG. 5. An example schlieren image with 3 s exposure time for hydrogeninjection case. While the unsteady features 共coherent structures兲 are averaged to zero, some of the weak shocks such as upstream separation shockwave and downstream recompression wave are emphasized.stream reattachment shock. The small instantaneous fluctuations of the bow shock are observed to average into asmoother and slightly thicker one.The barrel shock and the Mach disk are, however, notvery clear even in the long exposure schlieren images, mostprobably due to the unsteadiness of the shear layer vorticalstructures. Only the Prandtl-Meyer expansion fan of the underexpanded jet is observable 共the white region at the jetcore兲 indicating that the jet is indeed underexpanded. Wehave therefore attempted to estimate the location of the firstMach disk for our experiments by substituting an “effectiveback pressure” term in the Ashkenaz and Sherman18 correlation given in Eq. 共2兲. The effective back pressure introducedin earlier work is a notion that permits an analogy betweenthe very complicated flowfield of an underexpanded jetemerging into a supersonic crossflow and that for the simplerand well-understood case of a jet exhausting into a quiescentmedium. Among those previous studies, Schetz and Billig10suggested peb 0.8p , where p is the freestream pressurebehind a normal shock wave. Later, Billig et al.20 developeda correlation to predict the height of the Mach disk, y 1, assuming that the effective back pressure is equal to two thirdsof the freestream stagnation pressure behind a normal shockpeb 2 / 3ptot, . More recently Everett et al.21 measured thepressure distribution around a sonic jet injected transverselyinto a Mach 1.6 freestream using a pressure-sensitive-painttechnique. Their averaged surface pressure resulted in peb 0.35p 共for J 1.5兲 which differs greatly from the earlierwork. This discrepancy was attributed to the larger jet-tomomentum flux ratios, J used earlier. We have adopted theback pressure values of Everett 共peb 0.35p 兲, since thevalue of J in our experiments is small. Using Eq. 共2兲, theMach disk height for the current experiments was estimatedto be around y 1 1.7· d j which compares well with the jetbending location 共see discussion below兲.The freestream conditions behind the hydrogen bowshock could be estimated by measuring the average bowshock position. Figure 6 presents two plots; the first showsthe measured bow shock position and its angle 共 兲, while thesecond plot shows the bow shock-induced freestream velocity 共U2兲 and its turning angle 共 兲. Calculations are performedassuming a calorically perfect gas. In the region of 10 jetFIG. 6. 共a兲 Bow shock position and its angle at the centerline of the jet asmeasured from the long exposure schlieren image shown in Fig. 5. 共b兲 Thefreestream velocity behind the bow shock and the flow turning angle basedon the measured bow shock shape. For the calculations a calorically perfectgas has been assumed.diameters studied in this work, the bow shock starts almost at90 and weakens downstream as it angle decays continuously down to 20 –25 . Further downstream, the bow shockis expected to reach its minimum strength or a Mach wavewith an angle of 17.2 共M 3.38兲. The induced velocity ofthe freestream behind the bow shock is subsonic upstream ofthe location of the critical bow shock angle 共 cr 67.6 兲,defined as the maximum angle for an oblique shock to beattached to a wedge. It is interesting to see that the bowshock reaches this angle around 1.8–1.9 jet diameters abovethe wall at the expected height of the upper side of the Machdisk. Since the Mach disk occurs at a rather high Mach number on the jet centerline, the jet loses most of its momentum共owing to the rise on the static pressure across the Machdisk兲 and the subsequent trajectory of the jet turns nearlyparallel to the freestream direction. Consequently, beyondthe critical angle, the bow shock curves sharply downstreamand the shock-induced freestream velocity becomes supersonic varying from approximately 1050 m / s to 2260 m / s at9.5 jet diameters downstream 共note that the freestream veloc-Downloaded 29 Apr 2006 to 171.64.10.189. Redistribution subject to AIP license or copyright, see http://pof.aip.org/pof/copyright.jsp

026101-7Transverse jets in supersonic crossflowsPhys. Fluids 18, 026101 共2006兲FIG. 7. An example of eight consecutive schlieren images of underexpanded hydrogen injection 共d 2 mm兲into a supersonic crossflow 共nitrogen兲 obtained by highspeed-framing camera. Exposure time of each image is100 ns and interframing time is 1 s. Freestream conditions are U 2360 m / s, M 3.38, T 1290 K, p 32.4 kPa; and the jet-to-freestream momentum ratio isJ 1.4 0.1.ity is U 2360 m / s兲. In the following sections, this estimated freestream velocity behind the bow shock will becompared to the measured convection velocity of the largescale structures. Before that we will first discuss the temporalevolution of these structures.B. Large scale coherent structuresThe most interesting observations are related to the coherent structures, which are easily identified in instantaneousschlieren images. The large scale jet-shear layer vortices areconsidered important because of their role in the near-fieldmixing. These intermittently formed eddies appear to enlargeand engulf freestream fluid as they travel downstream withthe flow. We therefore studied the temporal evolution of thelarge eddies and their properties for both hydrogen and ethylene jets utilizing the high-speed-framing rate camera. Examples of instantaneous schlieren images are presented inFig. 7 for hydrogen injection and in Figs. 8 and 9 for theethylene case. While large-scale eddies are visible in theearly stages of the jet/freestream interaction, there are significant differences in their development for hydrogen andethylene injection.Hydrogen large scale coherent structures survive longdistances. Coherence of these shear layer eddies can be seenin Fig. 7, which constitutes consecutive schlieren imagesfrom a single experiment. Close to the jet exit, the circumferential rollers rise periodically creating gaps in between the

so-called "hanging vortices" that form in the skewed mixing layers mixing layers formed from nonparallel streams on each lateral edge of the jet leading to the formation of the CVP. The near-field mixing of transverse jets is dominated by the so-called "entrainment-stretching-mixing process" driven by large scale jet-shear layer vortices.

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