44th AIAA Aerospace Sciences Meeting and Exhibit

January 9–12, 2006/Reno, NV

Pulsed Jet Ejector Characteristics

Isaac M. Choutapalli∗, Anjaneyulu Krothapalli† and Luiz M. Lourenco‡

Department of Mechanical Engineering,

Florida A&M University and Florida State University, Tallahassee, FL 32310, USA

cisaac@eng.fsu.edu

An experimental study is carried out to investigate the flow field of a pulsed jet ejectorthrust augmenter operating at a mean primary jet exit mach number of MjMjMj = 0.30 and ata pulsing frequency of fff = 210 Hz. Phase-locked Particle Image Velocimetry (PIV) is usedto obtain the detailed spatio-temporal evolution of the ejector flow field. Experiments werecarried out at three different area ratios that help to define the conditions for maximumthrust augmentation. In the presence of the ejector duct, the pulsed jet primary vortexinduces a secondary vortex at the ejector wall. The induced vortex strength was foundto be maximum, as defined by its circulation, at the condition of the maximum thrustaugmentation. The presence of the counter rotating vortex pair results in enhanced mixingwhich in turn produces increased thrust augmentation. The measurements suggest that acompact ejector (L/D ≈ 3) with an area ratio (duct area/primary nozzle exit area) of about10 can produce a thrust augmentation ratio (total thrust/primary nozzle thrust) of about2.3.

I. Introduction

The use of thrust augmenting ejectors in Vertical Take off and Landing Aircraft (VTOL) has been asubject of research for over five decades (e.g. Krothapalli, A., 19791). Most of the earlier research wasdealt with steady jet ejectors that required large area ratio (duct area/primary nozzle exit area) to generatesufficient thrust to be of practical importance. Lockwood,2 in 1960’s showed that using pulse jets, a significantincrease in thrust augmentation ratio with compact ejectors can be achieved. The thrust augmentation ofthe pulsed jet ejector described here is defined as follows:

Φ =Measured total thrust

Reference thrust(1)

where measured total thrust is the combined thrust due to both the ejector and primary jet, and referencethrust is the thrust due to the steady primary jet alone.

A renewed interest in VTOL aircraft prompted the present study to provide a better understanding ofthe pulsed jet ejector thrust augmenter (PETA) performance using a carefully selected parametric range andwhole flow field diagnostics. The pulsed free jet characteristics as described by Alkislar et al.3 show thatthe initial region of the jet (within first five diameters) is dominated by the periodic large-scale energeticvortices that produce periodic turbulent stresses resulting in significant increase in mass entrainment. Thepressure at the core of these vortical structures is found to be significantly lower than the atmosphericpressure resulting in further enhanced entrainment of the ambient fluid. When such a jet is enclosed by acarefully designed duct, significant increase in thrust augmentation can be achieved as demonstrated in a

∗PhD Candidate, AIAA Student Member.†Don Fuqua Professor, AIAA Associate Fellow.‡Professor.Copyright c© 2006 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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44th AIAA Aerospace Sciences Meeting and Exhibit9 - 12 January 2006, Reno, Nevada

AIAA 2006-1019

Copyright © 2006 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

recent paper by Choutapalli et al.4 Not only does the mixing of the entrained air with the primary jet withinthe ejector contribute to an increase in the thrust of the system but also reduces the velocity, temperature,and noise of the jets. The resulting low-temperature and pressure footprint of this mixed flow would enablean aircraft to operate from unprepared and inhospitable terrains and in space constrained, tactical landingzones.

Experiments conducted at different jet exit Mach numbers (M < 1) and ejector area ratios indicate a weakdependance of the Mach number on the PETA performance (Choutapalli et al.4). For example changingthe jet exit Mach number from 0.3 to 0.8 will only result in an increase of about 5% in the maximumthrust augmentation ratio. Additionally, our recent investigations (Choutapalli, I., 20065) suggest that theprimary jet temperature, within the range of interest, does not significantly affect the PETA performance,consistent with the earlier results of Barankiewicz et al.6 With these observations in mind, the present studyis conducted at a nominal jet exit Mach number of 0.3 and at a stagnation temperature of 313K.

A number of studies in the past have shown that the performance of the pulsed jet ejector is superioras compared to a steady jet ejector of comparable dimensions. Thrust augmentation ratios between 1.6 and2.4 were obtained by Lockwood2 depending on the type of augmenter used - a straight duct vs. a divergentduct, with a heated pulsed jet. An experimental and theoretical investigation was carried out by Johnson7

to examine the effect of pulse frequency, duty cycle and primary jet exit Mach number on the pumping ratio(ejector exit mass flow rate/primary mass flow rate) of the ejector along with several ejector geometries.His studies showed that a high-energy pulse of a sufficiently small duration is required to achieve higherthrust-augmentation. Bernal and Sarohia8 examined the effect of jet modulation on thrust augmentationwith relatively small pulse amplitude of about 8% of the mean exit velocity. Their results indicate a modestimprovement of about 20% in thrust augmentation ratio over a corresponding steady jet ejector. Parikhand Moffat9 observed a substantial improvement in entrainment by pulsing the jet at organ-pipe resonantfrequency of the confined tube. More recently Paxson et al.10 investigated the effect of area ratio on thrustaugmentation and obtained a peak thrust augmentation of around 2.0. The enhanced thrust augmentationwas attributed to the properties, such as the circulation and size, of the vortex ring that is emitted. Aresonance tube was used in the experiments of Wilson and Paxson11 to generate the pulsed jet, where thethrust augmentation was found to be around 1.32. All of these experiments are indicative of an enhancedperformance of the pulsed jet ejector. There have been some attempts to study the pulsed flow field usingParticle Image Velocimetry. Opalski et al.12 carried out PIV measurements on the exhaust flow field of apulse detonation engine (PDE) and concluded that the emitted vortex is central to the observed high levelsof thrust augmentation. A similar investigation was carried out by Wentworth et al.13 to mainly investigatethe flow field at the ejector inlet. It was concluded from their study that the ratio of the vortex ring diameterto the ejector diameter is a factor in determining the optimal thrust augmentation.

Most of the previous investigations on pulsed jet ejectors focused mainly on thrust measurements and onlya very few have attempted PIV measurements of the flow field of the pulsed jet. Even in those investigations,a detailed study of the properties of the vortex ring are dealt with to a very limited extent. In addition,there is a dearth of literature related to flow field measurements inside the ejector. The primary objective ofthis study is to describe the dynamics of the flow field inside the pulsed jet ejector using PIV and to addressthe issue as to why the thrust augmentation of the pulsed jet ejector peaks at a certain area ratio and

(a) Schematic of the facility. (b) Cross sectional view A-A of the ejector (L=10d) for

different area ratios; a) 7.6 b) 11.0 c) 12.0.

Figure 1. The Pulsed Jet Facility.

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Figure 2. The effective chopper opening area changein a period; Shaded circles: rotating disk open-ing; Open circles: nozzle inlet opening.

Figure 3. Schematic arrangement of phase lockedPIV system.

begins to decrease as the area ratio is further increased. Towards this end, three area ratios were selected,i.e. 7.6, 11.0 and 12.0. Area ratio 11 is the case where maximum thrust augmentation is observed in thepresent investigation. The primary jet Mach number chosen for this study was 0.30 at a pulsing frequencyof 210 Hz. The total temperature in the stagnation chamber is kept at 313 K for all the cases mentionedhenceforth. The mean velocity at the nozzle exit is 100.5 m/s at the pulsing frequency of 210 Hz. The meanvelocity profile at the nozzle exit is a top hat profile and the state of the boundary layer is turbulent. Thejet Reynolds number based on the nozzle exit diameter and the mean jet exit velocity is 3.1 × 105.

II. Experimental apparatus and procedures

The experiments were conducted in the Pulsed Jet Facility (figure 1) of the Fluid Mechanics ResearchLaboratory located at the Florida State University. The facility was designed to produce pulsed jets at Machnumbers up to about 1.5 with stagnation temperatures of up to 600 K at flow rates of 1,000 SCFM. Thenozzle is attached to an axial force balance that measured the thrust. The ejector coupled to a linearlyfree support, is attached to the nozzle for thrust augmentation measurements. The rectangular ejector ductis of a fixed length (508 mm), width (152.4 mm) and a variable height (38.1 mm - 158.75 mm) giving theflexibility of having variable area. The ejector is designed with a super elliptic curve at the leading edgeto avoid separation for increased performance. The ejector is placed in such a way that the minimum areacoincides with the nozzle exit plane. This would contribute to an increase in the velocity of the fluid beingentrained into the ejector.

A schematic diagram of the pulse jet facility is given in figure 1(a). The air supply for the facility isprovided from high-pressure (15 MPa) storage tanks connected to a high displacement air compressor. Thetanks have a total capacity of 10 m3 and could drive jets with mass flow rates of 0.25 kg/s continuouslyup to 40 minutes. The plenum pressure and temperature were maintained steady by two pneumatic valvesand an inline electrical heater with automatic controllers. During the present experiments, the pressure waskept at its nominal value within a variation of ±2 kPa and the total temperature within ±0.5 K. The steadyflow supply to a settling chamber is converted into a pulsed flow by passing the high-pressure air through arotating circular-slotted disk. The rotating disk, which had six circular holes with size equal to the nominalinner diameter of the nozzle pipe, Dp = 76 mm, functioned as a flow chopper. The frequency of the pulsedflow is varied by varying the speed of the driving motor. The pulse frequency could be varied precisely upto 250 ± 1 Hz. The calculated opening area at the entrance of the pipe as a function of period is given infigure 2. The shaded circles represent the rotating disk opening and the open circles represent the nozzleinlet opening. This configuration gives a duty cycle of 50 %. The resulting pulsed flow propagates throughthe nozzle pipe of length, Lp = 450 mm, to reach to the circular nozzle exit with a diameter of d = 50.8 mm.The nozzle has a smooth cubic spline contraction before a 25.4 mm straight section at the exit. The nozzle is

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Figure 4. Total pressure vs. time signal at center ofthe nozzle exit.

Figure 5. Variation of thrust augmentation with arearatio; Mj =0.3, f =220 Hz.

connected to the stagnation chamber with a flexible stainless steel joint to allow axial thrust measurements.The joint is equipped with a liner inside to avoid excessive turbulence. A low spring constant of 30 kN/mensured precise thrust measurements.

The time evolution of the jet was captured by means of phase locked measurements. A schematic of theinstrumentation for the phase locked PIV measurements is shown in figure 3. A light sheet of approximately1 mm thickness was created by suitable combination of spherical and cylindrical lenses. The images arerecorded by two cross correlation CCD cameras (sharpVISION 1400 DE) fitted with a 55 mm (f/1.4) Nikonlens. The resolution of the camera was 1360 × 1024 and it was operated in double exposure mode. In thepresent experiments, the jet was seeded with sub-micron ( 0.3 µm) oil droplets generated by a modifiedWright Nebulizer, which delivered particles to the main jet. The flow rate of the seeding particles wascontrolled in such a way that there were enough particles in the jet. The ambient air was seeded withsmoke particles (1-10 µm) produced by a Rosco fog generator. The necessary signal for the synchronizationor phase locking of the PIV system and the pulsed flow was obtained from an incremental optical encoderattached to the rotating shaft. The encoder had 720 increments in order to precisely mark the phase ofthe pulses. The pulse trigger train from the encoder was synchronized with the fully closed pipe, whichcorresponds to the time at position ’0’ as shown in figure 2. This signal was divided appropriately so thatit corresponds to the frequency of the laser and the camera. A delay was applied to this signal to generatethe necessary phase trigger for the synchronized camera and laser strobe. All the time delays were accurateup to approximately 1 ns. The full cycle of the pulse was sampled with 60 phases with an interval of 6o

between phases. In each phase a total of 100 realizations were taken and the phase-averaged flow fields werecalculated by taking the mean of these realizations, giving an accuracy of 1% in the phase-averaged velocityand 10 % in the phase-averaged random turbulence measurements. The 6000 samples obtained from thephase-locked measurements were processed together to calculate the global mean quantities. For the phaseanalysis a triple decomposition of the variables was calculated in the same way explained in Alkislar et al.15

III. Results and Discussion

A. Unsteady nozzle exit flow and thrust measurements

The flow at the nozzle exit is characterized by measuring the unsteady total pressure using a Kulite pressuretransducer. Figure 4 shows a typical pressure-time signal obtained at the center of the nozzle exit. The jetpulsing frequency is 220 Hz. The corresponding Strouhal number is 0.11 (St = fd/Uj , f -pulsing frequency, d-nozzle exit diameter, Uj-mean nozzle exit velocity). The pressure-time signal is periodic and fluctuates abouta mean value. There is also a small amount of background flow present during the entire cycle. This is dueto the leak from the gap between the rotating flow-chopper and the fixed-slotted end-cap of the stagnation

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Figure 6. Global mean axial velocity contours from3D PIV data at the ejector exit plane, Mj = 0.3, f =210 Hz and area ratio=11.

Figure 7. Variation of normalized mass flow rate witharea ratio.

chamber. The shape of the variation is dictated by the geometry of the hole openings in the chopper. Inthe present experiments, since the openings are circular in geometry with 50 % duty cycle, the variation isnear sinusoidal although a more fuller profile would be ideal (a rectangular pulse with short duration). Theflow accelerates as the pulse begins and reaches a peak value. The amount of time it remains at the peakvalue depends on the frequency. At lower frequencies, there is a longer duration at the maximum value. Forhigher thrust augmentation, a high-energy pulse of sufficiently small duration is needed. As the hole closes,the flow decelerates and the next cycle begins. A different geometry of the opening in the rotating choppercan give rise to a different shape of the variation with time. The variation in the rms intensity of pressurep′

rms / (P0−Pa) with frequency is marginal.4 At a pulsing frequency of 220 Hz, the rms intensity of pressurefor Mach 0.30 is 55 %. The rms intensities of the periodic component of velocity, urms / Uj , measured atthe nozzle exit for three frequencies i.e. 60, 105 and 210 Hz for exit Mach number of 0.30 were found to be0.40, 0.42 and 0.42 respectively. The frequency of pulsations does not seem to affect the rms intensity of theperiodic component of velocity.4 This trend was also observed by Bernal and Sarohia.8

The main parameter characterizing pulsed jet flow is the thrust generated by the system. The thrust ofa free pulsed jet operating at a jet exit Mach number of 0.30 and pulsing frequency of 220 Hz is about 15%higher than that of an equivalent steady jet.4

Figure 8. Thrust augmentation ratio as a function ofnormalized mass flow rate.

Figure 5 shows the variation of the thrust augmen-tation ratio with area ratio for a nozzle exit Mach num-ber of 0.30 and a pulse frequency of 220 Hz. The arearatio is varied from 5.6 to 12.0. Also plotted in thesame figure is the data for a steady jet ejector. It isquite apparent that the performance of the pulsed jetejector is superior as compared to the steady jet ejec-tor of the same dimensions. As seen from the figure,the thrust augmentation of the pulsed case increasesfrom a value of around 1.35 at the area ratio 5.6 andreaches a peak value of 1.87 at area ratio of 11.0. Afurther increase in the area ratio to 12.0 results in areduction in the value of thrust augmentation to about1.67. This is particularly interesting when comparedto the behavior of a steady jet ejector which show anincreasing thrust augmentation with increasing arearatio. This indicates that the dynamics of the pulsedjet ejector are markedly different from that of a steadyjet ejector. Also plotted in the same figure is the datafrom the 3D PIV experiments conducted to verify the

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Figure 9. Ejector Flow field at Mj = 0.3, f = 210 Hz; a) Instantaneous PIV images b) Correspondinginstantaneous velocity-vorticity fields c) Phase averaged velocity-vorticity fields from 100 realizations; tophalf−vorticity field, bottom half−corresponding velocity field.

thrust augmentation values obtained from the load cell data. Phase-locked 3D PIV measurements were takenat ejector exit for a total of 60 phases with a phase difference of 6o. The global mean velocity field obtainedfrom 3D PIV measurements at the exit plane of the ejector is shown in figure 6. The thrust augmentationis calculated from the 3D PIV data as follows:

Φ =ρ

∫A

[u2e(t) dx dy]

ρU2j Aj

(2)

where the subscripts e denotes the ejector exit plane and j denotes the nozzle exit plane. The numerator isobtained by integrating the time-dependant momentum flux over the exit area, A, of the ejector. The termin the denominator, ρU2

j Aj represents the equivalent steady free jet thrust or momentum flux at the nozzleexit. It is seen from the figure 5 that there is fairly good agreement with the data obtained from load cellmeasurements and those from the 3D PIV measurements. Also plotted in the same figure are the theoreticalthrust augmentation curves obtained from a control volume analysis, the details of which can be found inreference [4]. The control volume analysis requires the fluctuation intensities at the ejector inlet as input,which is the reason for the two curves, one for area ratio 7.6 and the other for area ratio 11.0. The theoreticalcurves show an increasing trend of the thrust augmentation with area ratio for the pulsed jet ejector and donot account for the drop in thrust augmentation as shown by the experimental data. This is because thevortex dynamics have not been taken into account in the one-dimensional theoretical analysis. A reasonableagreement between both the methods of measurements i.e. 3D PIV and load cell, is also obtained for thesteady jet ejector. One of the main parameters that influences the thrust augmentation is the mass flow rate

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Figure 10. Phase averaged axial velocity contours with superimposed streamline pattern at different ejectorarea ratios for Mj =0.3, f =210 Hz, x / d = 2.3 and phase 144o; a) 7.6 b) 11.0 c) 12.0.

or the entrainment of the ambient air into the ejector. Figure 7 shows the variation of the normalized massflow rate with area ratio obtained from the control volume analysis for both the steady and pulsed cases.Also plotted in the figure is the data from 3D PIV measurements at area ratio 11. The normalized mass flowrate is calculated by integrating the velocity field based on the measurements at the ejector exit plane shownin figure 6 . The mass flow rate curves for a pulsed free jet from reference [14] show that a peak mass flowrate ratio of 6.85 is obtained when the jet is pulsed at a Strouhal number of 0.24. The theoretical curves offigure 8 indicate a possibility of an increase in the thrust augmentation to around 2.3 if the primary jet ispulsed at a Strouhal number of 0.24. Hence it is feasible that an increase of 52 % in entrainment by pulsingthe jet at a Strouhal number of 0.24 instead at 0.11 can possibly result in a net increase of around 30% inthrust augmentation up to 2.3 as suggested by figure 8.

B. Ejector Flow field

1. Phase Averaged Flow Field Characteristics

It has been observed from the results of the previous section that the thrust augmentation increases as thearea ratio is increased and reaches a peak value at 11. As the area ratio is further increased, the thrustaugmentation begins to decrease in magnitude . It is therefore of interest to examine the unsteady flow fieldinside the ejector in order to understand the thrust augmentation ratio variation with area ratio. Towardsthis end, three area ratios were chosen for the purpose of study i.e. 7.6, 11.0 and 12.0.

The characteristics of the flow field inside the ejector at an nozzle exit Mach number of 0.30 and at thethree above mentioned area ratios are investigated by using phase-locked Particle Image Velocimetry (PIV).The images are acquired using two cameras to cover the entire length of the ejector. The experiments areconducted at a pulsing frequency of 210 Hz. Figure 9(a) shows the typical double-exposed PIV images.The corresponding instantaneous velocity-vorticity flow field obtained by processing the image is shown infigure 9(b). The top half shows the vorticity field and the bottom half shows the corresponding velocityfield. The phase-averaged velocity-vorticity field obtained from 100 such instantaneous velocity fields isshown in Figure 9(c). The leading vortex ring followed by the trailing jet and the presence of reversed flowregion represented by the dark blue contour are seen. The remnants of the vortex from previous pulse ata downstream location in the ejector (x/d=8) can also be seen in the flow field. The phase-averaged flowfields enable the construction of different flow properties with time.

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Figure 11. Time evolution of the induced vortex inside the ejector for Mj = 0.3, f = 210 Hz and area ratio11; a) 72o b) 192o c) 288o d) (360+48)o

Figure 10 shows the phase-averaged axial velocity contours for the three different area ratios when thevortex ring is at an axial location of x/d=2.3. The streamline pattern is superimposed on the mean velocitycontours to highlight the flow features. The figures have been enlarged by showing only one half of theregion from the centerline up to the bottom edge of the ejector and from the ejector inlet to a downstreamlocation of 5d. At area ratio 11, shown by figure 10(b), a strong induced vortex is seen. This area ratiocorresponds to the maximum thrust augmentation condition. The streamlines are also closely spaced inthe vicinity of the vortex ring which is indicative of a higher flow velocity. The magnitude of the reversedflow velocity in this case reached a peak value of about 20 m/s. The curvature of the streamlines reflectsthe presence of pressure gradient in the flow field. Higher the curvature, greater is the pressure gradient.

Figure 12. Variation of maximum reverse flow ve-locity at different phases for ejector area ratio 11;Mj =0.3, f =210 Hz.

Similar phenomena occurs in a edge-tone flow field,where a large scale shear layer vortex induces a sec-ondary vortex on the wedge. The edge-tone flow fieldresults of Krothapalli and Horne16 suggest that thepresence of induced vortex results in a negative wallpressure. The regions of negative pressure convectsdownstream in congruence with the primary vortex inthe duct as shown in figure 11. The counter rotatingvortex pair establishes a pressure gradient in the axialdirection within the duct which reinforces the entrain-ment of the ambient flow. A higher entrainment thusresults in higher thrust augmentation. It appears thatthe strength of the primary jet vortex and its proxim-ity to the ejector wall will determine the strength ofthe induced vortex. In the case of area ratio 7.6 (fig-ure 10(a)), the strength of the primary vortex is notsufficient enough to induce a vortex before it touchesthe ejector wall. While at area ratio 12 (figure 10(c)),the distance of the duct wall from the primary vortexis too large to induce a strong vortex.

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Figure 13. Phase-averaged velocity profiles for con-fined and free pulsed jets at vortex location of x / d= 2.3, Mj =0.3, f =210 Hz.

Figure 14. Phase-averaged vorticity profiles for thecorresponding velocity profiles shown in figure 13.

A further analysis of the data for area ratio 11, as shown in figure 11 indicates that the strength ofthe induced vortex decreases with downstream distance. This is due to the primary vortex itself becomingweaker as it propagates through the ejector and hence cannot induce a stronger secondary vortex. Thisweakening of the induced vortex can be observed around 6.5d in figure 11(d). Also seen in the same figureis the formation of a new vortex as the new pulse cycle starts. The maximum reversed flow velocity us,max,caused by the induced vortex is plotted as a function of the phase angle in figure 12. The centerline velocityucl, in the plane of the vortex ring is used to normalize the reverse flow velocity. The figure shows that amaximum velocity ratio of around -15 % is reached at a phase angle of 144o which corresponds to a vortexlocation of x/d=2.3. As shown earlier in figure 10, it is at this same location there is a strong streamlinecurvature. At later times, as the vortex convects downstream, the magnitude of the velocity ratio begins todecrease.

The phase-averaged flow field for area ratio 11 at the maximum reverse flow velocity location (figure 10)was further examined by plotting the normalized velocity and vorticity profiles in the plane of the vortex ring.Figure 13 shows the normalized velocity profiles at the axial location of x/d=2.3 for the free and confinedjets. It can be seen from the figure that at area ratios 7.6 and 12, the profiles do not show any reverse flowvelocity. The reverse flow velocity is shown by the dark blue circles for area ratio 11. The pulsed free jetalso shows a reversed flow velocity, albeit it is smaller, as shown by the dark green circles in the velocityprofile. The gradient of velocity is related to the vorticity. The normalized vorticity profiles correspondingto the velocity profiles of figure 13 are shown in figure 14. It can be seen that area ratio 11 shows the highestmagnitude of normalized vorticity. A higher magnitude of vorticity represents higher mean shear. Thereforearea ratio 11 has the highest mean shear as compared to the other area ratios and the pulsed free jet.

The shear in the mean flow is a source of energy for turbulent velocity fluctuations. It causes moreturbulence production and hence turbulent mixing. As the mean shear is increased, the spreading of theshear layer also increases. Figure 15 shows the variation of the half velocity radius with the axial distance forthe free and confined jets calculated from the global mean velocity flow field. The half velocity radius, y0.5

at a given downstream location is defined as the radial location where the mean streamwise velocity is onehalf of the jet mean centerline velocity at that location. The half velocity radius is taken as a measure of thejet spreading. It can be seen from the figure that the jet spreading for the pulsed jet ejector cases is higherwhen compared to the others. Amongst the three area ratios of the pulsed jet ejector, the spreading for arearatio 11 is the highest. As discussed earlier, the mean shear for this area ratio is higher. The increase inthe mean shear is caused due to the reverse flow velocity as shown by its velocity profile in figure 13. Hencea higher jet spreading is observed. This is also indicative of higher mixing within the ejector. This is inaccordance with the previous studies17,18 which also show that in the presence of a counterflow shear layer,the turbulent mixing is enhanced and hence leads to a greater jet spreading. The pulsed free jet shows a

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Figure 15. Variation of half velocity radius with axialdistance for free and confined jets, Mj = 0.3; a) startof diffuser section b) ejector exit.

Figure 16. Normalized Reynolds stress profiles at x /d = 4, for free and confined jets; Mj =0.3, f =210 Hz.

lower jet spreading than the pulsed jet ejector cases because of lower mean shear as shown in figure 14. Forthe steady jet cases, it can be seen that the jet spreading of the free jet is higher when compared to thesteady jet ejector. The steady jet ejector shows the lowest jet spreading and hence a lower mixing. Thisis because the presence of co-flow in the steady jet ejector reduces the mean shear between the primary jetand the entrained fluid. It can also be noticed from figure 15 that the curves for all the confined jet casesshow an increase in the slope at 7.2d. This is caused due to the presence of the diffuser (a denotes the startof the diffuser section in the figure).

Figure 16 shows the radial variation of the normalized reynolds shear stress at an axial location of x/d=4.The data shown confirms the observations made in figure 15. It shows higher values of the normalized reynoldsshear stress for the pulsed jet ejector cases where the jet spreading is higher than that of the free pulsed jet.Area ratio 11 has the highest magnitude of the shear stress followed by 12 and 7.6. The pulsed free jet has alower magnitude and hence lower jet spreading. Hence the trend shown by the reynolds shear stress profilesis consistent with the trend shown by the half velocity radius variation with downstream distance.

2. Vortex Ring Characteristics

Previous studies have shown that the properties of the vortex ring like circulation and pinch-off play animportant role in the performance of the ejector. In order to study the vortex ring characteristics, the ring isidentified using the λ2 criterion19 i.e. if the sign of the second eigen value λ2, of the tensor S2+Ω2 is negativein the flow field, then that location is within a vortical structure. S and Ω are respectively the symmetricand antisymmetric components of the velocity gradient tensor. Once the boundaries of the individual vortexrings defined by λ2 = 0 are identified, different characteristics like size, location, convection velocity andcirculation can be calculated.

Figure 17 shows the normalized convection velocity of the vortex ring with downstream distance for thethree area ratios and the pulsed free jet. The vortex ring in the free jet attains a peak convection velocity ofaround 0.7Uj at about 1.5d. This peak location also corresponds to the end of the vortex formation region.The convection velocity for area ratio 7.6 is shown only up to 2d because the vortex ring touches the ejectorwalls once it reaches this location. It was not possible to identify the vortex ring beyond this location. Themagnitude of convection velocity of the confined vortex in the formation region is lower than that of a freejet. This is primarily due to the presence of the induced co-flow in the duct. The peak convection velocityis seen where the reverse flow is observed. The observations are also consistent with that of Dabiri andGharib.18 As the ring starts to diffuse, the convection velocity begins to decrease as seen from the figure.

The variation of normalized circulation at different downstream locations is shown in figure 18 for thefree and confined pulsed jets. The circulation magnitude seem to be independent of the area ratio up tox/d=2. As explained earlier, the data for area ratio 7.6 is shown only up to 2d. It is seen from the figure

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Figure 17. Normalized convection velocity of the vor-tex ring with downstream distance for free and con-fined pulsed jets, Mj =0.3, f =210 Hz.

Figure 18. Normalized ring circulation as a functionof downstream location for free and confined pulsedjets, Mj =0.3, f =210 Hz.

that the vortex ring at area ratios 11 and 12 attain maximum circulation at an axial location of about 4d. Atdownstream locations, the ring starts to diffuse and the circulation begins to decrease. It is also interestingto note that the maximum circulation attained by the ring at area ratio 12 is less than that obtained at arearatio 11. The strength of the vortex ring at area ratio 11 is higher than the vortex ring at area ratio 12. Atsmaller area ratios, the vortex ring itself becomes non-existent once it touches the ejector walls.

Figure 19 shows the normalized circulation for the free and confined pulsed jets as a function of anon-dimensional time. Here the convection velocity of the vortex ring and the nozzle exit diameter areused as the normalizing parameters for time. As seen from the figure, the data for all the cases collapsesfairly well on to a linear curve. The curve is linear because the as the time increases, the convectionvelocity decreases. Thus the decrease in convection velocity is compensated by an increase in time forthe different area ratios and for the free pulsed jet. Hence the variation in the magnitude of circulationwith downstream distance seen in figure 18 are only due to the varying primary vortex convection velocity.

Figure 19. Normalized ring circulation vs. non-dimensional time; Mj =0.3, f =210 Hz.

Figure 20 shows the variation of the total and ringcirculation for area ratios 11 and 12 with the non-dimensional time or the formation number as definedin Gharib et al.20 The formation number at pinch-off is defined as the non-dimensional time at whichthe maximum amount of circulation that can be ab-sorbed by the vortex ring is emanated by the jet. Theopen symbols connected by the solid line in the figureshows the total circulation and the filled symbols showthe ring circulation. In the present experiments, theformation number at pinch-off is found to be around2.8 and 2.5 for area ratios 11 and 12 respectively asshown in figure 20(a) and (b). For a pulsed free jet ata pulsing frequency of 210 Hz, the formation numberwas found to be also around 2.5.3 The variation inthe formation number at pinch-off for these two arearatios is within the experimental uncertainty. There-fore it may be argued that the pinch-off of the vortexring may not necessarily have a significant effect onthe thrust augmentation of the pulsed jet ejector.

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(a) Area ratio 11. (b) Area ratio 12.

Figure 20. Normalized circulation variation with non-dimensional time.

IV. Conclusions

The flow field characteristics of a pulsed jet ejector operating at a nozzle exit Mach number of 0.30and at a pulsing frequency of 210 Hz have been described using phase-locked Particle Image Velocimetry.The flow fields at three different area ratios have been examined to enable a better understanding of thethrust augmentation mechanism of the pulsed jet ejector. The results have illustrated the significance of theprimary and induced vortex pair with regard to the thrust augmentation ratio variation with the area ratio.At the area ratio where maximum thrust augmentation occurs, the phase-averaged velocity field showed thepresence of a strong induced vortex. At the other two area ratios, there was either no induced vortex or itis very weak. The time evolution of the phase-averaged velocity fields at the maximum thrust augmentationcondition has shown that the initially strong induced vortex progressively becomes weaker with time as itconvects downstream. This counter-rotating pair of the primary and induced vortex establishes an axialpressure gradient within the duct, as was indicated by the streamline curvature, that convects downstreamthus causing an enhanced mass flow rate into the ejector and hence resulting in higher thrust augmentationas observed. The results also showed a higher jet spreading and hence higher mixing at this area ratio. Theresults further indicate that the pinch-off phenomena of the vortex ring in a pulsed jet ejector may not have asignificant influence on the thrust augmentation process. The results also suggest a possibility that a thrustaugmentation ratio of around 2.3 is possible if the primary jet is pulsed at a Strouhal number of about 0.2with the area ratio being around 10.

Acknowledgements

The authors would like to thank Dr. Mehmet B. Alkislar for helping with the PIV measurements. Wewould like to thank DARPA for supporting the work under contract number MDA 972–03–C–0022.

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