Stereoscopic PIV Measurement of a Jet Flow with Vortex Generating Tabs

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1 The th International Symposium on Flow Visualization August -,, Kyoto, Japan F Stereoscopic PIV Measurement of a Jet Flow with Vortex Generating Tabs Hui HU* 1, Toshio KOBAASHI *, Tetsuo SAGA* and Nubuyuki TANIGUCHI* *1 Turbulent Mixing and Unsteady Aerodynamics Laboratory, Department of Mechanical Engineering, A, Research Complex Engineering, Michigan State University, East Lansing,, Michigan. U.S.A., Fax: , huhui@egr.msu.edu * Institute of Industrial Science, University of Tokyo, Komaba --1, Meguro-Ku, Tokyo -, Japan Abstract: A high-resolution stereoscopic Particle Image Velocimetry (PIV) system is used in the present study to conduct three-dimensional measurements at the near field (/D.) of a tabbed jet mixing flow. The measurement results reveal the great changes of the vortical and turbulent structures in the jet mixing flow due to the intrusion of the small tabs. The small tabs are found to generated very strong secondary flows in the tabbed jet flow to form streamwise vortices. The streamwise vortices pump the ambient flow into the core jet along the tab intrusion, and extract the core jet flow outward along the directions normal to the tab placement. Due to the pumping and extracting effect of the streamwise vortices, the tabbed jet flow is found to expand more rapidly in the directions normal to the tab intrusion. The bifurcation of the tabbed jet flow is found even at the two diameters of the test nozzle downstream. As the downstream distance increasing, more and more streamwise vortices were found to appear in the tabbed jet flow, which enhance the mixing between the core jet flow and ambient flow very efficiently. Keywords: Stereoscopic PIV, passive control, vortex flow, tabbed jet flow. 1. Introduction In an effort to increase mixing process in a jet flow, a passive control method, using vortex generators in the form of mechanical tabs or small protrusions at the exit of a nozzle, has been under investigation in the past several years. Bradbury and Khadem (1) were believed to be the first to report the effect of mechanical tabs on jet flows. They reported that, for a low speed jet flow, mechanical tabs or small protrusions at the exit of a conventional circular nozzle could increase the jet spread rate significantly, reduce the potential core length and even bifurcate the jet flow. Ahuja et al.(1, ) and aman et al.(,, and 1) began to investigate the mixing enhancement performance of mechanical tabs systematically. They found that mechanical tabs not only can increase the jet mixing in low speed jets, but also have good mixing enhancement performance in high speed and high temperature jet flows as well. Mechanical tabs have been proposed to suppress the jet noise of air breathe engines (Ahuja et al., 1 and aman et al., and 1). More recently, tabbed nozzles were also found to be used as fuel injector nozzles (Glawe et al., 1) in supersonic combustion chambers to enhance the mixing process of fuel with supersonic air. About the fundamental study of how and why mechanical tabs can enhance jet mixing process, aman et al.(1) suggested that the large-scale streamwise vortices generated by the Copyright by VSJ

2 mechanical tabs in jet flows are responsible for the enhanced mixing. They also postulated two sources of the streamwise vortex generating in their paper. In the research of molecular mixing in a jet mixing flow, hang and Scheider (1) found that mechanical tabs can reduce jet transitional Reynolds number and increase the molecular mixing about 3% at the downstream location of six diameters of the nozzle. The work of the Reeder and Samimy (1) revealed more detail about the evolution of the vortices and turbulent structures in tabbed jet flows based on the flow visualization and Laser Doppler Velocimeter (LDV) measurement results. They confirmed the existence of the large-scale streamwise vortices caused by the tab intrusion and reported the higher Reynolds stress levels in tabbed jet flows. Although many important results have been obtained in those previous studies, much work is still needed in order to understand the fundamental mechanism of the mixing enhancement in tabbed jet mixing flows more clearly. Previous studies have revealed the existence of unsteady vortical and turbulent structures in tabbed jet mixing flows by qualitative flow visualization. However, quantitative information about the evolution and interaction of these unsteady vortical and turbulent structures is still very limited. The experimental techniques used in most of the previous studies were Pitot probe, Laser Doppler Velocimetry (LDV) or Hot Film Anemometer (HFA). It is very hard to reveal the unsteady vortical and turbulent structures in the tabbed jet mixing flows instantaneously and globally due to the limitation of those experimental techniques. With the rapid development of modern optical techniques and digital image processing techniques, whole-field optical diagnostic technique like Particle Imaging Velocity (PIV) is assuming an ever-expanding role in the diagnostic probing of fluid mechanics. The advances of PIV technique in recent decades have lead it to become a matured technique for whole-field measurements in fluid flows. In earlier works of the authors (Hu et al. 1 and 1), both planar Laser Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV) techniques were used to study tabbed jet mixing flows in water channels. By using directly perceived LIF flow visualization images and quantitative velocity, vorticity and turbulence intensity distributions of the PIV measurement results, the evolution and interaction characteristics of various vortical and turbulent structures in the tabbed jet mixing flows were discussed. The PIV measurement results reported in the earlier work of the authors (Hu et al. 1) were obtained by using a conventional two-dimensional PIV system in a water channel. It is well known that a conventional PIV system is only capable of recording the projection of velocity into the plane of the laser sheet. That means the out-of-plane velocity component is lost while the inplane components may be affected by an unrecoverable error due to the perspective transformation (Prasad and Adrian, ). For the highly three-dimensional flow like tabbed jet mixing flows, the two-dimensional measurement results may not be able to reveal their threedimensional features successfully. A high-resolution stereoscopic PIV system is used in the present study to measure the near flow field of a tabbed jet mixing flow in order to reveal the three-dimensional features of the tabbed jet mixing flow more clearly. The characteristics of the mixing process in the tabbed jet mixing flow will be discussed based on the three-dimensional stereoscopic PIV measurement results.. Experimental Setup and the Stereoscopic PIV System.1 Test Nozzle and Mechanical Tabs Figure 1 shows the test nozzle and mechanical tabs used in the present research. The diameter of the circular nozzle at exit is 3 mm i. e, D=3. mm. The mechanical tabs used in the present study are triangular shaped tabs with apex angle and the orientation angle, just like the "delta tab" studied by aman et al.(1). Each tab has about 1.% blockage area after mounted on the nozzle exit. During the experiment, two tabs are placed diametrically opposed at the exit of the nozzle (Fig. 1(b)). Figure shows the air jet experimental rig used in the present study. A centrifugal compressor is used to supply air jet flows. A cylindrical plenum chamber with honeycomb structures is used to settle the airflow. Through a convergent connection (convergent ratio :1), the airflow is exhausted from the tabbed nozzle. The velocity range of the air jet out of the Copyright by VSJ

3 convergent connection (at the inlet of the test nozzle) can be varied from to 3 m/s. In the present study, a mean speed of the air jet flow at the exit of the test nozzle of U=1. m/s is used, which corresponds to a Reynolds Number of 3, (based on the nozzle diameter D=3mm). The air jet flow is seeded with 1~μm DEHS (Di--EthlHexyl-Sebact) droplets generated by a seeding generator. The DEHS droplets out of the seeding generator are divided into two streams; one is used to seed the core jet flow and the other for ambient air seeding. mm 1mm tabs a. mechanica tabs b. the tabbed nozzle Figure 1. Test nozzle and mechanical tabs Centrifugal compressor tabbed nozzle Cylindrical plenum chamber Convergent connection Two-dimensional translation mechanism Figure. The air jet experimental rig Copyright by VSJ

4 optics Host computer Laser sheet Double-pulsed Nd:AG Laser tabbed nozzle Synchronizer mm Measurement region mm by mm mm high-resolution CCD cameras Figure 3. The stereoscopic PIV system.. Stereoscopic PIV System Figure 3 shows the schematic of the stereoscopic PIV system used in the present study. The tabbed jet flow is illuminated by a double-pulsed Nd:AG laser set (NewWave, mj/pulse) with the laser sheet thickness being about.mm. The double-pulsed Nd:AG laser set can supply the pulsed laser (pulsed illumination duration ns) at a frequency of Hz. Two high-resolution (1K by 1K) cross-correlation CCD cameras (TSI PIVCAM-3) are used to perform stereoscopic PIV image recording. The two CCD cameras are arranged in an angular displacement configuration to get a large overlapped view. With the installation of tilt-axis mounts, the lenses and camera bodies are adjusted to satisfy the Scheimpflug condition (Prasad and Jensen, 1). In the present study, the distance between the illuminating laser sheet and image recording planes of the CCD cameras is about mm, and the angle between the view axial of the two cameras is about. The CCD cameras and double-pulsed Nd:AG lasers are connected to a workstation (host computer) via a synchronizer (TSI LaserPulse synchronizer), which controlls the timing of the laser sheet illumination and the CCD camera data acquisition. In the present study, the time interval between the two pulsed illuminations is 3μs. A general in-situ calibration procedure is conducted in the present study to obtain the mapping functions between the image planes and object planes (Soloff et al. 1). A target plate (mm by mm) with µm diameter dots spaced at intervals of. mm is used for the in-situ calibration. The front surface of the target plate is aligned with the center of the laser sheet and then calibration images are captured at three locations across the depth of the laser sheets. The space interval between these locations is.mm for the present study. The mapping function used in the present study is taken to be a multi-dimensional polynomial, which is fourth order for the directions ( and directions) parallel to the laser sheet plane and second order for the direction ( direction) normal to the laser sheet plane. The coefficients of the multi-dimensional polynomial are determined from the calibration images by using a least square method. The two-dimensional particle image displacements in each image plane is calculated separately by using a Hierarchical Recursive PIV (HR-PIV) software developed in-house. The HR-PIV software is based on hierarchical recursive processes of a conventional spatial correlation operation with offsetting of the displacement estimated by the former iteration step and hierarchical reduction of the interrogation window size and search distance in the next iteration step (Hu et al. ). Compared with conventional cross-correlation based PIV image processing methods, the Hierarchical Recursive PIV method has advantages in spurious vector suppression Copyright by VSJ

5 and spatial resolution improvement of the PIV result. Finally, by using the mapping functions obtained by the in-situ calibration and the two-dimensional displacements in the two image planes, all three components of the velocity vectors in the illuminating laser sheet plane are reconstructed. 3. Experimental Results and Discussions 3.1 The velocity and vorticity distributions in the tabbed jet Figure to Figure shows the stereoscopic PIV measurement results at four typical crossplanes of the tabbed jet mixing flow, which include typical instantaneous velocity vector plots, simultaneous vorticity distributions, ensemble-averaged velocity vector plots and ensembleaveraged vorticity fields. In the present study, the ensemble-averaged velocity is calculated by using frames of instantaneous stereoscopic PIV measurement results. The projections of the three-dimensional velocity vectors in the cross planes (- plane view) are also given in the figures in order to reveal the secondary flow (streamwise vortices) in the tabbed jet more clearly. The normalized instantaneous ( ϖ ) and ensemble-averaged streamwise vorticity( ϖ z ) shown in the figures are calculated based on the following equations. z D = U v u ( ) x ϖ (1) y D V U ϖ z = ( ) () U x y where the D=3 mm is the equivalent diameter of the test nozzle, U=1. m/s is the velocity of the core jet at the nozzle exit. u and v a are the instantaneous velocity components in x and y direction, while U and V are the ensemble-averaged velocity components. The test nozzle used in the present study is a conventional circular nozzle. Therefore, the jet flow out of the test nozzle is expected to be a conventional circular jet if there is not tab intrusion in the jet flow. From the stereoscopic PIV measurement results shown in Fig. to Fig., it can be seen that the vortical and turbulent structures in the jet flow has changed dramatically due to the intrusion of the small tabs at the exit of the circular nozzle. In the =3. mm (/D=1.) cross plane (Fig. ), the core jet flow is found to have two big inward indentations at the downstream of the two small tabs. The existence of strong secondary flows in the tabbed jet can be seen clearly in the - view of the typical instantaneous velocity vector plot (Fig. (a) and Fig. (b)). The secondary flows pump the ambient flow into the core jet along the tab intrusions and extracts the core jet flow outward along the directions normal to the tab placement. The inward indentations and the secondary flows due to the intrusion of the tabs are revealed more clearly in the ensemble-averaged velocity distribution shown in Fig. (c) and Fig. (d). The iso-velocity contours of the ensemble-averaged velocity in this cross plane shows that the maximum velocity of the tabbed jet is not at the center of the core jet flow. There are two velocity peak regions can be found in ensemble-averaged velocity distribution, which indicates that the tabbed jet flow begins to bifurcate even at the downstream location of =3.mm (/D=1.). The normalized distributions, corresponding to the typical instantaneous velocity and ensemble-averaged velocity fields, are given in Fig. (e) and Fig. (f). From the figures, it can be seen that strong secondary flows generated by the tabs in the tabbed jet result in several pairs of streamwise vortices in the tabbed jet flow. The instantaneous vorticity distribution in this cross planes shows that the instantaneous streamwise vortices appear in the flow field along the circular trailing edge of the test nozzle. Most of the streamwise vortices concentrate to the downstream regions of the tab intrusions. Based on the qualitative flow visualization and hot-wire quantitative measurements, aman et al. (1) suggested that there were two kinds of mechanism, which results in the generation of streamwise vortices in a tabbed jet flow. One is the upstream pressure hills generated by the tabs, which constitutes the main contributor of vorticity to the dominant pair. Another is due to z Copyright by VSJ

6 the vortex filaments shed from the sides of the tabs and reoriented downstream by the mean shear of the mixing layers. They also suggested that the latter source could produce a vortex pairing having a sense of rotation in the same direction or opposite to the dominant pair depending on the orientation of tabs. For the delta tabs, which are the same as the tabs used in the present study, the streamwise vortex pairs from the two sources would have the same rotating direction. The two streamwise vortex pairs from the two different sources conjectured by aman et al. (1) are revealed very clearly and quantitatively from the present high-resolution stereoscopic PIV measurement results in the cross plane of =3. mm (/D=1.). The ensembleaveraged distribution (Fig. (f)) shows that the streamwise vortices originating from the upstream pressure hill generated by the tabs have stronger vorticity than those due to the reorientation of the vortex filaments shed from the sides of the tabs. These two kind of streamwise vortices from the two different sources have the same rotating direction as the prediction of the aman et al. (1). Both of the two kinds of streamwise vortices pump the ambient flow into the core jet flow along the direction of the tab intrusion. Besides the two pairs of streamwise vortices conjectured by aman et al. (1), another two pairs of the streamwise vortices generated by each mechanical tabs can also be seen clearly from the ensemble-averaged vorticity distributions shown in Fig. (f). These streamwise vortices are located at the two sides of each tab along the interfaces between the outward ejecting core flow and inward-going ambient flow. These streamwise vortices are responsible for the outward ejecting of the core jet flow along the directions normal to the tab intrusion. Based on the measurement results of a Planar Doppler Velocimentry (PDV) system, Clancy and Samimy (1) claimed that they found a new kind of streamwise vortices in a tabbed supersonic jet besides the two reported by aman et al. (1). They named the third one as horseshoe vortices, and suggested that the horseshoe vortices was attributed to the wrapping of spanwise vortex lines around the tab. From the present stereoscopic PIV measurement results, it can be seen that the so-called horseshoe vortices are actually the inner parts of the streamwise vortices at the interfaces of the core jet and ambient flow. Since Clancy and Samimy (1) used the condensed water particles, which were formed naturally by the mixing between the warm and moist ambient air with the cold supersonic jet air, as tracers for their PDV measurement. Their PDV measurement results could be obtained only at the regions where the condensed water particles are formed. The outer parts of the streamwise vortice along the interfaces of the core jet and ambient flow locate at the ambient flow side, where the condensed water particles are hard to form. Therefore, Clancy and Samimy (1) could not get their PDV measurement results at those regions. It is supposed the reason why only the inner parts of the streamwise vortices along the interfaces of the core jet and ambient flow were found from the PDV measurement results of Clancy and Samimy (1). In the =.mm (/D=.) cross plane, the tabbed jet is found to be more turbulent. The inward indentations due to the intrusions of the tabs become bigger and deeper at this cross plane. The instantaneous secondary flows (streamwise vortices), which pump the ambient flow into core jet along tab intrusion directions and extract the core jet flow outward along the directions normal to the tab intrusion, also become stronger. The iso-velocity contours of the ensemble-averaged velocity show that the core jet flow expands rapidly along the directions normal to the tab intrusion, while the core jet flow shrink along the tab intrusions. There are two streamwise velocity peak regions in the tabbed jet flow, which are shifting away from the central line of the test circular nozzle. This indicates that the tabbed jet flow has bifurcated due to the intrusion of the small tabs even at =. mm (/D=.) downstream of the test nozzle exit. From the instantaneous distribution shown in Fig. (e), it can be seen that there are much more streamwise vortices appear in the tabbed jet flow. The appearance locations of these streamwise vortices in the tabbed jet flow become more random. The weaker streamwise vortices due to the reorientation of the vortex filaments shed from the sides of the tab can not be identified from the ensemble-averaged vorticity distribution anymore (Fig. (f)). The so-called horseshoe vortices, which is actually the inner parts of the streamwise vortice along the interfaces between the core jet and ambient flow with opposite rotating direction to the dominant streamwise vortices pairs, are also found to be dissipated out due to the intensive mixing in the tabbed jet flow. Only the stronger streamwise vortex pairs originated from the upstream pressure hill generated by the tabs and the outer parts of the streamwise vortice at the interfaces of the core jet and ambient flow can be seen from the ensemble-averaged vorticity Copyright by VSJ

7 distribution. These streamwise vortex pairs have the same rotating direction, and they move closer to each other to form bigger streamwise vortices. The vorticities of these streamwise vortices is found to dissipate very much compared with those in the =3mm(/D=1.) cross plane. When the downstream distance increases to =. mm (/D=., Fig. ), the tabbed jet flow becomes more and more turbulent, and more and more strong secondary flows appear in the tabbed jet flow. The core jet expands more dramatically along the directions normal to tab intrusion, and the cross-section geometry of the core jet at this cross plane is more rectangle other than circular. The two streamwise velocity peaks revealed from the iso-velocity contours of the ensemble-averaged velocity are found to be shifting away from the central line of the circular nozzle substantially. The tabbed jet flow looks like the combination of two parallel jets. The instantaneous vorticity distribution given in Fig. (e) shows that more and more instantaneous streamwise vortices appear in the tabbed flow, and the maximum vorticity level of these streamwise vortices is still as the same as those in the upstream cross planes. The ensemble-averaged vorticity distribution given in Fig. (f) shows that the ensemble-averaged streamwise vortices in the tabbed jet flow have dissipated so seriously that only four very weak streamwise vortex regions can be found in ensemble-averaged distribution. Figure shows the stereoscopic PIV measurement results in the =1. mm (/D=.) cross plane of the tabbed jet flow. The jet flow is found to become so turbulent that flow field is almost fully filled with the strong secondary flows (streamwise vortices) (Fig (a) and (b)). However, these strong secondary flows almost can not been seen from the ensemble-averaged velocity distribution shown in Fig. (c) and Fig. (d). It is because that the strong instantaneous secondary flows (streamwise vortices) are very unsteady and appear in the flow field very randomly. The information about these unsteady secondary flows is filtered out when the ensemble-averaged velocity field is calculated. This also indicates that the conventional measurement techniques like Pitot, HWA and LDV used in the previous studies of tabbed jet flows may not be able to reveal the evolution characteristics of these unsteady vortical structures properly. The ensemble-averaged velocity distribution of the present stereoscopic PIV measurements in this cross plane also shows that the tabbed jet have been round out very much due to the intensive mixing between the core jet flow and ambient flow. The iso-velocity contours of the ensemble-averaged velocity shows that two velocity peaks move away from the central line of the circular nozzle substantially. The magnetite of the velocity peaks has decrease to about. m/s ( u max / U =. ) at this downstream location. The instantaneous vorticity distribution in the =1. mm (/D=.) cross plane shows that so many streamwise vortices are found to appear in the tabbed jet flow that they almost fully filled the measurement window. The maximum vorticity value of these instantaneous streamwise vortices is found to be still at the same level of that in upstream cross planes. However, from the ensemble-averaged streamwise vorticty distribution in this cross plane, it can be seen that the strength of the ensemble-averaged steamwise vortices decreases substantially, only very vague streamwise vortical structures can be identified in the ensemble-averaged distribution. 3.. The distribution of the turbulent kinetic energy in the tabbed jet. The mixing process between the core jet flow and ambient flow in the tabbed can be represented more directly and quantitatively from the distribution of the turbulent kinetic energy distributions. The turbulent kinetic energy distributions of the tabbed jet flow in four selected cross planes are shown in Fig.. The turbulent kinetic energy values shown in these figures are calculated by using the following equation: 1 K( x, y, z) = (( r. m. s( u' )) + ( r. m. s( v' )) U 1 = U 1 ( N N t= 1 ( u t U ) 1 + N N t = 1 ( v + ( r. m. s( w' ) t V ) 1 + N ) N t= 1 ( w t W ) ) (3) Copyright by VSJ

8 mm mm m/s a. instantaneous velocity b. instantaneous velocity (- plane view) m/s mm m/s c. ensemble-avereged velocity d. ensemble-avereged velocity(- plane view) m/s e. instantaneous distribution f. ensemble-averged distribution Figure. Stereoscopic PIV measurement results in the =3. mm (/D=1.) cross plane Copyright by VSJ

9 mm mm m/s a. instantaneous velocity b. instantaneous velocity (- plane view) m/s mm m/s c. ensemble-avereged velocity d. ensemble-avereged velocity(- plane view) m/s e. instantaneous distribution f. ensemble-averged distribution Figure. Stereoscopic PIV measurement results in the =. mm (/D=.) cross plane Copyright by VSJ

10 mm mm m/s a. instantaneous velocity b. instantaneous velocity (- plane view) m/s mm m/s c. ensemble-avereged velocity d. ensemble-avereged velocity(- plane view) 1. m/s e. instantaneous distribution f. ensemble-averged distribution Figure. Stereoscopic PIV measurement results in the =. mm (/H=.) cross plane Copyright by VSJ

11 mm mm m/s a. instantaneous velocity b. instantaneous velocity (- plane view) m/s mm m/s c. ensemble-avereged velocity d. ensemble-avereged velocity(- plane view). m/s e. instantaneous distribution f. ensemble-averged distribution Figure. Stereoscopic PIV measurement results in the =1. mm (/H=.) cross plane Copyright by VSJ

12 normalized turbulent kinetic energy normalized turbulent kinetic energy a. =3. mm (/D=1.) b. =. mm(/d=.) normalized turbulent kinetic energy normalized turbulent kinetic energy c. =. mm(/d=.) d. =1. mm (/D=.) Figure. Turbulent kinetic energy distributions in four typical cross planes where N= is the number of the instantaneous stereoscopic PIV measurement frames used for the ensemble-averaged parameter calculation. ut, vt and wt are the instantaneous velocity components in x, y, and z direction, while U,V and W are the ensemble-averaged velocity components. In the =3. mm (/D=1.) cross plane, the contour of the turbulent kinetic energy distribution is found to be just like a doughnuts, which indicates that mixing of the core jet flow with ambient flow is conducted mainly at the interfaces of the two streams. There are two high turbulent kinetic energy regions at the downstream of the two tabs, which correspond to the strong secondary flows, are revealed in the instantaneous velocity vector plot given in Fig.. A low turbulent kinetic energy region is found to be in the center of the jet flow, which corresponds to the unmixed high-speed core jet flow. In the =. mm (/D=.) cross plane, the mixing area between the core jet flow and ambient flow is found to increase, and the mixing regions are found to expand outward and inward. Instead of circular form, the contour of the turbulent kinetic energy distribution is found Copyright by VSJ

13 to stretch more quickly along the directions normal to the tab intrusion, while shrink inward more seriously along the intrusion of the tabs. The size of the unmixed core jet flow region also is found to decrease. Instead of one low turbulent kinetic energy region in the center of the jet flow, two lowest turbulent kinetic energy peaks are found in this cross plane, which is shifting away from the central line of the test nozzle along the directions normal to the tab intrusion. This also indicates that the tabbed jet flow is bifurcated even at the downstream distance of =. mm (/D=.). This result consists with the above discussion of the ensemble-averaged velocity distribution. In the =. mm (/D=.) cross plane, the mixing region between the core jet flow and ambient flow increases substantially. Instead of a low turbulent kinetic energy region, a high turbulent kinetic energy region is found to appear in center of the tabbed jet flow at this cross plane. The big low turbulent kinetic energy regions revealed in the upstream cross planes is found to bifurcate into two smaller regions. The centers of the two smaller low turbulent kinetic energy regions are found to shift away from the central line of the test nozzle along the directions normal to the tab intrusion. The contour of the turbulent kinetic energy distribution in the tabbed jet flow looks like the combination of two separated circular jets with the centers of the two jets at the centers of the two smaller low turbulent kinetic energy regions. As the downstream distance increased to =1. mm (/D=.), the mixing between the core jet flow and ambient flow conducts more intensively. The high turbulent kinetic energy regions almost fully fill the measurement window.. Conclusion In an effort to increase mixing in a jet flow, a passive control method, using vortex generators in the form of mechanical tabs or small protrusions at the exit of a conventional circular nozzle, is investigated experimentally in the present study. A high-resolution stereoscopic PIV system is used to conduct three-dimensional measurement in the near field of an air jet flow exhausted from a tabbed nozzle. The stereoscopic PIV measurement results reveal the great changes of the vortical and turbulent structures in the jet mixing flow due to the intrusion of the small tabs. The small tabs are found to generated very strong secondary flows in the tabbed jet flow to form streamwise vortices. The streamwise vortices pump the ambient flow into the core jet along the tab intrusion, and extract the core jet flow outward along the directions normal to the tab placement. Due to the pumping and extracting effect of the streamwise vortices, the tabbed jet flow is found to expand more rapidly in the directions normal to the tab intrusion. The bifurcate of the tabbed jet flow is found even at the two diameters downstream of test nozzle. Two velocity peaks and two low turbulent kinetic energy regions are found in the downstream of four diameters of the test nozzle, and the tabbed jet flow looks just like the combination of two parallel circular jets. As the downstream distance increasing, more and more streamwise vortices are found to appear in the tabbed jet flow, which enhance the mixing between the core jet flow and ambient flow very efficiently. Copyright by VSJ

14 References Ahuja K.K., Manes J.P. and Massey K. C., 1, An Evaluation of Various Concepts of Reduction Supersonic Jet Noise, AIAA paper -3. Ahhja K. K., Mixing Enhancement and Jet Noise Reduction Through Tabs Plus Ejector, AIAA paper3-3. Bradbury L.J.S. and Khadem, A.H. 1, The Distortion of a Jet by Tabs, Journal of Fluid Mechanics. Vol., ppl-l3. Clancy P. S. and Samimy M. 1 Velocity and Vorticity Measurements in Supersonic Jets Modified with a Vortex Generating Tab, Proceedings of 1 ASME FED summer meeting (FEDSM-3). Glawe, D. D. Samimy, M., Najad A. S. and Chen, T. H., 1, Effects of Nozzle geometry on Parallel Injection into a Supersonic Flow, Journal of Propulsion and Power Vol. No., pp-. Hu H., Kobayashi T, Saga T., Taniguchi N. and Segawa S. 1, "Investigation on the Tabbed Jet Mixing Flows by using LIF and PIV", Proceeding of th International Symposium on Flow Visualization ( CD-Rom, No. ), Sorrento (NA), Italy, Sept. 1-, 1. Hu H., Kobayashi T., Wu SS. and Shen G-, "Research on the Vortical and Turbulent Structure Changes of Jet Flow by Mechanical Tabs", Proc. Inst. Mech. Engrs. Vol. Part C, Journal of Mechanical Engineering Science (U.K.), pp31-3, 1 Hu, H., Saga, T., Kobayashi, T., Taniguchi, N. and Segawa, S., Improve the Spatial Resolution of PIV Results by Using Hierarchical Recursive Operation, Proceedings of th International Symposium on Flow Visualization(paper No. ), Edinburgh, Scotland, UK, Aug.. Prasad, A. K. and Adrian, R. J.,, Stereoscopic Particle Image Velocimetry Applied to Fluid Flows, Experiments in Fluids, Vol. 1, No. 1, pp-. Prasad, A. K. and Jensen, K., 1 Scheimpflug Stereocamera for Particle Image Velocimetry in Liquid Flows, Applied Optics, Vol. 3, pp-. Reeder M. F. and Samimy M., 1, The Evolution of a Jet with Vortex Generating Tabs: Realtime Visualization and Quantitative Measurement, Journal of Fluid Mechanics, Vol. 3, pp33-. Soloff, S. M., Adrian, R. J. and Liu,. C., 1, Distortion Compensation for Generalized Stereoscopic Particle Image Velocimetry, Measurement Science and Technology, Vol., No., pp-1. aman K.B.M.Q., Reeder, M.F.and Samimy, M., Effect of Tabs on the Evaluation of an Axisymmetrical Jet, NASA-TM. aman K.B.M.Q., Reeder, M.F. and Samimy, M., Supersonic Jet Mixing Enhancement by Delta Tabs, AIAA paper-3. aman K.B.M.Q,, Streamwise Vorticity Generation and Mixing Enhancement in Free Jet by "Delta Tabs",AIAA paper aman K. B. M. Q., Reeder, M.F. and Samimy, M. 1, Control of an Axisymmetric Jet Using Vortex Generators, Physics of Fluids, Vol. No., pp-. hang S. and Scheider S.P. 1, Molecular-Mixing Measurements and Turbulent Structure Visualizations in a Round Jet with Tabs, AIAA paper -3. Copyright by VSJ