The 10th International Symposium on Flow Visualization
August 26-29, 2002, Kyoto, Japan
F0054
Stereoscopic PIV Measurement of a Jet Flow with Vortex
Generating Tabs
Hui HU*1, Toshio KOBAYASHI *2, Tetsuo SAGA*2 and Nubuyuki TANIGUCHI*2
*1 Turbulent Mixing and Unsteady Aerodynamics Laboratory, Department of Mechanical
Engineering, A22, Research Complex Engineering, Michigan State University, East Lansing,
48824, Michigan. U.S.A., Fax: 1-517-353-7179, E-mail: huhui@egr.msu.edu
*2 Institute of Industrial Science, University of Tokyo, Komaba 4-6-1, Meguro-Ku, Tokyo 1538505, 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 (Y/D ≤ 6.0) 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 (1975) 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.(1990, 1993) and Zaman et al.(1991, 1992, 1993 and 1994) 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.,
1990 and Zaman et al. 1992, 1993 and 1994). More recently, tabbed nozzles were also found to be
used as fuel injector nozzles (Glawe et al., 1996) 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, Zaman et al.(1994) suggested that the large-scale streamwise vortices generated by the
Copyright © 2002 by VSJ
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, Zhang and Scheider (1994) found that mechanical tabs can reduce jet
transitional Reynolds number and increase the molecular mixing about 35% at the downstream
location of six diameters of the nozzle. The work of the Reeder and Samimy (1996) 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. 1998 and 1999), 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. 1998)
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, 1993). 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.
2. Experimental Setup and the Stereoscopic PIV System
2.1 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 30 mm i. e, D=30.0 mm. The mechanical tabs used in the present
study are triangular shaped tabs with 900 apex angle and the orientation angle 1350, just like the
"delta tab" studied by Zaman et al.(1994). Each tab has about 1.5％ 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 2 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 50:1),
the airflow is exhausted from the tabbed nozzle. The velocity range of the air jet out of the
Copyright © 2002 by VSJ
convergent connection (at the inlet of the test nozzle) can be varied from 5 to 35 m/s. In the
present study, a mean speed of the air jet flow at the exit of the test nozzle of U0=18.0 m/s is used,
which corresponds to a Reynolds Number of 36,000 (based on the nozzle diameter D=30mm). The
air jet flow is seeded with 1~5μm DEHS (Di-2-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.
Y
tabs
8mm
X
1mm
Z
135
90
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 2. The air jet experimental rig
Copyright © 2002 by VSJ
optics
Host computer
Laser sheet
Double-pulsed Nd:YAG Laser
tabbed nozzle
Synchronizer
650 mm
25 0
650 mm
25 0
Measurement region
80mm by 80mm
high-resolution
CCD cameras
Figure 3. The stereoscopic PIV system
2.2. 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:YAG laser set (NewWave, 50mJ/Pulse) with
the laser sheet thickness being about 2.0mm. The double-pulsed Nd:YAG laser set can supply the
pulsed laser (pulsed illumination duration 6ns) at a frequency of 10Hz. Two high-resolution (1K
by 1K) cross-correlation CCD cameras (TSI PIVCAM10-30) 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, 1995). In the
present study, the distance between the illuminating laser sheet and image recording planes of
the CCD cameras is about 650mm, and the angle between the view axial of the two cameras is
about 500.
The CCD cameras and double-pulsed Nd:YAG 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 30μs.
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. 1997). A target plate (100mm
by 100mm) with 100µm diameter dots spaced at intervals of 2.5 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 0.5mm 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 (X and Y directions) parallel to the laser sheet plane and second order for the direction
(Z 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. 2000). Compared with conventional cross-correlation based PIV image processing
methods, the Hierarchical Recursive PIV method has advantages in spurious vector suppression
Copyright © 2002 by VSJ
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 4 to Figure 7 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 500 frames of instantaneous stereoscopic PIV measurement results. The projections of the
three-dimensional velocity vectors in the cross planes (X-Y 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 streamwise vorticity( ϖ z ) and ensemble-averaged streamwise
vorticity( ϖ z ) shown in the figures are calculated based on the following equations.
ϖz =
D ∂v ∂u
( − )
U 0 ∂x ∂y
(1)
ϖz =
D ∂V ∂U
(
−
)
U 0 ∂x ∂y
(2)
where the D=30 mm is the equivalent diameter of the test nozzle, U0=18.0 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. 4 to Fig. 7, 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 Z=30.0 mm (Z/D=1.0) cross plane (Fig. 4), 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 X-Y view of the typical instantaneous velocity
vector plot (Fig. 4(a) and Fig. 4(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. 4(c) and
Fig. 4(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 Z=30.0mm
(Z/D=1.0).
The normalized streamwise vorticity distributions, corresponding to the typical instantaneous
velocity and ensemble-averaged velocity fields, are given in Fig. 4(e) and Fig. 4(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, Zaman
et al. (1994) 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
Copyright © 2002 by VSJ
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
Zaman et al. (1994) are revealed very clearly and quantitatively from the present high-resolution
stereoscopic PIV measurement results in the cross plane of Z=30.0 mm (Z/D=1.0). The ensembleaveraged streamwise vorticity distribution (Fig. 4(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 Zaman et al. (1994). 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 Zaman et al. (1994), 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. 4(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 (1998) claimed that they found a new kind of streamwise vortices in a tabbed
supersonic jet besides the two reported by Zaman et al. (1994). 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
(1998) 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 (1998) 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 (1998).
In the Z=60.0mm (Z/D=2.0) 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 Z =60.0 mm (Z/D=2.0) downstream of the test nozzle exit.
From the instantaneous streamwise vorticity distribution shown in Fig. 5(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. 5(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 © 2002 by VSJ
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 Z=30mm(Z/D=1.0) cross
plane.
When the downstream distance increases to Z=120.0 mm (Z/D=4.0, Fig. 6), 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. 6(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. 6(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 streamwise vorticity distribution.
Figure 7 shows the stereoscopic PIV measurement results in the Z=180.0 mm (Z/D=6.0) 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 7(a) and 7(b)). However,
these strong secondary flows almost can not been seen from the ensemble-averaged velocity
distribution shown in Fig. 7(c) and Fig. 7(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
12.0 m/s ( u max / U 0 = 0.67 ) at this downstream location.
The instantaneous vorticity distribution in the Z=180.0 mm (Z/D=6.0) 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 streamwise vorticity
distribution.
3.2. 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. 8. The turbulent kinetic energy values shown in these figures are
calculated by using the following equation:
K ( x, y , z ) =
=
1
2U 0
2
1
2U 0
2
((r.m.s(u ' )) 2 + ( r.m.s (v' )) 2 + (r.m.s ( w' ) 2 )
(3)
1
(
N
N
1
(u t − U ) 2 +
å
N
t =1
N
1
(v t − V ) 2 +
å
N
t =1
N
å (w
t =1
t
− W )2 )
Copyright © 2002 by VSJ
Y
30
Z
5.0 m/s
X
20
20 m/s
15
18
9 10
5 4
5
8
4
5
4
13
7
14
18
17
61 3
6
71 18
6
10
Y mm
10
14 9
139
17
16
14
1312
12
Y mm
9
16
9
-20
-30
-30
0
-20
-10
0
10
20
30
40
X mm
10
Xm
m
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
7
6 10
-30
-10
5
54
-20
18
15
-10
118
17
18
11
15
-30
4
6
19
16
-20
0
161 2
-10
14
12
8
0
20
W m/s
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
1132
78 11 1 5
12
19
10
10
W (m/s)
5 4
20
7
30
20
30
a. instantaneous velocity
b. instantaneous velocity (X-Y plane view)
Y
30
30
Z
5.0 m/s
X
20
15 m/s
W (m/s)
45
6
17
7
9
11
14
1 1 12
4 6
9
15
13
17
Y mm
Y mm
7
7
-20
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
8
10
-30
-10
1 1 13
9
-20
0
17
-10
7 9 11 13
14 12 15 6
1
17
16
1 51 4 10
13 1 2
8 45
0
10
5
18
10
W m/s
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
6
8
10
4
20
5
8
4
6
-20
-30
-30
-10
-30
0
Xm
m
-20
-10
0
10
20
30
40
X mm
10
20
30
c. ensemble-avereged velocity
d. ensemble-avereged velocity(X-Y plane view)
30
30
-0.5
20
20
3
-0 .
5
- 0.
00..5
1
.1
0 .5
0 .1
-20
-0 .1
-20
1
-0
0.
-1.5
-0 .5
-2.5
-0 .1
3
-10
-0.5
-0.5 -0.5
0.
-10
1.100
0.900
0.700
0.500
0.300
0.100
-0.100
-0.300
-0.500
-0.700
-0.900
-1.100
7
1
0 .9
-0.5
1.5
-0 .1
0.5
-0
.
-0 .3
-0.1
-0.5
0.
0 .1
0.1
0
streamwise vorticity
0.5
-1 .1
0.7
Y mm
-0.5
0
10
0 .3
-0.5
-0.5
-0.5
0.5
-0.5
3.500
2.500
1.500
0.500
-0.500
-1.500
-2.500
-3.500
-0 .1
Y mm
10
0.5
0 .3
-0.5
-1.5
0.5
.1
-00.1
streamwise vorticity
-1.5
-0.3
0.5
0.5
-0.5
-30
-30
-30
-20
-10
0
10
X mm
20
30
40
-30
-20
-10
0
10
20
30
40
X mm
e. instantaneous streamwise vorticity distribution f. ensemble-averged streamwise vorticity distribution
Figure 4. Stereoscopic PIV measurement results in the Z=30.0 mm (Z/D=1.0) cross plane
Copyright © 2002 by VSJ
Y
30
Z
30
5.0 m/s
X
20
9
10
8
156
4
18
20
12
9
5
5
6
6
15
12
10
19
13
7
11
4
Y mm
Y mm
131 9
61
13
91 0 1 1
12 7
8
4
9
7
6 1114
19
-20
4
-30
-30
0
-20
-10
0
10
20
30
40
X mm
10
Xm
m
18
9
4
7
7
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
6
15
6
-30
-10
17
-20
6
64
5
5
-30
17
12
13
15
14 2
1
4
-20
14 1 3
10
-10
17
16
-10
10
3
15
16 1 2 1
15
11
12
9
8
5
0
9
11
14
6
0
76
11
W m/s
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
10
4
58
8
11 2
10
10
W (m/s)
5
4
20 m/s
20
20
30
a. instantaneous velocity
b. instantaneous velocity (X-Y plane view)
Y
30
30
Z
5.0 m/s
X
20
15 m/s
20
10
8 9
-10
12
65
12
16
0
91 8
7
4
6
5
4
Y mm
Y mm
17
11
-20
-30
-30
-10
-30
0
-10
20
30
40
30
d. ensemble-avereged velocity(X-Y plane view)
30
0.5
-0.1
1
-0 .
20
- 0.
streamwise vorticity
-20
-0.5
0 .1
-0
.3
.1
.1
0.3
.5
-0
0.1
.1
-0
1.5
-0.5 -1.5
-1.5
-0.5
-0
3
0.
0.5
0 .3
-1.5 0.5
-10
-1.5
-0.3
-0 .1
-0 .1
-0.5
1.100
0.900
0.700
0.500
0.300
0.100
-0.100
-0.300
-0.500
-0.700
-0.900
-1.100
-0 .3
0
0.1
1.5
-1.5
streamwise vorticity
-0
0.5
10
0 .1
0.5
0.5
3
0 .1
-0.5
-1.5
0.5
- 0 .1
0 .3
10
3.500
2.500
1.500
0.500
-0.500
-1.500
-2.500
-3.500
Y mm
-0.5
0 .1
-0.5
-0.5
-10
10
X mm
30
0
0
20
c. ensemble-avereged velocity
20
-20
10
Xm
m
Y mm
4
413
115
-20
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
5 7
8 9 11
-30
12
1 51134
76
-20
12
-10
10
0
5
0
10
4
6
10
W m/s
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
W (m/s)
65
7
89
11
13
14
15 1
17 6
-20
-0.5
-0
.1
-30
-30
-30
-20
-10
0
10
X mm
20
30
40
-30
-20
-10
0
10
20
30
40
X mm
e. instantaneous streamwise vorticity distribution f. ensemble-averged streamwise vorticity distribution
Figure 5. Stereoscopic PIV measurement results in the Z=60.0 mm (Z/D=2.0) cross plane
Copyright © 2002 by VSJ
Y
4
30
Z
5.0 m/s
X
7
30
5
6
20
W (m/s)
20 m/s
4
5
14
8
14
14
11
11
12
5
10
8
7
16
15
8
13
54
97
6
5
14 1
131
14
10
5
13
Y mm
4
54
4
8
9
5
6
12 9
11
13
10 12 8
7
6
57
14
13
11
12
9
12
8
5
4
-20
5
4
7
Y mm
4
12
13
6
6
-30
-30
0
Xm
m
10
-30
-10
15
10
12
11
11 1
0
11
6
-20
15
-30
-10
7
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
9
118
4
-20
9
7
9
17
13
-10
8
76
14
10
0
0
9
12
9
W m/s
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
9
12
10
10
4
56
7
20
-20
-10
0
10
20
30
40
X mm
10
20
30
a. instantaneous velocity
b. instantaneous velocity (X-Y plane view)
Y
30
30
Z
5.0 m/s
X
20
15 m/s
11
6
14
12
15
8
Y mm
Y mm
12
11
14
1
90
11
8
7
6
5
6
5
4
4
-20
8
9
10
14
-10
-30
-30
0
-10
0
10
20
30
40
X mm
20
30
c. ensemble-avereged velocity
d. ensemble-avereged velocity(X-Y plane view)
30
0.5
-0.5
-1.5
20
0.5
10
2.5
0.5
0.5
1.5
0
-1.5
0.5
1.5
0.5
0.5
1.5
-0.5
2.5
0 .1
10
streamwise vorticity
3
1
-0 .
.1
-0
0 .1
0
-10
0 .1
-0.5
0.
-0 .1
-1.5
-1.5
-1.5
-0.5
1.5
3.500
2.500
1.500
0.500
-0.500
-1.500
-2.500
-3.500
Y mm
0.5
0 .3
-0.5
1
-0 .
streamwise vorticity
0.5
-0 .3
20
0 .1
30
-10
-20
10
0.5
-0.5
-20
5
0.
0.
0.5
-30
-0.1
-0.1
1.100
0.900
0.700
0.500
0.300
0.100
-0.100
-0.300
-0.500
-0.700
-0.900
-1.100
0.1
Xm
m
Y mm
13
-30
-10
-20
11
13
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
6
-20
7
98
12
-30
0
7
-20
10
12
10
-10
10
9
0
6
5
10
W m/s
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
W (m/s)
4
5
7
4
7
20
1
-30
-30
-20
-10
0
10
X mm
20
30
40
-30
-20
-10
0
10
20
30
40
X mm
e. instantaneous streamwise vorticity distribution f. ensemble-averged streamwise vorticity distribution
Figure 6. Stereoscopic PIV measurement results in the Z=120.0 mm (Z/H=4.0) cross plane
Copyright © 2002 by VSJ
Y
30
4
8
8
97
17
15
4
10
14
7 11
4
4 6
5
6
5
-20
7
14
13
14
17
16
10
5
7
111
1
13
13
12
5
8
5
9 7
8
98
Y mm
9
12
7
6
67
4
4
4
4
5
10
89
8
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
7
10
10
14
Y mm
9
W (m/s)
7
9
16
12
11
7
4
-30
-30
-20
-10
0
10
20
30
40
X mm
10
Xm
m
13
5
6
0
9
10
9
1132
4
10
-10
8
1 41 2 12
15
11
13
-30
6
5
10 6
13
97
15
-30
10
4
0
4
111 5
14
7
-20
89
5
9
-10
12
11
W m/s
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
10
10
4
9
10
-10
6
6
5
20
20 m/s
-20
5.0 m/s
X
20
0
4
5
Z
30
20
30
a. instantaneous velocity
b. instantaneous velocity (X-Y plane view)
Y
30
Z
5.0 m/s
X
20
4
15 m/s
-30
-20
9
-10
8
11
9
7
6
7
6
5
4
-20
-30
-30
0
-20
-10
0
10
d. ensemble-avereged velocity(X-Y plane view)
30
-0 .1
-0.5
0.1
20
-2.5
0 .1
1
0.
- 0 .1
0.5
0.5
-0.1
.
-0
1
1.100
0.900
0.700
0.500
0.300
0.100
-0.100
-0.300
-0.500
-0.700
-0.900
-1.100
-0 .1
-20
0.5
1.5
.1
0 .3
-0.5
0 .1
-0 .1
-0.5
1.5
-0 .3
-0.1
-0
-10
0.5
1
-0.1
0.5
-0.50.5
0.
.1
0.5
1
-0
-1.5
0.
0 .1
0
streamwise vorticity
0 .1
1
1.5
0.3
0 .3
0.
-0.5
0.5
10
1
1
0.5
- 0.
-0.
3.500
2.500
1.500
0.500
-0.500
-1.500
-2.500
-3.500
1.5
0
0.1
streamwise vorticity
0.5
-0.1
0.5
0.5
0 .3
-0.5
Y mm
-0.5
10
-10
40
30
30
1.5
30
20
c. ensemble-avereged velocity
20
20
X mm
10
Xm
m
Y mm
10
8
-30
-10
-20
12
11
10
11
10
Y mm
Y mm
-20
10
0
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
9
11
-10
8
12
0
9
10
6
7
8
10
W m/s
20.00
19.00
18.00
17.00
16.00
15.00
14.00
13.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
W (m/s)
5
6
7
5
20
9
30
-0.5
0.5
-1.5
-30
-30
-30
-20
-10
0
10
X mm
20
30
40
-30
-20
-10
0
10
20
30
40
X mm
e. instantaneous streamwise vorticity distribution f. ensemble-averged streamwise vorticity distribution
Figure 7. Stereoscopic PIV measurement results in the Z=180.0 mm (Z/H=6.0) cross plane
Copyright © 2002 by VSJ
30
01
0.
-20
-30
-30
-20
-10
0
40
20
0 .0
5
25
0.0 25
0 .0
0
-20
-30
0 .0 1
0.02 0
-10
10
20
30
40
-30
-20
-10
0
10
20
30
40
X mm
a. Z=30.0 mm (Z/D=1.0)
b. Z=60.0 mm(Z/D=2.0)
30
30
0 .0
0 .0 1
20
05
0 .0 1
0.005
0
0.
0 .0
20
01
0.
normalized turbulent
kinetic energy
5
5
0.010
20
normalized turbulent
kinetic energy
03
-20
30
0
0.
-20
02
10
0.0 0.0
05
-10
0 .0 2
0
0
10
0
0
5 .0 3 0
0 .0 2
5
0.01 5
0 .0
-30
-20
03
0 .0 1 0
-30
-30
0.050
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0 .0
-10
0.
25
0.025
0.020
5
30
0 .0
25
0.015
0 .0
0 .0
0 .0
30
5
-10
0 .0 2
25
0.030
02
0
0.
03
35
0 .0
0
0.03
0.
0 .0
10
Y mm
0 .0 2 0
20
0 .0
0.0
25
30
5
0.050
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.030
0 .0 3 5
5
0 .0
0 .0 2
0 .02
0
10
Y mm
15
0..001 0
0
0 .0 1
0 .0 00
5
X mm
0
0.050
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
5
5
5
20
normalized turbulent
kinetic energy
03
.0 1
0 .0
0
0.030
0.010
0.0
2
0 .0 0
10
0.
02
0.
2 00
-10
0.01 5
05
0 .0
0 .0 2 0 0.0 15
0 .0
1
0
5
0 .0
5
0 .0 3
30
0 .015
.0
0
Y mm
00
0.
0
02
0
10
0.0 20
0.050
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.
0.020
0.0 05
45
normalized turbulent
kinetic energy
0.015
0 .0 2 5
10
Y mm
0.005
0 .0
15 0 .0 1 0
0.0
30
20
0.010
0 .0
20
0 .0
30
20
30
40
X mm
c. Z=120.0 mm(Z/D=4.0)
-30
-20
-10
05
0.00 5
0
10
20
30
40
X mm
d. Z=180.0 mm (Z/D=6.0)
Figure 8. Turbulent kinetic energy distributions in four typical cross planes
where N=500 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 Z=30.0 mm (Z/D=1.0) 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. 4. 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 Z=60.0 mm (Z/D=2.0) 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 © 2002 by VSJ
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 Z=60.0 mm
(Z/D=2.0). This result consists with the above discussion of the ensemble-averaged velocity
distribution.
In the Z=120.0 mm (Z/D=4.0) 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 Z=180.0 mm (Z/D=6.0), 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.
4. 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 © 2002 by VSJ
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Copyright © 2002 by VSJ