8TH INTERNATIONAL SYMPOSIUM ON FLOW VISUALIZATION (1998)
INVESTIGATION ON THE TABBED JET MIXING
FLOWS BY USING LIF AND PIV
Hu Hui, Toshio Kobayashi, Tetsuo Saga,
Nobuyuki Taniguchi, Shigeki Segawa
Keywords: Jet mixing flow, mixing enhancement, PIV technique and LIF technique
ABSTRACT
An experimental investigation of the vortical
and turbulent structure changes in the near
field of a jet mixing flow caused by
mechanical tabs placed at the exit of a
circular nozzle had been conducted. The
techniques of Laser Induced Fluorescence
(LIF) and Particle Image Velocimetry (PIV)
were used to accomplish flow visualization,
instantaneous quantitative concentration
measurements
and
velocity
field
measurements of the jet mixing flow.
Compared with a nature jet flow (a circular
jet flow without mechanical tabs intrusion),
the experimental results showed that,
besides the existence of the well known
streamwise
vortices
generated
by
mechanical tabs, the tabbed jet flow was
found to have shorter potential core length,
smaller size of the spanwise KelvinHelmholtz vortices, earlier appearance of
small scale turbulent structures and bigger
turbulent intensity. The bifurcation of the jet
flow was also found in the tabbed jet flow
due to the intrusion of the mechanical tabs.
Based on the LIF and PIV results, two
aspects of the mechanism of the jet flow
mixing enhancement caused by mechanical
tabs were suggested: The frist was that the
secondary instability of the jet flow was
enlarged by the intrusion of the mechanical
tabs. This accelerated the "cut and connect"
Author(s): Hu Hui, Toshio Kobayashi, Tetsuo Saga,
Nobuyuki Taniguchi, Shigeki Segawa
2nd department, Institute of Industrial
Science University of Tokyo, Roppongi 7-22-1
Tokyo 106 Japan.
process of the Kelvin-Helmholtz vortical
rings to transfer energy and vorticity from
large scale vortices to small scale
vortices.The second
was that the
interaction between the streamwise vortices
generated by the mechanical tabs and
spanwise Kelvin-Helmholtz vortical rings
also enhanced the "energy cascade" process
of the turbulence. Both of these resulted in
the creation of small scale intense
turbulence and enhanced the mixing of the
core jet with ambient flow.
INTRODUCTION
In an effort to increase mixing in jet
flows, a passive control method, using
vortex generators in the form of mechanical
tabs or small protrusions at the exit of a
nozzle had been under investigation in the
past several years. Bradbury and Khadem[1]
were the first to study the effect of
mechanical tabs on jet flows in detail. They
reported that, at low speed condition,
mechanical tabs or small protrusions at the
exit of a nozzle can increase jet spread rate
significantly, reduce the potential core
length (from 6D for the conventional jet to
3D, D is the diameter of the nozzle exit) and
even bifurcate the jet flows. Ahuja et al.[2,3]
and Zaman et al.[4,5,6,7,8] began to investigate
the mixing enhancement performance of
mechanical tabs systematically. They found
that mechanical tabs can not only increase
the jet mixing at low speed condition but
also have good mixing enhancement
performance at high speed and high
temperature condition as well. Ahuja [3]
reported that: for a circular jet flow of Mach
number 1.12 and total temperature 664K,
4.1
HU HUI, TOSHIO KOBAYASHI, TETSUO SAGA, NOBUYUKI TANIGUCHI, SHIGEKI SEGAWA
the potential core length of the jet flow
could be reduced from six diameters to
under two diameters by using two
diametrically opposed mechanical tabs, and
at the downstream location of five jet
diameters,
the
mixing
enhancement
produced by two mechanical tabs reduced
the temperature in the jet centerline from
655K to about 472K. It was also found that
mechanical tabs can reduce low frequency
noise of a supersonic jet flow up to seven dB
(Kobayashi [9]). More recently, tabbed
nozzle was also found to be used as a fuel
injector nozzle (Glawe et al.[10]) in
supersonic combustor to enhance the mixing
of fuel with supersonic air.
About the fundamental research of how
and why mechanical tabs can enhance jet
mixing process, Zaman et al.[7] reported that
large scale streamwise vortices can be
generated by mechanical tabs in jet flows
and two sources of the streamwise vortices
were postulated in their paper. In the
research of the molecular mixing in a jet
mixing flow with mechanical tabs, Zhang et
al.[11] found that mechanical tabs can reduce
jet transitional Reynolds number and
increase the molecular mixing about 35% at
the downstream location of six diameter of
the nozzle. The work of the Reeder et al.[12]
revealed more detail about the vortices and
turbulent structures in tabbed jet flows by
using flow visualization and Laser Doppler
Velocimeter (LDV) measurements. They
confirmed the existence of the large scale
streamwise vortices caused by mechanical
tabs and reported the higher Reynolds stress
levels in the tabbed jet flow.
Although many important results had
been got through these previous works much
work still need to be done to understand the
mechanism of the jet mixing enhancement
caused by mechanical tabs more clearly,
especially regarding to the vortical and
turbulent structure changes in a jet flow
caused by the intrusion of the mechanical
tabs and by what way the streamwise
vortices caused by mechanical tabs
enhanced jet mixing process. In the
meanwhile, most of the previous
experiments were conducted by using Pitot
probe, Laser Doppler Velocimeter (LDV) or
Hot Wire Anemometer which are very hard
to reveal the vortical and turbulent structures
of the flow field instantaneously and
globally due to the limitation of the
experimental techniques. In the present
study both the Laser Induced Fluorescence
(LIF) and Particle Image Velocimetry (PIV)
techniques which are modern fluid dynamics
experimental techniques and can reveal the
vortical and turbulent structures in flow
fields instantaneously and globally, were
used to conducted flow visualization and
instantaneous quantitative flow field
measurements (concentration field and
velocity field). By using the directly
perceived flow visualization images, the
quantity distributions of the scalar field
(concentration field) and vector field
(velocity field and vorticity field), the
evolution and interaction characters of the
Kelvin-Helmholtz vortices and streamwise
vortices generated by mechanical tabs in the
jet mixing flow are studied to reveal the
physics of the mixing flow and mechanism
of the mixing enhancement caused by
mechanical tabs.
EXPERIMENTAL
TECHNIQUES
SYSTEM
AND
1. Experimental system
water tank
Laser sheet
Laser beam
optical system
CCD camera
m ixing region
com b s t r u c t u r e
tested nozzle
flow m e t e r
laser
Im a g e
process
system
pump
jet supply tank
m onitor
H ost computer
Figure 1. Experimental Setup
Figure 1 shows the schematically
experimental facility used in the present
research. The tested nozzle was installed in
the
middle
of
the
water
tank
(550mm*550mm*600mm). Fluorescent dye
(Rhodamine B) for LIF or PIV tracers
(polystyrene particles of d=100um) was
premixed with the water in the jet supply
tank, and jet flow was supplied by a pump.
4.2
INSTIGATION ON THE TABBED JET MIXING FLOWS BY USING LIF AND PIV
The flowrate of the jet flow, which was used
to calculate the representative velocity and
Reynolds numbers of the jet flow, was
measured by a flow meter. A honeycomb
structure was installed in the entrance of the
tested nozzle to insure the uniform flow
entrance. A beam of argon ion laser
(wavelength is 457.9-514nm and output
power is 4w) passed an optical system to
form a plane sheet (thickness of the laser
sheet is about 1.5mm). The laser sheet can
be streamwise or normal cross the central
line of the jet flow. The investigated area
(180mm by 180mm) was focused on a CCD
camera (SONY-AVC-D7) and then,
recorded by Laser Videodisc Recorder
(SONY LVR-5000). The shutter of the CCD
camera was set at 1/250s, and the framing
rate was 30 frames per second. Each frame
consisted of two images, which resulted in
60 recorded images per second. The images
were digitized by a 8-bit gray-scale image
processing system (NEXUS 9000) at a
spatial resolution of 640 by 480 pixels
(corresponded about 3.0 pixel/mm) , which
also can be stored to a PC (host computer)
and displayed on a PC monitor.
1mm
8mm
135
90
a¡ tested mechanical tab
top view
screw
hole
in flow
Z
mechnical tab
Z
tab cover
Y
side
view
X
b.circlar nozzle used in the test
Figure 2. Tested circular nozzle
and mechanical tabs
Figure 2 shows the circular nozzle and
mechanical tabs used in the present research.
The diameter of the circular nozzle at exit
was 30 mm, and the tested mechanical tabs
were triangular shaped tabs with 900 apex
angle and the orientation angle 1350, just
like the "delta tab" studied by Zaman et
al.[7]. Each mechanical tab had about 1.5%
blockage area after mounted on the nozzle
exit. During the experiment, two mechanical
tabs were placed diametrically opposed at
the exit of the nozzle. In the present study,
the jet velocities were about 0.08m/s,
0.12m/s and 0.25m/s and the Reynolds
Number of the jet flows, based on the
circular nozzle exit diameter, were about
1,800, 3,000 and 6,000.
2. Experimental Techniques
Laser Induced Fluorescence (LIF) and
Particle
Image
Velocimetry
(PIV)
techniques are both modern fluid
experimental techniques. They can offer
many advantages for the study the fluid
mechanics
over
other
conventional
experimental techniques. In recent years, a
lot of research on fluid mechanics by using
LIF and PIV techniques had been conducted
and many promising results had been
obtained, such as Adrian [13], Liepmann et
al.[14], Willert et al.[15], Yoda et al.[16],
Southerland et al.[17], Coppeta et al.[18], Hu
Hui et al.[19] and Zhou et al.[20]. In the
present research, both of the LIF technique
and PIV technique were used to reveal the
physics of the mixing flow and mechanism
of the mixing enhancement caused by
mechanical tabs.
In the present research, Rhodamine B
was used as fluorescent dye. The peak of
absorption spectrum of Rhodamine B occurs
at La=5l7.5mn while fluorescence spectrum
peak is Lf=590nm. During the experiment,
the fluorescent light was separated from the
scattered light with an optical filter. It was
well known that, only the solution of the
fluorescent dye with low concentration can
insure the strength of fluorescent light being
linear with the concentration of the
fluorescent dye and the effect of laser light
attenuation is negligible as the laser light
sheet propagated through the investigated
area. So, in the present study, the
concentration of the Rhodamine B solution
in the jet supply box is 0.3mg/l.
To obtain fluid velocities by using PIV,
two or more images of seeded flow fields
are captured by CCD camera at successive
4.3
HU HUI, TOSHIO KOBAYASHI, TETSUO SAGA, NOBUYUKI TANIGUCHI, SHIGEKI SEGAWA
points in time, and comparison of these
images allows the velocity fields to be
constructed. In the present study, rather than
tracking individual particle the cross
correlation method was used to obtain the
average displacement of the particles in an
interrogation windows. In the PIV image
processing, the images were divided into 30
by 15 pixel interrogation windows. A 5
pixel in X direction (radical direction) and
10 pixel in Y direction (jet flow direction)
step size were used which resulted in 67%
overlapping of the adjacent interrogation
windows. The studied flow fields were
represented by discrete flow fields
containing about 2000 velocity vectors (40
by 52). The post-processing procedures
which including sub-pixel interpolation (Hu
Hui et al.[21]), velocity outliner deletion
(Zhou et al.[20]) and field smoothing (Willert
et al.[15]) were used to improve the accuracy
of the PIV measurements.
In the present study, both instantaneous
flow fields and mean flow fields were used
for
analysis
and
discussion.
The
instantaneous flow fields were the flow field
captured by CCD camera at frequency of
30Hz, and every instantaneous frame (two
images) was used to get the instantaneous
velocity fields. 100 continue frames were
used to compute the mean flow field.
concentration region) began to occur and
much small scale turbulent structures began
to appear in the flow field.
a. nature jet flow
b. side view slice of the tabbed jet flow
RESULTS AND DISCUSSIONS
1). LIF results
The results of the flow visualization
and
instantaneous
concentration
measurement for a nature jet flow (circular
jet flow without mechanical tabs intrusion)
and tabbed jet flow in the axial slices were
shown in Fig. 3. From these figures, it can
be seen that, for a nature jet flow (Fig. 3(a)),
three different regions, which are laminar,
transition and turbulent regions, can be
identified clearly in the figure. At the end of
laminar region (Y/D=1.5), spanwise KelvinHelmholtz vortices was found to roll up.
The pairing and combining of spanwise
Kelvin-Helmholtz vortices conducted in the
transition region. In the downstream of
Y/D=4.0, small scale mixing (low
c. top view slice of the tabbed jet flow
Figure 3. Flow visualization and instantaneous
concentration measurement by using LIF (Re=3,000)
Figure 3(b) is the LIF result in the side
view slice (Y-Z plane, Fig 2) of the tabbed
jet flow. Compared with the natural jet flow,
the laminar region of the tabbed jet flow in
this slice was not straight it became a
convergent region instead. This can be
explained by that there was a local
contraction at the exit of the nozzle due to
the existence of mechanical tabs. The length
of the laminar region (Y/D=0.5) was much
less than that in the nature jet flow
(Y/D=1.5). It also can be found that the
4.4
INSTIGATION ON THE TABBED JET MIXING FLOWS BY USING LIF AND PIV
scale of spanwise vortices rolled up by the
Kelvin-Helmholtz instability was much
smaller, and the transition region also
became much shorter (from Y/D=0.5 to
Y/D=1.5). The small scale vortices (low
concentration region) appeared earlier in the
tabbed jet flow (about at the downstream of
Y/D=1.5), and the flow field is a bit zigzag
instead of symmetrical in the nature jet flow.
Figure 3(c) is the results of the flow
visualizations
and
instantaneous
concentration measurements for tabbed jet
flow in the top view slice (X-Y plane,
Fig.2). It can be seen that, similar to the case
in the natural jet flow, there was also a
straight laminar instability region in the
tabbed jet flow in this slice, but it was much
shorter (Y/D=0.5). At the downstream of the
Y/D=2.5,
small-scale
mixing
(low
concentration region) occurred and smallscale turbulence structures appeared in the
jet flow. It also can be seen that the jet
spread angle of the tabbed jet was bigger in
this axial slice than that in the nature jet and
tabbed jet in the side view slice
Figure 4 is the flow visualization of the
tabbed jet flow and nature jet flow in the
several cross planes (Z-X plane, Fig.2). The
existence of the streamwise vortices
generated by mechanical tabs can be seen
clearly in the figures (Fig.4(a)). Due to the
"engulf effect" of the streamwise vortices,
an inward indentation of ambient flow into
core jet flow and the outward ejection of
core jet flow into ambient flow were
conducted, which resulted in that the jet
spread rate in the X-Y plane (top view slice)
was faster than that in the Z-Y plane (side
view slice). So, as the downstream distance
increased, the cross section form of the
tabbed jet flow changed from "round" to
"egg" gradually. It also can be seen that,
small scale vortices structures were
observed in the cross planes of the tabbed jet
flow
at
the
downstream
of
Y/D=2.0(Fig.4(a)), but this can not be
observed in the nature jet flow even in the
cross plane of Y/D=4.0 (Fig.4(b)).
Y/D=0.5
Y/D=1.0
Y/D=2.0
Y/D=4.0
a. tabbed jet flow
Y/D=2.0
Y/D=4.0
b. nature jet flow
Figure 4. Flow visualization in several cross planes
(Re=1,800)
showed in Fig. 5 to Fig. 7. Fig 5(a), Fig 6(a)
and Fig. 7(a) are the instantaneous velocity
and spanwise vorticity (Wz) distributions in
the nature jet and tabbed jet flow in the side
view slice and top view slice respectively.
While Fig. 5(b), Fig. 6(b) and Fig.7(b) show
the mean velocity fields and mean turbulent
intensity fields in these axial slices. As
motioned above, the mean velocity fields
showed on these figures were got by the
average of 100 frames of the instantaneous
velocity fields. The instantaneous spanwise
vorticity (Wzi,j,t), mean velocity (Ui,j, Vi,j)
and the mean turbulent intensity (Ti,j )
showed in these figures were defined as:
2). PIV results
The PIV measurement results of the
nature jet flow and tabbed jet flow were
4.5
HU HUI, TOSHIO KOBAYASHI, TETSUO SAGA, NOBUYUKI TANIGUCHI, SHIGEKI SEGAWA
100
U i , j = ∑ ui , j ,t
t =1
Wzi , j ,t =
∂ui, j ,t
100
Ti , j =
∑
t =1
−
2
∂v i, j ,t
∂x
2
u' i, j ,t + v ' i, j ,t
t =1
100
100
=
∂y
100
Vi,j = ∑ v i , j ,t
∑
( ui , j ,t − U i, j )2 + ( v i , j ,t − Vi , j ) 2
t =1
100
In which, ui,j,t and ui,j,t are the
instantaneous velocities in the X and Y
directions, while u'i,j,t and v'i,j,t are
instantaneous turbulent velocities.
From these figures, it can be seen that,
compared with the mean flow fields, the
instantaneous velocity and vorticity fields
showed that many small scale turbulent and
vortical structures appeared in the flow field,
which can not be observed in the mean flow
fields. This is also the advantage of the full
field experimental technique like PIV over
conventional measurement techniques.
From the comparison of the
instantaneous flow fields of the nature jet
and tabbed jet (Fig. 5(a), Fig. 6(a) and Fig.
7(a)), it can be seen that, much more
turbulence and more small scale vortices
were found in the tabbed jet flow than that
in the nature jet. Just like above LIF results,
the instantaneous flow field of the nature jet
is almost axisymmetrical in the near field
(Y/D<6.0), while in the tabbed jet flow,
some zigzag structures were found in the
flow fields.
From the mean velocity fields in these
slices, it can be seen that, in the top view
slice of the tabbed jet flow (Fig. 7(b)), two
local maxim velocity can be found in the jet
flow away from the central line of the
tabbed jet in the downstream of Y/D=2.0,
rather that the one maxim velocity in central
line for the nature jet case (Fig. 5(b)), which
indicates the bifurcation of the tabbed jet
clearly. The spread angle of the jet flow is
also found to be much bigger than that in the
nature jet and tabbed jet in the side view
slice (Fig. 6(b)), which also verified the
above LIF results.
From the mean turbulent intensity fields,
it also can be seen that, for the nature jet,
between two higher turbulent intensity
region
(red
color
region,
which
corresponding the mixing layers in the
nature jet flow), a low turbulent intensity
region (blue color region, which represent
the potential core region of the nature jet
flow) was found in the center of the nature
jet flow, and it last to the downstream of
Y/D=5.0. However, in the tabbed jet, the
low turbulent intensity region (blue color
region) was much shorter, which ended just
in the location of Y/D=l.5 and Y/D=2.0 in
the side view slice and top view slice,
respectively. In the meanwhile, it also
should be noted that, the maximum values
of the mean turbulent intensity in the tabbed
jet indicated in the color bars (which is
0.106m/s and 0.l20m/s in the side view slice
and top view slice respectively were much
bigger than that in the nature jet (which is
0.087m/s).
Fig. 8(a) to Fig. 10(a) show the
isometric views of l00 frames PIV
instantaneous
velocity
fields
and
instantaneous spanwise vorticity fields for
the nature jet and tabbed jet in the side view
slice and top view slice. In these figures, X
is direction the radical direction and Y
direction is the jet flow direction, while t
direction indicated the time step. The isosurface structures of vorticity Wz=1.0 (red)
and Wz=-l.0 (purple) also were showed in
the figures. From these figures, it can be
seen that, at the entrance of the
measurement region, for the iso-surface
structures of the spanwise vorticity Wz=l.0
(red) and Wz =-1.0 (purple), wave structures
were found along the time direction (Z
direction), which corresponded to the
periodically rolled up process of the
spanwise vortices due to the KelvinHelmholtz instability in the shear mixing
layer. The size of the iso-surface structures
decreased along the flow direction (Y
direction) due to the mixing process in the
jet flow, and in the exit of the measurement
region, the scale of the iso-surface structures
are much smaller than that in the entrance of
the measurement region.
4.6
INSTIGATION ON THE TABBED JET MIXING FLOWS BY USING LIF AND PIV
a. instantaneous velocity and spanwise vorticity (Wz) field
b. mean velocity and mean turbulent intensity field
Figure 5. PIV measurement results for nature jet flow (Re=6,000)
a. instantaneous velocity and spanwise vorticity (Wz) field
b. mean velocity and mean turbulent intensity field
Figure 6. PIV measurement results for tabbed jet flow in the side view slice (Re=6,000)
a. instantaneous velocity and spanwise vorticity (Wz) field
b.mean velocity and mean turbulent intensity field
Figure 7. PIV measurement results for tabbed jet flow of in the view slice (Re=6,000)
4.7
HU HUI, TOSHIO KOBAYASHI, TETSUO SAGA, NOBUYUKI TANIGUCHI, SHIGEKI SEGAWA
a. isometric view
b.the view from time direction (time average effect)
Figure 8. 100 frames of instantaneous velocity fields and iso-surface structures of spanwise vortices Wz=1.0 (red)
and Wz=-1.0 (purple) in the nature jet flow (Re=6,000)
a. isometric view
b.the view from time direction (time average effect)
Figure 9. 100 frames of instantaneous velocity fields and iso-surface structures of spanwise vortices Wz=1.0 (red)
and Wz=-1.0 (purple) in the side view slice for tabbed jet flow (Re=6,000)
a. isometric view
b. the view from time direction (time average effect)
Figure 10. 100 frames of instantaneous velocity fields and iso-surface structures of spanwise vortices Wz=1.0 (red)
and Wz=-1.0 (purple) in the top view slice for tabbed jet flow (Re=6,000)
4.8
INSTIGATION ON THE TABBED JET MIXING FLOWS BY USING LIF AND PIV
Fig. 8(b) to Fig. 10(b) are the views of
the Fig. 8(a) to Fig. 10(a) from t direction
(time direction, which can indicate the
average effect of the time). From Fig.8(b) it
can be seen that, two rows of the iso-surface
structures of the spanwise vorticity Wz=l.0
(red) and Wz=-1.0(purple) were found in the
shear layers of the circular jet flow. Between
these two structures are the core jet flow.
While for the tabbed jet, the distance
between two rows of the iso-surface
structures of Wz=l.0(red) and Wz=l.0
(purple) was found much smaller in the side
view slice (Fig.9(b)), which means stronger
interaction of these spanwise vortices and
more intensive mixing. In the top view slice
of the tabbed jet, additional two rows of the
iso-surface structure of Wz=1.0 (red) and
Wz=-l.0 (purple) were found to appear in
the jet flow, and the flow field is just like the
combination of two jets, which indicates the
bifurcation of the tabbed jet once more.
3). Mechanism of the mixing
enhancement by mechanical tab
From the above LIF and PIV results, it
can be seen that in comparison with a nature
jet flow, mechanical tabs can generate
streamwise vortices at downstream, reduce
the scale of Kelvin-Helmholtz vortices and
bifurcate the jet flow. Earlier appearance of
the small scale turbulent structures, bigger
turbulent intensity and shorter potential core
length of the jet flow were also detected in
the tabbed jet. All these indicate the mixing
enhancement performance of the mechanical
tabs. Why the mechanical tabs can cause the
changes of vortical and turbulent structures
in the jet flow so strongly and how did the
mechanical tabs enhance fluid mixing in jet
flows? In the paper of Reeder et al.[12] and
Zaman et al.[7], they suggested that such
strong changes in the jet flow are due to the
large scale streamwise vortices generated by
mechanical tabs. However, by what way
these streamwise vortices enhance the jet
mixing process? They did not give an
answer.
It was well known that, the mixing
process in a jet flow is also the transfer
processes of energy and vorticity from large
scale vortices to small scale vortices. In
order to understand the reason
mechanical tabs can enhance jet
mixing process, let us see how
processes conducted in a nature jet
first.
why
flow
these
flow
Figure 11. Idealization of the vortical evolution in a
nature jet flow conjectured by Hussain [22]
Just like the description in the article of
Hussain [22], for a circular jet flow, the
spanwise vortical rings will be rolled up due
to the Kelvin-Helmholtz instability (first
instability) existed at any shear layer. As
these spanwise vortical rings moved
downstream, they can not be two
dimensional vortical rings during their
evolutions due to the self-interaction effect
and cross-interaction effect of these vortical
rings. They will be the combinations of
many helical vortices, i.e. toroidal vortical
rings through the effect of an additional
instability (secondary instability or zimuthal
helical instability model) (Fig. 11). So the
two dimensional spanwise vortex rings
caused by the Kelvin-Helmholtz instability
will be wrapped and developed into three
dimensional structures through secondary
instability, and streamwise vortices will also
appear in the nature jet flow (Liepmam et
al.[14] ). Undergoing interaction, the large
scale toroidal vortical rings will be broken
down into many substructures through the
"cut and connect" process (Hussain[22])
which may be responsible for the avalanche
of three dimensional and smaller scale
motions and for the generation of high
turbulence and Reynolds stress. However,
for a nature jet, such process will need a bit
long streamwise distance to complete.
In a tabbed jet, because of the intrusion
of mechanical tabs, the streamwise vortices
will be generated directly in the jet flow
4.9
HU HUI, TOSHIO KOBAYASHI, TETSUO SAGA, NOBUYUKI TANIGUCHI, SHIGEKI SEGAWA
field. The streamwise vortices generated by
mechanical tabs have bigger strength and
longer life than those generated by the
secondary instability in the nature jet
(Zaman[8] ). The mechanical tabs can cause
big perturbation along the azimuth of the jet
flow (such as local constriction of the jet
flow due to the intrusion of the mechanical
tabs shown by above LIF results). The
streamwise vortices generated by the
mechanical tabs enlarge such big azimuthal
perturbations (such as streamwise vortices
can make an inward indentation of ambient
flow into core jet and outward ejection of
the core jet flow into ambient. All these
enhance the secondary instability of the jet
flow (the zigzag structures were found in the
tabbed jet flow revealed by above LIF and
PIV results), and the formation of the
complex three dimensional vortex structures
was accelerated. i.e. the "toroidal effect" of
the spanwise structures was enlarged. Then
the "cut and connect" process of the toroidal
vortical conducted more rapidly (such as the
bifurcation was occurred in the top view
slice of the tabbed jet shown by the above
PIV and LIF results). Which also means the
process of the large scale vortical structures
broken into smaller scale vortical structures
was conducted more rapidly, hence
enhanced the mixing of the jet flow with
ambient flow.
Besides this, the interaction of the
streamwise vortices produced by the
mechanical tabs and the spanwise vortical
rings produced by the Kelvin-Helmholtz
instability also made the spanwise vortical
rings stretched (Fig. 12). According to the
Helmholtz vorticity conservation law, the
scale of the vortices will be reduced when
the vortex was stretched, and the "energy
cascade" process of turbulence was
enhanced. (This is the reason why the scale
of spanwise Kelvin-Helmholtz vortices is
smaller in the tabbed jet flow than that in the
nature jet flow, also may be the reason why
the mechanical tabs can reduce the low
frequency jet noise in the supersonic jet
flow). This resulted in the rapid reduction of
the scale of the spanwise vortices, which
also resulted in the quicker energy transfer
from large-scale vortices to smaller scale
vortices. Both of these two aspects can
cause the creation of much small-scale
intense turbulence and enhance the mixing
of the core jet flow with ambient flows.
These may be the reasons why the tabbed jet
will be more turbulent and have bigger mean
turbulent intensity than the nature jet
revealed by above LIF and PIV results.
S t r e a m w ise vortices
K - H v or t e x r in g
Stretch points
Fig. 12 Stretch effect of the streamwise vortices
caused by mechanical tabs on the K-H spanwise
vortrica ring
CONCLUSION
The LIF and PIV results of the
investigation revealed the great changes of
the turbulent structures and vortices in the
near field of a jet flow by the intrusion of
the mechanical tabs. Compared with a
nature jet flow, tabbed jet flow was found to
have shorter potential core length, smaller
scale of the spanwise Kelvin-Helmholtz
vortices, earlier appearance of small scale
turbulent vortices and bigger mean turbulent
intensity. The bigger spread angle and
bifurcation of the jet were also found in the
top view slices of the tabbed jet flow. All
these indicated the mixing enhancement
performance of mechanical tabs.
Based on the LIF and PIV result, two
aspects of the mechanism of the jet mixing
enhancement caused by mechanical tabs
were suggested. The first was that
mechanical tabs can cause big azimuthal
perturbations in a jet flow, and these
azimuthal perturbations were enlarged by
the streamwise vortices generated by the
mechanical tabs. These enlarged the
secondary instability of a jet flow, so the
"cut and connect" process of the KelvinHelmholtz vortical rings was accelerated
which transferred the energy and vorticity
from large scale vortices to small scale
vortices. The second was that the interaction
between the streamwise vortices and Kelvin4.10
INSTIGATION ON THE TABBED JET MIXING FLOWS BY USING LIF AND PIV
Helmholtz vortical rings also enhanced the
"energy cascade" process of the turbulence.
Both of these resulted in the creation of the
much small scale intense turbulence and
enhanced the mixing of the jet flow with
ambient flow.
ACKNOWLEDGEMENTS
The authors wish to thank Ms. Hisako
Nagase and Mr. Akira Ono of University of
Tokyo for help in conducting the present
study. The authors also appreciate the
helpful
discussions
and
advisable
suggestions from Prof. Wu Shousheng and
Prof. Shen Gongxin of Beijing University of
Aeronautics and Astronautics. The research
fellowship provided by Japan Society for
Promotion of Science (JSPS) is also grateful
acknowledged. This study was supported by
the Original Industrial Technology R&D
Promotion Program from the New Energy
and Industrial Technology Development
Organization (NEDO) of Japan.
REFERENCES
[1] Bradbury L.J.S. and Khadem, A.H. "The
Distortion of a Jet by Tabs"J. of Fluid Mech.
,Vol.70, pp.80l-8l3, 1975.
[2] Ahuja, K.K., Manes J.P. and Massey, K. C., "An
Evaluation of Various Concepts of Reduction
Supersonic Jet Noise", AIAA90-3982, 1990.
[3] Ahhja K. K. "Mixing Enhancement and Jet
Noise Reduction Through Tabs Plus Ejector",
AIAA93-4347, 1993.
[4] Zaman K.B.M.Q., Reeder M.F.and Samimy M.
"Effect of Tabs on the Evaluation of an
Axisymmetrical Jet", NASA-TM104472, 1991.
[5] Zaman K.B.M.Q., Reeder M.F. and Samimy M.
"Supersonic Jet Mixing Enhancement by 'Delta
Tabs'", AIAA92-3548, 1992.
Zaman K.B.M.Q, "Streamwise Vorticity
[6]
Generation and Mixing Enhancement in Free Jet
by "Delta Tabs" AIAA93-3253, 1993.
[7] Zaman, K. B. M. Q., Reeder, M.F. and Samimy,
M. "Control of an Axisymmetric Jet Using
Vortex Generators", Phys. Fluid, Vol.6 No2.
1994, pp778-792.
[8] Zaman K. B. M. Q. "Axis Switching and
Spreading of an Asymmetrical Jet: the Role of
Coherent Structure Dynamics" J. Fluid Mech.
Vol.316, 1996, pp1-27.
[9] Kobayashi H., "Study on Tabs for Reducing
Noise from Hot and Cold Supersonic Jets" NAL
Research Progress, 1995, pp46-479.
[10] Glawe D. D. Samimy M., Najad A. S. and Chen
T. H., "Effects of Nozzle geometry on Parallel
Injection into a Supersonic Flow" Journal of
Propulsion and Power Vol. 12 No.6, 1996,
pp1159-1168
[11] Zhang Shucheng and Scheider S.P. "MolecularMixing Measurements and Turbulent Structure
Visualizations in a Round Jet with Tabs",
AIAA94-3082, 1994
[12] Reeder M. F. and Samimy M., " The Evolution
of a Jet with Vortex generating tabs: real-time
visualization and quantitative measurement", J.
Fluid Mech. Vol.31l, 1996, pp733-118
[13] Adrian R. J., "particle-Imaging Techniques for
Experimental Fluid Mechanics" Ann Rev. Fluid
Mech. Vlo.23, 1991, pp261-304.
[14] Liepmann D. and Gharib, M. "The Role of
Streamwise Vorticity in the Near Field
Entrainment of Round Jet", J. Fluid Mech.
Vol.254, 1992, pp643-668.
[15] Willert C.E. and Gharib M., "Digital Particle
Image Velocimetry" Exp. in Fluids Vol.l0 1991,
ppl8l-l993.
[16] Yoda M., Hesselink L. and Mungal M.
G.,"Instantaneous
Three
Dimensional
Concentration Measurement in the Self Similar
Region of a Round High-Schmidt Number Jet",
J. Fluid Mech., Vol.279, 1994, pp3l3-350.
[17] Southerland K.B and Dahm W. J.A. "Four
Dimensional Laser Induced Fluorescence Study
of the Structure of Molecular Mixing in
Turbulent Flow", AIAA94-0820, 1994.
[18] Coppeta J and Rogers C. "Mixing Measurements
Using Laser Induced Fluorescence" AIAA 950l67, 1995.
[19] Hu Hui, Wu Shousheng and Shen Gongxing,
"Experimental Investigation on the Jet Mixing
Flow of Lobed Nozzles by Using LIF" AIAA96l937, 1996.
[20] Zhou M. and Garner. C. P. "Particle image
velocimetry measurements of the flow field
within an enclosed rotating disk-stator system
and comparisons.", Optical Diagnostics in
Engineering , Vol.1 Part 2. 1996.
[21] Hu Hui, Saga T., Kobayashi T. and Taniguchi
N., "Evaluation of the Cross Correlation Method
by Using PIV Standard Images"accepted by the
Journal of Visualization. Jan. 1998
[22] Hussain A.K.M.F. "Coherence Structure and
Turbulence"J. Fluid Mech. Vol.l73, 1986,
pp303-356
4.11