Proceedings of 3rd International Workshop on PIV, Santa Barbara, USA, Sep.16-18, 1999
PIV and LIF Measurements on the Lobed Jet Mixing Flows
Hui HU, Toshio KOBAYASHI, Tetsuo SAGA, Shigeki SEGAWA and Nobuyuki TANIGUCHI
Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Tokyo 106, Japan
E-mail: huhui@cc.iis.u-tokyo.ac.jp
Abstract: The effect of lobed nozzle on the vortical and
turbulent structure chnages in the near field (X/D<3.0) of the
jet mixing flow had been conducted experimentally in the
present study. The techniques of planar Laser Induced
Fluoresce (LIF) and Particle Image Velocimetry (PIV) were
used to accomplish the flow visualization and velocity field
measurements. The experimental results showed that,
compared with a circular jet mixing flow, lobed jet mixing
flows were found to have shorter laminar region, smaller
scale of the spanwise Kelvin-Helmholtz vortices, earlier
appearance of small scale turbulent structures and thicker
mixing layers in the near field of the jet mixing flow. Bigger
high-turbulence intensity regions were found to concentrate at
the region of X/D<2.0 for the lobed jet mixing flows. All
these indicted the better mixing enhancement performance of
a lobed nozzle over a conventional circular nozzle at the near
field of a jet mixing flow.
Introduction
A lobed nozzle, which consists of a splitter plate with a
convoluted trailing edge, is an extraordinary fluid mechanic
device for efficient mixing of two co-flow streams with
different velocity, temperature and/or spices. It has been paid
great attention by many researchers in recent years and has
also been widely applied to the aerospace engineering. For
examples, on the commercial aero-engines, lobed nozzles had
been used to reduce take-off jet noise and Specific Fuel
Consumption (SFC) (Tillman et al. 1993, Presz et al. 1994,
and Hu et al. 1996). In order to reduce the infrared radiation
signals of military aircrafts to improve their survivability in
the modern war, lobed nozzles had also been used to enhance
the mixing process of the high temperature and high speed
gas plume from air-engine with ambient cold air (Power et al.,
1994). More recently, lobed nozzles had also emerged as
attractive approaches for enhancing mixing between fuel and
air in combustion chambers to improve the efficiency of
combustion and reduce the formation of the pollutant (Smith
et al., 1997).
The large scale streamwise vortices generated by lobed
nozzles had been suggested to be the reason for their good
mixing enhancement performance by many researchers, such
as Paterson, (1982), McCormick et al. (1993) and Belovich et
al., (1997). Although these previous researches had got many
important results, much work still need in order to understand
the mechanism of jet-mixing enhancement by using lobed
nozzles more clearly. In particular about the vortical and
turbulent structure chnages in the jet mixing flow caused by a
lobed nozzle. In the meanwhile, most of these previous
researches were conducted by using Pitot probe, Laser
Doppler Velocimetry or Hot film Anemeter, which are very
hard to reveal the vortical and turbulent structures in jet
mixing flows instantaneously and globally due to the
limitation of these experimental techniques. While, in the
present study, both planar Laser Induced Fluorescence (LIF)
and Particle Image Velocimetry (PIV) technique were used to
research the lobed jet mixing flows globally and
instantaneously. By using the directly perceived LIF flow
visualization images, velocity vectors, vorticity distributions
and turbulence intensity distributions got by PIV
measurement, the changes of the turbulent and vortical
structures in the near field of jet mixing flow caused by a
Overflow water tank
Laser sheet
Laser beam
water tank
optical system
CCD camera
For PIV
mixing region
Double
pulsed
YAG
laser
tested nozzle
Convergent section
and comb structure
flow
jet supply tank
For LIF
CCD camera
monitor
mage
process
I
system
Host computer
pump
lobed nozzle were studied in detail.
Figure 1. Experimental Setup
Experiment set-up
Figure 1 shows the schematically experimental facility
used in the present research. The test nozzles were fixed in
the middle of the water tank (600mm*600mm*1000mm).
Fluorescent dye (Rhodamine B) for LIF or PIV tracers
(polystyrene particles of d=20-30μm, density is 1.02) was
premixed with water in jet supply tank, and jet mixing flow
was supplied by a pump. The flowrate of the jet mixing flow,
which was used to calculate the representative velocity and
Reynolds numbers, was measured by a flow meter. A
convergent unit and honeycomb structures were installed at
the upstream of test nozzles to insure the turbulent levels of
jet mixing flows at the exit of test nozzles were less than 5%.
The pulsed laser sheet (thickness is about 1.0 mm) used for
LIF visualization and PIV measurement was supplied by a
Twin Nd:YAG Laser with the frequency of 10 Hz and power
of 200 mJ/pulse. The time interval between the two pulses
can be adjustable. A 1008 by 1016 pixels Cross-Correlation
CCD array camera (PIVCAM 10-30) was used to capture the
LIF and PIV images. The Twin Nd:YAG Laser and the CCD
camera were controlled by a Synchronizer Control System.
The LIF and PIV images captured by the CCD camera were
digitized by an image processing board, then transferred to a
work station (host computer, RAM 1024MB, HD20GB) for
image processing and displayed on a PC monitor.
Rhodamine B was used as fluorescent dye in the present
research. The fluorescent light was separated from the
scattered laser light by installation a high pass optical filter at
the head of the CCD camera. A low concentration Rhodamine
B solution (0.5mg/l) was used to insure the strength of
fluorescent light being linear with the concentration of the
fluorescent dye and the effect of laser light attenuation being
negligible as the laser light sheet propagated through the flow
(Hu et al. 1999).
For the PIV image processing, rather than tracking
individual particle, the cross correlation method (Willert et al.,
1991) was used in the present study to obtain the average
displacement of the ensemble particles. The images were
divided into 32 by 32 pixel interrogation windows, and 50%
overlap grids were employed. The resolution the PIV images
for the present research case is about 120μm/pixel. The
average displacement of the particles is about 4 to 6 pixels
between the two laser pulses (dt=1ms to 4ms). The
post-processing procedures which including sub-pixel
interpolation (Hu et al., 1998) and velocity outliner deletion
(Westerweel, 1994) were used to improve the accuracy of the
PIV result.
Z
Z
Y
X
lobe peak slice
lobe trough slice
a. circular nozzle
lobe width
W= 6mm
lobe side slice
lobe heigth
H=15mm
inner penetration
angle
220
140
outer penetration angle
c. three studied axial slices
b. lobed nozzle
Figure 2. The test nozzles and three studied axial slices
Figure 2 shows the two test nozzles used in the present
study: a baseline circular nozzle A and a lobed nozzle B with
six lobes. The height of the lobes is about 15mm and the
inner and outer penetration angles of the lobe structures are
about 220 and 140 respectively. The equivalent diameters of
these two nozzles at the exit were designed to be the same, i.e.
D=40mm. In the present study, the jet velocities were set as
about 0.1m/s and 0.2m/s. The Reynolds Number of the jet
mixing flows, based on the nozzle exit diameter, were about
3,000 and 6,000.
In the present study, the turbulent and vortical structure
changes caused by lobed nozzle in the three axial slices were
firstly investigated, which are the lobe trough slice, lobe peak
slice and lobe side slice (Figure 2(c).). Then, LIF
visualization and PIV measurement were also conducted at
several cross sections for the lobed jet mixing flow and
circular jet mixing flow.
For the PIV measurement results, the mean velocity
vectors, mean streamwise vorticity distributions and mean
turbulence intensity fields were used to compare the lobed jet
mixing flows with circular jet flow. The mean values of the
PIV measurements were calculated based on the average of
400 frames of the instantaneous flow fields, which were
obtained at the frequency of 10 Hz.
Results and discussion
1. In the axial slices
Figure 3 shows the LIF visualization and PIV
measurement result in the axial slice of a conventional
circular jet mixing flow at the Reynolds number of 6,000.
From the LIF flow visualization(Fig.3(a)), it can be seen that
there is a laminar region at the exit of the circular nozzle. At
the end of the laminar region (X/D ＝ l.5), spanwise
Kelvin-Helmholtz vortices were found to roll up. Pairing and
combining of these spanwise vortices and the transition of the
jet mixing flow to turbulence were found to conduct in much
downstream (X/D=4-6), which is out of the CCD camera
view of the present study. None of the small scale turbulence
and vortical structures can be found in the investigated region
(X/D<3.0). The LIF visualization got by the presnt study is
very similar as the result reported by Liepmann et al (1992).
From the mean velocity distribution (Fig.3(b)) and
turbulence intensity distribution (Fig.3(c)) at the same
Reynolds number level obtained by PIV measurement, it also
can be seen that the circular jet mixing flow began to
expended just after the spanwise Kelvin-Helmholtz vortices
rolling up at the downstream of X/D>1.5. A low turbulence
intensity region in the central of the circular jet mixing flow,
which is called the potential core region, can be found at the
jet mixing flow, which extent to the downstream of the
X/D>3.0 for the circular jet mixng flow.
Figure 4 shows the LIF visualization and PIV
measurement result in axial slice passing the lobe trough of
the lobed jet mixing flow. Compared with the circular jet
mixing flow, the laminar region of the lobed jet mixing flow
became much shorter in this axial slice (Fig.4(a)). The
spanwise Kelvin-Helmholtz vortices were found to roll up
almost just from the trailing edge of the lobed nozzle. It also
can be found that the size of these spanwise
Kelvin-Helmholtz vortices became much smaller compared
with that in the circular jet mixing flow. Some small scale
turbulent and vortical structures began to appear in the flow
field from the downstream of X/D>1.0.
From the PIV measurement results (Fig.4(b) and Fig.4(c))
at this axial slice, it can also be seen that the lobed jet mixing
flow began to expend almost just from the trailing edge of the
lobed nozzle. The expand angle of the lobe jet mixing flow
was found to be the angle of the outer penetration angle at the
lobed nozzle within the downstream region of X/D<2.0, and
then changed to the expand angle of circular jet mixing flow.
It can also be found that most of the high-turbulenceintensity regions for the lobed jet mixing flow concentrated at
the region of X/D<2.0. While, in the circular jet mixing flow,
these regions were found to be much downstream. It also can
be
found
that
the
potential
core
region
(low-turbulence-intensity) at the central of the lobed jet
mixing flow was much smaller and shorter than that of the
circular jet mixing flow.
Figure 5 shows the LIF flow visualization and PIV
measurement result in the axial slice passing the lobe peak
(Fig.2(c)) of the lobed jet mixing flow. The laminar region at
the exit of the lobed nozzle at this axial slice was not a
straight cylinder like that in the circular jet, and looked like a
expansive cut-off cone along the downstream of the lobe
structure instead. Compared with that in the lobe trough slice,
the laminar region in the lobe peak axial slice was a bit
longer (X/D=0.4), but it was still much shorter than that in
the circular jet mixing flow. This may be caused by the
different thickness of the boundary layer at the exit of the
lobed nozzle (the work of the Brink et al.(1993) had verified
that the thickness of the boundary layer at the lobe trough is
smaller than that in the lobed peak), and the thicker boundary
layer at the lobe peak need a longer streamwise distance to
roll-up the Kelvin-Helmholtz vortices (Hussain et al. 1989).
In this axial slice, it can also be seen that small-scale
turbulent and vortical structures were found to appear in the
flow field from the downstream of location X/D=1.0. Form
the PIV measurement results (Fig. 5(b) and Fig. 5(c)), it also
can be seen that most of the high-turbulence-intensity regions
were concentrated at the downstream of X/D<2.0. The
potential core region at the central of the lobed jet mixing
flow at this axial slice is also much smaller and shorter than
that in circular jet mixing flow, which is the same as that in
the lobe trough slice for the lobed jet mixing flow.
Figure 6 gives the LIF flow visualization and PIV
measurement result in the axial slice passing lobe side
(Fig.2(c)) of the lobed jet mixing flow. In this axial slice,
some streak flow structures can be seen clearly in the
downstream of the lobe structure trailing edge. These
structures were the Kelvin-Helmholtz vortical tubes shed
periodically from the lobe training edge, which was observed
and called “normal vortex” by McCormick et al.(1993). At
the downstream of the location X/D ＝ 1.0, small scale
turbulent and vortical structures were found to appear in the
flow field. As the same as that in the axial slices passing lobe
peak and lobe trough of the lobed jet mixing flow, most of the
high intensity region was found to concentrate at the region
of X/D<2.0. The potential core region at the central of the
lobed jet mixing flow in this axial slice was also much
smaller and shorter than that in the circular jet mixing flow.
2. In the cross planes
Figure 7 shows the LIF flow visualization of the lobed jet
mixing flow in six cross planes. From the figures it can be
seen that, at the location of X＝l0mm (X/D=0.25, Fig.7(a)),
the existence of the streamwise vortices in the form of 6 petal
“mushrooms” at the lobe peak can be seen clearly in the jet
mixing flow. The spanwise Kelvin-Helmholtz vortices rolled
up at the lobe trough parts were found to be as six crescents
at this cross section.
As the streamwise distance increased to X ＝ 20mm
(X/D=0.5), the “mushrooms” at the lobe peak grew up
(Fig.7(b)), which indicated the intensification of the
streamwise vortices generated by lobed nozzle.
As the streamwise vortices distance increased to X＝
30mm (X/D=0.75), the streamwise vortices generated by the
lobed nozzle in the form of “mushrooms” structure keep on
intensification. Six new counter-rotating streamwise vortex
pairs also can be found at six lobe trough. Though the
existence of the horseshoes structure at the trough of the lobe
structure had been suggested by Paterson (1992), this is the
best known visualization and provides unquestionable
evidence of their existence.
At the location of X=40mm (X/D=1.0, Fig. 7(d)), some
small scale turbulent and vortical structures began to appear
in the flow and the interaction between the streamwise
vortices and Kelvin-Helmholtz vortices made adjacent
“mushrooms” merging with each other, which indicated the
process that the streamwise vortices deform the
Kelvin-Helmholtz vortical tube into pinch-off structure
suggested by McCormick et al.(1993).
At the location of X＝60mm (X/D=1.5, Fig.7(e)) and X＝
80mm (X/D=2.0, Fig.7(f)), the “mushroom” shape structures
almost disappeared and the flow was almost fully filled with
small scale turbulent and vortical structures.
While for the circular jet mixing flow at the same
Reynolds number level, the jet mixing flow is still in this
laminar region at the location of X=40mm (X/D=1.0,
Fig.8(a)). The spanwise Kelvin-Helmholtz vortices were
found to roll up at the location of X=80mm (X/D=2.0, Fig.
8(b)). At the downstream location of X=120mm (X/D=3.0,
Fig.8(c)), some streamwise vortices due to the azimuthal
instability (Liepmann et al. 1992) were found to be appear in
the flow field. However, neither small-scale turbulent
sturtures nor small scale vortical structures can be found at
the cross planes of these locations for the circular jet mixing
flow.
Figure 9 shows the PIV measurement results at four typical
cross planes.As the same as LIF visaulization, basides the six
mushroom like large-scale streamwise vortices, the existance
of the counter rotating horseshoes vortices at the lobe trough
region can also be seen clearly in the lobed jet mixing flow.
As the distance inceased to the downstream of X=60mm, the
large scale streamwise vortices were found to break down
into many small scale vortices, and the flow field were fully
filled with small scale turbulent and vortical structures.
Conclusion
The LIF visualization and PIV measurement result
revealed the great differences of the turbulent structure and
vortex scale between the lobed jet mixing flow and circular
jet mixing flow. Compared with a circular jet mixing flow,
the lobed jet mixing flow was found to have shorter laminar
region, smaller scale of the spanwise Kelvin-Helmholtz
vorices, earlier appearance of small-scale turbulent and
vortical structures and bigger turbulence intensity in the very
near downstream. All these indicated the mixing
enhancement performances of a lobed nozzle over a circular
nozzle.
Based on the result of the present experimental research,
the analysis to reveal the mechanism of the mixing
enhancement performance of lobed nozzles and the
suggestions to do optimism design of the lobed nozzles will
be conducted in the future work.
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3.5
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a.LIF visulization
b.mean velocity distribution by PIV
c.turbulence intensity distribution
Figure 3. LIF visulization and PIV measurement result in the circular jet mixing flow (Re=6,000)
3.5
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a.LIF visulization
b.mean velocity distribution by PIV
c.turbulence intensity distribution
Figure 4. LIF visualization and PIV measurement results of lobed jet mixing flow in the lobe trough slice (Re=6,000)
3.5
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X
a.LIF visulization
b.mean velocity distribution by PIV
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Figure 5. LIF visaulization and PIV
measurement results of the lobed jet mixing3.5flow in the lobe peak slice (Re=6,000)
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Figure 6. LIF visualization and PIV measurement results of the lobed jet mixing flow in the lobe side slice (Re=6,000)
a.X=10mm
b.X=20mm
c.X=30mm
d.X=40mm
e.X=60mm
f.X=80mm
Figure 7. LIF visulization of the lobed jet mixing flow in the serveral corss planes (Re=3,000)
a.X=40mm
b.X=80mm
c.X=120mm
Figure 8. LIF visulization of the circular jet mixing flow in the serveral corss planes (Re=3,000)
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1 .7
X
0 .0
0.4
01
0.01 4
01
0.
-0.8
-0.5
.8
1.7
-0.8
-
8
0.
0.
-0.81 .7
7
1.
-1
0.4
a. mean velocity distribution
1.7
1
0.4
4
0.5
X
1 .7
0 .4
-1
-0.8
4
4
0.
-1
0.4
-0.75
-0 .8
0 .4
.8
-0.75
0.
-0 .8
-0.5
-0
0 .4
0.
-0.5
-0 .8
0.4
1.7
1.7
-0 .8
-0.8
-0.25
-0 .8
-0.25
.8
1
0.5
2.
9
7
.8
-0
7
1 . 0.4
0
1.
-0
0.4
0
0 .4
1 .7
0 .0 1
0.75
-0 .8
0.25
1
c.turbulence intensity distribution
0 .4
0 .4
1 .7
0.25
0 .4 -2.0
0.5
-0.8
0.5
0.5
0 .0
0.4
0.03 m/s
0
-1
X
-0
0.4
0.75
0.4
0.75
-0.5
1
b.streamwise vortices
C. X=60 mm
-0.8
a. mean velocity distribution
-1
0.5
X
0 .0 1
-0.5
4
-1
0 .0 1
1
0.01 7
0.5
-1
-2 .0 -2
.0
0.
4
0
X
0.01
0
-0.75
0.01 7
-0.6
20
7
-0.6
0.
-0
9
.6
0.0
0.0
0.
0 .9
0 .9
2 .4
9
10
01
0.
0.
-0 .6
17
0.007
.6
-0.5
0
-0.5
6
-0 .
-0
-1
01
7
0 .0
17
4
0 .0 1
0.9-0
.6
.2
-2
0.
01
-0 .6
1
0.
0.0
20
0.017
07
14
-0
4
0.9
0.9
0.5
0 .0
Y
2
4
12
0 .0
.6
Y
0.00
0
0.00 7
9
0.
-1
0.9
01
-0.25
.6
-1
-0
-0.75
0.9
2 .4
-0.6
-2 .2.7
-3
.2
-2
0.9
4
2.4
-0.75
2.
0.9
0.9
-2.2
-0.5
-2 .2
0.0
0
-2 .2
-0.5
-2 .2 -0.6
-0 .6
-
0.25
-0 .6
-0 .6
0.9
.6
-2.2
-0
2
2.
8
0.5
-0 .6
-2.2
-0
-0 .6
00
c.turbulence intensity distribution
0.
2 .4
.6
0
-0.25
0.
0.0 12
00
8
6
01
-0.5
0.75
0.9
9
0
0.01
0.
2.
0
2 .0
0.008
0 .0 1
6
-4
.6
-1 .3
2.0
-1 .3
01
0
2.
-1.3
0.004
5.
2
Y
Y
0.
3
-1
.
-1 .3
0.0
-1
0.004
0.
0 .9
-0.25
0.9
0.
20
X
-2 .2
-0.6
-0
0.9
0.0 04
1
0 .9
0.25
-0 .6
-2 .2
0.25
-0 .6
0.9 -0 .6
6
0 .0
-1
0.5
0.75
0.5
01
2
0.03 m/s
0.5
0
-0.75
5.
0
-0.6
0.
0.
02
12
0.0
08
0 .0
0 .0 04
-0.5
b.streamwise vortices
B. X=40 mm
0.75
1 6 20
0.0 0.0
0.01 2
0 .0
24
0.02
0
0.016
X
a. mean velocity distribution
8
0.020
4
00
00
-0.5
2.0
-1
2 .0
1
-1 .3 8 .5
-4 .6
0.
6
2.0
0.5
X
-0.25
5.2
-1.3-4 .6
5 .22 .0
-1
0
-1.3
-4 .6
-0.75
-1
2.
-7 .8.6
-4 .3
-1
-0.75
.6 4 .6
- .3
-1
0
.3
-0.5
2 .0
5.2
-1
-4
-1 .3
0
04
0.
.3
-0.25
-0.5
-1.3
0
2.
-1.3
02
0.0
0.
01
2
16
0 .0 2
0 .0 1 8
0 .0 0
-1
-0.25
0.25
.3
2 .0
0
0.008
0.0
1
0.
.9
-1 .3
-1
0
0.5
2 .0
-4 .6
0.012
2 .0
-2
.3
- 1.0
2
0.25
1
c.turbulence intensity distribution
01
6
0.012
0
-4 .6
-4 1 .3
.6
0.5
X
-1 .3
2 .0
-4 .6
2 .0
0.5
0
-0.5
-4.6
2.
0.75
0.03m/s
-0.5
-1
b.streamwise vortices
A.X=20 mm
0.25
0.01
0
0 .0
0.5
2.0
a. mean velocity distribution
-1
01
0.
5.6
1.6
.6
-1
0
1.6
-0.5
X
0.5
1
-1
-1
X
0.75
0.0
0.
01
4
0.0 10
1
0.
00
1 .6
9 .7
1 .6
-6 .6
1.6 -2.5
9 .7
1.6
Y
0.5
0.00
0
02
00
-0.5
-0.75
0
-1
0 .0
5.6
-1
-6.6
-0.75
-1
0.
00
0.
-6.6
2
0.01
0
0.0
0.0 1
6
1.
-6.6
.5
-2 .5
1 .6
-0.5
1
01.0
0 .0
0.0
01
0.
0.0 1
-0.25
-2 5 7
.5 .6.6
0
0.
1
-2 .5
1 .6
-2
.6
-1 0 5.6
-6 .6
-2.5
-0.5
-0.75
0
0. .0 1
00
0
-0.25
-0.5
0 .0
0 .0 1
0 .0
-0.25
0 .0 1 2
0.0
0 .0
1
0.
02
01 0 .02
Y
0
0.
0.0
0.25
0.01
-2 .5
6
1 . 5 .6
2 .5-8.6
1-.6
-2.5
1.6
1
0.02
1 .6
-6 .6 2.
5 .6 1 .6 5
Y
0
-2 .5
9.7
0 .0
0
0.
0.25
0 .0
0.0 0
0.25
0.5
1.6
1 .6
0.5
0
0.5
0 .0
.6
-2.5
0.03 m/s
0.75
-6
1.6
0.75
5 .6
0.75
0 .0
11
08
0.5
1
X
b.streamwise vortices
c.turbulence intensity distribution
D. X=80 mm
Figure 9. PIV measurement results in serveral cross planes of the lobed jet mixing flow (Re=3,000)