Proceedings
FEDSM’10
Proceedings of the ASME 2010 3rd Joint US-European Fluids Engineering
Summerof
Meetingand2010 onASME
Fluids Engineering
Divisionand
SummerMeeting
Nanochannels,
Microchannels,
Minichannels
8th International Conference
August 1-5, 2010,
Montreal, Canada
FEDSM-ICNMM2010
August 1-5, 2010, Montreal, Canada

FEDSM-ICNMM2010-31248
FEDSM-ICNMM2010-
LIQUID JET IMPINGEMENT ON A FREE LIQUID SURFACE: PIV STUDY OF THE
TURBULENT BUBBLY TWO-PHASE FLOW
Khaled J. Hammad
Dantec Dynamics, Inc.
Ramsey, NJ, USA

ABSTRACTThe turbulent two-phase flow arising from the normal
impingement of a round free-surface water jet on a horizontal
air-water interface was experimentally studied. Due to theweakly viscous nature of the flow system under consideration,
external perturbations or small variations in jet inflow
conditions can lead to drastically different flow fieldcharacteristics under seemingly similar test conditions. In thecurrent study, a fully developed turbulent jet, exiting a long
pipe, ensured properly characterized inflow conditions. The
study considered two jet inflow conditions; one entrained air
and created a bubbly two-phase flow field while the other did
not. Particle image velocimetry (PIV) was used to characterize
the flow field beneath the interface, with and without air
entrainment, for various nozzle-to-interface separation
distances. Turbulent velocity fields of the continuous-phase anddispersed-phase were simultaneously measured in thedeveloping flow region and presented using Reynolds
decomposition into mean and fluctuating components. The
mean and RMS velocities of the two-phase flow field werecompared with velocity measurements obtained under singlephase conditions.
NOMENCLATURE
d
H
L
Re
U
Ub
U ci
Uc
r, z

Nozzle internal diameter (m)Nozzle-to-interface separation distance (m)Pipe length (m)Reynolds number ( U b d  )Mean velocity magnitude (m/s)
Jet bulk velocity at nozzle exit (m/s)
Mean centerline velocity at interface
Mean centerline velocity at the nozzle exit (m/s)
Cylindrical coordinates (m)ur , u z

Instantaneous velocity in z and r directions (m/s)

uNormalized mean velocity u z uc in z directionv

Normalized mean velocity ur uc in r directionur , u z Velocity fluctuations in r and z directions (m/s)
ur2 , u z 2 Normal Reynolds stresses (m2/s2)
uru zShear Reynolds stress (m2/s2)u'Normalized RMS velocity

u z 2 U c in z directionv'

Normalized RMS velocityur2 U c in r direction

Greek Symbols
Density
Kinematic viscosity of jet fluid (m2/s)

Vorticity normalized by 2 U b d
Subscriptsb
BulkcCenterlineiInterfacej
Jetr
Radialz
Vertical
INTRODUCTION
The interaction of liquid jets with free surfaces can be
encountered in nature, e.g. waterfalls and waves in ocean [1],and in industrial applications. Air entrainment and subsurface
bubbly flow were found to take place once the jet velocity
impacting the free surface exceeded a critical value [2]. The
direct impingement of a free liquid jet on a liquid surface wasrecognized as an efficient method for producing gas bubbles in1

Copyright © 2010 by ASME

liquids and has been utilized in numerous industrial processes
for aeration and heat/mass transfer enhancement [3-5]. Due toit’s fundamental and practical importance, this problem has
been extensively studied in the past [6-15]. The bulk ofavailable research work focused on quantifying air entrainmentand size distribution and penetration depth of bubbles entrainedby the jet under various test conditions. The comprehensive
review by Bin [8] about air entrainment by plunging jets
presented a summary of the correlations established to predict
the onset of air entrainment based on a critical velocity of thejet, amount of entrained air, and diameter distributions within
the bubbly plum.
The reviews by Bin [8] and Chanson [10] highlighted thefact that most previous studies were qualitative and only asmall number of researchers studied the flow field below
impingement surface. McKeogh and Ervine [6] reported
velocity distributions primarily in the fully developed flow
region while Bonetto and Lahey [9] presented results obtainedin both developing and fully developed flow regions. Iguchi et.
al. [16] used LDV to study the liquid-phase motion in the
bubble dispersion region when injecting water verticallydownward through a straight circular pipe onto a water bath.
Turbulence levels were higher than the single-phase free jetflow case, mainly due to turbulence production in the wake of
bubbles. Reliable LDV measurements were not possible in theflow developing region and were restricted to >16 pipe
diameters beneath the bath surface. The more recent study by
Chanson et. al. [17] investigates air entrainment and bubble
dispersion in the developing flow region of vertical circularplunging jets. Clear water jet velocities and turbulent velocity
fluctuations were measured in the free-falling jet using Pitot
tubes and hot-film probes while air–water flow properties were
measured with single-tip conductivity probes.
Air entrainment and subsequent breakup into bubbles
depends on the physical properties of the fluids used, nozzle
geometry, nozzle-free surface separation, jet velocity andturbulence, and instabilities at the jet surface [6, 7, 11, 12].
The primary mechanisms leading to air entrainment are theinterfacial shear along the liquid jet interface which drags down
an air boundary layer and the developing interfacial
perturbations which leads to air entrapment at the point of
impact of the plunging jet with the free surface of the receiving
pool [13]. The annular air sheet dragged down by the liquid jet
beneath the free surface of the pool becomes unstable under theeffect of the shear and the entrapped pockets of air break up
into bubbles. The produced bubbles undergo a further break upprocess into smaller bubbles due to the turbulence generated by
the submerged jet within the shear-layer. The study by Oguz
[11] confirmed that the size of the largest bubbles produced
correlates to the wavelength of the jet surface roughness
resulting from boundary layer instability inside the nozzle. The
maximum bubble size was nearly equal to one quarter of the
wavelength of the jet surface disturbance which is consistent
with a breakup process of relatively large air pockets aroundthe jet.

The two-phase turbulent nature of the flow combined with
the importance of inertia, viscous, surface tension, and gravity
forces complicate the task of coming up with a single physicalmodel that can describe and predict the evolution of the flow
events. Studying and modeling the physical mechanisms at
work becomes even more difficult for weakly viscous flows.
Weakly viscous flow systems, like the turbulent air-water flow
field under consideration here, are sensitive to ambient
perturbations, such as free surface instability and external
vibrations, which can significantly affect the air entrapment
process and the subsequent flow evolution [14]. Previous
modeling work produced semi-empirical correlations thatpredict parameters such as the volume of entrained air, bubble
size distributions, or the depth of penetration of the bubbles.
These correlations were valid only over a restrictive range ofparameters.
Despite the available body of knowledge, the two-phasenature of flow remains poorly understood. More work is needed
to improve our understanding of the mechanisms that play arole in the air-entrapment process at the impinging point and at
the fluctuating interface of both the jet and the free surface ofthe receiving pool. An examination of the available literature
reveals a lack of air-water flow field information near the
impingement surface.
The current study investigates the turbulent two-phase flow
field characteristics in the developing flow region beneath theimpingement surface. A schematic of the current flow problem
is shown in Fig. 1. A fully developed turbulent jet with a mean
exit velocity Ub is ejected vertically downward from a pipe
nozzle of diameter d, located at a distance H from a poolsurface. The current work is based on well defined inflow
conditions and controlled experiments that minimized thepotential influence of external perturbation such as vibrations
or external aerodynamic forces.
EXPERIMENTAL SETUP
The closed-loop flow system used here comprised of awater tank, pump, flowmeter, pipe, pipe mounting andpositioning structure. The rectangular glass tank used was 508
mm long, 254 mm wide and 305 mm high. The tank walls wereapproximately 6 mm thick. The jet was produced using a
vertical type 316 stainless steel precision pipe placed on thecylinder centerline with an inner diameter d = 10.92 mm, a
length L = 914.4 mm and a wall thickness 0.889 mm. The
length-to-diameter ratio of the pipe, L/d = 84, assured a fully
developed turbulent flow conditions at the outlet. The jet was
positioned 127 mm, about 12d, away from the walls of the tank.
The filled height of the tank was 270 mm, 25d.
As shown in Fig. 1, the separation distance between thepipe outlet and the air/water interface is denoted by H. The
origin of the cylindrical r, z coordinate system coincides withintersection point of the jet centerline and the free surface. The
axial coordinate, z, is pointing downward away from the freesurface while the radial one, r, is measured along the horizontal
free surface.

2

Copyright © 2010 by ASME

Measurements were performed at four nozzle-to-interface
separation distances, H/d= -1, 0, 1, 2 and 3. Two constantvolumetric flow rates of 0.945 x 10-4 m3/s (1.5 GPM) and 1.575
× 10-4 m3/s (2.5 GPM) were used, resulting in a fixed pipe bulk
velocities, Ub, of 1.01 and 1.68 m/s. The kinematic viscosity of
the working fluid, water at 28C and 30C, were 8.38 × 10-7
m2/s and 8.00 × 10-7 m2/s. The corresponding Reynolds
numbers, Re  U b D  , were 22,962 and 13,153.

Hrz
Fig.1 Free surface impinging jet and coordinate system

PIV SYSTEM
A Dantec Dynamics two-dimensional Time-Resolved PIV
system was used to measure the flow field along each r-z planeof interest. It consisted of a diode pumped, dual cavity, Nd:YLF
laser system with an output energy of 15 mJ, 527nm per cavity
with a typical laser pulse duration of 150 ns @1kHz. A
maximum of 10 kHz firing frequency per cavity can beachieved. Laser beam divergence was <5 mrad, beam diameter
about 3 mm and pulse stability ±1%. The imaging systemallowed for the capture of un-interrupted 6,000 CMOS camera
images at a sampling rate of 2 kHz and a resolution of 1280 x
1024 pixels. Synchronization hardware for controlling the laserand camera and software running on a Windows-based platform
for data acquisition, management and post-analysis were also
used.
Light sheet forming optics mounted at the exit portal of thelaser system generated a thin, focusable light sheet by means ofa series of spherical and cylindrical lenses. A vertical divergent
light sheet was generated and used to illuminate the full extent
of the measurement region within the tank. The vertical lightsheet was aligned to pass through the center of the pipe. The
thickness of the light sheet at the imaging plane was 1.5 mm.
The spatial location of the illumination system was fixed during
all the tests. A micrometer was used to position the CMOS
camera perpendicular to the laser sheet and properly align andoptimize the imaged area. The imaging lens was a 60-mm f2.8
Nikkor. Nearly neutrally buoyant 50 m Polymide particles
having a density of 1.05 g/cm3 were used as tracer particles forthe liquid-phase. The density of the test fluid, water at about
30C, was 0.996 g/cm3.

DATA PROCESSING
Captured bubbly flow images, Fig. 2a, were pre-processedusing a sequence of low-pass, high-pass and morphology digital
filters for background subtraction, bubble detection and phaseseparation. The areas occupied by bubbles were identified using
median filtering, Fig. 2c, and then subtracted from enhanced
captured images, Fig. 2b, to produce the liquid-phase or
seeding particle image, Fig. 2d.
Both the liquid and bubble flow fields were evaluatedusing an adaptive correlation technique. For the present set ofmeasurements, a high-accuracy multi-pass adaptive crosscorrelation technique was utilized to evaluate the velocity field
determined from each pair of particle images. The signal
strength was further optimized by window off-set which
minimized particle drop-outs. A high sub-pixel accuracy that is
independent of correlation peak shape was achieved and further
minimized displacement estimate errors [18].
The final interrogation area, IA, was 64  64 pixels with
75% overlap in both the horizontal and vertical directions. An
initial IA of 256 × 256 pixels was sequentially reduced to thefinal IA. A dynamic second-order accurate method was used tospatially offset the two IA’s during each subsequent iteration
cycle by the displacement calculated from previous steps.
During each iteration cycle, checks on the correlation’s signalto-noise, as well as a local comparison of vector attributes withthe median value of 5  5 neighbors were applied to validate
and remove outliers. To remove spurious vectors, a minimum
peak height ratio (between the 1st and 2nd peak) of 1.2 wasselected, thereby putting more stringent conditions on peakidentification for the subsequent determination of vectors.
For each test case, 2000 PIV realizations at a sampling rateof 200 Hz were obtained and used to evaluate the mean and
RMS flow field information presented here. The focus of thecurrent work was to investigate the multiphase flow structure in
the vicinity of the submerged jet shear layer, the developing
flow region. The results shown represent approximately a 5D x
8D area, 51 x 84 mm, situated beneath the impingement
interface. Each field of view occupied 640 x 1040 Pixels andresulted in a total of 37 x 62 or 2294 vectors for each PIV data
set.
NOZZLE EXIT FLOW CONDITIONS
The pipe exit region was analyzed to investigate the flow
conditions 0.25d from the nozzle for a nozzle height H=0,
liquid-phase flow only case.

3

Copyright © 2010 by ASME

Fig. 2a Raw multiphase image

Fig. 2c Bubble-phaseFig. 2b Processed multiphase image

Fig. 2d Liquid-phase4

Copyright © 2010 by ASME

LIQUID-PHASE RESULTS
Mean velocity and turbulence characteristics of the liquidphase in the flow-developing region are shown in Fig’s 5, 6, 7,
and 8. Fig’s 5 and 6 correspond to Re=13,000 and represent theflow field in the absence of entrained air bubbles, i.e. singlephase impinging jet flow field. The Re=23,000 case, Fig’s 7 and
8, represent the higher jet velocity case that resulted in air
entrainment at jet impact. No air entrainment was observed forall studied Reynolds numbers when the nozzle was placed at
the air-liquid interface, i.e. H=0.
Bubble entrainment suppresses liquid-phase mean
velocities and enhances fluctuations in the streamwise
direction, as shown in Fig’s 7 and 8 for Re=23,000 and H/d=1
and 2. However, bubbles reduce radial RMS velocities of theliquid-phase and result in almost identical radial distributions
within z/d=2-6, H/d=2 case shown in Fig. 8. The significant
increase in the streamwise RMS velocities shown in Fig. 8 for
H/d=2 is due to the substantially higher entrained bubbles
population. Increasing the nozzle height to H/d=2 results in a
global transition in the air entrainment and bubble generation
mechanisms. This transition was visually observed and will be
discussed in the next section, bubble-phase results. The
streamwise RMS velocity values at the jet centerline, for
Re=23,000 and nozzle height H/d=2, are u ' =0.26, 0.177, 0.132
at z/d=2, 4 and 6, respectively.
Note that due to gravity, jet velocities at impact point, i.e.
air-liquid interface, are expected to be higher than the initial
ones out of the nozzle when H>0. For the simplified case of an
initial centerline velocity at nozzle exit, U c , accelerating under
the influence of gravity only, the jet centerline velocity at
impact point or interface, U ci , can be estimated using:

Re=13,000

u, v

0.8

u
v

0.6
0.4
0.2
0
-1

-0.5

0

r/d

0.5

1

0.2

Re=13,000

u' , v'

0.15

u'
v'

0.1

0.05
0
-1

-0.5

0

r/d

0.5

1

Fig. 3. Radial profiles of liquid-phase mean and RMS
velocity components obtained 0.25d downstream
the pipe nozzle for nozzle height of 0 at Reynolds
number = 13,000

1Re=23,000

0.8

u, v

law profile that is typical of fully developed pipe conditions.
The radial profiles of the RMS velocities display distinct peaks
at the edge of the jet. The lowest axial RMS component,u '  0.04 , was obtained at the jet center while the highest one,u '  0.15 , was obtained at r d  0.5 . Substantial differences
between the radial and axial components of the RMS velocities
confirm turbulence anisotropy.

1

u
v

0.6
0.4
0.2
0
-1

-0.5

0

r/d

0.5

1

0.2

Re=23,000
0.15

u' , v'

The centerline axial velocities were found to be Uc = 1.2
and 1.9 m/s for Re=13,000 and 23,000, respectively. Both Ucvalues were used to normalize reported flow field data. Radial
profiles of axial velocity component and Uc values for nozzle
heights H/d = 0 and -1 were identical.
Figures 3 and 4 show the radial profiles of the normalized
mean, u & v , and RMS, u ' & v ' , velocities at an axial station
located 0.25d downstream of the pipe nozzle for Re = 13,000
and 23,000, respectively.
All velocities are normalized by the centerline axial
velocity at the pipe exit, Uc, while axial and radial distances arenormalized by the pipe diameter, d. As expected, the radial
components of the mean velocity, v , are close to zero within
the jet, r d  0.5 , while the axial ones, u , display a power-

u'
v'

0.1

0.05
0
-1

-0.5

0

r/d

0.5

1

Fig. 4. Radial profiles of liquid-phase mean and RMS
velocity components obtained 0.25d downstream
the pipe nozzle for nozzle height of 0 at Reynolds
number = 23,000

5Copyright © 2010 by ASME

u, z=2dv, z=2du, z=4dv, z=4du, z=6dv, z=6d

Re=13,000Nozzle @ z=0

0.5

1.0

u, z=2dv, z=2du, z=4dv, z=4du, z=6dv, z=6d

Re=23,000Nozzle @ z=0

u, v

u, v

1.0

0.5

0.0

0.0
-1.5

-1

-0.5

0

0.5

1

1.5

-2

-1.5

-1

-0.5

r/d
u, z=2dv, z=2du, z=4dv, z=4du, z=6dv, z=6d

Nozzle @ z=-1d

0.5

-1

-0.5

0

0.5

1

0.5

1.5

-2

-1.5

-1

-0.5

u, z=2dv, z=2du, z=4dv, z=4du, z=6dv, z=6d

Nozzle @ z=-2d

0.5

-1

-0.5

0

0.5

1

-2

-1.5

-1

-0.5

0.08
0.04
-0.5

0

0.5

1

2

u' , z=2dv' , z=2du' , z=4dv' , z=4du' , z=6dv' , z=6d

0.08
0.04
0.00

1.5

u' , z=2dv' , z=2du' , z=4dv' , z=4du' , z=6dv' , z=6d

Nozzle @ z=-1d
0.12
0.08
0.04
0

1.5

Re=23,000Nozzle @ z=0

0.12

-2

-1.5

-1

-0.5

0.5

1

0.04

1.5

2

u' , z=2dv' , z=2du' , z=4dv' , z=4du' , z=6dv' , z=6d

Nozzle @ z=-1d
0.08
0.04
0.00

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

r/d
0.30

u' , z=2dv' , z=2du' , z=4dv' , z=4du' , z=6dv' , z=6d

Nozzle @ z=-2d

0.25

u' , v'

0.08

1

0.12

1.5

u' , z=2dv' , z=2du' , z=4dv' , z=4du' , z=6dv' , z=6d

Nozzle @ z=-2d
0.12

0.5

0.16

r/d
0.16

0

r/d

u' , v'

0.16

-0.5

1

0.16

r/d

-1

0.5

Fig. 7. Radial profiles of liquid-phase mean velocity
components, u and u , obtained 2d, 4d and 6d
beneath the impingement free surface for nozzle
heights of 0, 1d and 2d at Reynolds number =23,000u' , v'u' , z=2dv' , z=2du' , z=4dv' , z=4du' , z=6dv' , z=6d

Re=13,000Nozzle @ z=0

-1

0

r/d

0.16

u' , v'2u, z=2dv, z=2du, z=4dv, z=4du, z=6dv, z=6d0.5

1.5

Fig. 5. Radial profiles of liquid-phase mean velocity
components, u and u , obtained 2d, 4d and 6d
beneath the impingement free surface for nozzle
heights of 0, 1d and 2d at Reynolds number =13,000u' , v'

1.5

Nozzle @ z=-2dr/du' , v'

1

0.0
-1.5

0.00

0.5

1.0

0.0

-1.5

0

r/d

u, v

u, v

1.0

0.00

2

u, z=2dv, z=2du, z=4dv, z=4du, z=6dv, z=6d

r/d

-1.5

1.5

0.0
-1.5

0.00

1

Nozzle @ z=-1d

0.0

0.12

0.5

1.0

u, v

u, v

1.0

0

r/d

0.20
0.15
0.10
0.05

-1.5

-1

-0.5

0

0.5

1

0.00

1.5

Fig. 6. Radial profiles of liquid-phase RMS velocity
components, u and u , obtained 2d, 4d and 6d
beneath the impingement free surface for nozzle
heights of 0, 1d and 2d at Reynolds number =13,000

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

r/d

r/d

Fig. 8. Radial profiles of liquid-phase RMS velocity
components, u and u , obtained 2d, 4d and 6d
beneath the impingement free surface for nozzle
heights of 0, 1d and 2d at Reynolds number =23,0006

Copyright © 2010 by ASMEU ci  U c 2  2 gH

(1)u, z=2dv, z=2du, z=4dv, z=4du, z=6dv, z=6d

0.6

Re=23,000

u, v

0.2
0.0
-0.2

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

r/d
u, z=2dv, z=2du, z=4dv, z=4du, z=6dv, z=6d

0.6

Nozzle @ z=-2d
0.4
0.2
0.0

H/d

Uc
(m/s)

Uci
(m/s)

Uci/ Uc
(%)

Uc
(m/s)

Uci
(m/s)

Uci/ Uc
(%)

1
2
3

1.2
1.2
1.2

1.29
1.37
1.44

107
114
120

1.9
1.9
1.9

1.96
2.01
2.06

103
106
109

-0.2

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

r/d
u, z=2dv, z=2du, z=4dv, z=4du, z=6dv, z=6d

0.6

Nozzle @ z=-3d
0.4

u, v

Re=13,000

0.4

u, v

Using Eq. 1, the expected jet centerline velocity at impact
point, taking into account change resulting from gravity driven
acceleration only, is calculated and tabulated in Table 1 for all
studied Reynolds numbers and nozzle heights.
The percentage increase in centerline velocities at impact
point compared to those obtained at the nozzle exit for
Re=13,000 is approximately twice that of Re=23,000.

Re=23,000Nozzle @ z=-1d

0.2
0.0

mean axial velocities, u , of the bubble-phase shown in Fig. 9,
are substantially smaller than the liquid-phase ones shown in
Fig. 7. However, further away from the jet core, r d  2 , theflow field is dominated by the upward movement of thegenerated bubble population and the mean axial velocities, u ,are higher than the ones obtained in the liquid-phase only case,i.e. Re=13,000 case shown in Fig. 7. The axial component of
the mean velocity of bubbles, u , at r d  2 are tabulated inTable 2. They represent the bubble population rise and areshown to increase with nozzle height, H, reflecting larger
bubble size creation.

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

r/d

Fig. 9. Radial profiles of bubble-phase RMS velocity
components, u and u , obtained 2d, 4d and 6d
beneath the impingement free surface for nozzle
heights of 0, 1d and 2d at Reynolds number =13,000u' , z=2dv' , z=2du' , z=4dv' , z=4du' , z=6dv' , z=6d

0.25

Re=23,000Nozzle @ z=-1du' , v'

0.20
0.15
0.10
0.05
0.00

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

r/d
u' , z=2dv' , z=2du' , z=4dv' , z=4du' , z=6dv' , z=6d

0.25

Nozzle @ z=-2d

0.20

u' , v'

BUBBLE-PHASE RESULTS
Mean velocity and turbulence characteristics of the
bubble-phase are shown in Fig’s 9 and 10 for Re=23,000.
Recall that there was no air bubbles entrainment in the
Re=13,000 case.
For nozzle height H/d=1, individual air bubble
entrainment was observed. Increasing the nozzle height to
H/d=2 resulted in the development of an unstable air cavity
along the impingement perimeter. The air cavity position
fluctuated randomly, along the jet perimeter, with time. For
nozzle heights of H/d=3 and higher, the air cavity seemed toform around most of the jet perimeter and extended toapproximately z/d=2. Constant stretching and breakup of thecavity tip formed large air packets which were entrained below
the air cavity, close to the annular jet shear layer. The entrained
air packets broke up into smaller bubbles as they travelledfurther downstream.
Within the annular shear layer of the jet, r d  0.5 , the

-0.2

0.15
0.10
0.05
0.00

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

r/d
u' , z=2dv' , z=2du' , z=4dv' , z=4du' , z=6dv' , z=6d

0.25

Nozzle @ z=-3d

0.20

u' , v'

Table 1 Jet centerline velocities at nozzle exit and at impact
point for nozzle heights of 0, 1d and 2d andReynolds number = 13,000 and 23,000

0.15
0.10
0.05
0.00

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

r/d

Fig. 10. Radial profiles of bubble-phase RMS velocity
components, u and u , obtained 2d, 4d and 6d
beneath the impingement free surface for nozzle
heights of 0, 1d and 2d at Reynolds number =23,0007

Copyright © 2010 by ASME

z/d
2
4
6
Table 2

Bubble rise velocity (m/s),
from u at r/d=-2
H/d=1
H/d=2
H/d=3
0.053
0.103
0.245
0.073
0.124
0.272
0.073
0.127
0.269
Bubble rise velocity for nozzle heights of 1d, 2d
and 3d at Reynolds number = 23,000

Measured turbulent intensities in the radial direction forthe bubble-phase are substantially stronger than thecorresponding intensities of the liquid-phase, as shown in Fig’s
8 and 10.

REFERENCES[1] Chanson, H., Cummings, P., 1992, ‘Aeration of the ocean
due to plunging waves.’ Research report, No. CE142,
Department of Civil Engineering, University of
Queensland, Australia.
[2] Lara, P., 1979, ‘Onset of air entrainment for water jet
impinging vertically on water surface,’ Chem. Engng. Sci.
34, pp. 1164–1165.
[3] Burgess, J.M., Molly, N.A., Mc Carthy, M.J., 1972. A noteon plunging liquid jet reactor. Chem. Engng. Sci. 27, pp.
442–445.
[4] Iciek, J., 1982, ‘The hydrodynamics of a free liquid jet andtheir influence on direct contact heat transfer,’ Int. J.
Multiphase Flow 8 (3), pp. 239–249.

CONCLUSIONS
An experimental study of a fully developed turbulent jet
issuing from a long pipe and impinging normally a on ahorizontal air-water interface has been conducted. PIV
measurements were performed within the flow developing
region for two Reynolds numbers, Re= 13,000, 23,000, andfour nozzle-to-interface separation distances, H/d= -1, 0, 1, 2
and 3. In the case of Re= 13,000, a liquid-phase flow field wasobserved beneath the interface. However, increasing the
Reynolds number to Re= 23,000 resulted in the formation of abubbly two-phase flow field when the separation distance was
one jet diameter or higher. Simultaneous PIV measurements of
the liquid and bubble-phase velocities allowed for appropriate
and detailed evaluation of the flow features of the weakly
viscous system under consideration.
For nozzle height H/d=1, individual air bubble
entrainment was observed. Increasing the nozzle height to
H/d=2 resulted in the development of an unstable air cavity
along the impingement perimeter. The air cavity position
fluctuated randomly, along the jet perimeter, with time. For
nozzle heights of H/d=3 and higher, the air cavity seemed toform around most of the jet perimeter and extended toapproximately two jet diameters beneath the interface.
Strong modulation of liquid-phase turbulence due to thepresence of bubbles results in higher turbulent intensities in theaxial direction and lower ones in the radial direction. Bubble
entrainment suppresses liquid-phase mean velocities andenhances fluctuations in the streamwise direction. Measured
turbulent intensities in the radial direction for the bubble-phaseare substantially stronger than those of the liquid-phase. The
presence of bubbles reduces radial RMS velocities of theliquid-phase.

[8] Bin, A.K., 1993, ‘Gas Entrainment by plunging liquid jets,’
Chem. Eng. Sci, 48 (21), pp. 3585-3630.

ACKNOWLEDGEMENTS
The author is grateful to Mr. Craig Goulbourne for helping
with the experiment setup.

[13] El Hammoumi, M., Achard, J.L., Davoust, L., 2002,‘Measurements of air entrainment by vertical plunging
liquid jets,’ Experiments in Fluids 32, pp. 624–638.

[5] Kusabiraki, D., Murota, M., Ohno, S., Yamagiwa, K.,
Yasuda, M., and Ohkawa, A., 1990, ‘Gas entrainment rate
and flow pattern in a plunging liquid jet aeration systemusing inclined nozzles,’ J. Chem. Engng. Japan 23 (6), pp.
704–710.
[6] McKeogh, E.J., Ervine, D.A., 1981, ‘Air entrainment rateand diffusion pattern of plunging liquid jets,’ Chem.
Engng. Sci. 36, pp. 1161–1172.
[7] Lezzi, A.M., Prosperetti, A., 1991, ‘The stability of an air
film in a liquid flow,’ J. Fluid Mech. 226, pp. 319–347.

[9] Bonetto, F., Lahey, R.T., 1993, ‘An experimental study on
air carryunder due to a plunging liquid jet,’ Int. J.
Multiphase Flow 19 (2), pp. 281–294.
[10] Chanson, H., 1997. Air Bubble Entrainment in Freesurface Turbulent Shear Flows. Academic Press, London,
UK, 401pp.
[11] Oguz, H.N., 1998, ‘The role of surface disturbances in theentrainment of bubbles by a liquid jet,’ J. Fluid Mech. 372,
pp. 189–212.
[12] Zhu, Y., Oguz, H.N., Prosperetti, A., 2000, ‘On themechanism of air entrainment by liquid jets at a freesurface,’ J. Fluid Mech. 404, pp. 151–177.

8

Copyright © 2010 by ASME

[14] Davoust, L., Achard, J.L., El Hammoumi, M., 2002, ‘Air
entrainment by a plunging jet: the dynamical roughness
concept and its estimation by a light absorption technique,’Int. J. Multiphase Flow 28, pp.1541-1564.
[15] Chanson H, Manasseh R., 2003, ‘Air entrainment
processes in a circular plunging jet: void-fraction andacoustic measurements,’ J Fluids Eng. 125, 910-921.
[16] Iguchi, M., Okita, K. and Yamamoto, F., 1998, ‘Mean
velocity and turbulence characteristics of water flow in the
bubble dispersion region induced by plunging water jet,’Int. J. Multiphase Flow 24 (4), pp. 523-537.
[17] Chanson H, Aoki S, Hoque A, 2004, ‘Physical modellingand similitude of air bubble entrainment at vertical circularplunging jets,’ Chem Eng Sci, 59 (4), pp. 747–58.
[18] Westergaard, Madsen, Marassi, and Tomasini, 2003,‘Accuracy of PIV Signals in Theory and Practice,’ 5thInternational Symposium on Particle Image Velocimetry,
Busan, Korea, September 22-24, Paper 3301.

9

Copyright © 2010 by ASME