CIVIL ENGINEERING
Research
ISSN 0219-0370
No. 23 / 2010
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ENVIRONMENTAL AND WATER RESOURCES ENGINEERING
HYDRAULIC CHARACTERISTICS OF
JET-INDUCED INTERNAL CIRCULATION
IN A WATER COLUMN
Manh-Tuan Nguyen (nguy0108@ntu.edu.sg)
Soon Keat Tan (ctansk@ntu.edu.sg)
ABSTRACT: Particle Image Velocimetry (PIV) technique was used to study certain hydraulic characteristics of the turbulent jets and
demonstrated the effects of Froude number on the vertical mean velocity distributions in the flow development region. The results
show that an increase in Froude number results in the reduction in maximum velocity decay in the downstream direction. The findings
also revealed a distinct phenomenon termed as “dead” jet.
INTRODUCTION
The interaction of a turbulent surface jet with a free-surface
is an important topic, but relatively few works are focused
on the effects of Froude number on the turbulent structure
in free-surface jet flows. This paper is an attempt to fill
this gap.
Measurements of a developing planar surface jet were
made by [1], and the authors found a distinct decrease
in vertical velocity fluctuations near the free surface. An
investigation by [2] showed that instabilities in the nearfield region of a round jet are strongly influenced by the
presence of a free surface, and that the jet flow field is
altered through changes in the entrainment field near the
free surface. Based on the findings from experiments with
depths corresponding to h/d =1, 1.5, 2.5, 3.5 (where h, and
d were defined as the jet depth, and the jet exit diameter
respectively) using Hot-film velocity measurements, [3]
showed that the decay of the mean centerline velocity
is slower in the free-surface jet than that in the free jet.
Experiments by [4] using laser Doppler velocimetry (LDV)
measurements demonstrated that the velocity fluctuations
normal to the free surface were reduced nearer to the
surface while the tangential velocity components were
enhanced. The effects of Reynolds and Froude numbers
on the turbulence structure of round jets issuing beneath a
free surface were examined using LDV measurements [5]
who also provided a comprehensive review on this topic.
The researchers [5] showed that at high Froude number,
reduction in the interaction of the tangential vorticity
with its image was observed and near the free surface the
tangential velocity fluctuations were enhanced while the
surface normal fluctuations were reduced.
In this study, PIV technique is used as the tool to obtain
the complete velocity field in the near field of the flow
(0 < x/d < 16). The purpose of the investigation is to
examine the effects of Froude number on the vertical mean
velocity distributions within the development zone of the
free-surface jets.
Figure 1. Sketch of experimental setup.
EXPERIMENTAL SETUP
A series of experiments were carried out in a 5 m long,
re-circulating open water channel with a rectangular cross
section of 0.3 m wide × 0.45 m height. A number of
experiments were conducted at a jet exit velocity of 1 m/s
and jet diameter of 1cm (Re=9960) for depths corresponding
to h/d = 0.25, 0.62, and 1.87. The corresponding values of
Ue/√gh , where Ue is the jet exit velocity, for the free
surface jet cases are 20.39, 10.19, and 2.55. The coordinates
(x, y, and z) denote the streamwise, lateral, and vertical
directions, respectively. The origin is located at the jet exit
as shown in Figure 1.
PIV system (LaVision model) was used to obtain the results
in the vertical plane of the jet (xz plane). The view of field
was 116 mm x 155 mm. A Litro System double cavity
Nd:YAG laser was used to illuminate the flow field. The
particle images were recorded using a 12-bit charge-couple
device (CCD) camera which had a resolution of 1600x1200
pixels and a frame rate of 15 Hz.
Civil Engineering Research • January 2010
41
ENVIRONMENTAL AND WATER RESOURCES ENGINEERING
RESULTS AND DISCUSSION
Mean velocity profiles were plotted along the x-direction
as shown in Figure 2 for depths corresponding to h/d=0.25,
0.62, and 1.87, respectively. In all cases the measured mean
velocity profiles are normalized by the maximum velocity
at the nozzle (Ue). Figure 2 (c) shows the velocity profiles
measured at h/d=1.87 and various distance from the jet axial
plane. Eventually the jet reaches the free surface resulting
at a non-zero mean velocity at the surface. For x/d<8.28
the jet does not approach the free-surface and the velocity
distributions are very similar to the free jet velocity profiles.
Downstream of this location the normalized mean velocities
at those points close to free-surface increase. However, at
this depth and (0<x/d<16) the maximum mean velocity is
always found at the centre line of the jet (Fig. 4). As the
jet is located close to free-surface (h/d≤0.62) the interaction
occurs immediately from the jet exit (nozzle) as clearly
indicated in Figure 2 (a) and (b).
Figure 3. Downstream evolution of the maximum mean velocity
of the free surface jets. Ue = 1.0 m/s, Re= 9960. ®, h/d=0.25;
Ο, h/d=0.62; ∆, h/d=1.87.
Figure 2. Vertical mean velocity profiles for free surface jets.
Ue = 1.0 m/s, Reynolds number 9960. (a) h/d=0.25; (b)
h/d=0.62; (c) h/d=1.87. The flow is from right to left.
Our findings show that as Froude number increases, the
maximum mean velocity decay of the free-surface jets
associated with h/d=0.28 and h/d=0.63 is slower than that of
submerged jet (h/d=1.88) as shown in Figure 3. This means
that the closer the jet is to the free-surface, the smaller is
the decay in maximum velocity. This Phenomenon will be
investigated further.
The trajectory of the mean maximum velocity associated
with h/d=0.25 is significantly different from those of the
cases h/d =0.63 and h/d=1.88. In Figure 4 Z m/d is plotted as
a function of x/d for various cases examined. Z m is defined
as the distance from the location of the maximum mean
velocity to the free surface. For h/d=1.87 and h/d=0.62 the
location of the maximum mean velocity remains on the
jet centre line plane; meanwhile that of h/d=0.25 moves
downward beneath the jet centre line and fluctuates along
the downstream direction.
42
Civil Engineering Research • January 2010
Figure 4. The maximum mean velocity locations along the
streamwise coordinate. Ue = 1.0 m/s, Re= 9960. ®, h/d=0.25;
Ο, h/d=0.62; ∆, h/d=1.87.
A distinct feature of free-surface jet corresponding to the
case of h/d=0.28 is the clear revelation of the “dead” zone,
which corresponds to the part of water body enclosed by
the locus of the max jet velocity when the maximum mean
velocity decreases rapidly and significantly from x/d=0
to x/d=6.09, and recovers from then till x/d=8.28 before
decaying in the downstream direction. These phenomena
may be caused by the sharp increase in pressure at the
initial region at x/d<8.28.
ENVIRONMENTAL AND WATER RESOURCES ENGINEERING
CONCLUSIONS
REFERENCES
PIV measurement was used to investigate certain hydraulic
characteristics of the turbulent jets and demonstrated the
effects of Froude number on the vertical mean velocity
profiles in the flow development region. It was shown that
an increase in Froude number results in the reduction in
maximum velocity decay in the downstream direction. The
results also revealed a very interesting phenomenon termed
as “dead” jet which might be caused by the increase in
pressure along the jet centreline.
[1] Swean, T.F., Ramberg, S.E., Plesniak, M.W. & Stewart, M.B.,
1990. “Turbulent surface jet in channel of limited depth.”
Journal of Hydraulic Engineering, Vol. 115(12), pp. 15871606.
[2] Liepmann, D., 1990. “The Near-field Dynamics and
Entrainment Field of Submerged and Near-surface Jets.” PhD
Thesis, University of California, San Diego.
[3] Madnia, C.K. and Bernal, L.P., 1994. “Interaction of a turbulent
round jet with the free surface.” Journal of Fluid Mechanics,
Vol. 261, pp. 305-322.
[4] Anthony, D.G. and Willmarth, W.W., 1992. “Turbulence
measurements in a round jet beneath a free surface.” Journal
of Fluid Mechanics, Vol. 243, pp. 699-720.
[5] Walker, D.T., C.Y. Chen, and Willmarth, W.W., 1995.
“Turbulent structure in free-surface jet flows.” Journal of
Fluid Mechanics, Vol. 291, pp. 223-261.
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