October 10, 2007 / Vol. 5, No. 10 / CHINESE OPTICS LETTERS 609
Experimental inve stigation on wake prof ile detection
based on laser scattering by bubbles
Liping Su (
ÛÛÛ
), Weijiang Zhao (
), Xiaoyong Hu (
),
Deming Ren (
), and Xizhan Liu (
)
Institute of Opto-Electronics, Harbin Institute of Technology, Harbin 150080
Received April 16, 2007
The optical system for detecting wake profiles based on laser backscattering by bubbles at 180
◦
is reported,
in which the monostatic optical geometry is adopted and the p ower density estimation is used to process
bubble scattering signal. The profiles of wakes produced by a two-blade propeller with a diameter of
46 mm at 6000 and 8000 rpm are measured using this system. It is shown that the wake region can be
identified, the wake with different shapes can be distinguished, and the fine structure within wakes can
be detected. Also, the repeatability of the results is tested experimentally. Results show the feasibility of
this system in wake profile detection.
OCIS codes: 290.5820, 290.1350, 290.5850, 010.3640.
Recently, there is a great interest in studies on optical
scattering properties of wake bubbles
[1−5]
,whichisthe
base of the technique for detecting and tracing fish stocks
or ships by laser. Among the experimental investigations
on optical scattering properties of bubbles in water, it is
the most representative that Zhang et al. has measured
the volume scattering function in a range of scattering
angles of 10
◦
− 170
◦
for bubbles in natural water
[1]
.And
other researches were mainly focused on the small-angle
(< 1.5
◦
) scattering in the forward direction
[2]
.Nowa
few authors have been preliminarily studying bubble
backscattering at 180
◦[3,4]
. However, those researches
were limited on optical scattering properties of bubbles
in water. In this paper, based on bubble backscattering
at 180
◦
, we report our work on the optical system for
detecting wake profiles by laser from a wake-imaging
point of view.
In this work, the detection object was the wake pro-
duced by using the self-developed two-blade propeller
with a 46-mm diameter in the laboratory environ-
ment. The depth of water in the tank with size of
2(L)× 0.4(W)× 0.6(H) (m) was 41 cm. The center of the
propeller was 20 cm to the bottom of the water tank. The
driving system (as shown in Fig. 1) made the propeller
rotate at the speed of 6000 or 8000 rpm. The wake pro-
duced by the propeller rotating at 6000 rpm was axisym-
metric to spread out like a sector, but after about 10 s,
the water tank was full of bubbles so that the wake profile
Fig. 1. Driving system of the propeller.
was no longer clear. There was a very short cavity in the
wake near the propeller axis, around which the bubble
number rapidly increased, reaching to the maximum at
a distance of the propeller radius to its axis, and then
gradually decreased with the increase of the distance to
the axis. However, the wake produced by the propeller
at 8000 rpm was greatly different from that at 6000 rpm.
Its profile was very clear to last for a period, longer than
10 s. It obviously became longer and was approximately
a rectangular region where bubbles mainly located. But
the cavitation disappeared.
The experimental setup is shown in Fig. 2. The
optical system used a continuous wave (CW) single-
longitudinal-mode frequency-doubled Nd:YAG laser as
the light source, which emitted linearly polarized light
with an output power of 20 mW and a beam divergence
of 0.06
◦
. The beam diameter was enlarged to 5 mm and
the beam divergence reduced to 0.026
◦
by a collimating
and extender lens, which was composed of a 130-mm
focal-length plano-concave lens and a 300-mm focal-
length plano-convex lens. After expansion, the beam
was separated into two beams by the beam splitter (BS)
with transmissivity of 65%. The reflected beam entered
into a reference detector, and the transmitted one passed
through the 5-mm-diameter minipore at the center of the
Fig. 2. Experimental setup for simulating the wake profile.
1671-7694/2007/100609-04
c
2007 Chinese Optics Letters
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