PSEUDO IMAGE VELOCIMETRY
OF A REGULAR WAVE FLOW
NEAR A SUBMERGED BREAKWATER
Francisco Taveira-Pinto
Faculty of Engineering of the University of Porto, Rua do Dr. Roberto Frias, 4200-465 Porto, Portugal
Tel: + 351 22 508 1966, Fax: + 351 22 1952, E-mail: fpinto@fe.up.pt
Abstract
The installation of submerged breakwaters to protect the shore from the destructive effects that high water
waves might have, is becoming an important area of investigation. In a first step towards understanding the
actions of these waves, a wave tank experiment, containing a submerged breakwater model was set up. The
facility is described and special arrangements are out lined to yield a partial section in which twodimensional waves could be studied by laser Doppler anemometry (LDA). Two-dimensional LDA
measurements were performed, yielding information on the complex motion in the waves. Phase average
properties of the mean velocity field were recorded as well as phase average r.m.s. values of the velocity
fluctuations. The results are presented at 33 x-locations. The data provide an insight into the complex fluid
motion that occurs when a wave approaches the installed two-dimensional submerged breakwater. Examples
of the final results are provided in the form of diagrams. A complete set of results is available on a disc and
in an Appendix instructions to access the data are given.
1. Introduction
Several types of costal structures are now employed for shore protection, against wave action, such as
detached breakwaters, groins, wave energy - absorbing block mounds, etc. In some shore regions these
measures against wave - caused land erosion have been successful but in some cases they have had a
negative impact on the coastal natural environment. Some of the observed negative impacts are not fully
understood. This fact and the increasing concerns of society about the preservation of the natural
environment led to a search for new solutions for coastal protection. This has motivated to carry out research
and development work on new solutions to protect coastal shores from wave destruction, that show a
minimum of environmental impact. To carry out the necessary work, modern means of experimental and
numerical fluid mechanics are employed, as is the case in the research described in this paper.
Submerged breakwaters are one of the possible solutions that can be used for coastal protection against wave
destruction. As experience shows, they afford greater protection of the coastline and also decrease the risk of
erosion and help that sand retention could occur. It has also been shown that they permit long-lasting
solutions to be provided, but it is not entirely understood how they work and how they interact with the
oncoming waves. It is shown in this paper that they work by modifying the wave flow characteristics in a
fairly efficient way, and it was the aim of this work to demonstrate this and to document the submerged
breakwater interaction with the wave flow. As the present data show, the interaction is complex but very
efficient and it explains the high efficiency of submerged breakwaters in coastline protection.
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The entire experiments are described in detail, the LDA system employed is described and its employment
for detailed velocity measurements is explained. An impermeable breakwater model with a smooth surface
was used in the experiments. Measurements of horizontal and vertical velocity components, at different
locations around the structure, were defined, allowing phase-dependent flow patterns to be defined. Phase
averaged r.m.s. values of velocity fluctuations for both the horizontal and vertical velocity components were
also evaluated, allowing a detailed picture to be obtained of the wave interaction with submerged
breakwaters.
2. Laboratory Facilities and LDA Measuring Technique
The measurements were carried out in the unidirectional wave tank of the Hydraulics Laboratory of the
Faculty of Engineering of the University of Porto. The wave tank, represented schematically in Figure 1 is
4.8 m wide and 24.5 m long with a maximum water depth of 0.60 and 0.40 m near the wave generator and at
the test section, respectively. A window in the lateral wall of the tank allows visual access to the test section.
A piston-type wave generator installed on the tank allows wave generation of the used regular waves.
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3
5
2
1
SECTION AA’
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Figure 1. Hydraulics Laboratory wave tank.
(1 – wave generator; 2 – wave-absorbing beach; 3 – breakwater model; 4 – thin dividing wall;
5 – traversing system; 6 – system control area)
A wave-absorbing beach built with sand and gravel and placed at the opposite end to the wave generator was
used to reduce reflected waves to a minimum. A thin dividing wall placed on the test section was used in
order to avoid three-dimensional effects on the wave flow.
Wave probe modules allow water level measurements at different points of the test section. One of these
probes was always located at the section where velocity measurements were being carried out.
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For velocity measurements, the used light source was a Spectra-Physics Stabilité 2017S argon-ion laser
operating in single mode with a power of 2 W. The optical system consisted of a 55X modular LDA optics
based on a Dantec one-component fibre-optic system, with a 60 mm probe, working in the backscatter
configuration. A three-dimensional traversing table located in front of the tank window supported the fibreoptic probe and allowed the control volume to be positioned at each required point on the test section. To
improve the alignment of the optics and to reduce the size of the control volume, a 55
12 beam expander
(Dantec) was placed before the 600 mm front lens. The scattered light was collected by a photomultiplier
(PM). A burst spectrum analyser (BSA) processed the signal from the PM. An AT-MIO-10 card, interfaced
the BSA and the wave probe module located at the measuring section. In order to obtain A/D data coincident
with LDA samples, the analogue data acquisition was performed at a sampling frequency approximately 2-3
times the mean data rate of LDA samples. Coincidence filtering was used to match both AD and LDA data.
The main LDA characteristics are shown in Table 1.
Table 1. Main LDA characteristics
Laser wavelength
Measured half-angle of beams in air
514.5 nm
3.588º
Dimension of control volume in air:
Major axis
2.349 mm
Minor axis
0.1475 mm
Fringe spacing
4.111 m
3. Results
Measurements were carried out for a water depth of 22 cm and for regular waves, with height and period of
3.5 cm and 1.25 s, respectively, which when duly scaled correspond to common situations of the state of the
sea on the Portuguese coast. The celerity C of the wave, defined as the ratio between the wavelength and the
wave period, was equal to 1.3 m/s. A breakwater model with a smooth surface and trapezoidal section was
used. The breakwater model characteristics and dimensions and the location of the measured profiles in the
test section are shown in Figure 2.
For each of the 33 profiles and for each water level value (z), 50 velocity values corresponding to 50
intervals of the wave period were evaluated. For each profile the phase-dependent flow field was defined.
Water level and velocity values were normalized by the still water level (d) and by the wave celerity (C),
respectively. The data obtained allowed the definition of the velocity field around the submerged breakwater
in 50 different situations, corresponding to 50 different wave phase values.
In Figure 3 eight of these situations are presented. Velocity fluctuations were also evaluated for both
horizontal and vertical velocity components. The r.m.s. values (u’ and v’) of velocities at each profile and
each level were normalized by the mean local corresponding velocity value (u and v). As for the mean
velocity values, 50 different situations corresponding to 50 different wave phase values were analysed.
Flow motions near the submerged breakwater can be observed during the wave phase, allowing the
definition of recirculation regions in front of and behind the structure.
3
z
Profiles 20 to 22
(H , T )
Profiles 11 to 19
Profiles 23 to 31
h = 0.20 m
d = 0.22 m
d
h
B = 0.10 m
H = 3.5 cm
Submerged
Breakwater
Profiles 1 to 10
Profiles
21 and 33
1.00 m
0.95 m
T = 1.25 s
x
0.20 m
Thin wall
Lateral Window
LDV
Figure 2. Test section and position of measured profiles.
Velocity fluctuation flow fields show that the presence of the breakwater leads to an increase in turbulence,
the location of the higher turbulence intensity region being dependent on the wave phase value.
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Figure 3. Velocity flow field for eight different wave phase values.
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Figure 3. Velocity flow field for eight different wave phase values.
4. References
[1]
Ahrens, J.P., 1987. Characteristics of reef breakwaters, WES, CERC, Technical Report CERC-87-17,
Vicksburg, USA.
[2]
Allsop, N.W.H., 1983. Low-crested breakwaters, studies in random waves, ASCE, Proc. Specialty
Conference on the Design, Maintenance and Performance of Coastal Structures, Arlington, Virginia,
USA, 94-107.
[3]
D'Angremond, K; Van der Meer, J.W. and De Jong, R.J., 1996. Wave transmission at low-crested
structures, ASCE, Proc. of the 25th Int. Conference on Coastal Engineering, Orlando, Florida, USA,
Vol. 2, 2418-2427.
[4]
Seelig, W.N., 1983. Wave reflection from coastal structures, ASCE, Proc. of the Int. Conference on
Coastal Structures '83, Arlington, Virginia, USA, Vol. 1, 961-973.
[5]
Taveira Pinto, F., Proença, Maria Fernanda Proença and Veloso Gomes, F, 1998. Energy dissipation
study of submerged breakwaters using velocity measurements, Proc. of the 9th Int. Symposium on
Applications of Laser Techniques to Fluid Mechanics, Lisbon, 39.1.1-39.1.8.
[6]
Taveira Pinto, F., Proença, Maria Fernanda e Veloso Gomes, F, 1999. Dissipation analysis on
submerged breakwaters using laser Doppler velocimetry, for regular and random waves, Proc. of the
8th Int. Conf. on Laser Anemometry Advanced and Applications, Rome, Italy, 253-265.
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[7]
Taveira Pinto, F., Proença, M.F. and Veloso Gomes, F., 1999. Experimental analysis of the energy
reflected from submerged breakwaters, Iñigo J. Losada, Balkema, Proc. of the Int. Conf. On Coastal
Structures '99, Santander, Spain, Vol. 2, 683-688. ISBN 90 5809 092 2.
[8]
Troch, P. and De Rouck, J., 1999. A numerical wave flume for wave interaction with rubble mound
breakwaters, International Navigation Association (P.I.A.N.C.), Bulletim No. 101-1999, 15-20.
[9]
Van Der Meer, J.W., 1991. Stability and transmission at submerged structures, Delft Hydraulics
Publication No. 453, Delft, Netherlands.
[10]
Watanabe, Y., Wang, Y. Saeki, H. and Hayakawa T., 1999. Fluid motion with wave overtopping
behind a breakwater, Iñigo J. Losada, Balkema, Proc. of the Int. Conf. on Coastal Structures '99,
Santander, Spain, Vol. 2, 183-191. ISBN 90 5809 092 2.
Acknowledgements
The author wishes to thank to FCT (Foundation for Science and Technology), sponsor of the Project
PRAXIS/C/ECM/11303/1998.
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