STUDY OF THE FLOW PATTERN IN A PERFORATED
BREAKWATER USING PIV MEASUREMENTS
Jean-Marc Rousset
Research Associate
School of Mech Eng, Sanderson Building
The University of Edinburgh
EH9 3JL – Edinburgh (Scotland)
Tel: +44 131 650 5685
Fax: +44 131 667 3677
email: jrousset@eng.ed.ac.uk
ABSTRACT:
Introduction
Despite their increased complexity and cost of construction as compared to plain
caissons, perforated caisson breakwaters are becoming more and more popular. There are now
a large number of such structures in Europe and in Japan, built for anti-reflective quaywalls or
for external dykes. Mostly are composed of a vertical perforated (or slotted) screen in front of
a vertical plain wall, forming a chamber of typically about 10 m width. Their hydraulic
performance is a complex function of wave parameters (wave height, period, water depth) and
geometric parameters (porosity, number of screens, chamber width).
Perforated vertical breakwaters are intended to absorb part of the wave energy through
various mechanisms, such as turbulence, viscous friction and resonance (after analogy with
the interaction of acoustic waves and porous walls). However the process dissipating the wave
energy is still poorly described, mainly due to the lack of experimental data.
The present paper presents an experimental study of the flow patterns in the chamber of
a perforated breakwater and in the vicinity of its perforated wall.
Description of the experiments
The experiments are carried out in a 22 m long wave flume fitted with a computer
controlled wavemaker. Figure 1 shows a cross-section of the slotted caisson model and the
main experiment dimensions. Four wave periods are generated (T = 1.3 – 1.5 – 1.7 – 1.9 s)
with only one wave height (H = 0.05 m with 0.30 m water depth). Three wave gauges are also
disposed close the vertical screens to record the free surface evolution.
PIV is employed to obtain 2D velocity fields in front of the structure and in the caisson
chamber, the vertical laser sheet illuminating a section of the model, through the bottom of the
wave flume. The image acquisition is performed by a Kodak MegaPlus ES1 camera
(1008x1016 px) which is connected to Dantec Dynamics PIV 2100 processor. This equipment
is dedicated to transfer data from the camera to the computer, the image analysis being
performed by modified Matlab MatPiv toolbox [1]. This analysis uses cross-correlation and
advanced digital interrogation techniques (multi-grid interrogation process , subpixel accuracy
method, …) [2].
Two illumination systems are used: a scanning beam system with Argon-Ion laser [3] to
get the whole chamber velocity fields (∆t = 40 ms) or a Nd:Yag to study the jets generated by
the perforated screen.
1
Figure 1. Main dimensions of the experiments.
PIV temporal synchronisation is achieved using an acquisition board in an other
computer recording the free surface evolution. Zero-up-crossing of the S1 wave gauge signal
triggers the image acquisition for a sequence of 32 to 47 images, according to the wave
period.
Preliminary results
The flow through the perforated screen is quite complex with vortices moving around
the slots according to the wave phase, as shown by figures 2 (a, b). Figure 2a describes the
flow pattern as the PIV system is triggered by the acquisition board. The caisson chamber is
filled by horizontal jets. The flow is contracted and each horizontal beam produces one main
vortex. Figure 2b shows the opposite phase and the chamber emptying. This time, each bar
produces a pair of small counter-rotating vortices. Unfortunately the velocities of these
phenomena and their gradients can not be accurately analysed by a scanning beam PIV
system. Experiments are currently being carried out using a Nd:Yag to solve this problem.
Figure 2. Flow pattern in the vicinity of the slotted screen (T = 1.3 s):
a) filling of the chamber, b) emptying of the chamber (T/2 after Fig 2a)
However the flow field in the caisson chamber is well described by our method and lead
to original results. Figures 3 (a, b) show the velocity field for the same phases than the
previous figures. The perforated screen is on the left of each figure and the original image,
used for the cross-correlation calculations, lays as background. The experiment configuration
gives a field of 0.48x0.30 m and introduces a maximum velocity uncertainty around ±6 mm/s.
2
The filling of the chamber leads to the elevation of the free surface close the plain back
wall with a maximum ascending velocity of 0.20 m/s (Fig. 2a). This filling is produced by the
horizontal jets (with a velocity around 0.25 m/s) generated par the slots. During the opposite
phase (Fig. 2b), the wave is reflected by the back wall and the free surface level decreases.
The maximum velocities are lower (0.15 m/s in x-direction and 0.12 m/s in –y-direction) than
in the previous figure.
Figure 3. Velocity fields in the caisson chamber for the same wave
phases than Fig. 2 (T = 1.3 s).
The main preliminary conclusion is that the vortices are produced and are disappearing
in the vicinity of the perforated screen. They are not carried away from this wall and their
greatest diameter is of the horizontal bars thickness. Therefore we can admit than the wave
energy dissipation occurs mainly at the perforated wall and the next point of our study will be
its experimental assessment.
Our experimental study may be considered as a step forward to a better understanding
of the complex flows generated by coastal structures and a contribution to new improved
design guidance for perforated breakwaters.
Acknowledgements
This research was supported through a European Community Individual Marie Curie
Fellowship (contract HPMF-CT–2000–01010). The author is solely responsible for
information communicated and the EC is not responsible for any view or results expressed.
References
[1] Sveen J K (2000). "An introduction to MatPIV v1.4", University of Oslo, Norway, url:
www.math.uio.no/~jks/matpiv
[2] Raffel M, Willert C and Kompenhans J (2001). "Particle Image Velocimetry: a practical
guide (3rd edition)", Springer (Berlin), 253 p
[3] Gray C, Greated C A, McKluskey D R and Easson W J (1991). "An analysis of the
scanning beam PIV illumination system", Meas. Sci. Technol., vol 2, pp 717-724
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