Note: Descriptions are shown in the official language in which they were submitted.
~ t~
The present invention relates to a water wave absorber
and has particular but not exclusive application to laboratory
wave tanks where the space available for the installation of wave
absorbers is limited. Such laboratory wave tanks also generally
require that testing be carried out in variable water depths with-
out having to make major adjustments to the absorber and reflected
wave energy from the absorber must be minimized for a wide range
of water depths, wave heights and wave periods.
In laboratory wave tanks used for the modelling of
coastal and offshore structures, incident waves generated by a
wave generator must simulate open ocean conditions as realistical-
ly as possible. This requires the installation of wave absorbers
capable of providing adequate energy dissipation; otherwise, waves
reflecting off the absorber will propagate back to the test loca-
tion causing erroneous results. It is generally accepted that the
reflection coe~ficient (Cr) defined as the ratio of reflected
wave height to incident wave height (expressed as a percentage)
should be consistently less than 10~ and preferably less than 5%,
for all wave conditions that are to be simulated in the labor-
atory.
The most commonly used wave absorbers are beaches ofconstant slope which extend to the bottom and may be constructed
of concrete, sand, gravel or stones. However, other designs
include the use of transverse bars, solid or perforated plates,
wire mesh or fibrous materlals. In order to obtain good dissipa-
,
tion of wave energy, the slope of these beaches has to be mild,
usually less than 1:10. Thls results in using up valuable tank
~ ~ 7~ t~
space, particularly with increasing water depths. To reduce thelength of these beaches, a beach with a variable slope can be
installed. A parabolic slope is often used in conjunction with
surface roughness and porous materials; however, the position of
the parabolic profile relative to still water level has to be
adjusted and optimized each time the water depth is changed.
The concept of a progressive wave absorber (one in which
the porosity decreases towards the rear of the absorber) that
could be effective in dissipating wave energy over a short dist-
ance has been presented. A theory was developed on progressivewave absorption and some small scale tests were carried out on a
progressive wave absorber constructed of aluminum shavings which
became more compacted (less porous) in the direction of propaga-
tion of the incident waves. Relatively low reflection coeffi-
cients were measured and the test results seemed to substantiate
qualitatively the basic principle on which the progressive wave
absorption theory is based. However, the use of aluminum shavings
for a permanent wave absorber installation should be avoided.
With time, the compaction of the shavings change and the overall
efficiency of the absorber ~is affected.
In open water situations (as against test tank situa-
tions), an upright caisson breaXwater, having a perforated wall in
front of an impervious back wall, is occasionally used to reduce
the high reflections associated with solid wall breakwaters. It
has been shown analytically and experimentally that considerably
lower reflections over a wider~range of wave periods are possible
when two or three perforated walls are used instead of a single
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perforated wall. For varying wave periods, the reflection coeffi~
cients for a single perforated wall can vary from approximately
10% to 80%, while the two and three perforated wall configurations
generally can be designed to have lower reflection coefficients
ranging from 10~ to 50%. Although these reflections may be
acceptable for many open water situations, none of the above con-
figurations provides sufficiently low reflections for laboratory
use.
According to the present invention there is provided a
water wave absorption device comprising a plurality of absorption
elements each formed from at least one sheet of unflattened expan-
ded material having louvred apertures, the elements being posi-
tioned one behind the other in spaced relationship and arranged in
generally decreasing porosity from the front to the rear of the
device. This device provides wave reflection coefficients less
than 5%.
In a preferred form of the invention, the louvred aper-
tures are oriented with the louvres directed upwardly and forward-
ly in each sheet.
The invention has been found to be particularly efficac-
ious when the device is located in front of an impervious wall.
According to one preferred form of the invention each
element comprises a plurality of sheets arranged substantially end
~o end and the sheets in each element are arranged generally
parallel to corresponding eheets in the next adjacent element.
Certain advantageous results can be obtained, according
to the inventlon, by providing elements with generally decreasing
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porosity from the top to the bottom.
According to one preferred feature of the invention the
spacing between successive elements generally decreases from the
front to the rear of the device.
Ideally, some of the elements are essentially vertical
although it is not necessary in all instances that all elements
be vertical.
The elements may be covered with a sloping or curved
cover member extending upwardly towards the rear of the device and
the vertical heights of at least some of the essentially vertical
elements, preferably increase from the front to the rear of the
device. When the device is located in a laboratory tank, means
(such as a hoist) may be provided to adjust the vertical position
of the wave absorption device in the tank with respect to the
bottom thereof.
In the situation where the cover member extends upwardly
from the bottom towards the rear of the device, the cover member
is made porous.
The elements themselves may be arranged in a zig-zag
configuration when viewed In~plan.
According to another feature of the invention, fIoata-
tion means may be provided for the wave absorption device and
means may be provlded for mooring the device in a floating
condition.
According to another feature of the invention, a plur-
ality of secondary elements each formed from at least one sheet of
un1attened eYp~nd-d material having louvred apertures, are
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arranged substantially perpendicular to and between at least some
of the elements.
Preferably the lower porosity elements near the rear of
the device are located in the vicinity of the wave nodal points,
substantially one quarter of the wave lengths in front of the
impervious wall for the waves that are to be absorbed.
The invention further provides that the spacing between
and behind elements of higher porosity is greater than the spacing
between and behind elements of lower porosity.
According to a preferred feature, the distance from
front to rear of the device is from about 0.35L ma~ to 1.0
L max~ where L max is the length of the longest wave
that must be effect~vely absorbed in the tank.
By making the absorption elements from a sheet or sheets
of unflattened expanded material a great flexibility is imparted
to the device. ~It is readily assembled and disassembled, it lends
itself to ready alteration of configuration, increases or decreas-
es of size, ready alteration and selection of porosity and is
particularly suited for labora;tory tank purposes where it can be
built in modules that can~easl~1y be~moved. Furthermore, it is
strong, durable, light weight, portable, and clean when compared
with stone beaches~and maintains the same characteristics for a
long time.~ More important;ly, it produces an extremely efficient
level of wave absorption. The sharp edges encourage flow separa-
tion resultl~ng in turbulence and~energy dissipation.
A par~ticularly efficacious arrangement is obtained when
the louvred aperturee of the shests are or iented with the louvres
3~3~3
directed upwardly and forwardly in each sheet into the direction
of wave propagation of the incident waves.
The unflattened expanded material sheets are preferably
metal sheets of the type manufactured by Expanded Metal Corpora-
tion, 20 Fasken Drive, Rexdale, Ontario, Canada.
The following is a description by way of example of
certain embodiments of the present invention reference being had
to the accompanying drawings in which:
Figures la, lb and lc are front views of a sheet of
unflattened expanded metal having louvred apertures looking in the
direction of arrows a, b and c respectively as seen in Figure ld,
which itself is a cross-section on the line d-d in Figure lb;
Figure 2 is a schematic side elevation representation of
a device according to the invention, set-up as an upright wave
absorption device arranged in front of an impervious wall;
Figure 3 is a schematic plan view of the device seen in
Figure 2 showing that incident water waves may approach the front
of the~ device from various arbitrary directionsr here wave absorp-
tion elements are depicted as single sheet elements;
Figure 4 is:a schematic slde~elevation of a device with
a form of wave~absorpti:on elements not~shown in the other drawings
herein, the~porosity:of each element g~enerally decreasing from the
top to the bottom;
: : :
~ ~ Figure S is a schematic side:elevation:showing an
arrangement in~which`;the spacing between successive elements
generally:decreases~from the front to the rear of the device;
~ ~ Flgure 6 shows~an alternative oonfiguration in which the
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upright elements are followed by some horizontally arranged
elements;
Figure 7 is a schematic side elevation with fewer up-
right elements than in Figure 6 and some inclined elements posi-
tioned in front of and behind the essentially vertical elements;
Figure 8 is a schematic side elevation in which a large
number of elements are provided and in which the vertical height
of successive elements are generally increased from the front to
the rear, and in which the device is provided with a cover;
Figure 9 is a schematic side elevation of a device in
which the cover is parabolic in nature;
Figure 10 is a schematic side elevation of a device in
which the cover extends from the bottom, and is porous and in
which the elements are of generally uniform height;
Figure 11 is a schematic side elevation in which the
wave absorption device is modular in nature and is height adjust-
able within a tank;
Pigure 12 lS a schematic side elevation in which three
height adjustable modules such as seen in Figure ll are employed,
~0 one on top of another, the~top module being covered with a
covering means; ~ ~ ~
Figure 13 lS a schematic plan view showing multi sheet
elements arranged in zlg-zag formatlon; :
Plgur:e 14 is a schematic plan view in which the elements
are multi sheet:arranged parallel to each other with a small gap
being~provided betwean~individual sheets forming the elements;
Figure~lS~ a v~i-w similar to:Figure 14 in which the
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sheets forming the elements, while substantially being arranged in
end to end relationship, are permitted to overlap slightly;
Figure 16 is a schematic plan view similar to Figures 14
and 15 in which the sheets of the elements are completely aligned
and essentially abut each other;
Figure 17 is a schematic side elevation o~ an alterna-
tive construction in which the wave absorption device is provided
with floatation means so that it floats in open water situations
and is provided with anchors;
Figure 18 is a schematic plan view of the device shown
in Figure 17.
Figure 19 is a schematic plan view of a tank with end,
rear, corner, and side, wave absorbing devices;
Figure 20 is a detail of a corner wave absorbing device
unit;
Figure 21 is a schematic plan view with secondary wave
absorbing elements providea at right angles in the end and side
wave absorbing devices; and
Figure 22 is a detail of the end or side absorbing
device with secondary absorbing~elements.
Turning now to the drawings.
Figure 1 shows~metal sheets 30, which had been slit and
expanded~(drawn). For use in laboratory wave tanks hot dipped
galvanized steel~(1.2 mm to 1.5 mm thick) or aluminum expanded
metal sheets (1.6 mm to 3.2 mm thick) have been found suitable.
In open water situations metal sheets up to about 6.4 mm thicX may
be used. This type of perforated sheet is known as unflattened
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expanded metal. The slitting and expanding process provides each
sheet with a louvred design, which is very rigid. Figures la to
lc show three views of an expanded me~al sheet 30 designated as
having a porosity of 50~ where porosity (n) is defined as the
ratio of open area to total area expressed as a percentage. The
arrows a, b and c in Figure ld show the viewing angles that are
used for Figures la, lb and lc respectively when the louvres L in
the expanded metal sheet are directed upwards into the direction
of wave propagation of the incident waves. In Figure la the
porosity is considerably greater than 50%. For the normal view
Figure lb corresponding to the direction of the incident waves,
the porosity is approximately 50%; however, for Figure lc the
porosity is substantially less than 50~. These drawings illus-
trate how the porosity of expanded metal sheets is somewhat pro-
gressive, that is, as the viewing angle shown in Figure 1 changes
from la to lc there is a progressive decrease in porosity because
of the louvred design. Sheetg ranging in porosity from 5~ to 85%
in 5% increments may be used. Similar to Figure lb, each of the
designated porosities correspond to the approximate porosity when
viewed normal to the sheet. Although hot~dipped galvanized steel
or aluminum is the preferred mater1al used in test tanks, it is to
be understood that the term unflattened expanded material as used
hereinafter and in the claims is also intended to embrace other
suitable materials such as plastics, stainless steel and alloys
such as copper and nickel.
Turn1ng to Figure 2, the different symbols used therein
are described her~ nder, as ~r her terms used hereinafter in
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the application.
Cr = Reflection coefficient, Hr/H
h = Water depth
Hi = Incident wave height
Hr = Reflected wave height
Q = Length of wave absorber device
L = Wave length (Airy wave theory)
m = Number of elements in a given wave absorber device
n = Porosity of element (sheet) viewed normal to element
(sheet)
s = Spacing between and behind elements
T = Wave period
hlL = Relative water depth
Hi/L = Incident wave steepness
~/L = Relative length of wave absorber~device
SWL = Still water levPl :
In Figures 2, 3, 14, 15 ;and 16 there is shown a wate~
wave absorption~device A comprlsing a~plurality of absorption
elements 31,~32, 33, 34~and~35,:each~of which i~n~Figure 2 ~com~
prises~a single sheet 30 of unflattened expanded material having
louvred apertures L and each of which, in~Figures 14, 15:and 16,
comprises~a plurality of sheets 30~arranged~end to end to ~orm a
row with the sheets in each row being arranged generally parallel
to the corresponding ~sheet in:the~next ad~acent row. In Figure 16 : :
the sheets 30:in~each~row~abut each other, in Figure 14 they are
spaced apart~very s~light~ly~and~in Figure 15, they sl1ghtly overlap
each other~but:can be~sa~ld ~to~be generally ln end~to end relation-
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ship. If desired the absorption elements could be mounted inmodules with spaced and overlap combinations of sheets. The rows
of sheets are mounted on support frames (not shown) which conven-
iently may comprise metal piping in a scaffold-like structure,
which is highly transparent to wave motion. The absorption device
A is shown arranged in a laboratory test tank having an impervious
rear wall 40 and a tank bottom 41. The absorption elements 31
through 35 are arranged in generally decreasing porosity from the
front to the rear of the device with typical porosities being 40%
for element 31, 20% for element 32, 15% for element 33, 10% for
element 34 and 5% for element 35. Element 35, the rearmost
element in the device (as shown) is located in the vicinity of the
wave nodal points, substantially one quarter of the wave lengths
in front of the wall 40 for shorter waves that are to be absorbed.
In Figure 3 it will be seen that the incident waves may be inci-
:
dent on the front of the wave absorber from various arbitrary
directions (for example x, y and z).
Tests~on an upright wave absorber (or wave absorption
device) such as seen in~Figure 2 show that the alignment of the
louvres L in the;sheets 30 making up the absorption elements
affects the performance of the upright wave absorber ~of Figure 2.
By far the best overall results (lowest reflection coefficientsj
were obtained with the~ louvres in each sheet directed upwards into
the direction of wave propagation of the incident waves. Tests
conducted with the louvres directed upwards, often resulted in
reflection coefficient~s that were less than half of~those measured
with the louvres di~rected downwards.
:
Also the tests showed that the efficiency of the upright
wave absorber was improved by making the framework constituting
the supporting structure for the absorption elements as transpar-
ent as possible to wave propagation.
Tests have also shown that a constant porosity absorber
device with elements made from low porosity sheets (n=15%) is most
effective in absorbing the wave energy associated with low steep-
ness waves (Hi/L=0.01), while an absorber with elements made
from higher porosity sheets (n=30%) is most effective in dissi-
pating the wave energy related to higher steepness waves
(Hi/L=0.04). In general, a constant porosity wave absorber is
only effective in disslpating wave energy over a narrow range of
wave steepnesses depending on the specific porosity used. For
example, an absorber with five 30~ elements had a relatively low
reflection coefficient of 6% for Hi/L=0.04; however, for a wave
steepness of Hi/L=0.01, the reflection coefficient rose to 41%.
A numerical short-wave model and a simple theoretical solution
have shown similar trends for wave absorbers with constant
porosity.
The performance of~an upright wave absorber with con-
stant porosity is improved significantly when the porosity of the
elements is progressively decrsased towards the rsar of the ab-
~orber. The higher porosity elements at the ront of the absorber
provide optlmum~energy~dissipation of the higher waves. Low por-
osity slements nsar ths~frQnt would cause excessive reflection of
hlgh waves. ~The low waves pass quite freely through the high
porosity elements and are evéntually attenuated by the low
~ ~ - 12 -
~7g~9
porosity elements located near the rear of the absorber.
A high wave entering an efficient progressive wave
absorber undergoes successive stages of wave attenuation as lower
and lower porosity elements are encountered which are progressive-
ly more efficient in dissipating the energy associated with the
decreasing wave heights. Each element dissipates part of the
incident wave energy, while the balance is divided between reflec-
ted and transmitted energy. Multiple reflections between the
elements causes further energy dissipation.
For wave steepnesses ranging from 0.02 to 0.07, tank
tests indicated a strong oscillation of the reflection coeffi-
cient, for increasing relative lengths of the wave absorber. It
is evident that the length of the wave absorber (~) should be at
least 35% of the wave length (Q1L=0.35); otherwise, high reflec-
tions could result. When the length of the wave absorption device
exceeds the length of the wave (Q/L~1 ), the oscillating nature of
the reflection coefficient reduces significantly; however, some
reflection still occurs, even as the absorber becomes very long
(Q/L=2.0 to 2.9). In general terms, the experimental resuIts
indicate that a longer absorber~does not necessar1ly absorb more
energy than a shorter one for wave steepnesses between 0.~02 and
0.07. Generally if low reelection coefficients are essential over
a wide range of wave steepnesses, (including wave steepnesses less
than 0.02 and graater than 0.07j a wave absorber longer than the
minimum length of~0.35~L is required in order to aacommodate more
elements of suitable~porosity which are effectively located within
the absorption device. ~
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Test results show that the amount o~ energy dissipation
is dependent upon the location of each element with respect to the
locations of the nodes and antinodes in the standin~ wave system
set-up within the absorber. The predominant vertical water part-
icle motion that can develop near the antinodes renders the
elements placed in the vicinity of the antinodes ineffective in
providing energy dissipation. It was ~ound that essentially no
changes in the reflection coefficient resulted with the removal of
these elements. However, elements placed in the vicinity of the
nodes (regions of predominant horizontal water particle motion)
were found to be effective in attenuating wave energy. The exper-
imental results clearly illustrate that for effective attenuation
of the low wave heights, it is desirable not to locate the absorp-
tion elements of low porosity too close to the antinode at an
impervious wall located at the rear of the absorber. To be
effective these elements have to be located in the vicinity of the
nodal points, approximately one quarter of the wave lengths in
front of the impervious wall.
It was found that in general terms, the wider the range
of wave heights and wave periods (in regular and irr~egular wave
trains) that must be attenuated, the~greater the number of vari-
able porosity elements 30 ~that are required. Normally, a length
of absorber equivalent to the length of the longest wave to be
effectively absorbed in a wave tank (Q/LmaX=l.o) has been
found to be suf~ioient to~accommodate the required number of
elements to min1m~ize~the;ref1ection coefficients (Cr less than
5%) over a wide range of water depths, wave heights and wave
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periods.
It was found that the waves should have sufEicient space
for energy dissipation by turbulence before another element is
encountered; otherwise, excessive reflection may result. For very
efficient operation, it is important to have a wide spacing of the
elements at the front of the absorber for the case of high wave
heights and long wave periods for a given water depth (high Hi/L
and low h/L) where large horizontal displacements of water
particle motions are experienced throughout the water depth. As
the wave height and corresponding horizontal displacements of
water particle motions decrease with distance into the absorber, a
closer spacing of the elements can be used to optimize energy
dissipation by the increased turbulence of additional elements.
The most efficient absorber is considered to be one with progres-
sively decreasing spacing bétween the elements as well as decreas-
ing porosity of the elements from the front to the rear of the
absorber.
In a water depth of l.8m a wide spacing~(s=50 cm) of
the high porosity elements (n=55~ to 65~) near~the front of an
experimental absorber were effective in dissipating the wave
energy associated wi~th high waves~(Hi=50 cm to 64 cm)~and long
wave periods (for example T = 2.86~sec corresponding to a wave
length, L = 10.22 m). A narrower~spacing (s - 25cm) of the lower
porosity elements towards the~rear of the absorber provided
adequate energy diss1pation for the lower wave heights. The
number of~elements (m=20)~depended on the length of the absorber
devicé (Q=6.2s m) and the spacing~between and behind the elements
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(s=25 cm and 50 cm~.
Turning now to Figure 4, here the wave absorption device
A is shown to have eight absorption elements each made up from
rows of sheets which themselves are not of uniorm porosity
throughout. These sheets are manufactured by the aforementioned
Expanded Metal Corporation, and are made so that they have a
generally decreasing porosity from the top to the bottom of the
sheet. Typical porosities are indicated on Figure 4 for rows 31
through 38. Ideally, the porosity of each sheet forming each
element is the same as that of every other sheet in that element.
Typical values may in the case of element 31 vary from 45% near
the surface through 40% through 35%, 30~, 25% to 20% near the
bottom.
This type of variation in porosity throughout the height
of the elements provides a very efficient wave absorption device
where deep water waves must be dissipated.
In Figure 5, the devlce is shown with elements of con-
stant porosity throughout their heights" which elements are
arranged with the spacing 51 between and behind the higher
porosity elements 31,~3Z, 33, say 45% each, being greater than the
spacing s2 between and behind lower~porosity elements 34, 35, say
35% and the spacing s3 '~etween and behind still lower porosity
elements 36, 37, 38, say 25%, being less than the spacing s2.
Indeed in the optimum form, the distances between individual
::
eIements decreases progressively from the front to the rear of the
devlce in a~ fash~ion to match the decreasing porosity of the in-
dividual elements~ Figure~5 also illustrates that the present
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~2799~
invention includes constructions in which each successive element
need not be of lower porosity than the immediately preceding
element, only that the porosity generally decreases from front to
back.
Figure 6 shows an alternative construction in which the
front part of the device is arranged in an upright fashion, that
is to say elements 31 through 37 are as hereinbefore whereas
elements 38, 44, 45 and 46 are arranged at right angles to the
vertical elements 31 through 37.
In Figure 7 yet another form of absorber A is shown in
which elements 34, 35, 36 and 37 are upright, and elements 31, 32,
33, at the front of the device are inclined and elements 38, 44,
45 and 46 at the rear of the device are also inclined.
In Figure 8 there is shown another form of a vertical
water wave absorption device A in which the absorption elements
31 through 49 are vertical but of different height and in which a
cover member 50 covers the top of the wave absorption device. The
cover member may itself be porous or have a roughened surface.
Figure 9 shows an alternative configuration in which the
~0 cover means 50 is parabolic in configuration and the height of the
absorption elements selected accordingly.
Figure 10 shows yet an alternative arrangement in which
elements 31 through 36 are of equal helght and the cover member 50
extends upwardly from the bottom towards the rear. In this in-
stance becau~se the elements would otherwise not be exposed to wave
activity, the cover member of the device 50 is made porous.
In Figure 11 the w~ave absorption device A is made modu
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lar in configuration being connected by a bottom structure 55 as
well as the cover member 50. Any suitable lifting devices, for
example a hoist, schematically indicated at 56, on the rear wall
40 are capable of moving the wave absorption device up and down
within the tank to allow the device to be optimally positioned
relative to the still water level for different water depths in
the tank.
In Figure 12, three modular units similar to that shown
in Figure 11 are arranged one on top of each other with a cover
over only the uppermost modular unit.
Figure 13 is a plan view similar to Figures 14, 15 and
16 but here the elements 31 through 38 are arranged in zig-zag
configuration. By arranging the elements in zig-zag configura-
tion, energy dissipation is provided between the parallel sheets
and the build up of cross waves is minimized.
Figures 17 and 18 are a side elevation and plan view
respectively of the water wave absorption device A, again arranged
in modular form, but this time provided with floatation devices
60, anchors 62 and anchor chains 63. This type of device is
~0 particularly suitable for use ~in a lake, sheltered bay, river or
otherwise where it can provide a defence against damage by wave
action.
Figures 19 and 21 show structures in a test tank in
which an upright water wave;absorption unit A at one end of the
tank opposite a wave~generator G is flanked by two, similar con-
struction, side wave~absorption devices 65, 66 (165, 166). In
each instance the tDnk corners, between the end absorber device A
~ ~ - 18 -
9~3~g~3
and side absorber devices 65, 66 (165, 166) is filled by an up-
right corner absorber unit 67, 6~, details of which are shown
enlarged in Figure 20. In order to attenuate waves that may build
up in tha corners and propagate as cross waves in the end and side
absorbers A, 65, 66, (165, 166), secondary elements 71, 72, 73,
etc., (Figure 20), in the configuration shown made up of sheets of
un~lattened expanded material having louvred apertures, exactly
the same as is used in the elements of the end and side absorber
devices, are interposed perpendicularly to and between the primary
elements 31, 32, etc. which extend outwardly from the end
absorber device A. Sample porosity values (for the secondary
elements 71, 72, 73 etc.) are shown in Figure 20 and correspond
generally to the porosity values of the primary elements in the
side absorber device 65.
In Figure 21, the end wave absorption device A is pro-
vided with secondary elements 81, 82, 83, etc., as are the corner
absorbers and the side absorbers. Typical porosity for the
secondary elements in the end and side absorbers are shown in
Figure 22. By providing energy dissipation between the long
straight open chambers between the primary parallel elements,
these secondary elements help to minimize the build up of cross
waves.
For wet back wave generators, where water in the test
tank is behind the wave generating boards as well as in front of
them, a short upright rear wave absorber 91, and corner absorbers
92 and 93 similar to the~end absorber A and the corner absorbers
67 and 68 (see Figure 19) may be provided behind the wave genera-
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tor G. These absorbers 91, 92, 93 congtructed of unflattened
expanded material as before can be effective in attenuating the
waves generated behind the wave generator G and thus preventing
the build up of waves that might otherwisa overtop the rear wall
of the tank or reflect back to the wave generator G causing exces-
sive wave pressures on its wave boards. Alternatively the rear
absorber 91 may extend the full width of the tank.
Although not illustrated, it will be understood that
side absorbers with or without corner absorbers could be used in a
long tank or flume. Side absorbers are effective in dissipating
wave energy reflected off or diffracted by test structures in the
flume.
Although many of the Figures of the drawings only show a
total of 5, 6, 8, 11 elements in one absorber, it is to be under-
stood that an absorber could~have up to twenty elements or more
depending on the spe_iflc conditions to be satisfled.
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