NATIONAL RESEARCH LABORATORIES Ottawa, Canada
REPORT
Division of Mechanical Engineering. Ship Laboratory
Pages Prefacö - 3 Report: MB-252
Text - 6 Date: May 1963
App. - 3 Lab. Order: l3435A
Figures - 20 File; M2-29-2
For: Internal
Subject: EXPERIMENTAL DVELOPMENT OF A PERFORATED
WAVE ABSORBER OF SIMPLE CONSTRUCTION AND
MINIMUM LENGTH
Prepared by: P.A. Hamill Submitted by: S.T. Mathews
Heád
Ship Laboratory Approved by: D.C. MacPhail
Director SUMMARY
The principle of the perforated vertical. wall break-water (Ref. 1) has been used in an experimental programme to develop a new type of wave absorber. Model tests have shown
that low reflection coefficients can be obtained from a single layer construction of relatively short length. The new beach may be expected tó find application in model wave. testing facilities.
Page. - (ii) MB -25 TELE OF CONTENTS Paqe SUMMARY ,.: .-. . . .. ...
w
IIÏTRODUCTION .... . i DESiGN REQUIPEMENTS PRELIMINMY STUDY . 24 THE PERFORATED VERTICAL WALL BREAKWATER 3
5 EXPERIMENTAL DEVELOPMENT OF THE PERFORATED BEACH 4 CONCLUSION
REFERENCES ..
APPENDIX A:. MEASUREMENT 0F WAVE REFLECtION COEFFICIENT
LIST OF ÏLLUSTRATIONS
Model Beach Arrangement (After Ref. 4)
Thé Perforated Wall Breakwatèrin the Wave Test Flume Dimensions of the Pérforated Breakwàte Model
Reflèction Characteristics of the Perforated
Breakwater
Combinat,dn of. reakwater and Sloping Bar Beach
ModeL df Beach A
Modèi of, Beach B
F ig u e i .21 3 4 5 6
LIST OF ILLUSTRATIONS (Contid)
Figure Reflection Coefficient as a Function of Wave
Steepness, Wave Length = 3.7 ft. 8 = 4.3 ft. 9 = 5.4 ft. 10 = 6.5 ft. II 8.1 ft. 12 9.3 ft. 13 = 11,0 ft. 14
Average Reflection Coefficient as a Function of
Wave Length 15
Beach B in the Wave Flume - Still Water 16 Wave Approaching the Beach - Flow is Downward
Through Holes 17
Wave on the Beach - Start of Upsurge 18 The Breaking Wave - Intense Upsurge of Jets 19 Broken Wave is Soaked Down at End of Beach 20
Page - (iii) MB -252
Page - 1 MB-252
EXPERIMENTAL DEVELOPMENT OF A PERFORATED WAVE ABSORBER
OF SIMPLE CONSTRUCTION AND MINIMUM LENGTH.
1... INTRODUCTION
Future eve1opment of the Ship Section facilities fo
model seakeepirig te.st includes the installation of a bank of
wavemakers along the 200-ft. length Qf the 400-ft. by 2QO-ft. manoeuvring basin. The prototype pneumatic wavemaker unit,-now
in operation in the ship tank, thus presenìted a wave absorber requirement not only for the 25-ft. wide ship tank but also for the larger installation.
In view of this and because initial studie.s indicated that, to obtain the required high absorption, a conventional
rectangular bar beach would have to be a multi-layer cOnstruction
sOrne 25 to 30 ft. long, it was decided to examine the possibility of incorporating, in a beach, the mechariism of the perforated vertical wall breakwater. Model tests showed that this could be
done successfully and that a much shorter, less complicated, beach would meet the requirements.
No systematic optimizatiörì was criéd out but a riumbe.r of different arrangements were briefly examined in selecting two configurations for detailed study. .B.oth of these were found tO be suitable and the firal choice between them will depend largely
Qn ease of ôonstruction in the manoeuvring basin.
It is concluded that a perforated beach offers some advantage in simplicity of coOstruction and may find useful application in laboratory wave testing facilities, where space saving is important.
2. DESIGN REQUIREMENTS .
RefIecion óoefficient is defined as the ratio of the
reflected wave height to the incident wave height Since the wave energy is proportional to.the square of the wave height, 99 percent absorption of the incident wave energy results in a reflected wave of 10 percent of the incident wave height A
beach giving reflection coefficient's of 0.10 or less may be considered efficient and, in practice, (Ref 2), it appears
possible to achieve values in the range 0 04 to 0 08 The first design requirement of the. absorber is that'. for wave conditiöns
Page -MB-252
of primary interest, i.e. waves of length)'. = 7ft. to 30 ft. and of steepness (height/length hA) from 1/50 to 1/20 the
design must be such as togive minimum values of reflèctipn
coefficient. To set a requirement, values of 0.08 or less were considered acceptable.
In view of the 200-ft. length of the manoèuvring tank installation an economical structu'e is necessary. A related
requirement, which is also very imporarìt in its own right, is
that the beach shoUld occupy a minimum of space. Finally, the
design is subject to the special problem of supporting the
structure in the manoeuVring tank The floor, in this case, is simply a thin sheet of waterproofed asphalt on gravel so that, if possible, the beach should be supported entirely from the vertical wall. .
.3. PRELIMINARY STUDY
Consideration was. first givento conventional i.e.
sloping, gravel and rectangUlar bar constructions. Some
excellent experimental, data are available on absorbers of this
type. Ref'e±ences 2 and 3 are particularly comprehensive and
were most useful in the initial assessment. The rectangular bar construction appears to be preferable to other permeable raterials from the viewpoint of cost,, surface disturbance and uniformity o ref.lection ac±'oss the beach. However, the low reflections required would necessitate 'a rectangular bär béach some 27 ft. long and, for best results, having a permeable layer
thicknèss of 26' in. .
Some limited experimental work was carried out with
the beach geometry 'shown diagrammatically in Figure 1 (Ref. 4)
which, it was thought,: would ö.ffe.r some advantage in cost and simplicity' of structure. While no detailed measurements of
reflection coefficients were made, it' was,apparent that a beach of 20 ft..or longer would be required to give satisfactory absorption over the range of wave conditions.
Other possibilities for wave damping, with a minimum
los.s of tank space,were considered, Pneumatic and hydraulic
absorbers were evaluated as possible beach components, i.e.
supplementing a conventional short beach. In the. .present
application, however, it was found that neither of these offered the combinatiôn of simplicity and performance of the
perforated' vertical wall bre.akwatr recently develOped by the National Research C.ounbilts Hydraulics Section. A test
programme was then initiat.ed'to examine the.possibilitieS' of the breakWater as a wave absOrber. .
Page. 3 MB-252
THE PERFORATED VERTICAL WALL BREAKWATER
Details of the developmeh't' and the model.. studies of
the breakwater are availáble in Reference 1. A.3-ft. wide
model section was available from the Hydraulics Section. It is
shown in Figure 2 in. location in the two-dimens.onal wave flume. The perforated wall consists simply of a regular arrangement of circular holes. In Operation a reular ttai Of waves impinges
o'r the perforated wall. alternatively filling and emptying the'
enclosed chamber, the wave motion being transformed into a series of horizontal water jets in both directions The
impedance of the perforated wall is such that an appreciable phase difference and atténuation exists be.tween the outer wave motion and the oscillating water level within the chamber.
Thus,'in addition to the energy loss when the, wave crest spills through the holes into the òhamber, it also appears that as the
chamber, in turn, empties into thé waVe trough the jet diffusion
reates a surface current which opposes the incoming wave in the manner of an hydraulic breakwater.
The 'dimensions of the perforated brèakwater model used in the tests are given in Figure 3. In the first. instance a
test was made to establish roughly the reflection characteristics of the breakwater alone The experimental technique used to
measuré. reflection coefficient is given in detail in Appendix A. The solid curve on Figure,4 shows the results, reflection
co-efficient. being plotted as a function of wave leog,h. Of c.ou'se the. reflection coefficient also depends very lärgely on wave steepness, but this preliminary test. was made at a representa-tive rodel wave height of 3 in., the tentarepresenta-tive. mode.l scale for the present 'application being about 3. The results show that the perforated wall alone gives an ave.age reflection
coef-ficient of 0 20 over most of the wave length range It remained
to show that the absorptive properties of a corposi..te beach,, i e a conventional rectangular bar construction together with a perforated wall, would be additive. The configuration shown on Figure 5 was then tested and the broken line on Figure 4 shows the improvement due to the addition of the 2-ft. length of rectangular bar beach at 10 degr?es slope ahea.d'of the perforated wall Since this test was again of an exploratory
nature no systematic varitio'n of wave steepness was made, the
representative wave height of about 3 in. being maintained
throughout the test.. The results are sufficient to demonstrate
t'hat the perforated wall is .indèe.d a tiseful cothponent in a
composite beach arrangement The reflection coefficients are now seen to lie in the acceptable range of 0 08 or less for wave lengths from about 4 to 10 ft Since the over-all length of the
Page-4
MB -252composite model was only 3 ft., it was evident that the introduc-tion of the perforated' wall offered a simple and effective means of obtaining high absorption with. re.ltively short beach. The remaining thädel studies wére designed to determine the best means of using this result in the present application. '
5. EXPERIMENTAL DEVELOPMENT OF THE PERFORATED..BEACH
Cértan qualitative observations during the tests on the combination beach of Figure 5 p'ompted the next step in the'
development. . It was noticed that the breaking action of the
waves on the sloping rectangular bar section was cause.d not so
much by the friction of the barsthemselves as by an up-surge
of water through .th bars which appeared to act against the
on.-coming wave crest. This effect was particularly marked for wave lengths from 6 to 8 ft. where the system appeared to be correctly tuned and the.waves broke with a decisive, audible splash at a point somewhat below the still water level. It was thought that this effect would be increased if, instead of the bar section, another perforated section was introduced.
The upsurge would then be in the form of jèt which, initially, more cóncentrated, might e more effective in breaking the waves
and, possibly, may result in an additional loss of energy in the formation and diffusion of the jets.
A3-ft. long seëtion of 1/2-in, thick plywood was
prepare,d by boring a latticeof l-in, diameter holes at 2-in.
centres. This choice of perforation was arbitrary; it. matched
fairly closely the arrangement on the existing concrete vertical wall model. The 3-ft. length. was chosen to increase its
effectivéness and possibly to extend its performance over a
wider ránge of wave length and steepness than was obtained with
the 2-ft. rectangular bar section. Exploratory experiments
were made.to establish an optimum' slope and finally the geometry
of Figure 6, designated Beach A, with a slope of 10 degrees,
was elected for detailed study.
Another configuration, specifically. intended to ease the construction problem in the manoeuvring basin, was also tested in detail. This is Beach B, shown in Figure 7, and.
offers a simple solution to thé problem of supporting the
beach entirely from the. vertical wall. It. is also, of course, slightly shorter than Beach A.
Figures 8 to 14 show the results of these tests, reflection coefficient being plotted as a function of wave
Page -
5. MB -252curves refer to Beaches A and B, respectively.
The performance
of the two beaches are comparable, Beach A being slightly better
for the shorter wave lengths (3 to 4 ft
)and Beach B superior
fo
the longer vaves (8 to 11 ft.).
Beach B was found to
operate best with the still wate.r level at. the secoiid row of
holes, i.e. with a single row of holes out of the water.
Figure 15 is a summary of the results and shows average
reflec-tiön coefficient in the wave stéep,ness raìge of inte.rest,
10.e..1/50 to 1/20, as a function of wave length.
It is clear that
either arrangement considered as a model to scale of, say 3,
would meet the requirements.
The final decision ön which to
use will depend on cost ànd ease of construction in the
manoeuvririg tank.
The over-all length of the wave, absorber
wil.l be 12 ft. and 9 ft. respectively for Beaches A and B.
The series öf pictures, Figures 16 to 20, may help
to clarify thé mechanism of the perforated beach.
Figure 16
shows Beach B located in the wave fiume in stili water.
Theremaining plates show the progress and breaking of a wave of
length 8 ft. and steepness 1/30.
In Figure 17 the wave c.ret
has not yet ±eached the start of the beach and the flow at the
end half of the beach is downward thrQugh the holes.
As the
crest crosses the leading 'edge of the beach, F-Igure 18, the
upwell.inig jets ahead of the crest become
isible.
Figure 1,9
shows the. jets at peakintensity as the way? starts to break.
arid in Figure 20 the broken wave falls through the holes with
scarcely any run-up be''ond the stil.l wate.r level.
The water
droplets on the glass should not be taken as an indication of
splashing.
There is, in fact., very little splashing except for
waves of height 6 to 8 in.
A 3-in', wide horizontal board
located at a height of 4 in. above the still water level, see
Figure 7, was found to be most effective in preventing splash.
6.
CONCLUSIONThe specific wave absorber deign problem has been
solved.
A perforated beach, 9 ft..long and of suitably simple
construction, may be ex.pected to give highly satisfactory
absorption for the wave conditions o
interest.
In general it has been found. that the addition of a
per.forated vertical wall as a component, supplementing a
conventional sloping beach arrangement, has a beneficial effect
on the absorption and should permit considerably shorter and
less complicated beach structures.
Page - 6
M -252
An experimenta1tudy has shown that the two
spcifi.c
arrangements, Beaches A and B, for a
single layer perforated
beach give excellent absorption for a
wide range of wave
condi-tions
In this limited study no attempt was made to
optimize
the generl arrangement, the type of
p.erforatior., the thickness
or the length öf the. sloping
section.
A slope of l.
degrees to
the horizontal was foun
to be mòst suitable.
There is obviously
considerable scope for more extensive development,
both
experi-mentäl andtheoretical. .
4.
Vosp.e.r, A.
/AM
.A Review öf Facilities and
Ship-Model
Instrurrentation at the Admiralty
Experiment Works, Hasl.ar. (Paper No.
3).
Proceedings of the Symposium on Towin
Thnk Faci1ities
Instrumentation and
Measuring Techniques, Zagreb, 22-25
September 1959.
Published by Yogosla'.
Ship Hydrodynamics Institute, Zagreb,
1960.
(No. 9)
. .7.
REFERENCES1.
Jarlan, G.E.
A Perforated Vet±cal.Wall
Breakwatèr.
Dock arid Haur Aut:hoity, Vol..
XLI,
No. 486, April 1961, pp
394-399.
2.
Bowers, G.E.
An Experimeñtal Sttd.y of Wave
Absorbes.
Herbich, J.B.
St. Anthory Falls Hydraulic Laboratory,
Uiiversity Qf Minnesota
Project
Report No..
54., January 1957..
Herbich, J..B.
Expeinenta1 Studies. of. Wave Filters
and.Absol'bers.
St. Anthony Fall.s Hydraulic Labo.rat.ry,
University of Minnesota.
Project
\
\
TRAP\
WALL-\q\
18H
RECTANGULAR BAR SECTIONS - VARIOUS SLOPES AND BAR SIZES WERE USED
18'
\\ \
\\
\\\\\
36"
MODEL BEACH ARRANGEMENT
(AFTER REF. 4)
FIG. J
MB-252
7 STILL WATER LEVEL
THE PERFORATED WALL BREAKWATER
LATTICE OF HOLES
I-1/8" DIA. AT 2"
CENTRES
18"
V
DIMENSIONS OF THE PERFORATED BREAKWATER MODEL
FIG. 3
MB-252
0. 0.6 Q
I-z
La 0.4 wo
C-) 0.2 0.lFIG. 4
MB-252
o --I VERTICAL WALL OF VERTICAL BAR APPROX. 3 I ONLY WALL AND BEACH (FIG.5) INCHES--- ---
- COMBINATIONPERFORATED 2 T. LONG SLOPING WAVE HEIGHT o o o o o o o o --o- - - X
6 8 lO 12 i4 16 WAVE LENGTH - FT.BARS 1/2" z 1/2" AT I/2' SPACING
CONCRETE BLOCK
BASE j
STILL WATER LEVEL
5LOP O0
//////zz//z///
PERFORATED VERTICAL WALL BREAKWATER (SEE FIG. 3)
COMBINATION OF BREAKWATER AND SLOPING BAR BEACH
FIG. 5
MB-252
32'
-z-MODEL OF
BEACH A
PERFORATED VERTICAL WALL BREAKWATER (SEE FIG. 3) CONCRETE
BLOCK , BAS E
32"
N
1/2" THICK OF HOLES PLYWOOD WITH LATTICE I"DIA. AT 2' CENTRES
7 7
STILL WATER LEVEL
7
ø#ø#øøøø#.
STILL WATER LEVEL
MODEL OF BEACH
B1/2" THICK PLYWOOD WITH LATTICE OF HOLES I" DIA. AT 2" CENTRES
ZZZZZ/ //
FIG. 7
0.1 0.1 0.1
I-z
OJO w 0.05 J w oFIG. 8,9
MB-252
o 0 0 01 0.02 0.03 0.04 WAVE STEEPNESSFIG. .9
REFLECTION COEFFICIENT AS A FUNCTION OF WAVE STEEPNESS
0.05 0.06 WAVE LENGTH 3.7 FT. BEACH A BEACH B ..__.I X X X X X o o WAVE LENGTH = 4.3 FT. BEACH A BEACH B __X____...._...
ft._
-o 0.01 0D2 0.03 0.04 0.05 0.06 WAVE STEEPNESSW 5
z
w 0.05 w -J w 0 0.15z
wo
Li.. Li.. wo
o
z
o I-0.05-j
LL. w o FIG. IOII
MB-22
WAVE LENGTH : 54 FT. BEACH A BEACH B o-ooc.
-WAVE LENGTH 6.5 FT. BEACH A BEACH BN1
N
0 0.01 0.02 0.03 0.04 0.05 0.06 WAVE STEEPNESSFIG. IO REFLECTION COEFFICIENT AS A FUNCTION OF WAVE STEEPNESS
o ODI 0.02 0.03 0.04 0.05 0.06
WAVE STEEPNESS
0.15
z
Lio
0.10 wo
o
z
o
o
0.05 L. w 0.1 I-z Li O L. u-wo
O zo
I.-0.05 -J L.. w o WAVE STEEPNESS FIG. 12,13MB-252
FIG. 13 REFLECTION COEFFICIENT AS A FUNCTION
OF WAVE STEEPNESS
WAVE LENGTH 8.1 FT. -BEACH A BEACH B
01
o WAVE LENGTH = 9.3 FT. BEACH A o - X D 0.01 0.02 0.03 Ó.Ò4 005 0.01 0.02 0.03 0.04 0.05 WAVE STEEPNESS0.15
I-z
w Q ciic wo
o
z
o
I-Q LJ 0.05 -J w OFIG. I4I5
MB-252
WAVE LENGTH IN FEET
FIG. 15
AVERAGE REFLECTION COEFFICIENT AS A FUNCTION OF WAVE LENGTH
WAVE LENGTH 11.0 FT.
N
K---BEACH A
-
BEACH B-AVERAGE REFLECTION TAKEN
STEEPNESS RANGE 0.02 OVER WAVE TO 0.05
-x---
BEACH ABEACH B X -o 0.01 0.02 0.03 0.04 0.05 0.06 WAVE STEEPNESSFIG. 14 REFLECTION COEFFICIENT AS A FUNCTION OF WAVE STEEPNESS
0.1!
I.-z
wo
L. IL w8 o.c
z
o
I-Q w -J IL 0.05 w (D w > 0 6 8 Io 12BEACH B IN
THE WAVE FLUME
r 'J
.-'; _-.,.!1;-
-_z__?i- *J'
-WAVE APPROACHING THE BEACH
-FLOW IS DOWNWARD THROUGH HOLES
WAVE ON THE BEACH
THE BREAKING WAVE
BROKEN WAVE IS
APPENDIX A
MASJREMENT OF WAVE REFLECTION COEFFICIENT
For the case of sinusoidal waves let the incident wave be
= a sin (mx t)
and the reflected wave
= b sin (mx + t)
where m = 27T/L (L = wave length)
is the wave number, a and b the Wave amplitudes, x is the distance along a horizontal axis, = 27T/T (T = wave period) is the circular frequency and t is the time.
The standing wave, at the limiting condition of maximum disturbance due to reflection, is given by
n n. + n
'i r
= (a + b) sin mx cos cb
- (a - b) ços mx sin t
At the nodes x O, L/2, L, etc.,
= - (a- b) sin Ct
Page 'A-J MB-252
Page MB-252
and at the loöps .x = L/4, 3L/4 etc.,
= (a
cosdt
The envelopé of the tanding wave is developed as follows:I.
If HL is the envelope height at the loop ¿nd HN a the node then
2(a ± b);
The reflection öéfficien is then HL - HN
a - HL ±
¿n th? incident wave height
Page - A-3 MB-252
By making a longitudinal traverse with a capacitance wave probe the envelope was traced on a high speed pen recorder
and the reflection coefficient evaluated. This technique applies, strictly, only to sinusoidal waves but it was used throughout the present investigation.
