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OCR for page 319
Cavity Thickness on Rotating Propeller Blades
Measurements by Two Laser Beams
H.D. Stinzing (VWS, Berlin Model Basin, Germany)
ABSTRACT
A prediction of propeller-induced
vibratory hull pressures needs compu-
tation of the cavity volume, which de-
pends on blade geometry and operating
conditions of the propeller. In order
to verify or, if necessary, to improve
the theoretical model developed by the
Hamburg Ship Model Basin (HSVA), meas-
urements of thickness and extension of
the blade cavitation at two propeller
models of different blade geometry
were executed by a laser technique,
and the results were compared with
those following from computations.
Extensive preliminary studies for
optimizing the laser technique were
necessary in order to develop a pre-
cise and easily applicable method. The
essential feature of the new procedure
is the use of two laser beams of
constant intensity intersecting at the
surfaces of the cavity and of the pro-
peller blades, resp.
INTRODUCTION
As a result of the efforts to in-
crease the delivered power of ship
propellers, to build lighter hulls and
to use propellers with larger diam-
eters ship hull vibrations are an
increasing problem. These vibrations
are mainly caused by forces acting on
the propeller. The non-uniform wake
field of the ship results in period-
ically changing load of the propeller
blades which affects the hull through
the propeller shaft. In addition, the
propeller produces pressure fluctu-
ations which are transferred to the
hull through the water.
An essential reason for the pro-
peller-induced pressure fluctuations
is the nonsteady propeller cavitation.
The extension and the thickness of the
cavity on a propeller blade vary ac-
cording to the blade position and the
Hanns-Dieter Stinzing. Versuchsanstalt
Mueller-Breslau-Str. (Schleuseninsel)'
operating conditions of the propeller.
These volume variations cause pressure
fluctuations and thereby vibrations at
the afterbody.
The prediction of such propeller-
induced pressure fluctuations thus re-
quires the calculation of the cavity
volume. For this the Hamburg Ship
Model Basin (HSVA) has developed a
theoretical model that had to be con-
firmed and, if necessary, improved by
measurements executed by the Berlin
Model Basin (YWS).
The aim of the investigations
discussed below was to provide a meth-
od to measure the cavity thickness on
rotating propeller blades by using the
advantages of laser light and to apply
this technique to two different pro-
peller models operated in the small
cavitation tunnel of the VWS. The re-
sults had to be compared with calcu-
lations made by the HSVA.
MEASURING TECHNIQUES
Basic Methods
The simplest methods to "measure"
the cavity thickness are visual ob-
servation or simple photographs taken
at stroboscopic illumination of the
propeller. Both provide useful infor-
mation about the shape of the cavity,
its thickness, however, can only be
roughly estimated.
Slightly more accurate results
are achieved by stereo-photogrammetry
which gives the three-dimensional
shape of the cavity. This technique,
however, still may produce errors as
high as 100 percent (1). Another dis-
advantage is the troublesome eval-
uation of stereo-photographs.
The pin-gauge method uses stream-
lined, scaled pins normally fixed to
the blade surface (2). They allow an
easy estimation of the cavity thick-
ness. But great disadvantages of
fuer Wasserbau und Schiffbau
D-1000 Berlin 12, FR Germany
319
OCR for page 320
this method result from the dis-
turbance of the flow around the blades
and from cavitation that may be pro-
duced by the pins themselves.
The final breakthrough in cavity
thickness measurement was achieved by
the use of lasers utilizing the strong
beaming and the high intensity of
their light. The basic idea of this
technique was to measure the distance
between the light spots that appear
where the laser beam hits the surfaces
of the blade and the cavity, resp. To
make those spots as small as possible
the beam has to be pulsated like a
stroboscope. These pulsations are gen-
erated by means of an acousto-optic or
electro-optic modulator, triggered by
the propeller shaft. In case of an
acousto-optic modulator, which has the
same effect as an optical grating, for
the selection of the 1st order beam a
pinhole has to be attached in front.
By using an electro-optic modulator,
which rotates the plane of polar-
ization of the laser light, a polariz-
er is needed additionally.
If a single laser beam is used to
measure the cavity thickness, a sight
device is necessary to define the
measuring direction (3). By means of
this device a vertical virtual plane
containing the measuring direction is
selected, while the laser beam enters
the tunnel horizontally. In order to
hit the cavity where the measuring di-
rection penetrates the cavity surface,
the laser beam, which initially is po-
sitioned on the blade surface, has to
be shifted parallel with the propeller
shaft. The cavity thickness then fol-
lows from the shift with regard to the
measuring direction and the blade ge-
ometry (Fig.l).
In the beginning of a measurement
the laser beam, entering the tunnel
horizontally and normally to the pro-
peller shaft, is adjusted to the meas-
uring point on the blade surface while
the tunnel pressure is high, i.e.
while the propeller is not cavitating.
Then the measuring direction is de-
fined by means of the sight device and
the tunnel pressure is reduced accord-
ing to the desired cavitation number.
Finally the laser is shifted as far as
, that is visible on the
lies in the measuring
the light spot
cavity surface,
direction.
In the past useful results were
obtained by this method. That's why it
served as a basis for the investiga-
tions within the scope of our proj-
ect.
Another technique to measure the
cavity thickness by means of laser
light is to use two convergent and
synchronously pulsated laser beams in-
tersecting in a point. The beam inter-
section first is positioned in the
9
al SIGHT DEVICE
~\~
ACOUSTO - OPTIC
-T = _~ t~?
ret ~
( J `~^ t/~1 TRAVERSE SYSTEM
fOR LASER DOPPLER
VELCCIMETER
LIGH T ~STROBO
MODU LATCR S l G NAL SCOP_
5 ~ GNAt _ CoNDITION~ _ mL5
?ROC SSOR I GENERATOR
IPROPELLCR I
I DYNAMOME TER I
z
i> ~
a LASER
y
N
~ jig-AVERSE
N
Fig.1 Cavity thickness measurement according
to Ukon,Y. and Kurobe,Y.(3)
measuring point on the blade surface
when the propeller is not cavitating.
The cavity thickness there is then
calculated from the distance of the
two light spots observable on the cav-
ity surface, when the propeller is
cavitating. This method has been suc-
cessfully applied on large ships (4).
Method Applied By The VWS
Due to the good results obtained
with a single pulsated laser beam in
Japan, that technique initially was
selected to be used at VWS, too. To re-
examine the measuring principle, the
beam of a He-Ne laser of 5 mW power
was electro-optically modulated using
a Pockels-cell and a Glan-Taylor po-
larizer attached in front of it. All
the components were mounted on an op-
tical bench that was fixed to a tube
stand. So the laser beam could be
freely directed towards the propeller
in the cavitation tunnel. The 250 V
pulses required to control the
Pockels-cell, were supplied by a spe-
cial video amplifier that on his part
was controlled by a pulse generator
triggered by the propeller shaft. The
320
OCR for page 321
pulse generator could produce square-
wave pulses of any duty factor.
Preliminary tests showed that not
nearly all positions on the back of
the propeller can be reached by a
laser beam directed normally to the
tunnel window. That is because of the
limited height of the windows as well
as the geometry of the propeller.
That's why an inclined laser beam must
be used inspite of a number of
disadvantages: Since the tunnel window
is not a coated optical glass but
simple perspex, the reflection losses
at the outer surface may be quite
high. Moreover, the inclined beam is
refracted in the window as well as in
the tunnel water so that the direction
in which it strikes the propeller is
unknown. This means that the cavity
thickness can no longer be calculated
from the laser shift as described
above, but has to be measured as the
direct distance of the light spots,
using a glass scale for instance. This
technique is more complicated.
A further result of the prelim-
inary tests was that, even at 90°
angle of incidence, a laser power of
only 5 me is too low. Especially in
the case of a smooth cavity surface it
is difficult and over and above it
often impossible to recognize the
light spot generated by such a weak
laser. In addition, since long light
pulses are perceived as lines on the
blade or the cavity surface the de-
sired punctiform illumination requires
very short pulses (approx. 50 ps)
which complicates the visual obser-
vation. Finally, the intensity losses
in the tunnel water, mainly caused by
air bubbles, have to be taken into ac-
count, too.
The results of the preliminary
tests called for another measuring
principle and in addition for the in-
stallation of a more powerful laser.
Because of the considerably larger di-
mensions of the 35 mW He-Ne laser
presently used in the VWS and the very
restricted space on the portside of
the VWS cavitation tunnel it became
necessary to install the laser away
and to transport its light to the
measuring point through an optical
fiber. That means that now, instead
of the laser, only the much smaller
and lighter fiber positioner had to be
shifted. However, substantial losses
in light intensity, mainly caused by
the coupling into the optical fiber
(monomode with a 3 am core), are
disadvantageous.
The preliminary tests have shown
that the laser beam always can be ob-
served clearly and sharp-edged where
it is crossed by the propeller blades
and thus strikes the solid blade
surface - provided that adequately
long light pulses are used. This led
to the idea to use two permanent laser
beams intersecting in one point, in-
stead of a single pulsated beam, and
to utilize the beam intersection,
which in the interesting propeller
regions always can be clearly seen, as
a pointer tip. If this pointer tip is
moved parallel with the propeller
shaft from a position on the propeller
blade to the cavity surface, the cav-
ity thickness in this direction is ob-
tained directly from that shift. The
decisive advantage of this method is
the existence of a defined measuring
direction independent on the direction
of the laser beams.
A precondition for the posi-
tioning of the beam intersection point
is the visual fixation of the pro-
peller by additional stroboscopic il-
lumination. One can always adjust the
intensity of the stroboscope flashes
so that the laser light is not out-
shined, simply by adequate covering of
the reflector.
Using a stroboscope has the fur-
ther advantage that the complete
three-dimensional shape of the cavity
can be observed while it is measured.
This is important especially when
measurements have to be made in crit-
ical regions, namely near to the
leading edge of the blade or under the
bulge of the cavity close to the
trailing edge or when bubbles com-
plicate the observation of the cavity
surface.
MEASUREMENTS AND RESULTS
Measurements
The measurements were performed with
models of the six-bladed "Hongkong-
Express.' propeller (HS9A No. 2076,
211.11 mm diameter) and the five-
bladed "Sydney-Express" propeller
(HSVA No. 2054, 200.00 mm diameter).
The axial velocities adjusted by means
of sieves are shown in Figs.2 and 3.
To define the measuring points
the suction sides of the two measuring
hades (in both cases blade No. 1) have
been provided with rasters by polar
coordinates consisting of circular
arcs spaced in equidistances of 0.02 R
(where R designates the propeller ra-
dius) and with radii every 2° (Fig.4).
This raster was definitely fixed by
the point given by half the chord
length of the 0.7 R arc. The radius
going through this point was also used
to define the blade position.
In order to obtain realistic
measuring conditions for the models
data of the ships served as a basis.
These are the ship velocity vs=22.5
kn, the rate of revolutions n=92 mind
and the torque Q=2,750 kNm for the
321
OCR for page 322
[cm]
I I I
~/~/ ~ n ';~
-1
ho.
_.
CD
a
3
\ \\\
pot \`
~ I'd 7~
~ /
_ ~
/
St b.
F i 9 .2 Ax i a 1 i Of 1 ow of prope 1 1 er No . HSVA 2076
"Hongkong Express" and vs=22.0 kn,
n=110 min-t and Q=2,000 kNm for the
"Sydney Express". These values give
the torque coefficients KQ =0 . 045 and
KQ -O. 035, resp., and from the open
water diagrams result the thrust
coefficients KT =0. 250 and KT =0.215,
resp.
To define the caviation numbers a
depth of the propeller shafts of 7.0 m
for the "Hongkong Express" and 6.7 m
for the "Sydney Express" were assumed.
Thus the propeller diameters of 7.6 m
and 7.0 m, resp., and 1.0 m of height
of the stern wave give the cavitation
numbers co. B =0 . 340 and co.~=0.280,
resp., with regard to the highest po-
sition on 0.8 R.
The test setup is shown in Fig.5:
The beam of the He-Ne laser (1) is
coupled into a 10 m long optical fiber
(3) with the aid of a fiber coupler
(2). On the other end it is fed into
simple LDA-optics (5) using a fiber
positioner (4). The optical system,
consisting of a beam splitter, a beam
displacer and a front lens (focal
length 600 mm), divides the entering
laser beam into two convergent beams
intersecting in one point. For posi
.
hi
CD
-
_
o
4
~/
tat
Prop. No. HSVA 2076
\\\
_ ma/ ~ ~
~ \1
F i 9 e 3 Ax i a 1 i Of 1 ow of prone 1 1 er No . HSVA 2054
/
t
.~\
Prop. No. ItSVA 2054
Fig.4 Polar-coordinate raster for
measuring-point def inition
tioning this intersection, fiber po-
sitioner and optics are fixed to two
translation stages (7) with a rotary
stage (8) between them. These stages
322
OCR for page 323
,1 11 it,
/
~ I/
\ \
Fig.5 Test setup
1 ~ He-Ne- 1 aser ~ 6 ~ stroboscope 1 amp
2 ~ f i her coup 1 er ~ 7 ~ bans 1 at i on stage
~ 3 ~ opt i ca 1 f i ber ~ 8 ~ rotary stage
(4) fiber positioner (9) tube stand
(5 ~ LDA-optics
are mounted on a manifoldly adjustable
tube stand (9). The cavity thickness
is measured with the lower translation
stage which is aligned parallel with
the propeller shaft. With the strobo-
scope lamp (6), triggered by the pro-
peller shaft, the propeller is flashed
in proper phase relation and thus
visually fixed.
The tests were performed as shown
in Fig.6: At first thrust and torque
of the propeller are adjusted accord-
ing to the coefficients KT and KQ,
resp., by adjusting the flow velocity
at a given propeller speed. Then the
desired blade position is fixed by
changing the stroboscope trigger ade-
quately. After that the tunnel pres-
sure is reduced according to the cav-
itation number, to examine the ex-
tension of the cavity. Subsequently
the pressure is increased until the
cavitation disappears, this being nec-
essary for positioning the beam inter-
section in the measuring point on the
blade surface using the rotary and the
translation stages. For the same rea-
son it is necessary to reduce the in
,3
, ,
adjust thrust and
torque
~ b~;~: i; ~ ~ ~ ~;~f ~ i)
,
J __
_ ,
adj ust stroboscope
trigger
l
, ,
reduce tunne 1
pressure
. 1 _ ,
sketch cavity
extens ion
~ .
Crimea sur i ng po i nt:
I.
YeS
1
increase tunnel
pressure
~-
pos it ion beam i nter-
section and read
stage adj ustment
1 ~
reduce tunnel
pressure
,
, ~ _
sh if t beam i nter-
~ect i on and read
stage adjustment
1 .
Fig.6 Schematic of test procedure
tensity of the laser light which ex-
pediently can be achieved by means of
two rotating polarizing filters, in-
serted between fiber positioner and
LDA-optics. Following the positioning,
the light intensity is increased again
and the adjustment of the lower trans-
lation stage is noted down. Finally
the tunnel pressure is lowered again
down to the required value to get
cavitation and the beam intersection
is shifted into the cavity surface
with the aid of the lower stage. The
adjustment of which is noted down
again. The cavity thickness then fol-
lows directly from the beam shift.
According to this method.
points in 16 blade positions
measured on propeller HSVA No.
323
330
were
2076
OCR for page 324
/\\ N~A
~ =320°
. , .
(p =33oo
~ =34Oo
\, ,,~
~ =10°
(,t'4, ,/,\) ~
~ =20°
Fig.7 Measured cavity thicknesses of propeller No. USVA 2076 (isometric representation)
324
OCR for page 325
~ to
~(
At ~
:=50°
~.'
~ i] ,, I,:: To
~ =60°
'A \ A.'
~ W
Fig.8 Measured cavity thicknesses of propeller No. HSVA `076 (isometric representations
325
OCR for page 326
~ JO
~ ~ \~
~ =10°
And
~ =40
TV, \`,;~w
~ = Coo
Fig.9 Measured cavity thicknesses of propeller No. HSVA 2054 (isometric representation)
326
OCR for page 327
o
~ 2
._
_ ~
~i I _ _ _
0° 900
E
~ 4
a)
E \
>, 2- .
._
~ _
6
rat
~\ total propel ler / \
/ ~ single blade
Prop. No. HSVA 2076
~I_ _ - 1 - -
1 80° 270° 360°
b 1 ade pos it ion So .
Prop. No. HSVA 2054
A
\~4 Angle blade \~/ 4/ 4/ y
0° 90° 1 80° 270° 360
b 1 ade pos i tion go _
Fig.10 Cavity volume variation
and 139 points in 7 positions on
propeller HSVA No. 2054.
Results
The measured cavity thicknesses are
shown in Figs.7 to 9 in an isometric
representation, which gives a good
survey of the cavity geometry and its
changes according to the blade posi-
tion.
To facilitate the calculation of
the cavity volume as a step function,
the cavity thickness measured in di-
rection of the propeller shaft has
been converted to the direction normal
to the blade surface. The required
angles were determined experimentally
by reflecting a laser beam at the
blade surface.
The cavity volumes
_ appearing on a
single blade and on the complete pro-
peller during one revolution are shown
in Fig.10. It is conspicuous that the
"Hongkong-Express" propeller produces
a considerably greater cavity volume
than the "Sydney-Express" propeller
despite the higher cavitation number.
But the resultant volume fluctuations
are much smaller on the "Hongkong
in the course of one revolution
Express" than on the "Sydney-Express
propeller, which may be explained by
the larger number of blades, the dif-
ferent blade geometry and especially
the larger skew.
The calculations made by the HSVA
differ considerably from the results
of the VWS measurements as Fig.11
shows. This applies to the cavitation
inception, the extension of the cavity
and the cavity thickness likewise.
According to the calculations, on
the "Hongkong-Express" propeller the
cavitation incepts later and closer to
the blade root and it disappears much
quicker than is revealed by the meas-
urements. Correspondingly, the differ-
ences in cavity extension may be quite
large. At blade positions between
~=350° and ~=0°, measurements and cal-
culations agree fairly well, but out-
side this range the calculated exten-
sions are considerably smaller than
the measured values. Such differences
are inevitably connected with dif-
ferences in cavity thickness. Similar
values are only obtained for the blade
position of ~=350°. At the following
positions the measured cavity thick-
nesses may exceed the results obtained
327
OCR for page 328
art ~ lo
it' ~ ~:
Prop. No. HSVA 2076 >7.~'
I'm )
.~
~J~ /:
Prop. No. HSVI I54 \
Fig.ll Measured and calculated (hatched) cavity thicknesses
328
OCR for page 329
by linear calculation by a factor of
5. The greatest differences in each
case appear at the trailing edge of
The propeller blade.
On the "Sydney-Express" propeller
the calculated cavitation inception
appears much earlier than the measured
one, which is in contrast to the
"Hongkong-Express" propeller. Accord-
ingly, up to blade positions of ~-0°
and ~-10° the calculated cavity exten-
sions are larger than the measured
ones. For subsequent positions the
situation is vice versa. Furthermore
it can be stated that the measured
cavities are located closer to the tip
region than the calculated ones.
Concerning the cavity thickness
on the "Sydney-Express" propeller the
smallest differences belong to blade
positions near to ~=350°. After that
the measured values become consid-
erably larger and again the greatest
differences can be observed at the
trailing edge.
One possible reason for the dis-
crepancy between measurement and
calculation results might have been
the poor condition of the propellers.
Especially the "Hongkong-Express" pro-
peller, that is made of aluminum, has
a lot of corrosion pits at the leading
edge. Another reason may be that the
cavity thickness depends on the con-
centration of the cavitation nuclei
and macroscopic gas bubbles within the
tunnel water, but these parameters
have not been taken into account in
the calculations.
However, to be able at least to
estimate the effect of the gas bubbles
on the measured cavity thicknesses,
the number and size of the bubbles
(relative values) were determined by a
light scatter method at nearly all
measurements and shown as histograms.
In addition, at all tests the oxygen
content of the tunnel water was meas-
ured by means of an electrode.
The histograms didn't give any
significant differences in bubble-size
spectra. According to visual observa-
tion one may also suppose that the
propeller, working in a closed cir-
cuit, produces his own bubble spec-
trum. After a relatively short period
of about one minute the same situation
can be observed again and again. How-
ever, this does not exclude that dif-
ferent bubble spectra in fact result
in different cavity thicknesses.
The oxygen measurements proved
that the content of dissolved gas does
not affect the cavity thickness. A
test run over a whole day, constantly
revealed the same thickness for a
given position, while the oxygen con-
tent was greatly reduced by debasing.
The accuracy of these thickness meas-
urements was 0.1 to 0.2 mm. The same
can be assumed for most of the meas-
urements performed. Only if the cavity
is hard to be observed, or if meas-
urements must be taken under the bulge
of the cavity or if heavy fluctuations
or bubbles complicate positioning on
the cavity surface, the accuracy of
measurement is smaller. In such cases
it is estimated to be 0.5 mm.
CONCLUSIONS
The striking advantage of the new
method is its well-defined measuring
direction. Its try-out with two pro-
peller models has shown that it meas-
ures cavities fast and exactly. A
Japanese report (5) received after
closing the investigations described,
seems to confirm the practicability of
this technique.
The following task is to improve
the theoretical model. In addition ef-
forts should be made to modify the
measuring technique so that the cavity
outside the propeller blades, i.e.
within the range of the tip vortex,
can be measured, too. Finally a method
has to be developed to vary and record
size and number of the macroscopic gas
bubbles of the tunnel water in order
to clarify their effect on the pro-
peller cavitation.
REFERENCES
1. Sontvedt, T. and Frivold, H.,
"Low Frequency Variation of the Sur-
face Shape of Tip Region Cavitation of
Marine Propeller Blades and Corre-
sponding Disturbances on Nearby Solid
Bound-aries", Proceedinas of the llth
SYmoo-sium on Naval Hydrodynamics,
London 1976, pp. 717-729 e
2. Chiba, N., Sasajima, T., and
Hoshino, T., "Prediction of Propeller-
Induced Fluctuating Pressures and Cor-
relation with Full Scale Data", Pro-
ceedinas of the 13th Symposium on
Naval Hydrodynamics Tokyo 1980 pp.
t r
89-103.
3. Ukon r Y. and Kurobe, Y. r ''Meas-
urements of Cavity Thickness Distribu-
tion on Marine Propellers by Laser
Scattering Technique", Report of Shin
Research Institute, Vol. 19, No. 3,
1982, pp. 1-12.
4. Rodama, Y., Takei, Y., and A.
Rakugawa, "Measurements of Cavity
Thickness on a Full Scale Ship Using
Lasers and a TV Camera", Pacers of
Shin Research Institute, No. 73, Dec.
1983.
5. N.N., "Twenty Years of Ship Re-
search at IHI with the Ship Model
Basin 1966-1986", Research Institute
Ishikawajima-Harima Heavy Industries
Co., Ltd., Anniversary Publication,
Tokyo 1987.
329
OCR for page 330
DISCUSSION
.
Jlnzhang Feng
Pennsylvania State University, USA (China)
Cavitation measuring is difficult. It is more so near the blade tip
where the cavity bulb is usually fluctuating. The author, however,
has measured the bulb thickness remarkably close to the blade tip as
shown on Fig. 7 and Fig. 8. Is this because the author used time
average to smooth out the fluctuation or because the propeller the
author used in the test has a very little loading at the blade tip?
AUTHORS' REPLY
The operating conditions of both the propeller models were derived
from normal operating conditions of the full scale propellers (see
values). So, their loads should have been normal, too. At these
loads and the cavitation numbers mentioned, cavity fluctuations did
not occur at the blade tip but at the blade root boundary of the cavity
where its thickness was small. There, indeed, mean values were
measured. But due to the thin cavity these values contribute only
very little to the cavity volume, the quantity wanted.
DISCUSSION
Spyros A. Kinnas
Massachusetts Institute of Technology, USA
The paper offers valuable experimental information of unsteady cavity
shapes, which can be used to validate existing analytical techniques.
I would like to address the following two points though: (a) The
technique used in the paper measures the cavity thickness only on the
propeller blade. As it appears though from Figs. 7-9, the cavity
seems to extend beyond the trailing edge of the blade. Is the volume
of the cavity extending behind the trailing edge accounted for in the
computation of the cavity volume? (b) The analytical method that is
used appears to underpredict the cavity extend and volume
-substantially. A reference for this method would be nice to have
been given. Is it a quasi-steady vs unsteady method and is it a
stripwise 2-D method vs a completely 3-D method?
AUTHORS' REPLY
Figs. 7-9 indicate that the cavities indeed extended beyond the trailing
edges of the propeller blades. The technique described does not
enable cavities to be measured outside the blades. So the specified
cavity volumes only include the measured blade cavities. For the
calculation of the cavity thicknesses, a quasi-steady and 2-D method
was used. It is described in detail by K.-Y. Chao and H. Streckwall
in ~Kavitationsuntersuchungen, Druckschwankungsmessungen und
Vibrationsbewertung an schnellen Containerschiffen hoher Leistun,.
HSVA-Rep. No. 1569, August 1989.
330
Representative terms from entire chapter:
laser beam
