Though both the sensibility study and the CFD-analysis indicated the aft body of the vessel as probable
exciting area it was decided to include all areas where the CFD-analysis revealed vortex shedding in the
measurement of the following sea trials.
New test Trials
As the mode shape and frequency of the vibration in question were already known, only 3
accelerometers were applied to control and monitor its appearance. The analysis indicated different hull
areas as the possible source. Following the results of the vibration and CFD-analyses a total of 4
pressure gauges were installed in the outer shell plating covering the most likely excitation areas:
One pressure sensor was installed just behind the bow thruster tunnel, one aft of the exhaust gas outlet,
the 3
rd
and 4
th
sensor were placed in the shell above the outer shaft at frame 16 and 24 respectively.
High pressure fluctuations of one of the pressure signals would indicate the vicinity to the hydrodynamic
source in question. It is pointed out that the results of all measurement points are valid for both ship
sides (sb and ps) due to the symmetry of the hull structure and the propulsion system.
The possibility not to get a clear indication could not be excluded. It was therefore decided to precisely
monitor the surrounding and operational conditions, to re-run parts of the previous test program and to
add some new items.
To find out the relevant source the response of the different pressure gauges are decisive. It turned out
that only MP 08 (Pressure Outer Shaft) shows significant pressure fluctuations. This fluctuation
increases more or less continuously with ship speed and attains maximum amplitudes of more than 5
Feature 3
kPa as displayed in following Fig. 8.
FFT_08 Pressure Outer Shaft PS Frame 24
s kPa
5
700
600 4
500
3
400
300 2
200
1
100
0 0
2 4 6 8 10
Hz
Fig 7: Pressure fluctuations at MP08 (ship speed 10knots-20knots).
Fig. 8 Pressure fluctuations at MP 08 (Ship Speed 10-20 kn)
Fig.
Fig 5: Computed flow velocities around
5 Computed flow velocities around propeller shafts
Fig. propeller shafts.5 Computed flow velocities around
T
propeller
hese results already
shafts
prove that the excitation mechanism is acting in the vicinity of the location of MP 8
b(PS ow shaft thruoutlet) ster voas rteall x other sheddpressure ing wasgauges revealshow ed, not hsignificant e outer pressure shell plafluctintuation. g coveFollowing ring the Fig. mo9 st
b
displays
ut the
a
c
5
or
sec
res
time
pon
recor
ding
d
of
flu
the
ctu
vibration
ating f
signal
orce
in the
lik
ow
ely
ner’s
exci
cabin
tation
(MP
are
4)
a
s
a
:
s
o
w
n
e
e
ll
a
p
s
r
e
th
s
e
su
p
r
r
e
ess
se
u
n
re
s
or
a
signal
mplit
in
ud
the
es
area
occu
of
rr
the
ed
PS
at
shaft
a low
outlet
er fr
(MP
eque
8
n
).
c
F
y
o
.
r illu
w
st
a
ra
s
t
io
in
n
s
p
t
u
a
r
l
p
le
o
d
se
j
s
u
t
s
h
t
e
b
s
e
ig
h
n
i
a
n
ls
d
a
t
r
h
e
e
b
a
b
n
o
d
w
p
a
t
s
h
s
ru
filtered showing the tight correlation of the hydrodynamic source and the vibration response at a
ster
Th
frequency
e freque
of
n
about
cy of
3.6
the
Hz.
vortex shedding at the tunnel, one aft of the exhaust gas outlet,
exhaust pipes was computed to be about while the third and fourth sensors were
1.4Hz. placed in the shell above the outer shaft
As these results did not allow investigators frame 16 and 24 respectively.
to pinpoint the possible source of excitation, High pressure fluctuations of one of the
the numerical simulation was deviated to pressure signals would indicate the vicinity
the flow surrounding the ship’s stern in the to the hydrodynamic source in question.
vicinity of the propeller shafts. The results of all measurement points were
The computations showed that the flow valid from both ship sides (sb and ps) due
Fig. 5 Computed flow velocities around propeller shafts at the propeller tunnel and surrounding to the symmetry of the hull structure and
the propeller shafts was characterised by the propulsion system.
Fig. 6: Close-up of the flow velocities and pressur
the formati
e
o
oscillation
n of vortices.
s
To
in
h
a
elp
stern
visua
h
lis
o
e
rizontal
The
cut
po
ssibility of not getting a clear
Fig. 6: Close-up of the flow velocities and pressur
this vortex she
e
d
oscillation
ding, Figs 5
s
an
in
d 6
a
sh
stern
ow
h
i
o
nd
rizontal
ication
cut
could
not be excluded. It
samples of computed flow velocities in the was therefore decided to monitor the
neighborhood of the propeller tunnels. The surrounding and operational conditions
vortex shedding frequency was computed precisely, to re-run parts of the previous test
to be 3.8Hz for the outer tunnels, for the programme, and to add some new items.
V-brackets and the I-brackets, and for To find out the relevant source, the
the outer shafts. So, at least one of these response of the different pressure gauges
components was to be considered as a proved to be decisive. It turned out that
possible source of excitation for the one- only MP8 showed significant pressure
node torsional hull vibration of 3.6Hz. fluctuations. These increased more or less
Though both the sensibility study and continuously with ship speed and attained
the CFD analysis indicated the aft body of maximum amplitudes of more than 5kPa,
Fig. 6: Close-up of the flow velocities and pressur
Fig 6: Close-up of the flow velocities and
e oscillations in a stern horizontal
the
cut
vess
el as the probable area of excitation, as shown in Fig 7.
pressure oscillations in a stern horizontal it was decided to include all areas where These results already prove that the
cut. the CFD analysis revealed vortex shedding excitation mechanism is acting in the
in the measurement of the following sea vicinity of the location of MP08 (PS
trials. shaft outlet) as all other pressure gauges
of the bow, aft of the stern, and beneath the show no significant pressure fluctuation.
keel. New test trials Fig 8 displays a five second record of the
As the mode shape and frequency of vibration signal in the owner’s cabin (MP 4)
results of CFd analysis the vibration were already known, only as well as the pressure signal in the area of
The simulations revealed vortex shedding three accelerometers were applied to the PS shaft outlet (MP08). For illustration
at the bow thruster tunnels and at the control and monitor its appearance. The purposes the signals were band pass
Fiegx.h 7au: sTt igmase o huitsletotsr.i Aes s toaft itornanarsyv veorrstex f owracs e acatninalgy soisn inodnic-arotetda tidnigff epraenrtts houfl tl haer eoaus teas r profiplterlledr, sshohafwtsi ng the tight correlation of
(sohbipse srvpede dp o=r t1 6si dken oat st)h e trailing edge of the possible source. Following the results the hydrodynamic source and the vibration
the skeg near the keel. However, no vortex of the vibration and CFD analyses, a total response at a frequency of about 3.6Hz.
Fi
s
g
h
.
e
7
d
:
d
i
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i
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-r
a
o
u
t
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a
e
t
s
i
n
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g
er
p
e
a
i
r
n
t
s
s
t
a
o
lle
f t
d
h
e
in
outer
Th
pr
e
o
re
p
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el
a
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i
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t
ion is to pinpoint the
(ship speed = 16 knots)
The Naval Architect March 2008 67
NA Mar 08 - p64+65+67+
69.indd 67 10/03/2008 14:28:37
Fig. 7: Time histories of transverse force acting on non-rotating parts of the outer propeller shafts
(ship speed = 16 knots)
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