Ch04filt Ch08filt
mm/s^2 kPa
7
200
6
5
150
4
3
100
2
1
50
0
0
-1
-2
-50
-3
-4
-100 -5
-6
-150 -7
-8
576 577 578 579 580
s
Fig. 9 Correlation of Vibration in “Owner’s Cabin” and Pressure Fluctuation MP 8 at about 3.6 Hz
Feature 3
The remaining question is to pin point the source itself, not only the area of origin. The
Test Trials
detailed
after Counter Measures
CFD-
analysis revealed the shaft outlets together with the securing device as one of the p
The
ro
results
ba
of
b
these
le
trials
co
are
m
quickly
po
summarized:
nents
The problem has gone. Comparing following Fig. 13
to the original situation (Fig. 1) makes clear that the counter measures were perfectly effective. No trace
to generate
of the vortex phenomenon is left.
Ch04filt
vortex
Ch08filt
shedding. This was the reason for choosing the location of sensor MP 08.
Owners Bedroom Cabin Floor, y
Thus,
mm/s^2
the evaluation of the speed-up manoeuvres strongly indicates that the
kPa
outer shafts
s
with the
mm/s
650 1.5
securing device are to blame for the transverse vibrations mainly observed in the speed
600
range of 16-18
1.4
1.3
550
7 1.2
knots.
500
200 1.1
6 450
1.0
400
Though the problem was understood after the first items of the trials, the complete
5
150
program was
0.9
350 0.8
4 300
0.7
finalized to find out whether or not the excitation can be significantly influence
3
d by c
2
h
50
anging special
0.6
0.5
100
200
parameters. In this final state of the investigation the variation of operational param
0.4
2
eters
150
was deemed 0.3
100
1
0.2
helpful
50
for developing counter-measures. But again the vortex shedding mech
0
anism
50
turned out to be
0.1
0 0.0
very
0
stable, none of the operational changes led to a major effect. The remainin
-1
g task,
1
i.e.
2
to
3
cha
4
nge
5
the
6 7 8 9 10
Hz
Fig. 13 Vibration response after counter measures (Ship Speed 10-20 kn)
design of the shafts in a way that vortex shedding is avoided, was again supported
-2
Fig 11:
by CFD-analyses.
vibration response after
-50
-3
countermeasures (ship speed 10knots-
-4
Conclusion
Design
-100
Modifications
-5 20knots).
This trouble shooting example clearly demonstrates that it is worthwhile to consider the possible
-6
contribution of more complex and sophisticated technologies when deciding how to proceed with a tricky
vibration problem. The procedure selected here turned out to be both time and cost efficient and was
-150 -7
made possible by applying advanced methods of different technical domains. To rely on the trial and
error principle, would have meant loosing a lot of time and money.
To avoid vortex shedding induced vibration the design of the stern part was modified.
-8
The design
modifi
5
c
76
ation consis
5
t
7
e
7
d of adding
5

7
s
8
treamlined fairings
579
between
580
the propeller
for developing countermeasures. But, again,
s
shafts and the underside of
the vortex shedding mechanism turned out to
th
Fig. 9 Correlation
Fig 8: Correlation of vibration in the owner’s cabin and pressure fluctuation MP08 at about
e hull as shown in the close-up perspective view of the modified design. The design
be ve
modificat
ry stable, w
ion
hile none of the operational
consisting
of
of
Vibration
the added
in “Owner’s
streamlined
Cabin”
fa
and
irings
Pressu
between
re Fluctuation
the propeller
MP 8 at
sha
about
fts a
3.6
nd
Hz
3.6Hz.
the u
ch
n
a
d
n
e
g
r
e
s
s
i

d
le
e
d


o
to
f


a
th
m
e
a
h
jo
u
r
l

l
effect. The remaining
The remaining
were
qu
included,
estion is to
see
pin
Fig.10
point the source itself, not only the area of origin. The detailed CFD-
task, ie to change the design of the shafts in a
analysis revealed
.
the shaft outlets together with the securing device as one of
way that vortex shedding is avoided, was again
Fig 9: Close-up
the probable components
to generate vortex shedding. This was the reason for choosing the location of
supported by CFD analyses.
of the modified
sensor MP 08.
Thus, the evaluation of the speed-up manoeuvres strongly indicates that the out
design.
er shafts with the
securing device are to blame for the transverse vibrations mainly observed in the speed range of 16-18
Design modifications
knots.
To avoid vortex shedding-induced vibration,
Though the problem was understood after the first items of the trials, the complete program was
the design of the vessel’s stern part was
finalized to find out whether or not the excitation can be significantly influenced by changing special
modified. The design modification consisted
parameters. In this final state of the investigation the variation of operational parameters was deeme
o
d
f adding streamlined fairings between the
helpful for developing counter-measures. But again the vortex shedding mechanism turned out to be
p

ropeller shafts and the underside of the hull,
very stable, none of the operational changes led to a major effect. The remaining task, i.e. to changea sthe shown in Fig 9.
design of the shafts in a way that vortex shedding is avoided, was again supported by CFD-analyses. To demonstrate the effects of the modified
To demonstrate the effects of the modified design, Figs. 11 to 12 also show the flow velocities and the
design, Fig 10 shows the flow velocities and the
Design Modifications
time history of the pressure for the modified design. The improvement becomes obvious when
comparing these figures to the original state of Fig. 5 to 7.
time history of the pressure for the modified
Fig. 10 Close-up of the modified design
design. The improvement becomes obvious
To avoid vortex shedding induced vibration the design of the stern part was modified. The design when comparing these figures to the original
modification consisted of adding streamlined fairings between the propeller shafts and the underside stof ate, as shown in Figs 5 to 6.
the hull as shown in the close-up perspective view of the modified design. The design modification It can be concluded that the modifications
consisting of the added streamlined fairings between the propeller shafts and the underside of the huolfl the design, ie the installation of streamlined
were included, see Fig.10 fairings between the propeller shafts and
.
the underside of the hull, reduces the vortex
shedding in the shaft tunnels and will therefore
eliminate the vibration.
Comparing the following Fig 11 to the
original situation (Fig 1) makes clear that the
countermeasures were perfectly effective. No
Fig 10: Close-up of the flow velocities in a stern horizontal cut for the modified design. trace of vortex phenomenon is left.
Fig. 11 Close-up of the flow velocities in a stern horizontal cut for the modified design
This troubleshooting example clearly
demonstrates that it is worthwhile considering
source itself, not only its area of origin. The transverse vibrations mainly observed in the the possible contribution of more complex and
detailed CFD analysis revealed the shaft speed range of 16knots-18 knots. sophisticated technologies when deciding how
outlets together with the securing device as Though the problem was understood to proceed with a tricky vibration problem.
Fig. 10 Close-up
one of th
of
e p
t
r
h
o
e
b

a
m
bl
o
e
d
c
i
o
fi
m
ed
p

o
d
n
e
e
s
n
i
t
g
s
n
to

generate after the first items of the trials, the complete The procedure selected here turned out to be
vortex shedding. This was the reason for program had to be finalised to find out whether both time and cost efficient and was made
choosing the location of sensor MP08. Thus, or not the excitation could be significantly possible by applying advanced methods from
the evaluation of the speed-up manoeuvres influenced by changing special parameters. In different technical domains. To rely on the trial
strongly indicated that the outer shafts with this final stage of the investigation, the variation and error principle would have meant losing a
the securing device were to blame for the of operational parameters was deemed helpful lot of time and money. NA
The Naval Architect March 2008 69
NA Mar 08 - p64+65+67+69.indd 69 10/03/2008 14:28:38
Fig. 12 Computed time history of the pressure acting at a numerical sensor in the wake of the outer
shaft
It can be concluded that the modifications of the design, i.e., the installation of streamlined fairings
between the propeller shafts and the underside of the hull, reduces vortex shedding in the shaft tunnels
and therefore will eliminate the vibration.
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