Feature 3 | Noise aNd vibratioN
Advanced thinking on tricky excitations
The number of excitation sources potentially causing vibration problems can
be significantly reduced by CFD and vibration analyses. A report by Wolfgang
Menzel, Dr El Moctar, and Holger Mumm (all Germanischer Lloyd).
T
he source of unpleasant vibrations FFT_04 Owners cabin floor, y
onboard ships is sometimes easy
s mm/s
to detect but difficult to solve.
800 1.5
Normally, the respective frequency clearly
750 1.4
700
Vortex shedding induced excitation
demonstrates which part of the power train Depending on speed through water
1.3
650
1.2
is to blame, pointing to either the main
600
1.1
engine or the propeller. 550
Resonance condition
1.0
Less frequently, identifying the source
500
0.9
itself is part of the problem. Unfortunately,
450
0.8
400
with rare exceptions, this is the case for 0.7
350
hydrodynamic excitations, as the respective
0.6
300
frequency does not yield any information 250
0.5
about the area where, for instance, vortex
200
0.4
shedding is generated. Consequently, solving
150
0.3
100
Machinery induced excitation
this type of problem is often attempted by
Natural Frequency @ 3.6 Hz
0.2
Depending on speed of
50
‘the exclusion principle’, ie by changing all
rotational equipment
0.1
0 0.0
relevant operational parameters that may 1 2 3 4 5 6 7 8 9 10
have an influence.
interval around 3.6 Hz. The transverse response is illustrated in Fig. 2 exemplarily for some areas of
possible vortex shedding generation. The most significant responses arose from vertical
H
excitation
z
If not successful, the trial and error
Fig. 1 Vibration Level (Owner’s Cabin) vs. ship speed (frequency)
at
the
Fig 1:
outer
vibration level (owner’s cabin) vs ship speed (frequency).
shaft line, the transverse excitation at the aft edge of the skeg.
approach is applied, starting with alterations
Starting at about 2.5 Hz the excitation in question leads to high amplitudes in the owner’s cabin when
of the most likely appendages, such as V-
crossing the torsional hull mode shape at about 3.6 Hz, corresponding to 16-18 knots ship speed. At
brackets, fins, sea chests, or whatever appears
highe5,0r0 sE-h02ip speeds the vibration response is still present but lower in amplitude as the resonance area
promising in each special case.
has been passed.
outer I-BRACKET vert.exc. outer V-BRACKET vert.exc.
There is, however, another approach,
4,50E-02
through which the number of excitation
When 4,00comparing E-02 this response to the vibration response at the lower right corner of the figure the
sources potentially causing the problem was
difference in character becomes obvious: The excitation
S
in
KE
q
G
u
tra
e
n
s
sv
t
.
i
e
o
xc
n
.
is not of this mechanical type which
significantly reduced by additional vibration
clear
3
l
,
y
50

E
fo
-02
llows the stepwise re
outer
vo
ST
lu
E
tion
RN TUBE
incr
ver
ease
t.exc.
of the power train.
3,00E-02
and CFD analyses, allowing for one single
So, the knowledge about the vibration was rather complete: The mode shape (form) and the frequency
/
s
]
measurement trip to pinpoint the area of
are known
2,50E-02
[
m
as well as the fact that resonance at a certain ship speed occurs and -very important- that
hydrodynamic excitation. the vibration 2,00E-02 problem is reproducible. This knowledge would form a good basis to solve the problem; if
EXHAUST OUTLET vert.exc.
A yacht’s sea trials took place under rather
only the number of possible excitation mechanisms were more limited.
SEA CHEST transv.exc.
1,50E-02
rough weather conditions, not ideal for
BOW THRUSTER transv.exc.
vibration acceptance tests, but advantageous
It was
1,00
for
E-02
this reason that the yard preferred to order pre-investigations rather than rely on the trial and
error
to fully confirming the natural frequencies
5,
principle.
00E-03
predicted by the comprehensive FE analysis.
In a
0
first
,00E+0
step
0
a vibration sensibility study was performed on the basis of a FE-model aiming at those
0,85 0,90 0,95 1,00 1,05 1,10 1,15
The vibration level excited by the propellers
areas, where vortex shedding theoretically may exci_te _ the hull girder the way it has been measured.
compared sufficiently well to the results of
the forced vibration analysis, too. Hence, all
Fig.
Vibration Fig 2
2
: tr
Transverse
ansSensibilityverse re
Response
sp oStudy nse a
at
t o
Owner's
wner’s
Cabin
cabin
to
to
Unit
uni
Forces
t forces
acting
actin
at
g
different
at differe
areas
nt areas.
seemed to be in good order.
Only one moderate vibration response
The For avibration start thlevels e free due vibrto atithe onsunit of thforces e yacthen ht weyireld e cathe lculatequivalent ed using forces a 3-dimensional required to excite finite the element model. In
remained unclear, which could not be
mthe e
U
afrequency s
n
u
f
r
o
ed
rt
t
u
ra
n
n
a
s
t
range v
e
e
ly
rs
,
e
w
v
i
of
t
ib
h
rinterest,
i
a
n
ti

o
o
n
n

e
le
y
vel
e
the
a
of
r o
measured 1mm/s
f enter
at
in
t
g
h1-node e ow
g
n
e
e
n
rtorsional
e
’s
r

a
c
t
a
i
b
n
in
g
,


o
smode ee
r s
f
t
o
o
ll
p
shape ow
pi
in
n
g
g
F

at
t
i
h
g.3.6
e
3
t
.
r
Hz
ans
was
vers
clearly
e hull
retrieved.
addressed to the power train and, although service, the owner was repeatedly critical of vibration. The possible influences of rudder
the predominant sea condition was not a
Subsequently
n unpleasan
,
t
harmonic
transvers
unit
e vi
excitation
bration th
forces
at was
were
m
app
ov
lied
eme
at
nt
the
s,
locations
sea ches
of
ts
possible
, bow t
vortex
hruste
shedding
rs,
deemed a fully convincing excitation either, t
generation,
angibly pe
i.e.
rcep
at
ti
sea
ble i
chests,
n his c
exhaust
abin. He
gas
urg
outlets,
ed e
bow
xhau
thrusters,
st gas ou
stern
tlets,
tubes,
etc we
s
r
h
e
a

f
a
t
l
b
l
r
c
a
o
c
n
ke
si
t
d
s
e
a
r
n
e
d
d
the aft
this vibration component was believed to tehdeg ey aorfd th teo spkerogp. oTshee ceoxucnitatetiromn efoarscuerse ws etore fi ax pplieadt sdeparately ifferent efor ngieach ne splocation eeds. both in the transverse
vanish in still water conditions. Calmer tand he pvertical roblemship’s . direction. The forced vibration level Thwas e othen ffendcomputed ing vibraat tiotnhe t uorwnneedr’ so ucat btoin bine an
times will show whether or not it was caused As a result, tests were carried out on all one that had already been identified during
by the seaway. of the components that could possibly be the vessel’s sea trials – a one-node torsional
64 The Naval Architect March 2008
NA Mar 08 - p64+65+67+69.indd 64 10/03/2008 14:28:35
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