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Case Studies: |
Modal Analysis in the Presence of Harmonic Excitation
The case study
presented here
demonstrates how
the
harmonic
detection and
compensation
in the
Enhanced
Frequency Domain
Decomposition
algorithm makes
it easy to
estimate modes
from of a ship.
The measurements
and analysis
presented
here are
performed by
University of
Rostock,
Faculty of
Mechanical
engineering.
Chair of
Technical
Mechanics.
The ship tested
has the
following
technical
specifications:
-
Build at
Flensburger
Schiffbau
Gesellschaft.
-
Dimensions:
length over
all: 199.8 m
-
Speed: 22.5
kn
-
Main engine:
9L 60 MC-C
(MAN B&W, 9
cylinder)
-
Power: 20
070 kW
-
Speed: 123
rpm
-
Working
process:
two-stroke
-
4-bladed
propeller
with
controllable
pitch
The ship was
driven by
variable pitch
propeller,
therefore the
rotational speed
of the main
engine was more
or less
constant.
Several harmonic
excitation lines
is present in
the data because
of the dynamic
characteristic
of the engine.
Below the green
lines represent
the detected
harmonics and
especially it is
seen how the a
harmonic is
extremely close
to the resonance
frequency of the
second mode,
which has been
highlighted with
red.
Below the
corresponding
correlation
function of the
second mode is
shown together
with the
estimation
result for the
damping ratio.
Without the
harmonic
detection and
compensation
feature this
result would not
be possible,
because the
estimated
correlation
function would
have been
completed
dominated by the
harmonic. This
would have
caused severe
bias on both
frequency and
damping
estimates.
The results
using Enhanced
Frequency Domain
Decomposition
for the first
three modes are
given below:
|
Mode
No. |
Frequency
[Hz]
|
Damping
Ratio
[%] |
|
1 |
1.337 |
1.454 |
|
2 |
2.025 |
1.205 |
|
3 |
2.969 |
1.772 |
Below the mode
shape of the
second
(highlighted)
mode is shown:
Related
Information
N.J. Jacobsen, P.
Andersen, R. Brincker
Using Enhanced Frequency Domain Decomposition as
a Robust Technique to Harmonic Excitation in
Operational Modal Analysis
Proceedings of ISMA2006, Leuven, Belgium, 2006.
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