12/19/2005Shallow Water Hydrodynamics, Trondheim, Norway 1 Long-period Harbor Oscillations due to...
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Long-period Harbor Oscillations due to Short
Random Waves
Meng-Yi Chen & Chiang C. Mei
Massachusetts Institute of Technology
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Typhon Tim 1994: Hualien Harbor,Taiwan
(# 00)(# 00)
## 8 8
# 00# 00
# 05# 05
# 22# 22
# 10# 10
outsideoutside
insideinside
outsideoutside
insideinside
T (sec)0 200
H
inside
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Port of Hualien
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2
2
10
22
8
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Typhoon Longwang, Oct. 2nd 2005
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Past Works
• Harbor Oscillations- Linear theory
Miles & Munk (1961), Miles( 1971), Lee(1971),
Unluata & Mei (1973), (1978) , Carrier,Shaw & Miyata(1971)
– Nonlinear approximation -- narrow-bandedBowers(1977), Agnon & Mei (1989), Wu & Liu (1990)
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Standing waves near a cliff-Random sea
• Sclavounos (1992)-Stochastic theory
-Simple progressive and standing wave in deep water
-Incident waves: stationary, Gaussian
-Higher order spectrum depends on
first, second, and third-order
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21212 ),(
Pairs of frequencies
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0)(])()([
)()(
cosh
)(cosh)()(
112
12
12
1
11111
11
1111
1
hVhUCCkCC
deA
dekh
zhkigA
gg
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Frequency responses Frequency responses By Mild slope By Mild slope ApproximationApproximation
First-order First-order
Chamberlain & Porter (1995) Chamberlain & Porter (1995)
Far field : analytical solution +radiation Far field : analytical solution +radiation conditioncondition
Near field: FEMNear field: FEM
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Hybrid finite element method (Chen & Mei,1974)(HFEM)
Far Field
Analytical
Near Field
Finite element
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Square harbor, Normal incidence 300m by 300 m, depth h=20m
Effect of entrance(1) 60 m opening without protection
(2) 30 m opening without protection
(3) 30 m opening with protection
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Random sea: TMA Spectrum
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First-order average response
60m, no protection
30m, no protection
30m with protection
|)(| 1
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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 30
5000
10000
15000
20000
25000
30000
(rad/sec)2S
A(
) (
cm2 -s
ec)
Mean Linear spectrum
212 ||)()( ASS
60m, no protection
30m, no protection
30m with protection)(AS
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Second-order Mean: setup/down
60m
30m, with protection
30m, no protection
111212 ,,,, dyxStx I
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Nonlinear correction: long wave )(22 S
60m, no protection
30m, no protection
30m with protection
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 30
5000
10000
15000
20000
25000
30000
(rad/sec)2S
A(
) (
cm2 -s
ec)
)(AS
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Mean Harbor Spectrum
60m 30m,no
30m, protected
)(2 fS)(22 fS)(22 fS
)(22 fS
)(2 fS
)(2 fS)()()( 222 fSfSfS
)( 2O )( 4O
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Qualitative comparison with field data
outout
ininoutout
inin
30m, protected
)(22 fS )(2 fS
)()()( 222 fSfSfS )( 2O )( 4O
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Numerical Aspects
• For 2-nd order problem must be solved for a many pairs of frequencies by FEM
• Large sparse matrix for each pair -- for variable depth: modes are coupled
--10620 pairs, each pair need around 15 minutes, at least 100 days for ONE single computer,
--20-25 parallel computer (4G ram, 2.8G Hz), weeks
),( 212
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Summary- - Stochastic theory for long-period harbor resonance Stochastic theory for long-period harbor resonance by a broad-banded sea by a broad-banded sea
-Long-wave part of response spectrum is -Long-wave part of response spectrum is dominated by second-order correction, not dominated by second-order correction, not first or third-orderfirst or third-order
-Mild-slope equation for second order in wave -Mild-slope equation for second order in wave steepness is sufficient steepness is sufficient
--High-frequency part of response spectrum is High-frequency part of response spectrum is dominated by first-order wavedominated by first-order wave
-Extendable to Slow drift of floating structures-Extendable to Slow drift of floating structures