Francesco Fedele · 2015. 11. 12. · Boccotti 1989,2000 . Time t The crest height measured at a...
Transcript of Francesco Fedele · 2015. 11. 12. · Boccotti 1989,2000 . Time t The crest height measured at a...
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On Oceanic rogue waves
Francesco Fedele Associate Professor
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u Linear focusing (Boccotti 1989,2000)
u Nonlinear focusing (Janssen 2003)
u Space-time extremes (Fedele 2012)
DRAUPNER EVENT JANUARY 1995
Hmax=25.6 m h/Hs~1.63Hs H/Hs~2.15Hs
Possible physical mechanisms:
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What happens in the neighborhood of a point x0 if a large crest followed by large
trough are recorded in time at x0 ?
SPACE-TIME Covariance
),X(),(),X( 0000 TtxtxT ++=Ψ ηη
LINEAR FOCUSING CONSTRUCTIVE INTEFERENCE OF MANY ELEMENTARY WAVES BAD DAY AT THE TOWER Slepian model: Lindgren 1972, Adler 1981, Boccotti 1989, Piterbarg 1995)
Boccotti 1989,2000
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Time t The crest height measured at a fixed probe IS NOT the largest amplitude of the wave group The probability that the wave group passes by probe at the apex is zero
SPACE-TIME EXTREMES IN OCEAN SEAS (Fedele 2012 JPO)
Random fields, Euler Characteristics ( Adler 1981, Adler and Taylor 2000, Piterbarg 1995)
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ECMWF freak wave warning system (Janssen JPO 2003, Mori and Janssen JPO 2006, Janssen & Bidlot 2009 tm588, Mori, Onorato and Janssen JPO 2011)
Analytical large-time kurtosis for NLS turbulence
Benjamin-Feir Index Steepness over spectral bandwidth Analogous of Re in strong turbulence
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0 5 10 15 20 25 30ï0.1
0
0.1
0.2
0.3
0.4
0.5
C 4d / BF
I2
o=2/i2t / T0
1.31
2
0.4
R = 0
0.03
0.10.2
0.05
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• My refinement of Janssen’s (2003) theory implies that in typical multidirectional oceanic fields third-order quasi-resonant interactions do not appear to play a significant role in the wave growth (however, they can affect wave phases)
• The large excess dynamic kurtosis transient is a result of the unrealistic assumption that the initial wave field is homogeneous Gaussian with random phases. The ocean does not have paddles
• A random wave field forgets its initial conditions and adjusts to a
non-Gaussian state dominated by bound nonlinearities (Annenkov and Shrira JFM 2013).
• In this regime, statistical predictions of rogue waves can be based on the Tayfun (1980) and Janssen (2009) models to account for both skewness and bound kurtosis (Tayfun & Fedele 2007,Fedele 2015, arxiv.org)
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The Acqua Alta Project
PROJECT SITE
Gigi Cavaleri’s tower
‘ACQUA ALTA’
Venice, ITALY
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October 2009 event
• Duration 35’ • 21000 3D maps • Reconstructed area ~ 80x80 m2
Bora wind, Hs=1.3 m Mean wind speed ~ 10 m/s
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T ime
C= c0[1+(ak)2]1/2 phase speed c0=(gk)0.5
Steady (periodic) vs. unsteady waves
Linear Stokes wave
Nonlinear Stokes wave (Cnoidal waves)
Narrowband waves
Realistic ocean waves
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0.6 0.8 1 1.20.006
0.008
0.01
0.012
0.014
0.016
ak
0.6 0.8 1 1.2
0.006
0.008
0.010
0.012
0.014
0.016
c/c0
ak
S pace
WaveHeight
WaveHeight
E
D
C
B
AC
B
C
D
E
AC
A
A
E
D
B
E
B
D
Slowdown of crests in unsteady nonlinear wave groups explains reduced speed of breakers (Banner et al. 2014 PRL, Fedele 2015 EPL, Barthelemy et al. 2015, arxiv.org)
Growth phase A->B->C crest leans backwards as it slows down Decay phase C->D->E crest leans forward as it speeds up
The crest slowdown provides an explanation and quantifies the puzzling generic (O(20%)) slowdown of breaking wave crests
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Are there hidden physical mechanisms that delay the wave from breaking and lead to a rogue wave?
H1. (potential energy growth is inhibited) nonlinear dispersion reduction limits the crest slowdown of deep-water ocean waves leading to breaking (superharmonic instability Fedele JFM 2014)
H2. breaking and the associated kinetic energy growth are inhibited by enhancement of the crest slowdown, allowing waves to grow to larger amplitudes
For me H1 ….. Time irreversibility …..
H2 is for NLS turbulence ……
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Kinematic criterion for wave breaking Particle speed U= crest speed
Refined criterion: Particle speed=0.84 x crest speed Barthelemy et al. 2015 arxiv.org
Energy flux on the surface= U (ρgη+0.5ρU2)
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Hamiltonian structure of fluid particle kinematics
SYMPLECTICITY = VORTICITY [Fedele et al. 2015, arxiv.org] Different perspective drawing on differential geometry
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vort. = −2×K ×US − 2×dUN
ds
vorticity = −2×K ×US
T ime
US UN = 0
Particle speed=crest speed
Vorticity at crest =0
Vorticity must change sign if breaking occurs
Fedele et al. (2015) arxiv.org
Positive vorticity (curvature <0 at crest) (Longuet Higgings JFM 1992) waves don’t want to break
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• My refinement of Janssen’s theory implies that in typical multidirectional oceanic fields third-order quasi-resonant interactions do not appear to play a significant role in the wave growth.
• The large excess dynamic kurtosis transient is a result of the unrealistic assumption that the initial wave field is homogeneous Gaussian. (the ocean does not have paddles)
• A random wave field forgets its initial conditions and adjusts to a non-
Gaussian state dominated by bound nonlinearities
• Breaking has to be accounted for to obtain realistic extreme wave predictions
• The onset of breaking may be the kinematic manifestation of vorticity created on a free surface and it depends on the space-time evolution of energy fluxes
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• A wave alpha-times larger than the surrounding Na waves • Return period R(alpha,Na) = mean inter-time between two
unexpected waves • Unconditional period: harmonic mean over all unexpected waves of
any amplitude
ï60 ï40 ï20 0 20 40 60
ï1
0
1
2
t/Tm
dmax=1.62Hs
Hs=4.16 m Tm=6.6 s d=18 m kmd=1.43
WACSIS
dmax/2
_=2 twoïsided Na~60
_=2 oneïsided Na~50
Unexpected waves (Gemmrich & Garrett 2008)
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101 102 103 104 105 106 1070.4
0.6
0.8
1
1.2
1.4
1.6
h/H
s
Return period NR
100 101 102100
101
102
103
104
105
106
107
Retu
rn p
erio
d N
R
Na waves101 102 103 104 105 106 1070.4
0.6
0.8
1
1.2
1.4
1.6
Return period NR
_=2
Andrea
1.8u104
1.4
1.68
1.31
1.08
1.32
hNR
h(L)NR
1.0
1.2
3u106
1061.551.6
j=0, NR
h1/NR
h(L)1/NR
hα=2,Na
h(L)2,Na
h(L)max,NR
hmax,NR
Fedele 2015, arxiv.org
101 102 103 104 105 106 1070.4
0.6
0.8
1
1.2
1.4
1.6
h/H s
Return period NR
100 101 102100
101
102
103
104
105
106
107
Retu
rn pe
riod N
R
Na waves101 102 103 104 105 106 1070.4
0.6
0.8
1
1.2
1.4
1.6
Return period NR
_=2
Andrea
1.8u104
1.4
1.68
1.31
1.08
1.32
hNR
h(L)NR
1.0
1.2
3u106
1061.551.6
j=0, NR
h1/NR
h(L)1/NR
hα=2,Na
h(L)2,Na
h(L)max,NR
hmax,NR
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wavesofnumbertotalHthangreaterheightwithwavesofnumberHP =][
Hmax=25.6 m ! Crest height=16 m
L~300 m, T~13 s
Z=Crest_Height/standard deviation
observed extreme waves Z~6
Pr(crest height > Z ) = exp[− 12Z 2 ]
The Rayleigh distribution
Return Period R~T/P
z=4 R=0.5 days !
z=5 R=2 month
z=6 R=30 yrs
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21
Fedele 2012, JPO; Benetazzo, Fedele et al. 2012 Coastal Engineering; Fedele et al. 2011, OMAE; Gallego, Fedele et al. 2011, IEEE TGRS
…. terabytes of data …. Space-time wave manifold z-F(x,y,t)=0
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])88(64
1[])*211(*21exp[)Pr( 242
2 +−Λ
+++−−=> ZZZZheightcrest µµ
Fedele 2005, 2008, 2009, 2012, JFM, JPO, Physica D
Probability of exceedance for nonlinear crests :
The Tayfun-Fedele distribution
€
Pr(crest height > Z) = exp[ − 12Z 2 ]
The Rayleigh distribution
It requires skewness and kurtosis of the ocean field
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])88(64
1[])*211(*21exp[)Pr( 242
2 +−Λ
+++−−=> ZZZZheightcrest µµ
Fedele 2005, 2008, 2009, 2012, JFM, JPO, Physica D
Probability of exceedance for nonlinear crests :
The Tayfun-Fedele distribution
€
Pr(crest height > Z) = exp[ − 12Z 2 ]
The Rayleigh distribution
It requires skewness and kurtosis of the ocean field
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ï2 0 2
ï0.6
ï0.4
ï0.2
0
0.2
0.4
0.6
0.8
1
t/Tp
Draupner
η/η
max
ï2 0 2
ï0.6
ï0.4
ï0.2
0
0.2
0.4
0.6
0.8
1
t/Tp
Killard
ï2 0 2
ï0.6
ï0.4
ï0.2
0
0.2
0.4
0.6
0.8
1
t/Tp
Andrea
1995 2007
8oW 4oW 0o 4oE 8oE 51oN
54oN
57oN
60oN
63oN
66oN Hs [m]
2
3
4
5
6
7
8
2
2
2
2
4
4
4
4
6
6
6
6
8
8oW 4oW 0o 4oE 8oE 51oN
54oN
57oN
60oN
63oN
66oN
Tayfun NB steepness µ
0.0450.05
0.0550.06
0.0650.070.075
0.08
2
2
2
2
4
4
4
46
6
6
6
8
8oW 4oW 0o 4oE 8oE 51oN
54oN
57oN
60oN
63oN
66oN
Wave dimension ̀
2.75
2.8
2.85
2.9
2
2
2
2
4
4
4
46
6
6
6
8
8oW 4oW 0o 4oE 8oE 51oN
54oN
57oN
60oN
63oN
66oN
Boccotti ^*
0.5
0.55
0.6
0.65
0.7
0.75
ERA-Interim
ERA Hs map at the peak time of the Draupner event, North Sea