FREAK WAVES: BEYOND THE BREATHER SOLUTIONS OF NLS EQUATION

Transcription

1 UNIVERSITA DI TORINO Dipartimento di Fisica FREAK WAVES: BEYOND THE BREATHER SOLUTIONS OF NLS EQUATION M. Onorato Collaborators: A. Toffoli, A. Iafrati, G. Cavaleri, L. Bertotti, E. Bitner-Gregersen, A. Babanin, P. Janssen, N. Mori

2 OUTLINE OF THE PRESENTATION 1) Quick review on modulational instability and breather solutions of the NLS equation 2) New approach on studying the modulational instability using Navier-Stokes equation 3) The case of the Louis-Majesty accident: crossing seas conditions

3 MODULATIONAL INSTABILITY: Experimental result Amplitude (m) Amplitude (m) Amplitude (m) TIME (s) TIME (s) TIME (s)

4 BREATHERS: EXACT SOLUTION OF THE NLS N. Akhmediev,et al. (1987) Two remarks: 1) 2) The solution depends on steepness and N

5 MAXIMUM AMPLITUDE 1) It depends on the product εn 2) Maximum amplitude is 3 -> The Peregrine solution Such solutions have been tested experimentally in a number of wave tanks and fully nonlinear computations

6 THE AKHMEDIEV SOLUTION N=5 ε=0.1

7 OCEAN WAVES ARE CHARACTERIZED BY JONSWAP SPECTRUM Example: N=3, ε= 0.1 Wave group is stable BREATHERS ARE RARE OBJECTS

8 BASED ON THIS BASIC IDEA OF MODULATIONAL INSTABILITY, AFTER YEARS OF THEORETICAL W O R K, N U M E R I C A L S I M U L A T I O N S A N D EXPERIMENTAL WORK, THE FOLLOWING QUANTITY is NOW COMPUTED OPERATIONALLY AT THE E.C.M.W.F.: kurtosis = 3 + π 3 BFI 2 2D +18ε 2 NORMAL VALUE BOUND MODES BFI 2D = BFI 1+ α Δθ 2 /(Δω /ω 0 ) 2 FREE MODES α is a fitting constant (Mori et al. JPO 2011)

9 MODULATIONAL INSTABILITY FROM NAVIER-STOKES SIMULATIONS The numerical method has been developed by A. Iafrati (Iafrati 2010, JFM) Initial condition: Computational domain:

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13 DISSIPATION

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15 CONCLUSIONS 1) Modulational instability does not imply always rogue waves 2) Wave breaking due to modulational instability may result in a dissipation of energy larger in the air than in the water 3) During the breaking process, dipoles are formed 4) Dipoles can reach the height of the wave length

16 Rogue waves in crossing seas: The Louis Majesty accident THE ACCIDENT On March 03, 2010 at 15:20 the Louis Majesty has been hit by a wave at deck n. 5 which is 16 m from the undisturbed sea-level The wave broke the glass windshields in the forward section on deck five Cavaleri et al. JGR 2012

17 THE FORECASTING MODEL The wave fields are the result of the forecasting of Nettuno model from the Italian National Meteorological Service Resolution of the meteorological model: 7 km Resolution of the wave model in space (WAM): 1/20 0 Spectral Resolution of the wave model: number of frequencies: 30 number of directions: 36

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21 DIRECTIONAL SPECTRUM Time 12:00

22 DIRECTIONAL SPECTRUM Time 13:00

23 THE DIRECTIONAL SPECTRA Time 14:00

24 DIRECTIONAL SPECTRUM Time 15:00

25 THE DIRECTIONAL SPECTRA Time 16:00

26 THE DIRECTIONAL SPECTRUM Time 17:00

27 CROSSING SEAS: THE SIMPLEST CASE k y A=(k,l) θ k x B=(k,-l)

28 COUPLED NONLINEAR SHRODINGER EQUATION Zakharov equation i b o t = ω o b o + * T 0,1,2,3 b 1 b 2 b 3 δ( k o + k 1 k 2 k 3 )dk 1 dk 2 dk 3 consider the following decomposition b(k) = A(k k A )e iω(k A )t + B(k k B )e iω(k B )t with k A = (k,l) k B = (k, l) suppose that both spectral distribution are narrow banded

29 COUPLED NLS EQUATIONS A t + C x A x + C y A y i α 2 A 2 x + β A 2 y 2 γ 2 A x y + i[ ξ A 2 + 2ζ B 2 ]A = 0 B t + C x B x C y B y i α 2 B 2 x + β B 2 y + γ 2 B 2 x y + i[ ξ B 2 + 2ζ A 2 ]B = 0 Coefficients are a function of k and l

30 Consider perturbations only a function of k x k y θ k x A t iα 2 A x 2 + i ξ A 2 + 2ζ B 2 [ ] A = 0 B t iα 2 B x 2 + i ξ B 2 + 2ζ A 2 [ ]B = 0

31 AKHMEDIEV BREATHER SOLUTION Look for a solution of the form: A(x,t) = c 1 ψ(x,t) B(x,t) = c 2 ψ(x,t)exp[iδ] For the Louis Majestic case c 1 c 2 ψ(x,t) satisfies a standard NLS equation Parameters of the solution ψ 0 =1.7 m f A = f B = 0.1 Hz ε A = ε B = 0.07 N = 4

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34 SUMMARY OF THE RESULTS For θ <35.3 0, dispersive and both nonlinear terms have the same sign The ratio between nonlinearity and dispersion becomes larger as θ approaches (this is valid for both self-interaction and cross-interaction nonlinearity) The cross-interaction nonlinearity is stronger than the self Interaction one for angles between 0 0 and

35 ANALYSIS OF THE COEFFICIENTS α= dispersive term ξ= self-interaction term ζ=cross-interaction term

36 ANALYSIS OF THE COEFFICIENTS α= dispersive term ξ= self-interaction term ζ=cross-interaction term

37 ANALYSIS OF THE COEFFICIENTS α= dispersive term ξ= self-interaction term ζ=cross-interaction term

38 DISPERSION RELATION FOR PERTURBATION Ω = αk 2 [ 2(ξ + 2ζ)A 2 + αk 2 ] 0

39 AMPLIFICATION FACTOR FOR BREATHER SOLUTIONS A max A 0 = α ξ + 2ζ κ 2 εn 2 1/ 2 N = number of waves under the envelope ε = initial steepness κ = modulus of the wave number

40 AMPLIFICATION FACTOR GROWTH RATE

41 SUMMARY OF THE RESULTS: The maximum amplification is for θ -> and large N The maximum growth rate is for θ=0 0 and N 3 CONSIDERATIONS: Extreme waves are the result of a maximum amplification factor in a reasonable time scale WE EXPECT LARGE EXTREME WAVE ACTIVITY AT ANGLES OF θ

42 EXPERIMENTS: MARINTEK FACILITY

43 DESCRIPTION OF THE EXPERIMENT SUM OF TWO JONSWAP SPECTRA: E(ω,θ) = E 1 (ω,θ) + E 2 (ω,θ) with E 1 (ω,θ) = αg2 ω Exp 5 ω p 5 4 ω E 2 (ω,θ) = αg2 ω Exp 5 ω p 5 4 ω 4 4 γ Exp [ (ω ω p ) 2 /(2σ 2 ω 2 p )] δ(θ θ0 ) γ Exp [ (ω ω p ) 2 /(2σ 2 ω 2 p )] δ(θ + θ0 )

44 NUMERICAL SIMULATIONS HIGHER ORDER SPECTRAL METHOD (THIRD ORDER IN NONLINEARITY) BOX PERIODIC IN x AND y COORDINATES INITIAL CONDITIONS PROVIDED BY TWO JONSWAP SPECTRA TRAVELLING AT AN ANGLE

45 RESULTS ON MAXIMUM KURTOSIS θ

46 PROBABILITY OF EXCEEDENCE AT THE TIME OF THE ACCIDENT Linear case:

47 PROBABILITY OF EXCEEDENCE AT THE TIME AND PLACE OF THE ACCIDENT Exceedence probability RAYLEIGH MODIFIED RAYLEIGH H (m) Hs=5.11 m

48 WAITING TIME AT THE TIME AND PLACE OF THE ACCIDENT WAITING TIME (MINUTES) RAYLEIGH MODIFIED RAYLEIGH Hs=5.11 m H (m)

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50 Limitations of the computation