Periodic Solutions of the Serre Equations. John D. Carter. October 24, Joint work with Rodrigo Cienfuegos.

Transcription

1 October 24, 2009 Joint work with Rodrigo Cienfuegos.

2 Outline I. Physical system and governing equations II. The Serre equations A. Derivation B. Justification C. Properties D. Solutions E. Stability

3 Physical System Consider the 1-D flow of an inviscid, irrotational, incompressible fluid. z Η, free surface a 0 z h 0 undisturbed level Η l 0 h 0 z x z 0 at the bottom

4 Governing Equations Consider the 1-D flow of an inviscid, irrotational, incompressible fluid. Let η(x, t) represent the location of the free surface u(x, z, t) represent the horizontal velocity of the fluid w(x, z, t) represent the vertical velocity of the fluid p(x, z, t) represent the pressure in the fluid ɛ = a 0 /h 0 δ = h 0 /l 0 Λ = 2a 0 /l 0 = 2ɛδ (a measure of nonlinearity) (a measure of shallowness) (a measure of steepness)

5 Governing Equations The dimensionless governing equations are u x + w z = 0 for 0 < z < 1 + ɛη u z δw x = 0 for 0 < z < 1 + ɛη ɛu t + ɛ 2 (u 2 ) x + ɛ 2 (uw) z + p x = 0 for 0 < z < 1 + ɛη δ 2 ɛw t + δ 2 ɛ 2 uw x + δ 2 ɛ 2 ww z + p z = 1 for 0 < z < 1 + ɛη w = η t + ɛuη x at z = 1 + ɛη p = 0 at z = 1 + ɛη w = 0 at z = 0

6 Derivation of the Serre Equations Derivation of the Serre Equations

7 Depth Averaging The Serre equations are obtained from the governing equations by (Serre 1953, Su & Gardner 1969, Green & Naghdi 1976) 1. Depth averaging The depth-averaged value of a quantity f (x, z, t) is defined by f (x, t) = 1 h(x, t) h(x,t) 0 f (x, z, t) dz where h(x, t) = 1 + ɛη(x, t) is the location of the free surface. 2. Assuming that δ << 1 3. No restrictions are made on ɛ

8 Governing Equations After depth averaging, the dimensionless governing equations are η t + ɛ(ηu) x = 0 u t + η x + ɛu u x δ2 ( η 3( u xt + ɛu u xx ɛ(u x ) 2)) 3η = x O(δ4, ɛδ 4 )

9 The Serre Equations Truncating this system at O(δ 4, ɛδ 4 ) and transforming back to physical variables gives the Serre Equations η t + (ηu) x = 0 u t + gη x + u u x 1 ( η 3( u xt + u u xx (u x ) 2)) 3η = 0 x where η(x, t) is the dimensional free-surface elevation u(x, t) is the dimensional depth-averaged horizontal velocity g is the acceleration due to gravity

10 Further justification of the Serre Equations Seabra-Santos et al. 1988: Compare range of validity of the Serre equations with other equations Dingemans 1997: Wave and current interactions Guizein & Barthélemy 2002: Soliton creation experiments Barthélemy 2003: Experiments of solitons over steps Cienfuegos et al. 2006: Serre equations for uneven bathymetries El & Grimshaw 2006: Serre equations modeling undular bores Lannes & Bonneton 2009: Serre equations are appropriate for nonlinear shallow water wave propagation

11 Properties of the Serre Equations Serre equation conservation laws

12 Serre Equation Conservation Laws I. Mass II. Momentum t (η) + x (ηu) = 0 t (ηu) + x ( 1 2 gη2 1 3 η3 u xt + ηu η3 u 2 x 1 3 η3 u u xx ) = 0 III. Energy ( 1 t 2 η( gη+u η2 u 2 ) ) ( x + x ηu ( gη+ 1 2 u η2 u 2 x 1 3 η2 (u xt +uu xx ) )) = 0 IV. Irrotationality ( t u ηη x u x 1 ) 3 η2 u xx + x (ηη t u x +gη 1 3 η2 u u xx + 1 ) 2 η2 u 2 x = 0

13 Translation Invariance The Serre equations are invariant under the transformation η(x, t) = ˆη(x st, t) u(x, t) = û(x st, t) + s ˆx = x st where s is any real parameter. Physically, this corresponds to adding a constant horizontal flow to the entire system.

14 Solutions of the Serre Equations Solutions of the Serre Equations

15 Solutions of the Serre Equations η(x, t) = a 0 + a 1 dn 2( κ(x ct), k ) ( u(x, t) = c 1 h ) 0 η(x, t) κ = c = 3a1 2 a 0 (a 0 + a 1 )(a 0 + (1 k 2 )a 1 ) ga0 (a 0 + a 1 )(a 0 + (1 k 2 )a 1 ) h 0 h 0 = a 0 + a 1 E(k) K(k) Here k [0, 1], a 0 > 0, and a 1 > 0 are free parameters.

16 The water surface if k (0, 1): 1 a 0 a 1 1 k 2

17 Constant Solution of the Serre Equations If k = 0, the solution simplifies to η(x, t) = a 0 + a 1 u(x, t) = 0 The water surface if k = 0: a 0 a 1

18 Soliton Solution of the Serre Equations If k = 1, the solution reduces to η(x, t) = a 0 + a 1 sech 2( 3a1 (x ) g(a 0 + a 1 )t) 2a 0 a0 + a 1 u(x, t) = ( g(a 0 + a 1 ) 1 a ) 0 η(x, t) The water surface if k = 1: a 1 a 0

19 Stability of Solutions of the Serre Equations Stability of The soliton solution of the Serre equations is linearly stable (Li 2001).

20 Stability of Solutions of the Serre Equations Transform to a moving coordinate frame The Serre equations become χ = x ct τ = t η τ cη χ + ( ηu ) χ = 0 u τ cu χ + u u χ + η χ 1 ( η 3( u χτ cu χχ + u u χχ (u χ ) 2)) 3η = 0 χ and the solutions become η = η 0 (χ) = a 0 + a 1 dn 2( κχ, k ) ( u = u 0 (χ) = c 1 h ) 0 η 0 (χ)

21 Stability of Solutions of the Serre Equations Consider perturbed solutions of the form η pert (χ, τ) = η 0 (χ) + ɛη 1 (χ, τ) + O(ɛ 2 ) u pert (χ, τ) = u 0 (χ) + ɛu 1 (χ, τ) + O(ɛ 2 ) where ɛ is a small real parameter η 1 (χ, τ) and u 1 (χ, τ) are real-valued functions η 0 (χ) = a 0 + a 1 dn 2 (κχ, k) ( ) u 0 (χ) = c 1 h 0 η 0 (χ)

22 Stability of Solutions of the Serre Equations Without loss of generality, assume η 1 (χ, τ) = H(χ)e Ωτ + c.c. u 1 (χ, τ) = U(χ)e Ωτ + c.c. where H(χ) and U(χ) are complex-valued functions Ω is a complex constant c.c. denotes complex conjugate

23 Stability of Solutions of the Serre Equations This leads the following linear system ( ) ( ) H H L = Ω M U U where ( u L = 0 + (c u 0 ) χ η 0 η ) 0 χ L 21 L 22 ( ) 1 0 M = 0 1 η 0 η 0 χ 1 3 η2 0 χχ and prime represents derivative with respect to χ.

24 Stability of Solutions of the Serre Equations where L 21 = η 0(u 0) 2 cη 0u cη 0u 0 + η 0u 0 u η 0u 0u η 0u 0 u 0 + ( η 0 u 0 u 0 g η 0 (u 0) 2 cη 0 u 0) χ L 22 = u 0 + η 0 η 0u η2 0u 0 + ( c u 0 2η 0 η 0u η2 0u 0) χ + ( η 0 η 0u 0 cη 0 η η2 0u 0 ) χχ + ( 1 3 η2 0u cη2 0 ) χχχ

25 Stability of Solutions of the Serre Equations L ( ) ( ) H H = Ω M U U We solved this system using the Fourier-Floquet-Hill Method (Deconinck & Kutz 2006). This method allows the computation of eigenvalues corresponding to eigenfunctions of the form ( ) ( ) H = e iρχ H P U U P where H P and U P are periodic in χ with period 2K/κ ρ [ πκ/(4k), πκ/(4k)). If ρ = 0, then the perturbation has the same period as the unperturbed solution.

26 Stability of Solutions of the Serre Equations We conducted three series of numerical simulations I. Fixed a 0 and k, varying a 1 II. Fixed a 0 and a 1, varying k III. Fixed a 0 and wave amplitude, varying k and a 1 simultaneously wave amplitude=a 1 k 2

27 Case I: Fixed a 0 and k a 0 = 0.3, k = 0.75, N = 75, P = 1500 Case I: Fixed a 0 and k a 1 δ ɛ Λ

28 Case I: Fixed a 0 and k m a m a e e a m e m a e

29 Case I: Fixed a 0 and k Observations: If a 1 is small enough, there are no Ωs with positive real part. Therefore the solution is linearly stable. If a 1 is large enough, then there are Ωs with positive real part. Therefore the solution is linearly unstable. The cutoff between stability and instability is at a The maximum growth rate increases as a 1 increases. All instabilities are oscillatory instabilities. The rate of instability oscillation increases as a 1 increases. For each value of a 1, there is only one band of instabilities (for these parameters). Generally, the instability with maximal growth rate corresponds to a perturbation with nonzero ρ.

30 Case II: Fixed a 0 and a 1 a 0 = 0.3, a 1 = 0.1, N = 75, P = 1500 Case II: Fixed a 0 and a 1 k δ ɛ Λ

31 Case II: Fixed a 0 and a 1 m k e m k e m k e m k e

32 Case II: Fixed a 0 and a 1 Observations: If k is small enough, there is no instability. If k is large enough, there is instability. The cutoff between stability and instability occurs at k As k increases away from zero, the maximum growth rate increases until k Above this value, the maximum growth rate decreases. At k = 0.947, the maximum growth rate is All instabilities are oscillatory instabilities. As k increases, the number of bands of instability increases. Generally, the instability with maximal growth rate corresponds to a ρ 0 perturbation.

33 Case III: Fixed a 0 and wave amplitude a 0 = 0.3, a 1 k 2 = 0.1, N = 75, P = 1500 Case III: Fixed a 0 and wave amplitude k δ ɛ Λ

34 Case III: Fixed a 0 and wave amplitude m k 0.5 m k e e m k e m k e

35 Case III: Fixed a 0 and wave amplitude Observations: If k is small enough, there is no instability. If k is large enough, there is instability. The cutoff between stability and instability occurs at k As k increases away from zero, the maximum growth rate increases until k Above this value, the maximum growth rate decreases. At k = 0.86, the maximum growth rate is All instabilities are oscillatory. As k increases, the number of bands of instabilities increases. Generally, the instability with maximal growth rate corresponds to a ρ 0 perturbation.

36 Summary Waves with sufficiently small amplitude/steepness are stable. Waves with sufficiently large amplitude/steepness are unstable. Series of simulations in which Λ was unbounded, did not exhibit a maximal growth rate. Series of simulations in which Λ was bounded, exhibited a maximal growth rate.