Revision as of 10:10, 24 July 2006

Introduction

We present here the solution to the Infinite Array based on an infinite image system of Free-Surface Green Functions

Problem Formulation

We begin by formulating the problem. Cartesian coordinates [math]\displaystyle{ (x,y,z) }[/math] are chosen with [math]\displaystyle{ z }[/math] vertically upwards such that [math]\displaystyle{ z=0 }[/math] coincides with the mean free surface of the water. An infinite array of identical bodies are periodically spaced along the [math]\displaystyle{ y }[/math]-axis with uniform separation [math]\displaystyle{ l }[/math]. The problem is to determine the motion of the water and the bodies when plane waves are obliquely-incident from [math]\displaystyle{ x=-\infty }[/math] upon the periodic array of bodies.

The bodies occupy [math]\displaystyle{ \Delta_m }[/math], [math]\displaystyle{ -\infty \lt m \lt \infty }[/math]. Periodicity implies that if [math]\displaystyle{ (x,y) \in \Delta_0 }[/math], then [math]\displaystyle{ (x,y+ml) \in \Delta_m }[/math], [math]\displaystyle{ -\infty \lt m \lt \infty }[/math].

We assume that we have the Standard Linear Wave Scattering Problem. The incident wave potential given by

[math]\displaystyle{ \phi^{{\rm in}} = \frac{A}{k} e^{ik (x\cos\theta+y\sin\theta)}\,e^{kz}, }[/math]

where [math]\displaystyle{ A }[/math] is the dimensionless amplitude and [math]\displaystyle{ \theta }[/math] is the direction of propagation of the wave (with [math]\displaystyle{ \theta = 0 }[/math] corresponding to normal incidence.

Transformation to an Integral Equation

We now Floquet's theorem (Scott 1998) (also called the assumption of periodicity in the water wave context) which states the displacement from adjacent plates differ only by a phase factor. If the potential under the central plate [math]\displaystyle{ \Delta_{0} }[/math] is given by [math]\displaystyle{ \phi( \mathbf{x}_{0},0) }[/math], [math]\displaystyle{ \mathbf{x}_{0}\in\Delta_{0} }[/math], then by Floquet's theorem the potential satisfies

[math]\displaystyle{ \phi(\mathbf{x}_{m},0) = \phi(\mathbf{x}_{0},0) e^{im\sigma l}, }[/math]

and the displacement of the plate [math]\displaystyle{ \Delta_{m} }[/math] satisfies

[math]\displaystyle{ w(\mathbf{x}_{m}) = w(\mathbf{x}_{0}) e^{im\sigma l}, }[/math]

where [math]\displaystyle{ \mathbf{x}_{m} \in \Delta_{m} }[/math], [math]\displaystyle{ -\infty \lt m \lt \infty }[/math] and the phase difference is [math]\displaystyle{ \sigma = k\sin\theta }[/math] (see, for example, Linton 1998).

A standard approach to the solution of the equations of motion for the water is the Green Function Solution Method in which we transform the equations into a boundary integral equation using the Free-Surface Green Function. In doing so we obtain

[math]\displaystyle{ \phi(\mathbf{x}) = \phi^{\rm in} (\mathbf{x},0) +\sum_{m=-\infty}^{\infty} \int_{\Delta_{m}} \left(G_{n_\xi}(\mathbf{x},\xi) \phi(\xi) - G(\mathbf{x},\xi) \phi_{n_\xi}(\xi) \right) d\xi }[/math]

[math]\displaystyle{ G(\mathbf{x},\xi) }[/math] is the Free-Surface Green Function This can be written alternatively as

[math]\displaystyle{ \phi(\mathbf{x}) = \phi^{\rm in}(\mathbf{x}) +\int_{\Delta_{0}} \sum_{m=-\infty}^{\infty} \left(G^{\mathbf{P}}_{n_\xi}(\mathbf{x},\xi+(0,ml,0))e^{im\sigma l} \phi(\xi) - G^{\mathbf{P}} (\mathbf{x},\xi)e^{im\sigma l} \phi_{n_\xi}(\xi) \right) d\xi }[/math]

where the kernel [math]\displaystyle{ G_{\mathbf{P}} }[/math] (referred to as the periodic Green function) is given by

[math]\displaystyle{ G^{\mathbf{P}} (\mathbf{x};\xi) = \sum_{m=-\infty}^{\infty} G(\mathbf{x},\xi+(0,ml,0))e^{im\sigma l}. }[/math]

Accelerating the Convergence of the Periodic Green Function

The spatial representation of the periodic Green function given by equation ((G_periodic)) is slowly convergent and in the far field the terms decay in magnitude like [math]\displaystyle{ O(n^{-1/2}) }[/math]. In this section we show how to accelerate the convergence. We begin with the asymptotic approximation of the Three-dimensional Free-Surface Green Function far from the source point,

[math]\displaystyle{ G(\mathbf{x},\xi) \sim -\frac{ik}{2} \,H_{0}( k |\mathbf{x}-\xi|), |\mathbf{x}-\xi| \to \infty }[/math]

Wehausen and Laitone 1960 where [math]\displaystyle{ H_0 \equiv H_{0}^{(1)} }[/math] is the Hankel function of the first kind of order zero Abramowitz and Stegun 1964. In Linton Linton 1998 various methods were described in which the convergence of the periodic Green functions was improved. One such method, which suits the particular problem being considered here, involves writing the periodic Green function as

[math]\displaystyle{ G_{\mathbf{P}} (\mathbf{x};\xi) = \sum_{m=-\infty}^{\infty} \left[ G\left(\mathbf{x};\xi)+(0,ml)\right) + \frac{ik}{2} H_{0} \Big(k\sqrt{\left( X+cl\right)^2 + Y_{m}^2}\Big) e^{im\sigma l} \right] -\sum_{m=-\infty}^{\infty} \frac{ik}{2}H_{0} \Big(k\sqrt{ (X+cl)^2 + Y_{m}^2 }\Big) e^{im\sigma l} }[/math]

where [math]\displaystyle{ c }[/math] is a numerical smoothing parameter, introduced to avoid the singularity at [math]\displaystyle{ \mathbf{x} = \xi }[/math] in the Hankel function and

[math]\displaystyle{ X = x-\xi,\quad \mathrm{and} \quad Y_{m} = (y-\eta)-ml. }[/math]

Furthermore we use the fact that second slowly convergent sum in ((near_accelerated)) can be transformed to

[math]\displaystyle{ (help_accelerated) -\frac{i}{l} \sum_{m=-\infty}^{\infty} \frac{\exp^{ik \mu_{m} |X+c| }\,\exp^{i \sigma_{m} Y_0}}{\mu_{m}} }[/math]

Linton98,Jorgenson90,Singh90 where [math]\displaystyle{ \sigma_{m} = \sigma + 2 m \pi/l }[/math] and

[math]\displaystyle{ \mu_m = \left[ 1-\left(\frac{\sigma_{m}}{k} \right)^{2} \right]^{\frac{1}{2}}, }[/math]

where the positive real or positive imaginary part of the square root is taken. Combining equations ((near_accelerated)) and ((help_accelerated)) we obtain the accelerated version of the periodic Green function

[math]\displaystyle{ \begin{matrix} G_{\mathbf{P}} (\mathbf{x};\mbox{\boldmath\lt math\gt \xi }[/math]})

&\! = &\!\!\!\! \displaystyle{\sum_{m=-\infty}^{\infty}} \!
\left[ G\left(\mathbf{x};\mbox{\boldmath[math]\displaystyle{ \xi }[/math]}+(0,ml)\right)  
+ \frac{ik}{2} H_{0} \Big(k \sqrt{\left( X+cl\right)^2 + Y_{m}^2}\Big)
\right] \exp^{im\sigma l}
\\
&\! &\! -\frac{i}{l}\displaystyle{\sum_{m=-\infty}^{\infty}
\frac{\exp^{ik \mu_{m}|X+cl| }\exp^{i\sigma_{m}Y_0}}{\mu_{m}}}.
 (Gp_fast)

\end{matrix}</math>

The convergence of the two sums in ((Gp_fast)) depends on the value of [math]\displaystyle{ c }[/math]. For small [math]\displaystyle{ c }[/math] the first sum converges rapidly while the second converges slowly. For large [math]\displaystyle{ c }[/math] the second sum converges rapidly while the first converges slowly. The smoothing parameter [math]\displaystyle{ c }[/math] must be carefully chosen to balance these two effects. Of course, the convergence also depends strongly on how close together the points [math]\displaystyle{ \mathbf{x} }[/math] and [math]\displaystyle{ \mbox{\boldmath }[/math]\xi[math]\displaystyle{ } }[/math] are.

Note that some special combinations of wavelength [math]\displaystyle{ \lambda }[/math] and angle of incidence [math]\displaystyle{ \theta }[/math] cause the periodic Green function to diverge ( Scott98,Jorgenson90). This singularity is closely related to the diffracted waves and will be explained shortly.

The scattered waves (modes)

We begin with the accelerated periodic Green function, equation ((Gp_fast)) setting [math]\displaystyle{ c=0 }[/math] and considering the case when [math]\displaystyle{ X }[/math] is large (positive or negative). We also note that for [math]\displaystyle{ m }[/math] sufficiently small or large [math]\displaystyle{ i\mu_m }[/math] will be negative and the corresponding terms will decay. Therefore

[math]\displaystyle{ (G_p_accelerate) G_{\mathbf{P}} (\mathbf{x};\mbox{\boldmath\lt math\gt \xi }[/math]})

\sim - \frac{i}{l} \sum_{m=-M}^{N} \frac{\exp^{ik\mu_{m}|X|}\, \exp^{i\sigma_{m}Y_0}}
{\mu_{m}}, \qquad \mbox{as [math]\displaystyle{ X \to \pm \infty }[/math]}
 (new_G_p_trunc)

</math>

where the integers [math]\displaystyle{ M }[/math] and [math]\displaystyle{ N }[/math] satisfy the following inequalities

[math]\displaystyle{ (M_N) \left. \begin{matrix} [c]{c} \sigma_{-M-1}\lt -k\lt \sigma_{-M},\\ \sigma_{N}\lt k\lt \sigma_{N+1}. \end{matrix} \right\} (cut_off) }[/math]

Equations ((M_N)) can be written as

[math]\displaystyle{ \frac{l}{2\pi}\left(\sigma+k-2\pi \right) \lt M \lt \frac{l}{2\pi}\left( \sigma+k \right), (diffracted_M) }[/math]

and

[math]\displaystyle{ \frac{l}{2\pi}\left( k - \sigma \right) \gt N \gt \frac{l}{2\pi} \left( k-\sigma - 2\pi \right) (diffracted_N) }[/math]

Linton98. It is obvious that [math]\displaystyle{ G_{\mathbf{P}} }[/math] will diverge if [math]\displaystyle{ \sigma_m = \pm k }[/math]; these values correspond to cut-off frequencies which are an expected feature of periodic structures.

The diffracted waves

The diffracted waves are the plane waves which are observed as [math]\displaystyle{ x \to \pm \infty }[/math]. Their amplitude and form are obtained by substituting the limit of the periodic Green function ((new_G_p_trunc)) as [math]\displaystyle{ x\to\pm\infty }[/math] into the boundary integral equation for the potential ((bem_eq_2)). This gives us

[math]\displaystyle{ \lim_{x\to\pm\infty} \phi^{s} ( \mathbf{x},0 ) = - \frac{i}{l} \sum_{m=-M}^{N} \int_{\Delta_0} \frac{\exp^{ik\mu_{m} |X| } \exp^{i\sigma_{m}Y_0}}{\mu_{m}} \left[ k\phi(\mbox{\boldmath\lt math\gt \xi }[/math]},0)

- w(\mbox{\boldmath[math]\displaystyle{ \xi }[/math]}) \right] 
d\mbox{\boldmath[math]\displaystyle{ \xi }[/math]},
 (phi_s)

</math>

where [math]\displaystyle{ \phi^{s} = \phi-\phi^{\rm in} }[/math] is the scattered wave which is composed of a finite number of plane waves. For [math]\displaystyle{ x \to -\infty }[/math] the scattered wave is given by

[math]\displaystyle{ \lim_{x\to-\infty}\phi^{s} (\mathbf{x},0) = A_{m}^{-}\,\exp^{ik\mu_{m}x}\exp^{i\sigma_{m}y}, (phi_m_min) }[/math]

where the amplitudes [math]\displaystyle{ A_{m}^{-} }[/math] are identified from ((phi_s)) as

[math]\displaystyle{ A_{m}^{-} = -\frac{i}{\mu_{m}l} \int_{\Delta_0} \exp^{ik\mu_{m}\xi } \exp^{-i\sigma_{m}\eta} \left[ k\phi\left( \mbox{\boldmath\lt math\gt \xi }[/math]}\right)

- w (\mbox{\boldmath[math]\displaystyle{ \xi }[/math]}) \right] 
d\mbox{\boldmath[math]\displaystyle{ \xi }[/math]}.
 (Ad_m)

</math>

Likewise as [math]\displaystyle{ x \to \infty }[/math] the scattered wave is given by

[math]\displaystyle{ \lim_{x\to\infty}\phi^{s} (\mathbf{x},0) = A_{m}^{+} \exp^{-ik\mu_{m}x} \exp^{i\sigma_{m}y}, (phi_m_plus) }[/math]

where [math]\displaystyle{ A_{m}^{+} }[/math] are

[math]\displaystyle{ A_{m}^{+} = -\frac{i}{\mu_{m}l}\int_{\Delta_0} \exp^{-ik\mu_{m}\xi }\exp^{-i\sigma_{m}\eta} \left[ k\phi (\mbox{\boldmath\lt math\gt \xi }[/math]},0)

- w(\mbox{\boldmath[math]\displaystyle{ \xi }[/math]}) \right]
d\mbox{\boldmath[math]\displaystyle{ \xi }[/math]}.
 (Ad_p)

</math>

The diffracted waves propagate at various angles with respect to the normal direction of the array. The angles of diffraction, [math]\displaystyle{ \psi_{m}^{\pm} }[/math], are given by

[math]\displaystyle{ \psi_{m}^{\pm} = \tan^{-1}\left( \frac{\sigma_{m}}{\pm k\mu_{m}}\right). (psi_m) }[/math]

Notice that for [math]\displaystyle{ m=0 }[/math] we have

[math]\displaystyle{ \psi_{0}^{\pm}=\pm\theta, (psi_0) }[/math]

where [math]\displaystyle{ \theta }[/math] is the incident angle. This is exactly as expected since we should always have a transmitted wave which travels in the same direction as the incident wave and a reflected wave which travels in the negative incident angle direction.

The fundamental reflected and transmitted waves

We need to be precise when we determine the wave of order zero at [math]\displaystyle{ x\to\infty }[/math] because we have to include the incident wave. There is always at least one set of propagating waves corresponding to [math]\displaystyle{ m=0 }[/math] which correspond to simple reflection and transmission. The coefficient, [math]\displaystyle{ R }[/math], for the fundamental reflected wave for the [math]\displaystyle{ m=0 }[/math] mode is given by

[math]\displaystyle{ R = A_{0}^{-} = -\frac{i}{\mu_{0}l}\int_{\Delta_0}\exp^{ik (\xi\cos\theta -\eta\sin\theta)}\left[ k\phi(\mbox{\boldmath\lt math\gt \xi }[/math]},0)

- w(\mbox{\boldmath[math]\displaystyle{ \xi }[/math]})\right]
d\mbox{\boldmath[math]\displaystyle{ \xi }[/math]}.
 (R)

</math>

The coefficient, [math]\displaystyle{ T }[/math], for the fundamental transmitted wave for the [math]\displaystyle{ m=0 }[/math] mode is given by

[math]\displaystyle{ T = 1 + A_{0}^{+} = 1 - \frac{i}{\mu_{0}l} \int_{\Delta_0} \exp^{-ik(\xi\cos\theta +\eta\sin\theta)}\left[ k\phi(\mbox{\boldmath\lt math\gt \xi }[/math]},0)

- w(\mbox{\boldmath[math]\displaystyle{ \xi }[/math]}) \right]
d\mbox{\boldmath[math]\displaystyle{ \xi }[/math]}.
 (T)

</math>

Conservation of energy

The diffracted wave, taking into account the correction for [math]\displaystyle{ T }[/math], must satisfy the energy flux equation. This simply says that the energy of the incoming wave must be equal to the energy of the outgoing waves. This gives us

[math]\displaystyle{ \cos\theta = \left( |R|^2+|T|^2 \right) \cos\theta + \sum_{\stackrel{\scriptstyle{m=-M} }{m ~ \neq 0}}^{N} \left( |A_{m}^{-}|^2 \cos\psi_{m}^{-} +|A_{m}^{+}|^2 \cos\psi_{m}^{+} \right). (NRGbal) }[/math]

The energy balance equation ((NRGbal)) can be used as an accuracy check on the numerical results.

Results

We tested the convergence of our accelerated version of the Green function and we use [math]\displaystyle{ c = 0.05 }[/math] and [math]\displaystyle{ 44 }[/math] terms in the first sum (the spatial term) and [math]\displaystyle{ 46 }[/math] terms in the second sum (the spectral term) of ((Gp_fast))). These values were determined by a convergence study, the details of which can be found in Wang wangphd04. These values will be used in all our subsequent calculations. We consider four geometries for the plates which are shown in Figure (floes4jfs).

Scattering from a Dock

Aside from the energy balance equation or wide spacing, it is difficult to compare our results to establish their validity. However, there is one case in which we can make comparisons. If we consider the case when we have the dock boundary condition under the plate (so that [math]\displaystyle{ w=0 }[/math]) and the plates are square and joined then the problem reduces to a two dimensional dock problem which is discussed extensively in \cite[Chapter 2]{Linton_mciver01}. To impose the condition of a dock we simply solve equation ((phi_plate)) setting [math]\displaystyle{ \vec{w} }[/math] to zero (we do not require equation ((comp_eqn))), choosing a square plate (geometry 1) and setting the plate separation to [math]\displaystyle{ l=4 }[/math].

Figure (fig_RnT4stiff) shows the reflection and transmission coefficients for a square plate (geometry 1) with the plate separation [math]\displaystyle{ l=4 }[/math] and the dock boundary condition (crosses) and the solution to the two-dimensional dock problem using the method of Linton_mciver01 (solid and dashed lines). As expected the results agree.

Table (table_stiff) shows the values of the coefficient

[math]\displaystyle{ A_{m}^{\pm} }[/math] for a dock of geometry 1 with [math]\displaystyle{ {\lambda=4} }[/math],

[math]\displaystyle{ {l=6} }[/math] and [math]\displaystyle{ {\theta=\pi/6} }[/math]. These results are given to assist in numerical comparisons. \begin{table} \begin{tabular}{ccc} \hline [math]\displaystyle{ m }[/math] & [math]\displaystyle{ A_{m}^{-} }[/math] & [math]\displaystyle{ A_{m}^{+} }[/math] \\ \hline [math]\displaystyle{ -2 }[/math] & [math]\displaystyle{ -0.214-0.042i }[/math] & [math]\displaystyle{ 0.232+0.023i }[/math] \\ [math]\displaystyle{ -1 }[/math] & [math]\displaystyle{ 0.266-0.268i }[/math] & [math]\displaystyle{ -0.185+0.349i }[/math] \\ [math]\displaystyle{ 0 }[/math] & [math]\displaystyle{ 0.631-0.210i }[/math] & [math]\displaystyle{ -0.702-0.141i }[/math] \\ \end{tabular} \caption[]{The coefficients [math]\displaystyle{ A_{m}^{\pm} }[/math] for the case of a dock of geometry 1 with [math]\displaystyle{ {\lambda=4} }[/math], [math]\displaystyle{ {l=6} }[/math] and [math]\displaystyle{ {\theta=\pi/6} }[/math].} (table_stiff) \end{table}

Scattering from Elastic Plates

We begin with a short table of numerical results. Table (flexible_table) is equivalent to Table (table_stiff) except that the plate is now elastic with [math]\displaystyle{ \beta = 0.1 }[/math] and [math]\displaystyle{ \gamma = 0 }[/math]. As expected the reflected energy is less because the waves can propagate under the elastic plates. \begin{table} \begin{tabular}{ccc} \hline [math]\displaystyle{ m }[/math] & [math]\displaystyle{ A_{m}^{-} }[/math] & [math]\displaystyle{ A_{m}^{+} }[/math] \\ \hline [math]\displaystyle{ -2 }[/math] & [math]\displaystyle{ 0.001+0.014i }[/math] & [math]\displaystyle{ -0.040-0.016i }[/math] \\ [math]\displaystyle{ -1 }[/math] & [math]\displaystyle{ -0.016-0.008i }[/math] & [math]\displaystyle{ -0.070-0.099i }[/math] \\ [math]\displaystyle{ 0 }[/math] & [math]\displaystyle{ -0.058-0.072i }[/math] & [math]\displaystyle{ -0.209-0.582i }[/math] \\ \end{tabular} \caption[]{The coefficients [math]\displaystyle{ A_{m}^{\pm} }[/math] for the case of an elastic plate of geometry 1 with [math]\displaystyle{ {\beta=0.1} }[/math], [math]\displaystyle{ {\gamma=0} }[/math], [math]\displaystyle{ {\lambda=4} }[/math], [math]\displaystyle{ {l=6} }[/math] and [math]\displaystyle{ {\theta=\pi/6} }[/math].} (flexible_table) \end{table}

Figures (fig_square_vartheta_beta0p1) and

(fig_triangle_vartheta_beta0p1) show the amplitudes of the diffracted waves due to the array as a function of the incident angle for plates of geometry one and two respectively with [math]\displaystyle{ \beta=0.1 }[/math], [math]\displaystyle{ \gamma=0 }[/math], [math]\displaystyle{ k=\pi/2 }[/math], and [math]\displaystyle{ l=6 }[/math]. There are 3 pairs of diffracted waves (including the reflected-transmitted pair) for any angle. [math]\displaystyle{ G_{\bf P} }[/math] diverges if [math]\displaystyle{ \sigma_n = \pm k }[/math] which for our values of [math]\displaystyle{ l }[/math] and [math]\displaystyle{ k }[/math] means that [math]\displaystyle{ \theta = \pm 0.3398 }[/math]. As [math]\displaystyle{ \theta }[/math] moves across these points one of the diffracted waves disappears (at [math]\displaystyle{ \pm\pi/2 }[/math]) and an other appears (at [math]\displaystyle{ \mp\pi/2 }[/math]). In the plots we have plotted [math]\displaystyle{ A^{\pm}_{-2} }[/math] and [math]\displaystyle{ A^{\pm}_{1} }[/math] with the same line style and also [math]\displaystyle{ A^{\pm}_{-1} }[/math] and [math]\displaystyle{ A^{\pm}_{2} }[/math] since they represent diffracted waves which appear and disappear together. Interestingly the result of doing this is to produce smooth curves for [math]\displaystyle{ -\pi/2\lt \theta\lt \pi/2 }[/math]. Figures (fig_square_vartheta_beta0p1) and (fig_triangle_vartheta_beta0p1) show that there is a very strong dependence on the amount of reflected energy as a function of incident angle with small angles giving the smallest reflection and large angles giving the greatest reflection.

Figures (fig_disp_p_square) to (fig_disp_p_trapezoid) show the real part of the displacement for five plates ([math]\displaystyle{ \Delta_{j} }[/math], [math]\displaystyle{ j=-2,-1,0,1,2 }[/math]) of the array for plates of geometry one to four respectively, with [math]\displaystyle{ \beta=0.1 }[/math], [math]\displaystyle{ \gamma=0 }[/math], and [math]\displaystyle{ l=6 }[/math]. The angle of incidence is [math]\displaystyle{ \theta=\pi/6 }[/math]. We consider two values of the wavenumber, [math]\displaystyle{ k=\pi/2 }[/math] (a) and [math]\displaystyle{ k=\pi/4 }[/math] (b). The complex response of the elastic plates is apparent in these figures as is the coupling between the water and the plate. It is also clear that there is a great deal of difference in the individual behaviour of plates of different geometries. This has practical implications for experiments which might be performed on individual ice floes. For example, from these figures it appears that it will be very difficult to make

measurements of  wave spectra using an accelerometer

deployed on an ice floe if the floe size is comparable to the wavelength.

In Figure (energy_vs_k) we consider the total reflected energy [math]\displaystyle{ E_R }[/math] given by

[math]\displaystyle{ E_R = |R|^2 \cos\theta + \sum_{\stackrel{\scriptstyle{m=-M} }{m ~ \neq 0}}^{N} |A_{m}^{-}|^2 \cos\psi_{m}^{-} }[/math]

as a function of [math]\displaystyle{ k }[/math] for [math]\displaystyle{ l= 6 }[/math], [math]\displaystyle{ \beta=0.1 }[/math], and [math]\displaystyle{ \gamma=0 }[/math] for floes of geometry 1 and 2 and for angles of [math]\displaystyle{ \theta = 0 }[/math], [math]\displaystyle{ \pi/6 }[/math], and [math]\displaystyle{ \pi/3 }[/math]. In Figure (energy_vs_k) we have divided [math]\displaystyle{ E_R }[/math] by [math]\displaystyle{ \cos\theta }[/math] to normalise the curves, since if all the energy is reflected [math]\displaystyle{ E_R/\cos\theta=1 }[/math]. This figure shows the kind of important results which we can produce from our model even with some simple calculations. The results show that for [math]\displaystyle{ k\lt 1 }[/math] there is no scattering of energy at all. While it is to be expected that long wavelength waves will not be strongly scattered this figure gives us a quantitative value for the [math]\displaystyle{ k }[/math] below which there is negligible scattering. As [math]\displaystyle{ k }[/math] increases the reflection increases and this increase is much more marked for waves incident at a large angle. This is to be expected since waves which are normally incident can be expected to pass through the plates even for short wavelengths. However, the effect of angle appears to be much stronger than might be expected on a simple geometric argument (which is also what was found in Figures (fig_square_vartheta_beta0p1) and (fig_triangle_vartheta_beta0p1)). Interestingly, the effect of geometry is much less significant than either [math]\displaystyle{ \theta }[/math] or [math]\displaystyle{ k }[/math]. This implies that for practical purposes it may be sufficient to take one or two representative geometries. This seems surprising given the differences in the responses of plates of different geometries shown in Figures (fig_disp_p_square) to (fig_disp_p_trapezoid). Figure (energy_vs_k) can be regarded as a preliminary figure, showing the kinds of calculations which are required to develop a model for wave scattering in the marginal ice zone from the present results.

Concluding Remarks

Motivated by the problem of modelling wave propagation in the marginal ice zone and by the general problem of scattering by arrays of bodies we have presented a solution to the problem of wave scattering by an infinite array of floating elastic plates. The model is based on the linear theory and assumes that the floe submergence is negligible. For this reason it is applicable to low to moderate wave heights and to floes whose size is large compared to the floe thickness. The solution method is similar to that used to solve for a single plate except that the periodic Green function must be used. The periodic Green function is obtained by summing the free-surface

Green function for all [math]\displaystyle{ y }[/math]. However the periodic Green function is slowly

convergent and therefore a method, which is commonly used in Optics, is devised to accelerate the convergence. The acceleration method involves expressing the infinite sum of the Green function in its far-field form. From the far-field representation of the periodic Green Function, we calculate the diffracted wave far from the array and the cut-off frequencies. We have checked our numerical calculations for energy balance and against the limiting case when the plates are rigid and joined where the solution reduces to that of a rigid dock. We have also presented solutions for a range of elastic plate geometries.

The solution method could be extended to water of finite depth using a similar periodic Green function, but in this case based on the free surface Green function for finite depth water. The same problems of slow convergence would arise and a similar method for convergence of the series would be required. We believe that a method similar to the one presented here could be used to accelerate the convergence of the periodic Green function for water of finite depth. Another extension would be to consider a doubly periodic lattice in which a doubly periodic Green function would be required. This should also be able to be computed quickly by methods similar to the ones presented here.

The most important extension of this work concerns using it to construct a scattering model for the marginal ice zone. Such a model would be based on the solutions presented here but would be a far from trivial extension of it. For a model to be realistic the effects which have been induced by the assumption of periodicity would have to be removed. One method to do this would be to consider random spacings of the ice floes and to average the result over these. This should lead to a scattered wave field over all directions rather than discrete scattering angles as well as a reflected and transmitted wave. These solutions would then have to be combined, again with some kind of averaging and wide spacing approximation, to compute the scattering by multiple rows of ice floes.

\bibliography{/home/groups/seaice/bibdata/mike,/home/groups/seaice/bibdata/others,/home/meylan/Papers/Periodic_Cynthia/ALMOSTALL,/home/meylan/Papers/Periodic_Cynthia/Cynthia}

\pagebreak \begin{figure} [p] \begin{center} \includegraphics[scale = 0.7] {array} \caption{A schematic diagram of the periodic array of floating elastic plates.}

(fig_array)

\end{center} \end{figure}

\begin{figure}[ptb] \begin{center} \includegraphics[scale = 0.7

] {floes4jfs} \caption{Diagram of the four plate geometries for which we will calculate solutions. }

(floes4jfs)

\end{center} \end{figure}

\begin{figure} [p] \begin{center} \includegraphics[scale = 0.7]{stiff_compare} \caption{The reflection coefficient [math]\displaystyle{ R }[/math] (solid line) and the transmission coefficient [math]\displaystyle{ T }[/math] (dashed line) as a function of [math]\displaystyle{ k }[/math] for a two-dimensional dock of length 4 for the incident angles shown. The crosses are the same problem solved using the three-dimensional array code with the dock boundary condition and using plates of geometry 1 with [math]\displaystyle{ l=4 }[/math].}

(fig_RnT4stiff)

\end{center} \end{figure}