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Next: Definition of an ideal Up: Joint statistics of amplitudes Previous: Introduction

Fields with Random Phases and Amplitudes.

Let us consider a wavefield $a({\bf x}, t)$ in a periodic cube of with side $L$ and let the Fourier transform of this field be $a_l(t)$ where index $l {\in } {\cal Z}^d$ marks the mode with wavenumber $k_l = 2
\pi l /L$ on the grid in the $d$-dimensional Fourier space. For simplicity let us assume that there is a maximum wavenumber $k_{max}$ (fixed e.g. by dissipation) so that no modes with wavenumbers greater than this maximum value can be excited. In this case, the total number of modes is $N = (k_{max} / \pi L)^d$. Correspondingly, index $l$ will only take values in a finite box, $l \in {\cal B}_N \subset
{\cal Z}^d$ which is centred at 0 and all sides of which are equal to $k_{max} / \pi L = N^{1/3}$. To consider homogeneous turbulence, the large box limit $N \to \infty $ will have to be taken. 1

Let us write the complex $a_l$ as $a_l =A_l \psi_l $ where $A_l$ is a real positive amplitude and $\psi_l $ is a phase factor which takes values on ${\cal S}^{1} $, a unit circle centred at zero in the complex plane. Let us define the $N$-mode joint PDF ${\cal P}^{(N)}$ as the probability for the wave intensities $A_l^2 $ to be in the range $(s_l, s_l +d s_l)$ and for the phase factors $\psi_l $ to be on the unit-circle segment between $\xi_l$ and $\xi_l + d\xi_l$ for all $l \in {\cal B}_N$. In terms of this PDF, taking the averages will involve integration over all the real positive $s_l$'s and along all the complex unit circles of all $\xi_l$'s,


$\displaystyle \langle f\{A^2, \psi \} \rangle
= \left(
\prod_{ l {\cal 2 B}_N }...
... S}^{1} }
\vert d \xi_l\vert \right) \; {\cal P}^{(N)} \{s, \xi \}
f\{s, \xi \}$     (1)

where notation $f\{A^2,\psi\}$ means that $f$ depends on all $A_l^2 $'s and all $\psi_l $'s in the set $\{A_l^2,
\psi_l; l {\cal 2 B}_N \}$ (similarly, $\{s, \xi \}$ means $\{s_l,
\psi_l; l \in {\cal B}_N \}$, etc). The full PDF that contains the complete statistical information about the wavefield $a({\bf x}, t)$ in the infinite $x$-space can be understood as a large-box limit

\begin{displaymath}{\cal P} \{ s_k, \xi_k \} = \lim_{N \to \infty}
{\cal P}^{(N)} \{s, \xi \},
\end{displaymath}

i.e. it is a functional acting on the continuous functions of the wavenumber, $s_k$ and $\xi_k$. In the the large box limit there is a path-integral version of (1),
\begin{displaymath}\langle f\{A^2, \psi \} \rangle =
\int {\cal D}s \oint
\vert{\cal D} \xi\vert \; {\cal P} \{s, \xi \} f\{s, \xi \}
\end{displaymath} (2)

The full PDF defined above involves all $N$ modes (for either finite $N$ or in the $N \to \infty $ limit). By integrating out all the arguments except for chosen few, one can have reduced statistical distributions. For example, by integrating over all the angles and over all but $M$ amplitudes,we have an ``$M$-mode'' amplitude PDF,
\begin{displaymath}
{\cal P}_{j_1, j_2, \dots , j_M} = \left(
\prod_{ l \ne j_1,...
...1} }
\vert d \xi_m\vert \; \right) {\cal P}^{(N)} \{s, \xi \},
\end{displaymath} (3)

which depends only on the $M$ amplitudes marked by labels $j_1, j_2, \dots , j_M {\cal 2 B}_N$.



Subsections
next up previous
Next: Definition of an ideal Up: Joint statistics of amplitudes Previous: Introduction
Dr Yuri V Lvov 2007-01-17