# Code Basics

The code solves partial differential equations of the form

$$$\partial_t u = Lu + N(u) \ ,$$$

using Fourier transforms on periodic domains. Above, $u(\bm{x}, t)$ is the state variable. On the right-hand-side, term $L u$ is the 'linear' part of the equation. The term $N(u)$ is, in general, a 'nonlinear' part.

In FourierFlows.jl, $L u$ is specified by the various modules by prescribing the linear operator $L$ as an array of the same dimension as $u$. The nonlinear term $N(u)$ is specified via a function that takes $u$ as its argument.

Boundary conditions in all spatial dimensions are periodic. That allows us to expand all variables using a Fourier decomposition. For example, if $u$ depends only in one spatial dimension, $x$, defined over the domain $x \in [0, L_x]$, then:

$$$u(x, t) = \sum_{k_x} \hat{u}(k_x, t) \, e^{i k_x x} \ ,$$$

where wavenumbers $k_x$ take the values $\tfrac{2\pi}{L_x}\{0,\pm 1,\pm 2,\dots\}$. When we further consider that $x$ takes discrete values over $[0, L_x]$, e.g., $x_j$, $j = 0, 1, \dots, n_x$, then only $n_x$ wavenumbers are independent. By denoting $u_j(t) \equiv u(x_j, t)$ and $\hat{u}_k(t) \equiv \hat{u}(\tfrac{2\pi}{L_x} k, t)$ with $k$ an integer, then the Fourier sum above truncates to:

$$$u_j(t) = \sum_{k=-n_x/2}^{n_x/2-1} \hat{u}_k(t)\,e^{2\pi i k x_j / L_x} \ .$$$

(We assumed here that $n_x$ is even.)

Applying the Fourier transform as above, the partial differential equation transforms to:

$$$\partial_t \hat{u} = \hat{L} \hat{u} + \widehat{ N(u) } \ ,$$$

where $\hat{L}$ above denotes the linear operator in Fourier space.

Equations are oftentimes time-stepped forward in Fourier space. Doing so, $\hat{u}_k(t)$ becomes now our state variable, i.e., the array with all Fourier coefficients of the solution $u$. Although time-stepping in Fourier space is by no means a restriction of the code, it usually enhances performance because it requires less transformations back and forth from physical to Fourier space and vice versa.