# Time-stepping

FourierFlows.jl includes several time-stepping algorithms.

Most of the time-stepping algorithms are fully explicit schemes: ForwardEulerTimeStepper, AB3TimeStepper, RK4TimeStepper, and LSRK54TimeStepper. Also we have implemented an ETDRK4TimeStepper scheme with the improvements described by [2].

The Problem constructor expects the chosen time stepper as as string that includes the corresponding name of the time stepper without the ending TimeStepper. For example, "RK4" for the Runge-Kutta 4th-order time stepper.

## High-wavenumber filtering

Most of the time steppers also come with their Filtered equivalents: FilteredForwardEulerTimeStepper, FilteredAB3TimeStepper, FilteredRK4TimeStepper, FilteredLSRK54TimeStepper, and FilteredETDRK4TimeStepper.

The filtered time steppers apply a high-wavenumber filter to the solution at the end of each step. The motivation behind filtering is to preclude enstrophy from accumulating at high wavenumbers and creating noise at grid-scale level.

The high-wavenumber filter used in the filtered time steppers is:

$$$\mathrm{filter}(\boldsymbol{k}) = \begin{cases} 1 & \quad |\boldsymbol{k}| ≤ k_{\textrm{cutoff}} \, ,\\ \exp{ \left [- \alpha (|\boldsymbol{k}| - k_{\textrm{cutoff}})^p \right]} & \quad |\boldsymbol{k}| > k_{\textrm{cutoff}} \, . \end{cases}$$$

For fluid equations with quadratic non-linearities it makes sense to choose a cut-off wavenumber at 2/3 of the highest wavenumber that is resolved in our domain, $k_{\textrm{cutoff}} = \tfrac{2}{3} k_{\textrm{max}}$ (see discussion in Aliasing section).

Given the order $p$, we choose coefficient $\alpha$ so that the filter value that corresponds to the highest allowed wavenumber in our domain is a small number $\delta$, usually taken to be close to machine precision. That is:

$$$\alpha = \frac{- \log\delta}{(k_{\textrm{max}} - k_{\textrm{cutoff}})^p} \ .$$$

The above-mentioned filter form originates from the book by [3]. In geophysical turbulence applications it was used by [4] and later by [5].

Not too steep, not too shallow

Care should be taken if one decides to fiddle with the filter parameters. Changing the order $p$ affects how steeply the filter falls off. Lower order values imply that the filter might fall off too quickly and may lead to Gibbs artifacts; higher order value implies that the filter might fall off too slow and won't suffice to remove enstrophy accumulating at the grid scale.

The filter using the default parameters provided by the filtered time steppers (see FourierFlows.makefilter) is depicted below. The same plot also compares how the filter changes when we vary the order parameter $p$ while keeping everything else the same.

using CairoMakie
using FourierFlows: makefilter

K = 0:0.001:1 # non-dimensional wavenumber k * dx / π

fig = Figure()
ax = Axis(fig[1, 1], xlabel = "|𝐤| dx / π", ylabel = "filter", aspect=2.5, xticks=0:0.2:1)

vlines!(ax, 2/3, color = (:gray, 0.4), linewidth = 6, label = "cutoff wavenumber |𝐤| dx / π = 2/3 (default)")

lines!(ax, K, makefilter(K), linewidth = 4, label = "order 4 (default)")
lines!(ax, K, makefilter(K, order = 1), linestyle = :dash, label = "order 1")
lines!(ax, K, makefilter(K, order = 10), linestyle = :dot, label = "order 10")

axislegend(position = :lb)