19: Explicit time stepping

For the time integration, Trixi.jl uses the package OrdinaryDiffEq.jl from the SciML ecosystem. The interface to this package is the solve(...) function. It always requires an ODE problem and a time integration algorithm as input parameters.

solve(ode, alg; kwargs...)

In Trixi.jl, the ODE problem is created by semidiscretize(semi, tspan) for a semidiscretization semi and the time span tspan. In particular, semidiscretize returns an ODEProblem used by OrdinaryDiffEq.jl.

OrdinaryDiffEq.jl provides many integration algorithms, which are summarized in the documentation. Particularly interesting for Trixi.jl are their strong stability preserving (SSP) methods and low-storage methods. There are some differences regarding the choice of the used time step.

Error-based adaptive step sizes

First, we treat time integration algorithms with adaptive step sizes, such as SSPRK43. It is used in some elixirs, like elixir_euler_colliding_flow.jl or elixir_euler_astro_jet_amr.jl.

Other error-based adaptive integration algorithms are for instance RDPK3SpFSAL35, RDPK3Sp35, RDPK3SpFSAL49, RDPK3Sp49, RDPK3SpFSAL510, RDPK3Sp510.

They already contain an error-based adaptive step size control and heuristics to guess a starting step size. If this heuristic fails in your case, you can specify an appropriately small initial step size as keyword argument dt=... of solve.

If you run Trixi in parallel with MPI you need to pass internalnorm=ode_norm and you should pass unstable_check=ode_unstable_check to enable MPI aware error-based adaptive step size control. These keyword arguments are also included in ode_default_options.

CFL-based step size control

The SciML ecosystem also provides time integration algorithms without adaptive time stepping on their own, such as CarpenterKennedy2N54. Moreover, you also can deactivate the automatic adaptivity of adaptive integration algorithms by passing adaptive=false in the solve function.

These algorithms require another way of setting the step size. You have to pass dt=... in the solve function. Without other settings, the simulation uses this fixed time step.

For hyperbolic PDEs, it is natural to use an adaptive CFL-based step size control. Here, the time step is proportional to a ratio of the local measure of mesh spacing $\Delta x_i$ for an element i and the maximum (local) wave speed $\lambda_{\max}$ related to the largest-magnitude eigenvalue of the flux Jacobian of the hyperbolic system.

\[\Delta t_n = \text{CFL} * \min_i \frac{\Delta x_i}{\lambda_{\max}(u_i^n)}\]

We compute $\Delta x_i$ by scaling the element size by a factor of $1/(N+1)$, cf. Gassner and Kopriva (2011), Section 5.

Trixi.jl provides such a CFL-based step size control. It is implemented as the callback StepsizeCallback.

stepsize_callback = StepsizeCallback(; cfl=1.0)

A suitable CFL number depends on many parameters such as the chosen grid, the integration algorithm and the polynomial degree of the spatial DG discretization. So, the optimal number for an example is mostly determined experimentally.

You can add this CFL-based step size control to your simulation like any other callback.

callbacks = CallbackSet(stepsize_callback)
alg = CarpenterKennedy2N54(williamson_condition=false)
solve(ode, alg;
      dt=1.0 # solve needs some value here but it will be overwritten by the stepsize_callback
      callback=callbacks)

You can find simple examples with a CFL-based step size control for instance in the elixirs elixir_advection_basic.jl or elixir_euler_source_terms.jl.

Package versions

These results were obtained using the following versions.

using InteractiveUtils
versioninfo()

using Pkg
Pkg.status(["Trixi", "OrdinaryDiffEq"],
           mode = PKGMODE_MANIFEST)

Julia Version 1.10.6
Commit 67dffc4a8ae (2024-10-28 12:23 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 4 × AMD EPYC 7763 64-Core Processor
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-15.0.7 (ORCJIT, znver3)
Threads: 1 default, 0 interactive, 1 GC (on 4 virtual cores)
Environment:
  JULIA_PKG_SERVER_REGISTRY_PREFERENCE = eager
Status `~/work/Trixi.jl/Trixi.jl/docs/Manifest.toml`
⌃ [1dea7af3] OrdinaryDiffEq v6.66.0
  [a7f1ee26] Trixi v0.9.6 `~/work/Trixi.jl/Trixi.jl`
Info Packages marked with ⌃ have new versions available and may be upgradable.

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