elixir_euler_free_stream.jl

using OrdinaryDiffEqLowStorageRK
using Trixi

###############################################################################
# semidiscretization of the compressible Euler equations

equations = CompressibleEulerEquations2D(1.4)

initial_condition = initial_condition_constant

# Up to version 0.13.0, `max_abs_speed_naive` was used as the default wave speed estimate of
# `const flux_lax_friedrichs = FluxLaxFriedrichs(), i.e., `FluxLaxFriedrichs(max_abs_speed = max_abs_speed_naive)`.
# In the `StepsizeCallback`, though, the less diffusive `max_abs_speeds` is employed which is consistent with `max_abs_speed`.
# Thus, we exchanged in PR#2458 the default wave speed used in the LLF flux to `max_abs_speed`.
# To ensure that every example still runs we specify explicitly `FluxLaxFriedrichs(max_abs_speed_naive)`.
# We remark, however, that the now default `max_abs_speed` is in general recommended due to compliance with the 
# `StepsizeCallback` (CFL-Condition) and less diffusion.
solver = DGSEM(polydeg = 3, surface_flux = FluxLaxFriedrichs(max_abs_speed_naive))

# Mapping as described in https://arxiv.org/abs/2012.12040 but reduced to 2D
function mapping(xi_, eta_)
    # Transform input variables between -1 and 1 onto [0,3]
    xi = 1.5 * xi_ + 1.5
    eta = 1.5 * eta_ + 1.5

    y = eta + 3 / 8 * (cos(1.5 * pi * (2 * xi - 3) / 3) *
                       cos(0.5 * pi * (2 * eta - 3) / 3))

    x = xi + 3 / 8 * (cos(0.5 * pi * (2 * xi - 3) / 3) *
                      cos(2 * pi * (2 * y - 3) / 3))

    return SVector(x, y)
end

###############################################################################
# Get the uncurved mesh from a file (downloads the file if not available locally)

# Unstructured mesh with 48 cells of the square domain [-1, 1]^n
mesh_file = Trixi.download("https://gist.githubusercontent.com/efaulhaber/a075f8ec39a67fa9fad8f6f84342cbca/raw/a7206a02ed3a5d3cadacd8d9694ac154f9151db7/square_unstructured_1.inp",
                           joinpath(@__DIR__, "square_unstructured_1.inp"))

# Map the unstructured mesh with the mapping above
mesh = P4estMesh{2}(mesh_file, polydeg = 3, mapping = mapping, initial_refinement_level = 1)

# Refine bottom left quadrant of each tree to level 2
function refine_fn(p4est, which_tree, quadrant)
    quadrant_obj = unsafe_load(quadrant)
    if quadrant_obj.x == 0 && quadrant_obj.y == 0 && quadrant_obj.level < 3
        # return true (refine)
        return Cint(1)
    else
        # return false (don't refine)
        return Cint(0)
    end
end

# Refine recursively until each bottom left quadrant of a tree has level 2.
# The mesh will be rebalanced before the simulation starts.
refine_fn_c = @cfunction(refine_fn, Cint,
                         (Ptr{Trixi.p4est_t}, Ptr{Trixi.p4est_topidx_t},
                          Ptr{Trixi.p4est_quadrant_t}))
Trixi.refine_p4est!(mesh.p4est, true, refine_fn_c, C_NULL)

semi = SemidiscretizationHyperbolic(mesh, equations, initial_condition, solver,
                                    boundary_conditions = Dict(:all => BoundaryConditionDirichlet(initial_condition)))

###############################################################################
# ODE solvers, callbacks etc.

tspan = (0.0, 1.0)
ode = semidiscretize(semi, tspan)

summary_callback = SummaryCallback()

analysis_interval = 100
analysis_callback = AnalysisCallback(semi, interval = analysis_interval)

alive_callback = AliveCallback(analysis_interval = analysis_interval)

save_solution = SaveSolutionCallback(interval = 100,
                                     save_initial_solution = true,
                                     save_final_solution = true,
                                     solution_variables = cons2prim)

stepsize_callback = StepsizeCallback(cfl = 2.0)

callbacks = CallbackSet(summary_callback,
                        analysis_callback, alive_callback,
                        save_solution,
                        stepsize_callback)

###############################################################################
# run the simulation

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