# Changelog

Trixi.jl follows the interpretation of
[semantic versioning (semver)](https://julialang.github.io/Pkg.jl/dev/compatibility/#Version-specifier-format-1)
used in the Julia ecosystem. Notable changes will be documented in this file
for human readability.


## Changes when updating to v0.17 from v0.16.x

#### Added

#### Changed
- The `NonConservativeJump` terms now require `normal_direction_ll` and
  `normal_direction_rr` as function arguments instead of the previous averaged
  `normal_direction` ([#2890]).
  This is necessary because the averaged `normal_direction` did not yield a consistent
  jump term.
- Renamed the hyperbolic right-hand side function `rhs!` to `rhs_hyperbolic!` at
  the semidiscretization and solver levels for consistency with `rhs_parabolic!` ([#2921]).
  The internal `default_rhs` helper now rejects `SemidiscretizationHyperbolicParabolic`,
  which has separate hyperbolic and parabolic right-hand sides and thus lacks a single
  default right-hand side function.
- The internal ODE right-hand side functions such as `rhs_hyperbolic!` and
  `rhs_parabolic!` now dispatch additionally on the backend type ([#3113]).
  The public interface `rhs_hyperbolic!(du_ode, u_ode, semi, t)` is unchanged.
- Trixi.jl now uses t8code v4 through T8code.jl v0.9 ([#2706]).
  This results in some changes to the t8code API in examples and internally in Trixi.jl.
  Moreover, the `T8codeMesh` constructors gained the keyword argument
  `partition_allow_for_coarsening`, which keeps same-level sibling elements together
  during mesh partitioning to allow later coarsening.
- The function `cons2prim` for `CompressibleNavierStokesDiffusion` has been changed ([#3125]).
  It now returns the primitive variables `(rho, v1, v2, v3, p)` (identical to the
  `CompressibleEulerEquations`) instead of `(rho, v1, v2, v3, T)`.
  The latter functionality is now provided by `cons2prim_temperature` instead
  (although it may change in future releases since it is labeled as experimental for now).


#### Deprecated

#### Removed

- The unnecessary method of `flux_nonconservative_chan_etal` accepting two normal
  directions was removed ([#3147]). The other methods are still there and unchanged.
- The keyword argument `n_cells_max` has been removed from the `TreeMesh`
  constructor and from `load_mesh` ([#3021]). Previously it set the initial
  capacity of the internal tree data structure; since `TreeMesh` now auto-resizes
  as needed, the argument is no longer necessary. To migrate, simply remove any
  `n_cells_max = ...` argument from existing code. `TreeMesh` mesh files no longer
  store the internal `capacity` HDF5 attribute; new versions derive storage from
  `n_cells` when loading, while older files that still contain `capacity` remain
  readable.


## Changes in the v0.16 lifecycle

#### Added
- Extended `PositivityPreservingLimiterLiuZhang` to `P4estMesh`, and added an example using `CompressibleNavierStokesDiffusion` ([#3100]).
- Extended `PositivityPreservingLimiterLiuZhang` to `CompressibleEulerEquations1D` and `CompressibleEulerEquations2D` ([#3066]). When combined with `PositivityPreservingLimiterZhangShu`, the density and pressure are guaranteed to be positive at nodal points. The limiter is currently implemented for `TreeMesh` and supports lower bounds on both density and internal energy (or both density and pressure).
- Added `PositivityPreservingLimiterLiuZhang` for `TreeMesh`, which enforces global positivity of cell averages through an iterative algorithm ([#3063]). Currently only implemented for `LinearScalarAdvectionEquation`.
- Added experimental support for block-structured finite volume methods on 1D and 2D `TreeMesh`es via the new `BlockFV` solver, `UniformFiniteVolumeBasis`, and `VolumeIntegralFiniteVolume`, together with example elixirs ([#3067]). Check the progress in <https://github.com/trixi-framework/Trixi.jl/issues/3068>.
- The `BlockFV` solver now supports mortars on the `TreeMesh` in 2D ([#3104]).
- Added support for plotting 1D solutions with Makie.jl, matching the existing Plots.jl interface ([#3035]).
- `VolumeIntegralAdaptive` is now also available with `VolumeIntegralSubcellLimiting` for `TreeMesh` in 2D and 3D using the heuristic a-priori indicator `IndicatorHennemannGassner` ([#2924], [#2986]).
- A new EOS type `AbstractHelmholtzEOS`, with concrete implementation `HelmholtzIdealGas`. This implementation roughly follows Klein et al.'s approach in
  ([arXiv:2603.15112](https://arxiv.org/abs/2603.15112)).
- A new semidiscretization type `SemidiscretizationParabolic` has been added to support purely parabolic equations with no hyperbolic part.
  The new equation types `LinearDiffusionEquation1D` and `LinearDiffusionEquation2D` have been implemented to demonstrate this functionality ([#2874]).
- A new AMR indicator `IndicatorNodalFunction` is introduced, which allows AMR depending on the solution, space, and time. This can be useful, for example, for testing AMR implementations, but also when the solution behavior is known a priori ([#2881]).
- GPU support extended to include AMD GPU with a buildkite workflow using `TRIXI_TEST=AMDGPU` ([#2834]).
- Support for 3D subcell limiting was extended by local limiting for nonperiodic `TreeMesh`es ([#2878]).
- Support for user-defined RHS splitting for IMEX methods via SemidiscretizationHyperbolicSplit ([#2518]). The splitting follows the form `y_t = f_1(y) + f_2(y)`, allowing users to define separate solvers for the stiff (`f_1`) and non-stiff (`f_2`) parts of the right-hand side. Boundary conditions and source terms can be specified independently for the stiff and non-stiff parts.
- Added postprocessing for kinetic energy spectral analysis via `compute_kinetic_energy_spectrum` for `AbstractCompressibleEulerEquations` on `TreeMesh`/`DGSEM` and on `DGMultiMesh`/`DGMultiSBP` in 2D and 3D; the routine returns matching integer wavenumber shells and the isotropic 1D spectrum `E(k)`.
- Added the wrapper `FluxTurbo` that enables SIMD optimized implementations for conservative systems. Precomputation of primitive variables or other more efficient sets of variables can be enabled by specifying the number of precomputed variables, `nturbovars`; the transformation from conservative to precomputed variables, `cons2turbo`; and the `volume_flux_turbo`, which computes the numerical flux in terms of the precomputed variables. ([#3090])
- `FluxTurbo` support was extended to nonconservative systems (mainly for the `P4estMesh` in 3D). ([#3094])
- For the `NonIdealCompressibleEulerEquations` with a non-ideal equation of state a new Newton solver interface for the
  temperature has been added.
  This is required for nonideal equations of state, where one cannot explicitly solve for temperature given two other thermodynamic variables.
  The new function `temperature_given_Vp` computes temperature given the specific volume `V` and pressure `p` using Newton's method,
  analogous to the existing `temperature` function for `AbstractEquationOfState` which takes in `V` and specific internal energy `e_internal` ([#3093]).
- Added support to apply the positivity-preserving limiter after coarsening and refinement steps in AMR via the keyword argument `limiter!` in `AMRCallback` ([#2396]).
- To model a calorically imperfect but thermally perfect gas, a new equation of state `ThermallyPerfectGas9PolyFit` has been added.
  This is a concrete implementation for `AbstractThermallyPerfectGas` that uses a 9th order polynomial fit to the NASA polynomials for specific heat capacities, as described in the corresponding [NASA Technical Publication](https://ntrs.nasa.gov/citations/20020085330).
  This EOS allows for temperature-dependent specific heat capacities (`c_p(T)`, `c_v(T)`) and ratio of specific heats (`\gamma(T)`), while obeying the ideal gas law to relate pressure, density, and temperature ([#3079]).
  This equation of state needs to be supplied to `NonIdealCompressibleEulerEquations`.
- Added support for one-block periodic SBP operators (finite differences, CGSEM, etc.) on `DGMultiMesh`es with nonconservative terms ([#3144]).

#### Changed
- Fixes a bug in `IndicatorEntropyCorrectionShockCapturingCombined` where blending was only applied when the entropy residual was positive. The intended behavior is to apply blending when either entropy correction or shock capturing are activate ([#3131]).
- For performance, `LaplaceDiffusionEntropyVariables` parabolic fluxes for `CompressibleEulerEquations1D`, `CompressibleEulerEquations2D`, and `CompressibleEulerEquations3D` now use explicit Jacobian formulas from Barth 1999 instead of AD ([#3028]). Other equation types continue to use an automatic differentiation fallback.
- Removed the requirement to pass `uEltype = real(dg)` to the `AnalysisCallback` for `DGMulti` solvers ([#3143]).


## Changes when updating to v0.16 from v0.15.x

#### Changed

- The implementation of the local DG (`ViscousFormulationLocalDG`) `solver_parabolic` has been changed for the `P4estMesh`.
In particular, instead of computing the `ldg_switch` as the dot product of the normal direction with ones,
i.e., summing up the normal components, the `ldg_switch` is now selected as
the sign of the maximum (in absolute value sense) normal direction component,
which corresponds to the dominant direction of the interface normal.
This might change results slightly for some meshes where the sum of the normal might be close to zero,
thus introducing some spurious switch assignments ([#2871]).
- The word "viscous" is now used only where it refers specifically to fluid viscosity.
The word "parabolic" is used in more general contexts.
In particular, viscosity is no longer used as a proxy for any parabolic/diffusive process such as heat conduction.
For example, `ViscousFormulationLocalDG` is now `ParabolicFormulationLocalDG` and
`ViscousFormulationBassiRebay1` is now `ParabolicFormulationBassiRebay1`.
For consistency, `cfl_advective` and `cfl_diffusive` have also been renamed `cfl_hyperbolic` and `cfl_parabolic` ([#2868]).
Moreover, some internal functions have been renamed accordingly, including the results shown by the timer outputs after running a simulation.

#### Added

- Introducing GPU support: Based on work by Jan Kraus and Lars Christmann, Trixi.jl can
  now partly be executed on GPUs. This includes simulations with flux differencing on
  `P4estMesh` in 2D and 3D. Adaptive mesh refinement, multi-GPU, source terms, and callbacks
  are not available, yet. Offloading is achieved via KernelAbstractions.jl kernels,
  which, at the moment, execute the same code as usually run on CPUs. A backend is selected
  by passing an appropriate data type as keyword argument `storage_type` to
  `semidiscretize`. See the
  [heterogeneous](https://trixi-framework.github.io/TrixiDocumentation/dev/heterogeneous/)
  section for some instructions on how to port kernels. This is however still preliminaray
  and will change.
  GPU kernels are currently CI-tested on NVIDIA GPUs in a buildkite workflow using
  `TRIXI_TEST=CUDA` ([#2590]).


## Changes in the v0.15 lifecycle

#### Added

- Added shock capturing support for `DGMulti` solvers with `SBP()` approximation types on triangular (`Tri`), tetrahedral (`Tet`), quadrilateral (`Quad`), and hexahedral (`Hex`) elements. `GaussSBP()` approximation types are also supported on `Quad` and `Hex` elements.
- It is now possible to use `ViscousFormulationLocalDG()` as the `solver_parabolic` for non-conforming `P4estMesh`es.
This is useful for (locally) diffusion-dominated problems.
This enables in particular adaptive mesh refinement for that solver-mesh combination ([#2712]).
- Added functionality to `ScalarPlotData2D` allowing visualization a field provided by a user-defined scalar function ([#2796]).
- Added `NonIdealCompressibleEuler2D` ([#2768]).
- Generalization of `VolumeIntegralShockCapturingHG` and `VolumeIntegralShockCapturingRRG` to support different volume integrals on the
  non-stabilized and stabilized elements/cells.
  The generalized volume integral is called `VolumeIntegralShockCapturingHGType` and takes the three keyword arguments `volume_integral_default`,
  `volume_integral_blend_high_order`, and `volume_integral_blend_low_order` besides the usual `indicator` argument.
  In particular, `volume_integral_default` may be e.g.  `VolumeIntegralWeakForm` or `VolumeIntegralAdaptive`, i.e.,
  the non-stabilized elements/cells are no longer restricted to `VolumeIntegralFluxDifferencing` only ([#2802]).
- Added `IndicatorEntropyCorrection`. When combined with `VolumeIntegralAdaptive`, this blends together a stabilized and non-stabilized
  volume integral based on the violation of a volume entropy condition. `IndicatorEntropyCorrectionShockCapturingCombined` additionally
  guides the blending by taking the maximum of the entropy correction indicator and a shock capturing indicator ([#2764]).
- The second-order subcell volume integral is no longer limited to reconstruction in primitive variables.
  Instead, it is possible to reconstruct in custom variables, if functions `cons2recon` and `recon2cons` are provided to
  `VolumeIntegralPureLGLFiniteVolumeO2` and `VolumeIntegralShockCapturingRRG`([#2817]).
- Add Legendre-Gauss basis for DGSEM and implement solver (`VolumeIntegralWeakForm` and `SurfaceIntegralWeakForm` only) support for conforming 1D & 2D `TreeMesh`es ([#1965]).
- Extended 3D support for subcell limiting was added ([#2763], [#2846], [#2844], [#2848]).
  In the new version, local (minimum and maximum) limiting for conservative variables (using the
  keyword `local_twosided_variables_cons` in `SubcellLimiterIDP()`) with `P4estMesh` is supported.
  The support was extended to nonperiodic `P4estMesh{3}`es.
  Moreover, initial support for `TreeMesh{3}` including positivity limiting and local limiting on periodic meshes was added.

## Changes when updating to v0.15 from v0.14.x

#### Changed

- Boundary conditions are now passed as a `NamedTuple` consistently across all mesh types, i.e., boundary conditions, which
  were previously passed as a `Dict` for the `P4estMesh`, `T8codeMesh`, and `UnstructuredMesh2D` mesh types have to be converted to `NamedTuple`s. Also passing boundary conditions as a `Tuple` is no longer supported. This change also makes `boundary_condition_default` obsolete, which is why it was removed ([#2761]).
- The `TreeMesh`, `StructuredMesh` as well as the hypercube constructors of `P4estMesh` and `T8codeMesh` are now non-periodic by default, i.e., you need to
  explicitly set the `periodicity` keyword argument to `true` to obtain periodic meshes. This change was made to be consistent with other mesh types and to avoid confusion ([#2770]).
- There are no default boundary conditions anymore in `SemidiscretizationHyperbolic` and `SemidiscretizationHyperbolicParabolic`.
  Users now have to explicitly provide boundary conditions via the `boundary_conditions` keyword argument, which were periodic by default before ([#2770]).
- Renamed `energy_internal` for `NonIdealCompressibleEulerEquations1D` to `energy_internal_specific` ([#2762]). This makes `energy_internal` for `NonIdealCompressibleEulerEquations1D` consistent with the rest of Trixi.jl, where `energy_internal` refers to the specific internal energy scaled by density.
- Changed `cons2prim(u, ::NonIdealCompressibleEulerEquations1D)` so that it returns `rho, v1, p`. Previously, `cons2prim` returned `V, v1, T`; this functionality is now provided by the non-exported function `cons2thermo` ([#2769]).
- The variable name (printed to the screen and files) of the total energy in the compressible Euler equations has been changed from `rho_e` to `rho_e_total` to avoid confusion with the internal energy `rho_e_internal` ([#2778]).
- `convergence_test` now returns the complete convergence orders and the full errors matrix. To obtain the mean convergence rates, use `Trixi.calc_mean_convergence` on the convergence orders ([#2753]).
- The serial and parallel mesh types have been renamed from `SerialTreeMesh`, `ParallelTreeMesh`, `SerialP4estMesh`, `ParallelP4estMesh`, `SerialT8codeMesh`, and `ParallelT8codeMesh` to `TreeMeshSerial`, `TreeMeshParallel`, `P4estMeshSerial`, `P4estMeshParallel`, `T8codeMeshSerial`, and `T8codeMeshParallel`, respectively ([#2787]).

#### Added

- Added `PengRobinson` equation of state ([#2769]).

## Changes in the v0.14 lifecycle

#### Added

- Added `NonIdealCompressibleEulerEquations1D`, which allows users to specify a non-ideal equation of state. Currently `IdealGas` and `VanDerWaals` are supported ([#2739]).
- Added the APEC (approximate pressure equilibrium preserving with conservation) fluxes of `flux_terashima_etal` and `flux_central_terashima_etal` from [Terashima, Ly, Ihme (2025)](https://doi.org/10.1016/j.jcp.2024.113701) ([#2756]).
- Support for second-order finite volume subcell volume integral (`VolumeIntegralPureLGLFiniteVolumeO2`) and
  stabilized DG-FV blending volume integral (`VolumeIntegralShockCapturingRRG`) on
  3D meshes for conservative systems ([#2734], [#2755]).
- Extended 3D support for subcell limiting with `P4estMesh` was added ([#2733]).
  In the new version, local (minimum or maximum) limiting for nonlinear variables (using
  the keyword `local_onesided_variables_nonlinear` in `SubcellLimiterIDP()`) is supported.

#### Changed

## Changes when updating to v0.14 from v0.13.x

#### Changed

- van Leer's limiters changed to snake case: `vanLeer` => `vanleer` ([#2744])
- A couple `struct`s have been made completely immutable, or only a couple fields thereof ([#2640]). Most notably, `save_solution.condition.save_initial_solution` where `save_solution isa SavesolutionCallback` can no longer be directly changed. Instead, the `@reset` macro from [Accessors.jl](https://github.com/JuliaObjects/Accessors.jl) is used in the elixirs.

## Changes in the v0.13 lifecycle

#### Added

- Initial 3D support for subcell limiting with `P4estMesh` was added ([#2582], [#2647], [#2688], [#2722]).
  In the new version, IDP positivity limiting for conservative variables (using
  the keyword `positivity_variables_cons` in `SubcellLimiterIDP()`) and nonlinear
  variables (using `positivity_variables_nonlinear`) is supported.
- Optimized 2D and 3D kernels for nonconservative fluxes with `P4estMesh` were added ([#2653], [#2663]).
  The optimized kernel can be enabled via the trait `Trixi.combine_conservative_and_nonconservative_fluxes(flux, equations)`.
  When the trait is set to `Trixi.True()`, a single method has to be defined, that computes and returns the tuple
  `flux_cons(u_ll, u_rr) + 0.5f0 * flux_noncons(u_ll, u_rr)` and
  `flux_cons(u_ll, u_rr) + 0.5f0 * flux_noncons(u_rr, u_ll)`.
- Added support for `source_terms_parabolic`, which allows users to specify gradient-dependent source terms when solving parabolic equations ([#2721]).
- Support for second-order finite volume subcell volume integral (`VolumeIntegralPureLGLFiniteVolumeO2`) and
  stabilized DG-FV blending volume integral (`VolumeIntegralShockCapturingRRG`) on
  1D & 2D meshes for conservative systems ([#2022], [#2695], [#2718]).

## Changes when updating to v0.13 from v0.12.x

#### Changed

- The `polyester` preference got merged with the `native_threading` preference and the `Trixi.set_polyester!`
  function got renamed to `Trixi.set_threading_backend!` ([#2476]).
- Default wave-speed estimate used within `flux_lax_friedrichs` changed from `max_abs_speed_naive` to
  `max_abs_speed` which is less diffusive.
  In v0.13, `flux_lax_friedrichs = FluxLaxFriedrichs(max_abs_speed = max_abs_speed)`
  instead of the previous default
  `FluxLaxFriedrichs(max_abs_speed = max_abs_speed_naive)` ([#2458]).
- The signature of the `VisualizationCallback` constructor changed.
  In the new version, it is mandatory to pass the semidiscretization `semi` to
  determine the default plotting type (1D for 1D simulations, 2D for 2D and 3D simulations).
  This can further be customized via the keyword argument `plot_data_creator`, which had
  the default value `plot_data_creator = PlotData2D` before the change ([#2468]).

#### Removed

- Deprecations introduced in earlier versions of Trixi.jl have been removed.


## Changes in the v0.12 lifecycle

#### Added
- Initial support for adapting data-structures between different storage arrays was added. This enables future work to support GPU with Trixi ([#2212]).

#### Deprecated


## Changes when updating to v0.12 from v0.11.x

#### Added

- Arbitrary solution-dependent quantities can now be saved in the `SaveSolutionCallback` (and thus later on visualized) ([#2298]).
- Added support for nonconservative terms with "local * jump" formulation in `VolumeIntegralSubcellLimiting` ([#2429]).

#### Changed

- When using the `VolumeIntegralSubcellLimiting` with the `SubcellLimiterIDP` the
  `:limiting_coefficient` must be explicitly provided to the `SaveSolutionCallback` via
  ```julia
  save_sol_cb = SaveSolutionCallback(interval = 42,
                                     extra_node_variables = (:limiting_coefficient,))
  ```
  i.e., is no longer automatically saved ([#2298]).

#### Deprecated

#### Removed

- The shallow-water equation types `ShallowWaterEquations1D`, `ShallowWaterEquations2D`, and
  `ShallowWaterEquationsQuasi1D` have been removed from Trixi.jl and are now available via
  [TrixiShallowWater.jl](https://github.com/trixi-framework/TrixiShallowWater.jl/).
  This also affects the related functions `hydrostatic_reconstruction_audusse_etal`,
  `flux_nonconservative_audusse_etal`, and `FluxHydrostaticReconstruction`. ([#2379])
- The additional `ìnitial_cache` entries in the caches of `SemidiscretizationHyperbolic`
  and `SemidiscretizationHyperbolicParabolic`, and the corresponding keyword arguments of
  their constructors have been removed. ([#2399])

## Changes in the v0.11 lifecycle

#### Added

- Added symmetry plane/reflective wall velocity+stress boundary conditions for the compressible Navier-Stokes equations in 2D and 3D.
  Currently available only for the `P4estMesh` mesh type, `GradientVariablesPrimitive`, and `Adiabatic` heat boundary condition ([#2416]).
- Added `LaplaceDiffusionEntropyVariables1D`, `LaplaceDiffusionEntropyVariables2D`, and `LaplaceDiffusionEntropyVariables3D`. These add scalar diffusion to each
  equation of a system, but apply diffusion in terms of the entropy variables, which symmetrizes the viscous formulation and ensures semi-discrete entropy dissipation ([#2406]).
- Added the three-dimensional multi-ion magneto-hydrodynamics (MHD) equations with a
  generalized Lagrange multipliers (GLM) divergence cleaning technique ([#2215]).
- New time integrator `PairedExplicitRK4`, implementing the fourth-order
  paired explicit Runge-Kutta method with [Convex.jl](https://github.com/jump-dev/Convex.jl)
  and [ECOS.jl](https://github.com/jump-dev/ECOS.jl) ([#2147])
- Passive tracers for arbitrary equations with density and flow variables ([#2364])

#### Deprecated

- The (2D) aerodynamic coefficients
  `DragCoefficientPressure, LiftCoefficientPressure, DragCoefficientShearStress, LiftCoefficientShearStress` have been renamed to
  `DragCoefficientPressure2D, LiftCoefficientPressure2D, DragCoefficientShearStress2D, LiftCoefficientShearStress2D`. ([#2375])

## Changes when updating to v0.11 from v0.10.x

#### Added

#### Changed

- The `CallbackSet` from the OrdinaryDiffEq.jl ecosystem is `export`ed from Trixi.jl ([@2266]).
- The examples switched from OrdinaryDiffEq.jl to its sub-packages such as
  OrdinaryDiffEqLowStorageRK.jl and OrdinaryDiffEqSSPRK.jl ([@2266]). The installation
  instructions for Trixi.jl have been updated accordingly.
- The output of the `SummaryCallback` will automatically be printed after the simulation
  is finished. Therefore, manually calling `summary_callback()` is not necessary anymore ([#2275]).
- The two performance numbers (local `time/DOF/rhs!` and performance index `PID`)
  are now computed taking into account the number of threads ([#2292]). This allows
  for a better comparison of shared memory (threads) and hybrid (MPI + threads) simulations
  with serial simulations.

#### Deprecated

#### Removed


## Changes when updating to v0.10 from v0.9.x

#### Added

#### Changed

- The numerical solution is wrapped in a `VectorOfArrays` from
  [RecursiveArrayTools.jl](https://github.com/SciML/RecursiveArrayTools.jl)
  for `DGMulti` solvers ([#2150]). You can use `Base.parent` to unwrap
  the original data.
- The `PairedExplicitRK2` constructor with second argument `base_path_monomial_coeffs::AbstractString` requires
  now `dt_opt`, `bS`, `cS` to be given as keyword arguments ([#2184]).
  Previously, those where standard function parameters, in the same order as listed above.
- The `AnalysisCallback` output generated with the `save_analysis = true` option now prints
  floating point numbers in their respective (full) precision.
  Previously, only the first 8 digits were printed to file.
  Furthermore, the names of the printed fields are now only separated by a single white space,
  in contrast to before where this were multiple, depending on the actual name of the printed data.
- The boundary conditions for non-conservative equations can now be defined separately from the conservative part.
  The `surface_flux_functions` tuple is now passed directly to the boundary condition call,
  returning a tuple with boundary condition values for both the conservative and non-conservative parts ([#2200]).

#### Deprecated

#### Removed


## Changes in the v0.9 lifecycle

#### Added

- New time integrator `PairedExplicitRK3`, implementing the third-order paired explicit Runge-Kutta
  method with [Convex.jl](https://github.com/jump-dev/Convex.jl), [ECOS.jl](https://github.com/jump-dev/ECOS.jl),
  and [NLsolve.jl](https://github.com/JuliaNLSolvers/NLsolve.jl) ([#2008])
- `LobattoLegendreBasis` and related datastructures made fully floating-type general,
  enabling calculations with higher than double (`Float64`) precision ([#2128])
- In 2D, quadratic elements, i.e., 8-node (quadratic) quadrilaterals are now supported in standard Abaqus `inp` format ([#2217])
- The `cfl` value supplied in the `StepsizeCallback` and `GlmStepsizeCallback` can now be a function of simulation
  time `t` to enable e.g. a ramp-up of the CFL value.
  This is useful for simulations that are initialized with an "unphysical" initial condition, but do not permit the usage of
  adaptive, error-based timestepping.
  Examples for this are simulations involving the MHD equations which require in general the `GlmStepsizeCallback` ([#2248])

#### Changed

- The required Julia version is updated to v1.10.


## Changes when updating to v0.9 from v0.8.x

#### Added

- Boundary conditions are now supported on nonconservative terms ([#2062]).

#### Changed

- We removed the first argument `semi` corresponding to a `Semidiscretization` from the
  `AnalysisSurfaceIntegral` constructor, as it is no longer needed (see [#1959]).
  The `AnalysisSurfaceIntegral` now only takes the arguments `boundary_symbols` and `variable`.
  ([#2069])
- In functions `rhs!`, `rhs_parabolic!`  we removed the unused argument `initial_condition`. ([#2037])
  Users should not be affected by this.
- Nonconservative terms depend only on `normal_direction_average` instead of both
  `normal_direction_average` and `normal_direction_ll`, such that the function signature is now
  identical with conservative fluxes. This required a change of the `normal_direction` in
  `flux_nonconservative_powell` ([#2062]).

#### Deprecated

#### Removed


## Changes in the v0.8 lifecycle

#### Changed

- The AMR routines for `P4estMesh` and `T8codeMesh` were changed to work on the product
  of the Jacobian and the conserved variables instead of the conserved variables only
  to make AMR fully conservative ([#2028]). This may change AMR results slightly.
- Subcell (IDP) limiting is now officially supported and not marked as experimental
  anymore (see `VolumeIntegralSubcellLimiting`).

## Changes when updating to v0.8 from v0.7.x

#### Added

#### Changed

- The specification of boundary names on which `AnalysisSurfaceIntegral`s are computed (such as drag and lift coefficients) has changed from `Symbol` and `Vector{Symbol}` to `NTuple{Symbol}`.
  Thus, for one boundary the syntax changes from `:boundary` to `(:boundary,)` and for `Vector`s `[:boundary1, :boundary2]` to `(:boundary1, :boundary2)` ([#1959]).
- The names of output files like the one created from the `SaveSolutionCallback` have changed from `%06d` to `%09d` to allow longer-running simulations ([#1996]).

#### Deprecated

#### Removed

## Changes in the v0.7 lifecycle

#### Added
- Implementation of `TimeSeriesCallback` for curvilinear meshes on `UnstructuredMesh2D` and extension to 1D and 3D on `TreeMesh` ([#1855], [#1873]).
- Implementation of 1D Linearized Euler Equations ([#1867]).
- New analysis callback for 2D `P4estMesh` to compute integrated quantities along a boundary surface, e.g., pressure lift and drag coefficients ([#1812]).
- Optional tuple parameter for `GlmSpeedCallback` called `semi_indices` to specify for which semidiscretization of a `SemidiscretizationCoupled` we need to update the GLM speed ([#1835]).
- Subcell local one-sided limiting support for nonlinear variables in 2D for `TreeMesh` ([#1792]).
- New time integrator `PairedExplicitRK2`, implementing the second-order paired explicit Runge-Kutta
  method with [Convex.jl](https://github.com/jump-dev/Convex.jl) and [ECOS.jl](https://github.com/jump-dev/ECOS.jl) ([#1908])
- Add subcell limiting support for `StructuredMesh` ([#1946]).

## Changes when updating to v0.7 from v0.6.x

#### Added

#### Changed

- The default wave speed estimate used within `flux_hll` is now `min_max_speed_davis`
  instead of `min_max_speed_naive`.

#### Deprecated

#### Removed
- Some specialized shallow water specific features are no longer available directly in
  Trixi.jl, but are moved to a dedicated repository: [TrixiShallowWater.jl](https://github.com/trixi-framework/TrixiShallowWater.jl). This includes all features related to wetting and drying, as well as the `ShallowWaterTwoLayerEquations1D` and `ShallowWaterTwoLayerEquations2D`.
  However, the basic shallow water equations are still part of Trixi.jl. We'll also be updating the TrixiShallowWater.jl documentation with instructions on how to use these relocated features in the future.


## Changes in the v0.6 lifecycle

#### Added
- AMR for hyperbolic-parabolic equations on 3D `P4estMesh`
- `flux_hllc` on non-cartesian meshes for `CompressibleEulerEquations{2,3}D`
- Different boundary conditions for quad/hex meshes in Abaqus format, even if not generated by HOHQMesh,
  can now be digested by Trixi in 2D and 3D.
- Subcell (positivity) limiting support for nonlinear variables in 2D for `TreeMesh`
- Added Lighthill-Whitham-Richards (LWR) traffic model


## Changes when updating to v0.6 from v0.5.x

#### Added
- AMR for hyperbolic-parabolic equations on 2D `P4estMesh`

#### Changed

- The wave speed estimates for `flux_hll`, `FluxHLL()` are now consistent across equations.
  In particular, the functions `min_max_speed_naive`, `min_max_speed_einfeldt` are now
  conceptually identical across equations.
  Users, who have been using `flux_hll` for MHD have now to use `flux_hlle` in order to use the
  Einfeldt wave speed estimate.
- Parabolic diffusion terms are now officially supported and not marked as experimental
  anymore.

#### Deprecated

#### Removed

- The neural network-based shock indicators have been migrated to a new repository
  [TrixiSmartShockFinder.jl](https://github.com/trixi-framework/TrixiSmartShockFinder.jl).
  To continue using the indicators, you will need to use both Trixi.jl and
  TrixiSmartShockFinder.jl, as explained in the latter packages' `README.md`.


## Changes in the v0.5 lifecycle

#### Added

- Experimental support for 3D parabolic diffusion terms has been added.
- Non-uniform `TreeMesh` available for hyperbolic-parabolic equations.
- Capability to set truly discontinuous initial conditions in 1D.
- Wetting and drying feature and examples for 1D and 2D shallow water equations
- Implementation of the polytropic Euler equations in 2D
- Implementation of the quasi-1D shallow water and compressible Euler equations
- Subcell (positivity and local min/max) limiting support for conservative variables
  in 2D for `TreeMesh`
- AMR for hyperbolic-parabolic equations on 2D/3D `TreeMesh`
- Added `GradientVariables` type parameter to `AbstractEquationsParabolic`

#### Changed

- The required Julia version is updated to v1.8 in Trixi.jl v0.5.13.

#### Deprecated

- The macro `@unpack` (re-exported originally from UnPack.jl) is deprecated and
  will be removed. Consider using Julia's standard destructuring syntax
  `(; a, b) = stuff` instead of `@unpack a, b = stuff`.
- The constructor `DGMultiMesh(dg; cells_per_dimension, kwargs...)` is deprecated
  and will be removed. The new constructor `DGMultiMesh(dg, cells_per_dimension; kwargs...)`
  does not have `cells_per_dimension` as a keyword argument.

#### Removed

- Migrate neural network-based shock indicators to a new repository
  [TrixiSmartShockFinder.jl](https://github.com/trixi-framework/TrixiSmartShockFinder.jl).


## Changes when updating to v0.5 from v0.4.x

#### Added

#### Changed

- Compile-time boolean indicators have been changed from `Val{true}`/`Val{false}`
  to `Trixi.True`/`Trixi.False`. This affects user code only if new equations
  with nonconservative terms are created. Change
  `Trixi.has_nonconservative_terms(::YourEquations) = Val{true}()` to
  `Trixi.has_nonconservative_terms(::YourEquations) = Trixi.True()`.
- The (non-exported) DGSEM function `split_form_kernel!` has been renamed to `flux_differencing_kernel!`
- Trixi.jl updated its dependency [P4est.jl](https://github.com/trixi-framework/P4est.jl/)
  from v0.3 to v0.4. The new bindings of the C library `p4est` have been
  generated using Clang.jl instead of CBinding.jl v0.9. This affects only user
  code that is interacting directly with `p4est`, e.g., because custom refinement
  functions have been passed to `p4est`. Please consult the
  [NEWS.md of P4est.jl](https://github.com/trixi-framework/P4est.jl/blob/main/NEWS.md)
  for further information.

#### Deprecated

- The signature of the `DGMultiMesh` constructors has changed - the `dg::DGMulti`
  argument now comes first.
- The undocumented and unused
  `DGMultiMesh(triangulateIO, rd::RefElemData{2, Tri}, boundary_dict::Dict{Symbol, Int})`
  constructor was removed.

#### Removed

- Everything deprecated in Trixi.jl v0.4.x has been removed.


## Changes in the v0.4 lifecycle

#### Added

- Implementation of linearized Euler equations in 2D
- Experimental support for upwind finite difference summation by parts (FDSBP)
  has been added in Trixi.jl v0.4.55. The first implementation requires a `TreeMesh` and comes
  with several examples in the `examples_dir()` of Trixi.jl.
- Experimental support for 2D parabolic diffusion terms has been added.
  * `LaplaceDiffusion2D` and `CompressibleNavierStokesDiffusion2D` can be used to add
  diffusion to systems. `LaplaceDiffusion2D` can be used to add scalar diffusion to each
  equation of a system, while `CompressibleNavierStokesDiffusion2D` can be used to add
  Navier-Stokes diffusion to `CompressibleEulerEquations2D`.
  * Parabolic boundary conditions can be imposed as well. For `LaplaceDiffusion2D`, both
  `Dirichlet` and `Neumann` conditions are supported. For `CompressibleNavierStokesDiffusion2D`,
  viscous no-slip velocity boundary conditions are supported, along with adiabatic and isothermal
  temperature boundary conditions. See the boundary condition container
  `BoundaryConditionNavierStokesWall` and boundary condition types `NoSlip`, `Adiabatic`, and
  `Isothermal` for more information.
  * `CompressibleNavierStokesDiffusion2D` can utilize both primitive variables (which are not
  guaranteed to provably dissipate entropy) and entropy variables (which provably dissipate
  entropy at the semi-discrete level).
  * Please check the `examples` directory for further information about the supported setups.
    Further documentation will be added later.
- Numerical fluxes `flux_shima_etal_turbo` and `flux_ranocha_turbo` that are
  equivalent to their non-`_turbo` counterparts but may enable specialized
  methods making use of SIMD instructions to increase runtime efficiency
- Support for (periodic and non-periodic) SBP operators of
  [SummationByPartsOperators.jl](https://github.com/ranocha/SummationByPartsOperators.jl)
  as approximation type in `DGMulti` solvers
- Initial support for MPI-based parallel simulations using non-conforming meshes of type `P4estMesh`
  in 2D and 3D including adaptive mesh refinement

#### Removed

- The `VertexMappedMesh` type is removed in favor of the `DGMultiMesh` type.
  The `VertexMappedMesh` constructor is deprecated.

#### Changed

- The required Julia version is updated to v1.7.
- The isentropic vortex setups contained a bug that was fixed in Trixi.jl v0.4.54.
  Moreover, the setup was made a bit more challenging. See
  https://github.com/trixi-framework/Trixi.jl/issues/1269 for further
  information.

#### Deprecated

- The `DGMultiMesh` constructor which uses a `rd::RefElemData` argument is deprecated in
  favor of the constructor which uses a `dg::DGMulti` argument instead.

## Changes when updating to v0.4 from v0.3.x

#### Added

- Experimental support for artificial neural network-based indicators for shock capturing and
  adaptive mesh refinement ([#632])
- Experimental support for direct-hybrid aeroacoustics simulations ([#712])
- Implementation of shallow water equations in 2D
- Experimental support for interactive visualization with [Makie.jl](https://makie.juliaplots.org/)

#### Changed

- Implementation of acoustic perturbation equations now uses the conservative form, i.e. the
  perturbed pressure `p_prime` has been replaced with `p_prime_scaled = p_prime / c_mean^2`.
- Removed the experimental `BoundaryConditionWall` and instead directly compute slip wall boundary
  condition flux term using the function `boundary_condition_slip_wall`.
- Renamed `advectionvelocity` in `LinearScalarAdvectionEquation` to `advection_velocity`.
- The signature of indicators used for adaptive mesh refinement (AMR) and shock capturing
  changed to generalize them to curved meshes.

#### Deprecated

#### Removed

- Many initial/boundary conditions and source terms for typical setups were
  moved from `Trixi/src` to the example elixirs `Trixi/examples`. Thus, they
  are no longer available when `using Trixi`, e.g., the initial condition
  for the Kelvin Helmholtz instability.
- Features deprecated in v0.3 were removed.


## Changes in the v0.3 lifecycle

#### Added

- Support for automatic differentiation, e.g. `jacobian_ad_forward`
- In-situ visualization and post hoc visualization with Plots.jl
- New systems of equations
  - multicomponent compressible Euler and MHD equations
  - acoustic perturbation equations
  - Lattice-Boltzmann equations
- Composable `FluxPlusDissipation` and `FluxLaxFriedrichs()`, `FluxHLL()` with adaptable
  wave speed estimates were added in [#493]
- New structured, curvilinear, conforming mesh type `StructuredMesh`
- New unstructured, curvilinear, conforming mesh type `UnstructuredMesh2D` in 2D
- New unstructured, curvilinear, adaptive (non-conforming) mesh type `P4estMesh` in 2D and 3D
- Experimental support for finite difference (FD) summation-by-parts (SBP) methods via
  [SummationByPartsOperators.jl](https://github.com/ranocha/SummationByPartsOperators.jl)
- New support for modal DG and SBP-DG methods on triangular and tetrahedral meshes via [StartUpDG.jl](https://github.com/jlchan/StartUpDG.jl)

#### Changed

- `flux_lax_friedrichs(u_ll, u_rr, orientation, equations::LatticeBoltzmannEquations2D)` and
  `flux_lax_friedrichs(u_ll, u_rr, orientation, equations::LatticeBoltzmannEquations3D)`
  were actually using the logic of `flux_godunov`. Thus, they were renamed accordingly
  in [#493]. This is considered a bugfix
  (released in Trixi.jl v0.3.22).
- The required Julia version is updated to v1.6.

#### Deprecated

- `calcflux` → `flux` ([#463])
- `flux_upwind` → `flux_godunov`
- `flux_hindenlang` → `flux_hindenlang_gassner`
- Providing the keyword argument `solution_variables` of `SaveSolutionCallback`
  as `Symbol` is deprecated in favor of using functions like `cons2cons` and
  `cons2prim`
- `varnames_cons(equations)` → `varnames(cons2cons, equations)`
- `varnames_prim(equations)` → `varnames(cons2prim, equations)`
- The old interface for nonconservative terms is deprecated. In particular, passing
  only a single two-point numerical flux for nonconservative is deprecated. The new
  interface is described in a tutorial. Now, a tuple of two numerical fluxes of the
  form `(conservative_flux, nonconservative_flux)` needs to be passed for
  nonconservative equations, see [#657].

#### Removed