2D_NS_Solver

2D compressible Euler / Navier–Stokes finite-volume solver on body-fitted structured grids. This repository is mid-refactor: the original single-binary C++98 code (src/*.cpp legacy tree, target ns_legacy → binary 2D_NS_Solver) is being rebuilt as a modern C++23 stack (targets ns_core, ns_solver, ns_config) with TOML configuration, Google Test coverage, and optional MPI parallelization. See REFACTORING_PLAN.md for the full roadmap, audit findings, and per-phase verification gates.

Status

Phases 0–7 complete: audit & bug fixes, CMake+GTest scaffolding, TOML config layer, ghost-aware fields/mesh/IO infrastructure, physics + inviscid scheme ports with bit-for-bit legacy parity goldens, single-rank end-to-end driver, and j-slab MPI decomposition.

Quick summary of what runs today:

Target	What it is
ns_legacy / 2D_NS_Solver	Frozen pre-refactor solver (parity reference)
ns_core	Fields, mesh/metrics, numerics, physics, IO, MPI library
ns_config	Typed TOML config loader + legacy .cfg importer
ns_solver	New driver: ns_solver <config.toml> <grid.dat> <out-dir> [iters] [--restart file]
cfg2toml	Converts legacy Input*.cfg + GridTop*.dat pairs to canonical TOML

Building

cmake --preset release          # or: debug / asan
cmake --build --preset release -j
ctest --preset release          # unit + parity tests

MPI flavour (j-slab domain decomposition):

cmake -S . -B build-mpi -D CMAKE_BUILD_TYPE=Release -D NS_ENABLE_MPI=ON
cmake --build build-mpi -j
mpirun -np 4 build-mpi/ns_solver configs/RampEuler.toml \
    input/RampGrid24080.dat out-dir 200

NS_ENABLE_NATIVE_ARCH=ON adds -march=native; note this enables FMA contraction and shifts last-bit results — parity gates must run without it.

Configuration

Canonical schema lives in configs/*.toml (migrated from input/Input*.cfg by scripts/migrate_configs.sh). Boundary segments keep the legacy node-index convention; edges are named imin/imax/jmin/jmax. Restart:

ns_solver cfg.toml grid.dat out 100            # writes nothing special
ns_solver cfg.toml grid.dat out 10 --restart checkpoint.rst   # continue

Checkpoints are written by adding solver.write_restart(path) calls (see src/solver/euler_solver.hpp).

Testing & parity

• tests/ — 66 GTest cases: math utilities, config load/validation, legacy import round-trips, fields/ghost semantics, metrics vs hand-computed geometry, restart bit-exactness, kernel-level bitwise golden parity with the legacy solver (physics, LLF/Roe/NKFDS-MOVERS chains), MPI halo exchange.
• scripts/capture_regression.sh / compare_regression.sh — full-file regression baselines against the frozen legacy binary.
• scripts/check_e2e_parity.sh — serial new-vs-legacy trajectory check.
• scripts/check_mpi_parity.sh [iters] — np=1/2/4 vs legacy.
• scripts/bench.sh [iters] — timings: legacy vs new serial vs MPI.

Known numerical caveat (documented in REFACTORING_PLAN.md §6): the legacy NKFDS-MOVERS scheme quantizes wave speeds to 3 decimals; sub-roundoff codegen differences therefore amplify to ~1e-4 relative field deviations over long runs. Kernel-level parity gates remain bitwise.

Layout

src/config/    TOML loading, validation, legacy importer
src/core/      error type
src/fields/    Field<T>/MultiField<T>: contiguous, ghost-aware, View2D
src/mesh/      StructuredMesh, legacy grid reader (incl. slab loader), metrics
src/numerics/  face flux data, LLF, NKFDS-MOVERS, Roe schemes
src/physics/   IdealGas EOS, Sutherland transport, dependent variables,
               Green-Gauss face gradients, viscous flux
src/io/        VTK writer, binary restart, (legacy-format writers in solver)
src/parallel/  SlabDecomposition, HaloExchange
src/solver/    EulerSolver driver (serial + MPI)
tools/cfg2toml config migration tool
legacy refs:   src/*.cpp (frozen), input/*.cfg|*.dat (cases)

Scripts

Script	Purpose
scripts/migrate_configs.sh	Convert all convertible legacy inputs to TOML
scripts/capture_regression.sh	Capture golden outputs from legacy binary
scripts/check_e2e_parity.sh	Serial new-vs-legacy comparison
scripts/check_mpi_parity.sh	np=1/2/4 vs legacy residuals
scripts/bench.sh	Timing comparison across implementations