CBS3D++ is a modern C++20 finite-element solver for three-dimensional incompressible fluid dynamics. It is based on the semi-implicit Characteristic-Based Split (CBS) formulation and is being developed for research-scale laminar flow, turbulence modelling, conjugate heat transfer and broader multiphysics applications.
The project modernises a legacy Fortran CBSFlow development while preserving the numerical method, finite-element conventions and benchmark lineage. The new implementation introduces modular C++ architecture, reproducible build systems, OpenMP acceleration, PETSc pressure-solver integration, distributed-mesh infrastructure and modern VTU/PVD output.
Important
The validated production path is currently the serial/PETSc or OpenMP shared-memory solver. Distributed mesh ownership, halo exchange and MPI solver integration are under active development. Fully distributed numerical acceleration is not yet claimed as a released capability.
| Capability | Status |
|---|---|
| Three-dimensional incompressible Navier–Stokes equations | Available |
| Semi-implicit four-stage CBS velocity-correction scheme | Available |
| Linear P1 tetrahedral finite elements | Available |
| Laminar steady/pseudo-transient flow | Available |
| Conjugate heat transfer and scalar energy transport | Development path available |
| Spalart–Allmaras turbulence model | Active development and validation |
| Native matrix-free preconditioned conjugate gradient | Available |
| PETSc KSP/HYPRE pressure solve | Available in the current serial/OpenMP path |
| OpenMP element assembly | Available |
| MPI mesh distribution, ownership and halo infrastructure | Active development |
| Fully distributed CBS Steps 1–4 | Not yet production-ready |
| ParaView VTU/PVD output and residual histories | Available |
Detailed limitations and development gates are recorded in the parallelisation status and the internal solver audit under docs/.
The active solver uses a pressure-projection form of the Characteristic-Based Split method:
- Momentum predictor: compute a pressure-free intermediate velocity.
- Pressure solve: assemble and solve the pressure-Poisson system.
- Velocity correction: apply the pressure-gradient correction.
- Energy/scalar update: advance temperature or the active transported scalar.
The spatial discretisation uses linear tetrahedral elements with element-wise constant shape-function gradients and lumped nodal mass terms. See the numerical formulation for the governing equations, discrete assumptions and implementation scope.
- C++20 compiler
- CMake 3.16 or newer, or GNU Make
- OpenMP implementation for threaded builds
- MPI implementation for the distributed-development build
- PETSc with a compatible compiler/MPI toolchain for the PETSc pressure path
git clone https://github.com/prajwal1798/CBS-3D.git
cd CBS-3D
cmake -S . -B build \
-DCMAKE_BUILD_TYPE=Release \
-DCBS3D_ENABLE_OPENMP=ON \
-DCBS3D_ENABLE_MPI=OFF \
-DCBS3D_ENABLE_PETSC=OFF
cmake --build build -jmake USE_PETSC=1 USE_MPI=0 USE_OPENMP=1 \
PETSC_DIR="$PETSC_DIR" -jThe generated executable name depends on the selected build configuration. See installation for the supported variants and compatibility rules.
A case is identified by a common base name. Depending on the selected physics, the solver reads:
<case>.plt
<case>.bco
<case>.par
<case>.material # material-enabled cases
<case>.matprop # material-enabled cases
Typical execution:
./cbs3dpp_si path/to/case_namePETSc-enabled execution:
./cbs3dpp_si_petsc path/to/case_name \
-pc_type hypre \
-pc_hypre_type boomeramg \
-ksp_converged_reasonStart with the quick-start guide. The complete file contract is described in case files.
CBS3D++ validation is organised into four evidence classes:
- Legacy regression: C++ results are compared against the corresponding legacy Fortran implementation on matched meshes, parameters and boundary conditions.
- Independent benchmarks: results are compared with analytical, experimental or published reference data.
- Discretisation verification: mesh, time-step and solver-tolerance sensitivity are quantified.
- Parallel verification: distributed results must reproduce the accepted serial solution before scalability is reported.
Current and planned cases include:
- lid-driven cavity;
- internal/channel flow;
- flow around a cylinder;
- conjugate heat-transfer configurations;
- Spalart–Allmaras flat-plate boundary layer;
- strong- and weak-scaling studies after the distributed solver is complete.
See the validation programme and legacy-regression protocol.
CBS3D++ is a new C++ implementation derived from the numerical and operational lineage of the legacy CBSFlow 3D finite-element solver. The public Zeta Computational Resources material describes CBSFlow 3D as an incompressible Navier–Stokes finite-element solver using characteristic-based split convection stabilisation and a fractional-step solution procedure.
This repository does not treat the legacy source as redistributable material. Zeta states that its source code is copyrighted and may not be redistributed without written permission. The public project therefore documents provenance and numerical equivalence while keeping licensing boundaries explicit.
- CBSFlow 3D description
- Zeta software terms
- Zeta 3D benchmark data
- O. C. Zienkiewicz, R. L. Taylor and P. Nithiarasu, The Finite Element Method for Fluid Dynamics, 7th edition.
The full provenance statement is maintained in legacy provenance.
include/cbs/ Public solver headers
src/ Active C++ implementation
input/ Case definitions and development inputs
tools/ Pre-processing, monitoring and scaling utilities
docs/ Numerical, user, validation and developer documentation
.github/ Continuous integration and repository templates
The documentation is intentionally separated from this landing page:
- Documentation index
- Installation
- Quick start
- Case-file reference
- Numerical formulation
- Parallelisation status
- Validation programme
- Legacy provenance
- Sunbird HPC workflow
- Troubleshooting
The same Markdown tree is configured for a future MkDocs documentation website.
Development contributions should preserve numerical behaviour, document changed equations and assumptions, and include an appropriate regression case. Read CONTRIBUTING.md before opening a pull request.
Citation metadata is supplied in CITATION.cff. A permanent release DOI will be added when the first public research release is archived.
An explicit open-source licence has not yet been selected. Until a LICENSE file is added, copyright law applies and no general permission to copy, modify or redistribute this repository is granted. This will be resolved before the first formal public release.