ANSYS vs OpenFOAM Comparison
#+CATEGORY: ansys
ANSYS Fluent vs OpenFOAM :: Feature Comparison for Industrial and Academic CFD
This note compares ANSYS Fluent and OpenFOAM side by side. It covers the dimensions that matter most to practical CFD users: solver capabilities, turbulence models, multiphase flows, combustion, meshing, workflow, performance, and licensing. The comparison helps researchers and engineers choose the right tool. It fits their application, constraints, and resources.
The core difference is philosophical. Fluent is a fully integrated commercial product with extensive support. OpenFOAM is an open-source framework for extensibility and customization.
Solver Architectures
| Feature | ANSYS Fluent | OpenFOAM |
| --------- | ------------- | ---------- |
| Solver types | Pressure-based (segregated/coupled), Density-based (implicit/explicit) | Pressure-based (SIMPLE/PISO/PIMPLE), Density-based (rhoFoam family) |
| Pressure velocity coupling | SIMPLE, SIMPLEC, PISO, Coupled | SIMPLE, PISO, PIMPLE |
| Compressible solvers | Density-based (implicit/explicit) | rhoSimpleFoam, rhoPimpleFoam, sonicFoam (each separate solver) |
| Incompressible solvers | segregated or coupled pressure-based | simpleFoam, pimpleFoam, pisoFoam, icoFoam (each separate solver) |
| Implicit/Explicit modes | Both (pressure-based is implicit; density-based can be explicit) | Mostly implicit (some explicit in specialized solvers) |
| Preconditioning | Pressure-based with low-Mach preconditioning | Limited (some compressible solvers, not standard) |
| Riemann solvers | AUSM+, Roe, HLLC, JST (density-based) | Limited (only in some compressible solvers) |
Fluent's coupled pressure-based solver solves U and p simultaneously in a single matrix system. This speeds up high-speed compressible and highly coupled problems. OpenFOAM uses a segregated approach. It solves U, then p, then corrects U. This uses less memory and parallelizes more easily. It may require more iterations for convergence.
Turbulence Models
| Model | Fluent | OpenFOAM | Notes |
| ------- | -------- | ---------- | ------- |
| Spalart-Allmaras | ✓ (RANS) | ✓ (RASModel) | Fluent has better wall function integration |
| Standard k-epsilon | ✓ | ✓ | Fluent RNG option available |
| RNG k-epsilon | ✓ | ✓ | Fluent has better RNG implementation (derived from RNG theory, not just a modified coefficient) |
| Realizable k-epsilon | ✓ | ✓ | Fluent's implementation is considered more robust for industrial cases |
| k-omega SST (baseline) | ✓ (default RANS) | ✓ (default RANS) | OpenFOAM's SST has additional blending options |
| k-omega SST (IDDES) | ✓ | ✓ | Fluent has IDDES variant; OpenFOAM has IDDES in OpenFOAM-v2006+ |
| Reynolds Stress Model (RSM) | ✓ | ✓ | Fluent supports all six Reynolds stress components |
| v2-f | ✓ | ✗ | Fluent-only (v2-f model not available by default in OpenFOAM) |
| LES | ✓ (Smag, DynSmag, WALE, Vreman) | ✓ (Smag, DynSmag, WALE, Vreman) | Fluent uses filtered Navier-Stokes; OpenFOAM uses filtered Navier-Stokes in pisoFoam/pimpleFoam |
| DES / DDES | ✓ (SST-DES, SST-DDES, SST-IDDES) | ✓ (SST-DES, SST-DDES, SST-IDDES) | Fluent's IDDES offers improved near-wall modeling |
| SAS | ✓ (SSTSAS) | ✓ (kOmegaSSTSAS) | Scale-Adaptive Simulation |
Fluent includes the v2-f model. OpenFOAM does not. The v2-f model improves near-wall treatment for attached flows with strong curvature. Fluent has a unique turbulence option here.
Fluent's turbulence models have extensive industrial validation. The solver includes recommended y+ ranges and practical guidance (via wall functions or low-Re treatments). OpenFOAM needs manual configuration. The user selects wall treatment (wall functions vs low-Re) through boundary conditions.
Multiphase Flows
| Model | Fluent | OpenFOAM | Notes |
| ------- | -------- | ---------- | ------- |
| VOF | ✓ (GeoReconstruct, CICSAM, Compressive) | ✓ (MULES, hrIC, geoReconstruct) | Fluent has geometric reconstruction built-in; OpenFOAM requires interFoam or similar |
| Level Set | ✓ | ✗ | Fluent-only |
| Mixture | ✓ | ✗ | Fluent-only (not standard in OpenFOAM) |
| Eulerian-Eulerian | ✓ | ✓ (twoPhaseEulerFoam) | Fluent supports up to 5 phases; OpenFOAM supports multiple (no hard limit) |
| DPM (Eulerian-Lagrangian) | ✓ | ✓ (DPMFoam) | Fluent's DPM has more advanced injection types and collision models |
| Sprays | ✓ (DPM with evaporation) | ✓ (sprayFoam) | Fluent's DPM has more advanced liquid breakup and evaporation models |
| Population Balance Model (PBM) | ✓ | ✗ | Fluent-only (not available in OpenFOAM by default) |
| DEM (Discrete Element Method) | ✓ (coupled with DPM) | ✗ | Fluent-only (not available in OpenFOAM) |
| Eulerian-Lagrangian with FSI | ✓ | ✗ | Fluent-only |
Fluent's exclusive multiphase features (v2-f, PBM, DEM coupling) provide capabilities that OpenFOAM lacks natively. OpenFOAM's multiphase framework is highly extensible. MULES handles VOF. Custom models exist for other needs. The community has developed custom models for PBM, DEM, and other advanced multiphase scenarios.
Combustion
| Model | Fluent | OpenFOAM | Notes |
| ------- | -------- | ---------- | ------- |
| Non-Premixed (PDF) | ✓ | ✗ (custom) | Fluent's PDF is the most widely used non-premixed model |
| Flamelet | ✓ | ✓ (flameWP, flameletModels) | Fluent's flamelet model is more mature (FPV) |
| Eddy Dissipation (EDM) | ✓ | ✓ | Standard both platforms |
| Eddy Dissipation Concept (EDC) | ✓ | ✓ (edc) | Fluent has better EDC implementation |
| Partially-premixed | ✓ | ✓ (partiallyPremixedFoam) | OpenFOAM has separate solver; Fluent has multiphase approach |
| Detailed Chemistry | ✓ (CHEMKIN, Cantera, Chemkin-Pro) | ✓ (thermodynamic and kinetic packages) | Fluent has better integration with detailed chemistry solvers |
| LES Combustion | ✓ | ✓ | Both support filtered Navier-Stokes |
| Radiation (coupled) | ✓ (P-1, DO, S2S, FLRG) | ✓ (DoD, S2S) | Fluent has more radiation models |
Meshing
| Feature | Fluent (ANSYS Meshing) | snappyHexMesh |
| --------- | ---------------------- | --------------- |
| CAD import | Native (STEP, IGES, Parasolid, SolidWorks, NX, Creo) | STL only |
| Hex-dominant meshing | ✓ (Fluent Meshing hex-dominant, ICEM CFD) | Limited (blockMesh for background, snappyHexMesh for hex-dominant) |
| Automatic prism layers | ✓ | ✓ (snappyHexMesh addLayers) |
| Automatic polyhedral conversion | ✓ (one-click) | ✗ (user must develop or use third-party tool) |
| Adaptivity | ✓ (solution-adaptive refinement) | ✗ (static mesh only, no automatic refinement on solution) |
| Advanced wall treatment | ✓ (sizing, body of influence, inflation) | Limited (manual level-based refinement) |
| Surface mesh quality control | ✓ (SpaceClaim) | Manual |
| Automatic surface cleanup | ✓ (Watertight workflow) | Manual (mesh quality depends on STL quality) |
| Mesh generation automation | ✓ (highly automated for industrial geometry) | Lower (snappyHexMesh requires manual tweaking) |
| Octree mesher | ✓ (Cartesian-based) | ✗ |
Fluent's meshing is more automated and integrated than snappyHexMesh. Industrial applications with complex CAD geometry (automotive, aerospace, HVAC) run faster in ANSYS Meshing. It produces higher-quality meshes with less user intervention. OpenFOAM's snappyHexMesh offers fine-grained control. Users can adjust refinement levels, prism layer thickness, and adaptive refinement.
Post-Processing
| Feature | Fluent | OpenFOAM (ParaView) |
| --------- | -------- | ------------------- |
| Built-in | ✓ | ✗ (external) |
| Field visualization | ✓ (native GUI) | ✓ (ParaView native reader) |
| Streamlines/Pathlines | ✓ | ✓ |
| Custom field functions | ✓ (built-in expression language) | ✓ (Python, C++ function objects) |
| Report definitions | ✓ (automated force, lift, drag, etc.) | ✓ (Python, function objects) |
| Probe points | ✓ | ✓ (ParaView) |
| Parameterized post-processing | ✓ (Expressions, Report Definitions) | ✓ (Python scripting, ParaView) |
| Automation | ✓ (journal files, ACT Python) | ✓ (postProcess, Python (pythonFoam, foamlib)) |
| GPU accelerated visualization | ✗ (ParaView has limited GPU) | ✓ (ParaView) |
| Scripting interface | ✓ (journal files, Python (PyFluent)) | ✓ (Python (pythonFoam, foamlib)) |
Fluent's post-processing is more integrated and user-friendly for engineers. Its Expression language lets users compute custom fields without external tools. ParaView excels at advanced visualization and script-based analysis.
Mesh Quality and Validation
| Metric | Fluent | OpenFOAM |
| -------- | -------- | ---------- |
| Mesh check tools | ✓ (built-in "mesh check") | ✓ (checkMesh utility) |
| Orthogonal quality (0-1) | ✓ | Skewness (0-1) |
| Non-orthogonality correction | ✓ (built-in) | ✓ (fvSchemes: corrected) |
| Mesh validation | ✓ (mesh check, mesh statistics) | ✓ (checkMesh -lint) |
| Mesh quality export | ✓ | ✓ (checkMesh, meshQuality utility) |
Computational Performance
| Metric | Fluent | OpenFOAM |
| -------- | -------- | ---------- |
| Parallel scaling | Good (up to ~2000 cores for RANS) | Good (up to ~10000 cores for large RANS cases) |
| GPU acceleration | ✓ (Fluent GPU for linear solvers) | ✗ (GPU solvers in development; limited production use) |
| Memory efficiency | Good (especially separated solver) | Good (segregated solvers are memory-efficient) |
| Solver convergence | Generally smoother, more robust | Can require tuning (turbulence models, under-relaxation) |
| Runtime optimization | Commercial optimization, vendor support | Community-developed, user-tuned |
OpenFOAM runs faster on parallel clusters with 10⁴+ cores. Its distributed linear solvers scale well. Fluent's commercial solver is faster on moderate parallel runs (100-1000 cores). Vendor optimization and aggressive parallel optimizations drive this.
Licensing, Cost, and Support
| Feature | Fluent (ANSYS) | OpenFOAM |
| --------- | --------------- | ---------- |
| License cost | ~$10,000-20,000+ (perpetual or subscription), varies by modules | Free (open-source, LGPL/GPL) |
| User count | Typically per-core or floating license | Unlimited (open-source) |
| Support | Vendor support (ANSYS) | Community support (OpenFOAM Foundation, ESI/CE, CFD Direct, etc.) |
| Training | Formal training available (ANSYS University, partner training) | Informal (CFD Online, YouTube, CFD University) |
| Updates | Regular updates with new features (ANSYS releases) | Community-driven development |
Use Case Recommendations
| Use Case | Recommended Tool |
| ---------- | ----------------- |
| Industrial RANS for HVAC, automotive, aerospace | Fluent (integrated workflow, validation) |
| Complex multiphase with PBM/DEM | Fluent (PBM, DEM coupling) |
| Research requiring custom physics/modifications | OpenFOAM (source code access, easy modification) |
| DNS/LES with custom numerical schemes | OpenFOAM (FVM with custom discretization) |
| Rapid prototyping with complex CAD | Fluent (integrated CAD-to-mesh) |
| High-fidelity simulation (turbulence modeling) | Both (Fluent has more validated models) |
| Multi-physics coupling (FSI, thermo-structural) | Fluent (ANSYS Workbench coupling) |
| Academic research with limited budget | OpenFOAM (free, no license costs) |
| Custom combustion models, plasma, microfluidics | OpenFOAM (easy to implement custom models) |
Summary
1. Fluent: integrated commercial product with extensive validation, excellent CAD handling, better wall treatment, and vendor support 2. OpenFOAM: open-source framework with unlimited customization, free licensing, better for research requiring novel physics or numerical methods 3. Both platforms cover the core CFD physics (Navier-Stokes, turbulence, combustion, multiphase) comprehensively. 4. The choice depends on: budget, required features, customization needs, team expertise, and industry vs academic context 5. Fluent's exclusive features (PBM, DEM, v2-f, Level Set) are not available in OpenFOAM by default 6. OpenFOAM's extensibility is its greatest strength for research. Users can modify every line of source code
See Also
- ANSYS Fluent Overview
- Governing Equations
- OpenFOAM Solver Selection
- ANSYS Turbulence Models
- OpenFOAM Multiphase Flows
- ANSYS Multiphase Features