ANSYS Fluent Meshing & Post-Processing

#+CATEGORY: ansys

ANSYS Fluent Meshing & Post-Processing :: Meshing Approaches, Case Setup, and Visualization

ANSYS provides several meshing tools for Fluent. Each targets specific geometry complexity and meshing requirements. OpenFOAM uses external meshing tools (snappyHexMesh, Gmsh, Pointwise). ANSYS has a more self-contained ecosystem. The meshing tools require licenses. This note covers the meshing ecosystem, Fluent's mesh handling, case setup workflow, and post-processing approaches.

ANSYS Meshing Ecosystem

Modern Fluent workflow emphasizes the "Watertight Geometry workflow" for most applications: 1. Import CAD (STEP, IGES, Parasolid, or native SolidWorks/NX) 2. Clean geometry in SpaceClaim (identify regions of interest, remove small features) 3. Generate hybrid mesh in ANSYS Meshing (tetrahedral core + prismatic boundary layers) 4. Convert or export mesh to Fluent format 5. Run Fluent

Mesh Types in Fluent

Fluent supports:

• Hexahedral: the highest quality, most efficient cell type
• Tetrahedral: robust for complex geometry
• Polyhedral: automatically generated from tet/hex, typically 50-70% fewer cells than tet with better accuracy
• Prism/Wedge: for boundary layer resolution
• Hybrid: combination of cell types (typically poly/tet with prism layers)

Polyhedral mesh is the standard workflow: 1. Generate a tetrahedral + prism layer mesh 2. Convert to polyhedral (Fluent can do this automatically with one click) 3. The polyhedral conversion does three things:

• Merge adjacent tet cells into polyhedral cells
• Reduce face count per cell from ~12-20 (polyhedrons) vs ~4 (tetrahedrons)
• Reduce total cell count by 50-70% compared to tet
• Improve numerical accuracy (better gradient reconstruction, less spurious diffusion)

Mesh Generation in Fluent

Fluent's built-in mesher (Fluent Meshing, recently enhanced) supports:

• Surface Mesher: triangulates surfaces for flow simulation
• Volume Mesher: generates tetrahedral or polyhedral volume mesh
• Prism Mesher: adds prismatic boundary layers
• Octree Mesher: generates unstructured meshes (tet/hex/poly) on an octree grid
• Hex-dominant mesher: creates hexahedral-dominant meshes for simple geometries

The Octree mesher works well for complex geometries. Traditional structured methods struggle with them. The tool produces high-quality hybrid tet/hex/poly meshes.

Mesh Quality Requirements (ANSYS/Fluent)

Fluent's mesh quality metrics use Orthogonal Quality (0 to 1, where 1 is perfectly orthogonal):

For LES/DNS in Fluent:

• Orthogonal quality > 0.9 (ideally > 0.95)
• Non-orthogonality correction enabled in Fluent solver
• Aspect ratio < 10

Fluent Case Setup Workflow (GUI-based)

Fluent's GUI setup follows a step-by-step physics-driven process:

1. File → Read → Mesh: Import the mesh file 2. Check: Verify mesh quality (the mesh check prints min/max cell volume, skewness, etc.) 3. General → Solver: Select the solver type (pressure-based or density-based), absolute/relative pressure, gravity, time 4. Materials: Define fluid/solid materials (pre-defined or user-defined) 5. Cell Zone Conditions: Assign materials and models to cells (e.g., turbulence, combustion) 6. Boundary Conditions: Set inlet, outlet, wall, symmetry conditions (same concept as OpenFOAM) 7. Operating Conditions: Set operating pressure, gravity, Mach number reference 8. Models: Enable physical models (viscous/turbulence, energy, species, radiation, multiphase) 9. Solution Methods: Select discretization schemes 10. Solution Controls: Set under-relaxation factors, convergence criteria 11. Initialization: Choose standard/fixed-value initialization 12. Run Calculation: Start the solver (steady or transient)

Discretization Schemes (Fluent)

Match OpenFOAM's discretization philosophy:

• Momentum: Second-order upwind (standard)
• Pressure: PRESTO! (pressure staggering) or Second-order
• Turbulence: First or Second-order upwind
• Energy: Second-order upwind
• Species: First or Second-order upwind
• Radiation: Second-order upwind (when enabled)
• Discrete Phase: Second-order (when available)

Fluent Parallel Computing

Fluent parallel execution uses MPI. The mesh gets decomposed by the parallel mesher or by Fluent's built-in decomposition:

Fluent parallel setup: 1. Start Fluent in parallel mode: `fluent 3d -t16 -cnf=hosts` or via Workbench 2. Fluent decomposes the mesh automatically for parallel execution 3. Each core gets a subdomain. The code exchanges data at processor interfaces 4. Graph-based decomposition balances cells + boundary faces for load balancing

Users can enable GPU acceleration in Fluent:

• Fluent GPU: accelerated linear solver (available in recent Fluent versions)
• CUDA-compatible GPU cards: for pressure and momentum solves
• Speedup: 3-5× reported for GPU vs CPU

ANSYS Fluent Post-Processing

Built-in Fluent post-processing:

• Field visualization: contours, vectors, streamlines, pathlines, particle tracks, animations
• Data sampling: point, line, plane, surface, volume probes
• Calculated quantities: area-weighted average, mass-weighted average, integral (area, volume, surface)
• Custom field functions: user-defined computed fields
• Report definitions: force, moment, pressure, mass flow rate, area average, minimum/maximum

ANSYS Data Processing supports these post-processing steps: 1. Field Function Creation → compute derived quantities on-the-fly 2. Expressions → parameterized expressions for force, lift, drag coefficients, etc. 3. Report Definitions → automated monitoring of key quantities 4. Surface/Line Probes → data extraction at points, lines, or planes 5. Animation → time-dependent video generation 6. Export → export data and images to various formats (CSV, VTK, Tecplot, Ensight, CGNS, PNG, JPG)

Use ANSYS Workbench environment for post-processing:

• CFX-Post or Academic Research (Fluent-compatible post-processors)
• ACT (Automation Tool for CFD Post Processing) → scriptable post-processing via Python
• PyFluent → Python scripting interface for Fluent automation and post-processing

Post-Processing Comparison (Fluent vs OpenFOAM)

Summary

This summary covers the key points:

1. ANSYS meshing tools (SpaceClaim, ANSYS Meshing, Fluent Meshing) provide an integrated workflow. OpenFOAM uses external meshing. 2. Polyhedral meshing cuts cell count by 50-70% vs tet with better accuracy 3. Fluent parallel uses MPI with built-in graph-based decomposition 4. Fluent includes robust built-in post-processing (expressions, report definitions, probe points) 5. ANSYS provides Python scripting (ACT, PyFluent) for automation and post-processing 6. Fluent GPU acceleration delivers 3-5x speedup for supported solvers

See Also

• ANSYS Fluent Overview
• OpenFOAM Mesh Generation
• OpenFOAM Post-Processing
• OpenFOAM Parallel Computing