In CFD simulation, how to handle warped faces in mesh ?

In CFD simulations, warped faces (non-planar faces) in a mesh can lead to inaccuracies in the solution, convergence issues, or even solver failures. Here's how to handle them:

1. Understanding Warped Faces

• A warped face is a face where the vertices do not lie on the same plane (common in quad/hex elements or high-curvature regions).
• Solvers often assume planar faces for flux calculations, so warped faces can introduce errors.

2. How to Detect Warped Faces

• Most meshing tools (ANSYS Fluent, OpenFOAM, STAR-CCM+, etc.) provide mesh quality metrics, including:
  • Warpage angle (angle between face normals at different points).
  • Skewness (deviation from ideal shape).
  • Face planarity (distance from face centroid to vertices).
• Tools like Check Mesh in OpenFOAM (checkMesh) or Mesh Quality in Fluent can identify warped faces.

3. How to Fix Warped Faces

A. Preprocessing (Before Simulation)

1. Remesh the Problematic Region

   • Use triangles/tets instead of quads/hexes in highly curved regions (triangles are always planar).
   • Apply local refinement to reduce face warpage.
   • Use pave/biased meshing near curved surfaces.

2. Splitting Quads/Hexes into Triangles/Tets

   • Many solvers (e.g., Fluent) allow converting quads → tris (improves planarity).
   • In OpenFOAM, use snappyHexMesh with splitHexWarpedFaces true.

3. Adjust Mesh Smoothing & Optimization

   • Use Laplacian smoothing (but avoid excessive distortion).
   • Apply mesh optimization (ANSYS Fluent's "Improve Mesh" tool).

4. Use High-Order Elements (if supported)

   • Some solvers (e.g., STAR-CCM+) support quadratic elements, which better capture curvature.

B. Solver-Specific Adjustments

1. ANSYS Fluent

   • Enable "Warped Face Gradient Correction" (in Solution → Methods → Spatial Discretization).
   • Use "Node-based gradient" instead of cell-based for better accuracy.
   • Try "High-Order Term Relaxation" for better convergence.

2. OpenFOAM

   • Use adjustPhi to correct face flux imbalances due to warpage.
   • Enable correctWarpedFaces in snappyHexMeshDict.
   • Consider leastSquares gradient scheme for better stability.

3. STAR-CCM+

   • Enable "Aspect Ratio Control" in meshing.
   • Use "Polyhedral Mesh" for complex geometries (avoids quad/hex warping).

4. Post-Processing Checks

• Monitor residuals and mass imbalance (warped faces often cause flux errors).
• Check local velocity/pressure anomalies near warped regions.

5. When to Ignore Minor Warping?

• If warping is < 5° (some solvers tolerate slight non-planarity).
• If the solution is converged & mass imbalance is < 1%.

Conclusion

• Best practice: Avoid warped faces during meshing (use tris/tets in curved zones).
• If unavoidable: Use solver corrections (gradient schemes, warped face corrections).
• For critical cases: Remesh with finer resolution or polyhedral elements.