Q: How is orthogonal quality calculated in CFD meshes?

Orthogonal quality is computed as the cosine of the angle θ between the face normal vector and the vector connecting adjacent cell centroids. A value of cos(θ) = 1 (θ = 0°) means perfect orthogonality. ANSYS Fluent requires orthogonal quality > 0.1 (minimum), with values < 0.2 flagged for remeshing. OpenFOAM's checkMesh reports non-orthogonality angle directly, recommending maximum angles below 70°.

Q: What are the mesh quality differences between OpenFOAM, Fluent, and Star-CCM+?

While all three solvers share similar underlying mesh quality concepts, their tolerances differ due to numerical scheme implementations. OpenFOAM uses non-orthogonal correction schemes allowing up to ~70° non-orthogonality. ANSYS Fluent requires skewness 0.1. Star-CCM+ uses its own quality metrics but similarly targets skewness < 0.85. Always check mesh quality using the solver's built-in diagnostic tools: checkMesh (OpenFOAM), Mesh Quality dialog (Fluent), or Mesh Diagnostics (Star-CCM+).

Question 1

What is mesh skewness in CFD and why does it matter?

Accepted Answer

Mesh skewness measures how much a cell deviates from an ideal equilateral shape. The equiangle skewness formula is S = (θmax − θequi) / (180 − θequi), where 0 is perfect and 1 is degenerate. High skewness causes numerical diffusion, poor convergence, and inaccurate results, especially in regions with strong gradients. OpenFOAM and ANSYS Fluent both recommend keeping skewness below 0.85 for reliable simulations.

Question 2

What aspect ratio is acceptable for CFD meshes?

Accepted Answer

For general internal flow regions, an aspect ratio (AR = Lmax/Lmin) below 5 is ideal and below 100 is generally acceptable. In boundary layer regions, high-AR cells aligned with the flow (AR up to 1000) are intentionally used to resolve near-wall gradients efficiently. The key is that the high-AR direction must be aligned with the flow direction, where gradients are small. ANSYS Fluent recommends AR < 100 for interior cells.

Question 3

How is orthogonal quality calculated in CFD meshes?

Accepted Answer

Orthogonal quality is computed as the cosine of the angle θ between the face normal vector and the vector connecting adjacent cell centroids. A value of cos(θ) = 1 (θ = 0°) means perfect orthogonality. ANSYS Fluent requires orthogonal quality > 0.1 (minimum), with values < 0.2 flagged for remeshing. OpenFOAM's checkMesh reports non-orthogonality angle directly, recommending maximum angles below 70°.

Question 4

What are the mesh quality differences between OpenFOAM, Fluent, and Star-CCM+?

Accepted Answer

While all three solvers share similar underlying mesh quality concepts, their tolerances differ due to numerical scheme implementations. OpenFOAM uses non-orthogonal correction schemes allowing up to ~70° non-orthogonality. ANSYS Fluent requires skewness < 0.85 and orthogonal quality > 0.1. Star-CCM+ uses its own quality metrics but similarly targets skewness < 0.85. Always check mesh quality using the solver's built-in diagnostic tools: checkMesh (OpenFOAM), Mesh Quality dialog (Fluent), or Mesh Diagnostics (Star-CCM+).

Metric	OpenFOAM	Fluent	Star-CCM+
Skewness	<0.85	<0.85	<0.85
Aspect Ratio	<1000	<100	<1000
Non-orth. [°]	<70	—	<85
Orth. Quality	>0.01	>0.1	>0.1
Smoothness	<1.2	<1.2	<1.15
Expansion	<1.3	<1.2	<1.2

CFD Mesh Quality Metrics Calculator

Quality Radar Chart

Acceptable Ranges by Software

Theory