BPS5234A Chap.6 Grid Generation and Mesh Quality Control
Grid Generation and Mesh Quality Control
What a mesh is for
Grid generation breaks the domain into cells, and on each cell the conservation laws turn into algebraic equations. The choice of grid affects accuracy, stability, convergence and cost, and interacts with turbulence and heat transfer modelling.
It is also the main driver of how long a study takes to run.
Choosing a mesh family
Structured meshes are regular and indexed, add little artificial diffusion when their lines run with the flow, and allow precise control of near-wall spacing, but are laborious for complex urban geometry. Unstructured meshes conform to any shape and allow targeted refinement.
Hybrid meshes combine prism layers on walls with tetrahedral or polyhedral cells in the core and are standard practice for building and urban work. Local refinement, solution adaption, overset grids and moving meshes place resolution where gradients are steep or let parts of the geometry move.
Measuring quality
Cell quality is measured by skewness, orthogonality, aspect ratio and growth between neighbours.
The course gives bands for skewness from excellent to degenerate, limits of 0.85 for quadrilaterals, hexahedra and triangles and 0.9 for tetrahedra, Fluent’s recommendation of minimum orthogonal quality above 0.1 and maximum skewness below 0.95, and a growth ratio of at most 1.2 between adjacent cells.
The hidden cost of a poor mesh
Numerical viscosity is artificial diffusion from discretisation.
It under-predicts peak pedestrian wind speeds, suppresses stratification, over-dilutes plumes and shortens wakes, all of which can look like good news in a design report.
What this chapter covers
- 01
Structured, unstructured and hybrid mesh families
- 02
Local refinement and solution-based adaption
- 03
Overset and moving meshes for devices and openings
- 04
Skewness bands and quality limits
- 05
Aspect ratio, orthogonality and growth rate
- 06
Numerical viscosity and its effect on results
Worked example · free
How many cells does a smooth transition need?
- 1Write the size of the n-th cell as a geometric progression.
- 2Solve for the number of cells to reach 8 m.
- 2Sum the series to find the transition distance.
Key terms
- Hybrid Mesh
- A mesh that combines prism or hexahedral layers near walls with tetrahedral or polyhedral cells elsewhere.
- Local Mesh Refinement
- Increasing resolution only in regions with steep gradients while keeping the rest of the domain coarser.
- Overset Grid
- An independently generated body-fitted mesh that overlaps a background mesh and exchanges data by interpolation.
- Orthogonal Quality
- A measure of how well cell face normals align with the lines joining neighbouring cell centres.
- Growth Rate
- The size ratio between adjacent cells, kept at or below 1.2 for smooth transitions.
- Aspect Ratio
- The ratio of a cell's longest to shortest dimension, acceptable when the long side follows the flow or the wall.
- Inflation Layer
- A stack of thin prism cells grown from a wall to resolve the boundary layer.
- Solution Adaption
- Refining or coarsening the mesh during a run according to solution values, gradients or regions.
Grid Generation and Mesh Quality Control FAQ
Why is the hybrid mesh standard practice for buildings?
It combines thin prism layers that resolve boundary layers on walls and ground with flexible tetrahedral or polyhedral cells that fit complex architecture. Real building and urban models value fast set-up and adaptability over the small efficiency advantage of a fully structured mesh.
What skewness should a building mesh stay below?
The course gives 0.85 as the maximum for hexahedral, quadrilateral and triangular cells and 0.9 for tetrahedra, and cites Fluent’s general advice of maximum skewness below 0.95 with minimum orthogonal quality above 0.1. Cells in high-gradient regions such as walls and jets should be well inside those limits.
How can numerical viscosity make a design look better than it is?
It smears gradients, so peak pedestrian wind speeds are under-predicted, stratification looks like good mixing, pollutant peaks are diluted and wakes look shorter. Each of these errors flatters a natural ventilation or wind-comfort result.
When are overset or moving meshes worth the effort?
When part of the geometry moves or is changed repeatedly: oscillating fans, louvres at different angles, or windows opening. Overset grids let a body-fitted mesh be placed and replaced without remeshing the background; moving meshes deform one connected grid in time.
How big is a typical problem?
The lecture describes about ten thousand cells as small, a hundred thousand as intermediate and a million as large enough to need several processors. The urban morphology lab targets three to eight million cells.
Assessment move
Every time you mesh, export the quality report and copy three numbers into your notes: maximum skewness, minimum orthogonal quality and total cell count. Over a few runs you will see how refinement choices move those numbers, and the report will already contain the evidence a marker wants.
