Airfoil CFD Mesh Design: Boundary Layers, Inflation Layers and Grid Independence

The layer is the mesh

Profile drag and separation live inside a thin boundary layer. A mesh that looks fine on the airfoil outline and has a few huge cells next to the wall cannot see that layer. It will still print a lift, because lift is an outer-flow integral. It will invent a drag. The near-wall spacing is the first design choice, not a cosmetic inflation picture.

If the turbulence model integrates to the wall, the first cell needs a y+ of order 1 or below, so the cell sits inside the viscous sublayer. If the model uses wall functions, the first cell is supposed to sit farther out, often at a y+ of a few tens, and the sublayer is modelled rather than resolved. Mixing those — a wall-function model on a y+ of 0.5, or an integration-to-the-wall model on a y+ of 50 — is a different and wrong boundary condition. Say which treatment you are using before you quote CD.

A sane starting layout

• A structured or hybrid inflation stack growing off the entire surface, including the trailing edge and a stretch of the wake. Growth ratio around 1.2 keeps the stack from jumping. The stack should be thicker than the boundary layer you expect at the trailing edge, which you can estimate from a panel method before you build the volume mesh.
• Clustering at the nose, where the radius and the suction peak live, and at the trailing edge, where a blunt face or a sharp cusp is only a few cells wide if you are careless.
• A far field many chords away, on the order of tens of chords for a low-speed case, with boundary conditions that are actually far-field conditions. A small domain with a hard velocity on the boundary will change the lift.
• Wake refinement downstream of the trailing edge if you intend to trust a drag integrated in the wake rather than on the surface.

Independence is a table

Publish the y+ you achieved, the growth ratio and the independence delta next to the coefficient. A single mesh with a picture of inflation layers is not a grid study. It is a mesh.

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Where this sits in the series

In simple words: what this page is for

A RANS airfoil mesh has to resolve the boundary layer, not only the outline. Independence means lift and drag stop moving when you refine, including the wake.

This lesson exists because section decisions fail when geometry, method labels and selection reasons are scattered. Airfoil CFD Mesh Design: Boundary Layers, Inflation Layers and Grid Independence helps you keep a clean trail from shape to polar to choice.

Why the topic matters

Engineers lose time when a title sounds clear but the page only sketches the idea. This article expands Airfoil CFD Mesh Design: Boundary Layers, Inflation Layers and Grid Independence so you can answer three questions without jargon: what is the idea, when do you use it, and how do you check it in a HENDOI product.

Read the sections above for the core definitions. Use the steps below to move from reading into a real project trail you can share with a teammate.

How to apply this in HENDOI

In practice: create a HENDOI account or log in, open HENFOIL, bring a section in (create, import DAT, or digitize), pass the geometry gate, climb the analysis ladder only as far as the decision needs, then compare and archive. Every calculated number should keep its method label.

When the section choice is ready for the aircraft, continue in HENSYNTH for mission, compliance and trades. Same account. Same plans.

What this page will not claim: a teaching polar or geometry note is not a flight-Re certificate. HENFOIL labels methods so you know what you bought. Tunnel data and professional judgement still sit outside any single software run.

Checklist before you leave this page

• Can you state the idea of Airfoil CFD Mesh Design: Boundary Layers, Inflation Layers and Grid Independence in one sentence?
• Do you know which product owns the next action — HENFOIL or HENSYNTH?
• Will every number you keep carry a method label or a stored-run source?
• If you are new here, have you opened Register or Login?

Related reading on this site

• Active Flow Control on Airfoils: Suction, Blowing and Synthetic Jets
• What Is the Aerodynamic Center of an Airfoil? Location, Meaning and Applications
• How to Identify Numerical Convergence Problems in Airfoil Analysis
• How to Build an Airfoil Analysis Database with Consistent Simulation Conditions
• All HENDOI blog articles
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• HENSYNTH product page
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• Index: all HENFOIL airfoil lessons

These pages teach the ideas. They are not wind-tunnel data and not a certification analysis. When a number matters, compute it at a stated Reynolds number, Mach number and transition setting, and keep the method label on the result. In HENFOIL that production polar is a VIC polar — see the analysis ladder and the polar field guide.