y+ is a non-dimensional wall distance. It tells you how far the first near-wall solution point sits from a solid wall when that physical distance is scaled by the local viscous and wall-shear conditions. It is one of the key quantities used to judge whether your near-wall mesh is compatible with the turbulence model and wall treatment.
What does y+ actually mean?
Near a no-slip wall, velocity changes rapidly from zero at the surface toward the outer-flow velocity. The relevant physical length scale in that region is not simply a millimetre or a micrometre; it depends on viscosity and on the wall shear generated by the flow.
The wall coordinate y+ converts the physical wall-normal distance y into a dimensionless quantity:
y — wall-normal distance
uτ — friction velocity = √(τw/ρ)
ν — kinematic viscosity = μ/ρ
NASA's CFPOST documentation uses this same definition and derives the friction velocity from wall shear stress. The important consequence is that y+ depends on both the mesh and the solved flow field. A geometric spacing alone does not determine y+.
The near-wall regions behind the number
The boundary layer is commonly described in wall units. The exact boundaries are not sharp physical discontinuities, but the following engineering picture is useful when planning a CFD mesh.
Wall treatments may blend between formulations, and solver-specific implementations differ. Think in terms of matching the mesh to the chosen near-wall model rather than chasing one magic number.
Should I target y+ = 1, 30, 50 or 100?
The correct answer starts with the near-wall modelling strategy.
Wall-resolved approach
Use a first solution point very close to the wall when you intend to resolve the viscous sublayer. This usually requires several well-controlled prism or inflation layers through the boundary layer, not merely one tiny first cell.
- Higher mesh cost
- Useful when wall shear or heat transfer needs detailed resolution
- Requires good near-wall cell quality and gradual growth
Conventional wall-function approach
Place the first near-wall point in the logarithmic region and let the wall function represent the unresolved inner layer. Current OpenFOAM documentation describes high-Re wall-function operation as intended for approximately 30 ≤ y+ ≤ 300.
- Lower near-wall mesh cost
- Common for high-Re engineering RANS
- Must remain consistent with the selected wall functions
For conventional non-blended wall functions, this buffer range is often avoided because the first point is neither cleanly wall-resolved nor well placed for a log-law wall function. Some modern wall treatments are designed to blend across regimes, but that is a solver/model feature—not a reason to ignore mesh quality.
From target y+ to first-cell distance
Before a solution exists, the wall shear stress is unknown. That means the pre-mesh first-cell distance has to be estimated using an engineering correlation or a value from a previous/related simulation.
Re = ρ U L / μCf → τw = ½ρU²Cfuτ = √(τw/ρ)y = y+ ν / uτUse the Abecator y+ & First Cell Height Calculator
It exposes every intermediate value so you can see the Reynolds number, Cf, wall shear and friction velocity behind the estimate.
Important: wall distance vs. prism-layer thickness
In the y+ equation, y is a distance from the wall to the near-wall solution location. In a cell-centred finite-volume method this is associated with the first cell centre. A meshing tool may instead ask for the geometric thickness of the first prism/inflation layer. These are related but are not automatically identical.
For a simple orthogonal prism of uniform thickness, the centroid lies roughly halfway through that first layer. Real meshes, polyhedral cells and mesher conventions can differ, so check what your meshing software means by “first layer thickness” or “first cell height” before copying a number directly.
A practical y+ workflow before and after meshing
- Choose the turbulence model and near-wall treatment first.
Your mesh target should follow the physics model, not the other way around.
- Choose a target y+ appropriate to that treatment.
For example, near 1 for a wall-resolved strategy or in the log-layer range for a conventional high-Re wall function.
- Estimate the wall-to-first-cell distance.
Use representative local velocity and fluid properties. For complex flows, calculate more than one operating condition or location.
- Build the complete prism-layer package.
First-cell distance is only one parameter. Layer count, growth rate, total thickness, surface resolution and cell quality all matter.
- Run the CFD solution and inspect the achieved y+ field.
Look at the distribution, not only a single average. High-shear zones, stagnation regions, separation and recirculation can produce very different local values.
- Remesh if the solved y+ conflicts with the modelling strategy.
Near-wall planning is iterative. A pre-processing estimate is a starting point, not proof that the final mesh is correct.
What to check in STAR-CCM+ and OpenFOAM
Verify the wall treatment and solved wall y+
Keep the target consistent with the turbulence model and wall-treatment settings used in your physics continuum. After the solution develops, inspect the local wall-y+ distribution on the surfaces that matter rather than relying on the pre-mesh estimate alone.
Use yPlus post-processing and compatible wall functions
OpenFOAM provides a yPlus function object for computing near-wall y+ for turbulence models. Its wall-function documentation explicitly ties mesh requirements to the chosen high- or low-Re operation.
In OpenFOAM, the yPlus function object can be added to controlDict or evaluated with the post-processing utilities. This is the type of post-solution verification you should perform in any CFD package.
Common y+ mistakes that create bad meshes
The target is not an independent mesh preference. It follows the turbulence/wall modelling strategy.
A single freestream or inlet velocity may poorly represent jets, accelerating passages, rotating machinery or separated flows.
Boundary-layer resolution depends on the full wall-normal mesh: layer count, growth and total thickness.
The final wall shear comes from the numerical solution. Always inspect the solved field.
Real wall shear varies spatially. Design for a useful distribution and focus on engineering-critical surfaces.
When wall heat flux and thermal boundary layers matter, near-wall treatment can affect more than drag. Check the thermal wall-function or wall-resolved requirements as well.
y+ FAQ
What is a good y+ value for CFD?
There is no universal value. A wall-resolved approach commonly starts near y+ = 1, while conventional high-Re wall functions generally want the first point in the logarithmic region. Use the requirement of your turbulence model and near-wall treatment.
Is y+ = 1 always more accurate?
No. A fine first cell does not automatically improve a simulation. If the turbulence model, wall treatment, prism layers, surface mesh or numerical setup are inconsistent, a nominal y+ of 1 is not a guarantee of accuracy.
Can I use y+ = 30?
It can be an appropriate lower-end target for some conventional wall-function strategies, but targeting the boundary of a range leaves little tolerance for spatial variation. Many engineers choose a target comfortably inside the intended log-layer range and then check the achieved distribution.
Why did my solved y+ differ from the calculator?
The calculator estimates wall shear before the CFD solution exists. The actual geometry, pressure gradient, transition, separation, roughness, rotation and local velocity field determine the solved wall shear and therefore the final y+.
Do I need y+ for laminar simulations?
y+ is primarily used as a near-wall coordinate in turbulent wall modelling and assessment. For genuinely laminar flow, the meshing decision is instead driven by resolving the velocity and thermal gradients required by the laminar solution.
Technical references
This Abecator guide is written as practical engineering guidance. For solver-specific implementation details, use the official documentation for the version you run.
Estimate your first-cell distance now.
Choose your target y+, enter representative flow conditions and use the result as the starting point for your near-wall mesh.
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