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Dimensionless wall distance (y plus)

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A non-dimensional wall distance for a wall-bounded flow can be defined in the following way:

y^+ \equiv \frac{u_* \, y}{\nu}

Where u_* is the friction velocity at the nearest wall, y is the distance to the nearest wall and \nu is the local kinematic viscosity of the fluid.

y^+ is often refered to simply as y plus and is commonly used in boundary layer theory and in defining the law of the wall.

Estimation of y plus

The following online tools can be used to estimate the necessary grid spacing in order to obtain a desired y plus:

A y+ wall distance estimation script provided by CFD Online. Uses the free-stream velocity and gas-data to estimate the wall distance needed to obtain a desired y+ value at a certain boundary layer length. This is one of those wonderful little tools that saves you from learning a formula!
A JavaScript that helps you to estimate at what normal distance from a wall you should place your first grid-line. Variants suitable for compressible flow using total conditions and Mach number also available.
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