Dimensionless wall distance (y plus)
From CFD-Wiki
A non-dimensional wall distance for a wall-bounded flow can be defined in the following way:
Where 
is the friction velocity at the nearest wall, 
is the distance to the nearest wall and 
is the local kinematic viscosity of the fluid.
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.
