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 JavaScript that helps you to estimate at what normal distance from a wall you should place your first grid-line. This is one of those wonderful little tools that saves you from learning a formula!
- Another y+ estimation script by CHAM. Uses a slightly different approximation formula but also works great.
