# Dimensionless wall distance (y plus)

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 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.