Dimensionless wall distance (y plus)
A non-dimensional wall distance for a wall-bounded flow can be defined in the following way:
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!