# Dimensionless wall distance (y plus)

(Difference between revisions)
 Revision as of 09:53, 17 December 2008 (view source)Peter (Talk | contribs)m (Reverted edits by C4tdaRlile (Talk) to last version by Jola)← Older edit Revision as of 21:36, 12 June 2011 (view source)Peter (Talk | contribs) Newer edit → Line 11: Line 11: The following online tools can be used to estimate the necessary grid spacing in order to obtain a desired y plus: The following online tools can be used to estimate the necessary grid spacing in order to obtain a desired y plus: - *[http://geolab.larc.nasa.gov/APPS/YPlus/ Viscous Grid Spacing Calculator] + *[http://www.cfd-online.com/Tools/yplus.php Y+ Wall Distance Estimation] - :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! + : 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! - *[http://www.simuserve.com/cfd-shop/GOODIES/YPLUS.HTM Viscous Grid Spacing Calculator] + *[http://geolab.larc.nasa.gov/APPS/YPlus/ Viscous Grid Spacing Calculator] - : Another y+ estimation script by CHAM. Uses a slightly different approximation formula but also works great. + :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. [[Category:Dimensionless parameters]] [[Category:Dimensionless parameters]]

## Revision as of 21:36, 12 June 2011

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.