Cfd Online Y+

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Cfd Online Y+

The formulas used are give in CFD-Wiki here. The Schlichting skin-friction formula, as desribed here (equation 21.16 footnote in Schlichting), is used to estimate the local skin-friction for a turbulent boundary layer on a smooth flat plate. The correlation is valid for Re < 10^9. The density and viscosity given by default in the form is for air at sea-level and room temperature (20°C).

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:

  • Y+ Wall Distance Estimation

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!

  • Viscous Grid Spacing Calculator

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.

  • Y+ Calculator

An online calculator written in Javascript for computing the distance of the first grid point off the wall to achieve a desired Y+ value for given flow conditions. iOS and Android versions also available.

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