yPlus utility in OpenFOAM

This post is about how to calculate yPlus in OpenFOAM.

For RANS

Note that yPlusRAS calculates y* and not y+

1.     With wall function;  

·        type ‘yPlusRAS’ for incompressible flow

·        type ‘yPlusRAS’ –compressible for compressible flow

2.     Without wall function;

·        type wallShearStress if it is incompressible. If not, add "-compressible" flag

·        calculate manually: sqrt(wallShearStress/rho) * y/nu

y = the distance to first cell from the wall

nu = kinematic viscosity

Be careful: You should use the cell center (height of the cell devided by two) for y

For LES

type ‘yPlusLES’

Note:
Y-star = rho*Cmu^0.25*sqrt(k)*y/mu
y-plus = rho*sqrt(tau_w/rho_w)*y/mu
Y-star is related to the turbulent kinetic energy and y-plus to the wall shear stress.

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. [Ref]

Check these conversations on CFD Online:

1. wallShearStress, wallGradU are inconsistent
2. Problems with YPlusRAS and wallShearStress
3. yPlusRAS Question