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.
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. [Ref]

Check these conversations on CFD Online:
  1. wallShearStress, wallGradU are inconsistent
  2. Problems with YPlusRAS and wallShearStress
  3. yPlusRAS Question

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