# Y plus wall distance estimation

When meshing it is often useful to be able to estimate the wall-distance needed to obtain a certain Y+ value. To estimate this you can do the following:

1. Compute the Re number:

$Re = \frac{\rho \cdot U_{freestream} \cdot L_{boundary\,layer}}{\mu}$

2. Estimate the skin friction using one of the formulas given here, for example, using the Schlichting skin-friction correlation:

$C_f = [ 2 \, log_{10}(Re_x) - 0.65 ] ^{-2.3} \quad \mbox{for} \quad Re_x < 10^9$

3. Compute the Wall shear stress:

$\tau_w = C_f \cdot \frac{1}{2} \, \rho \, U_{freestream}^2$

4. Compute the Friction velocity:

$u_* = \sqrt{\frac{\tau_w}{\rho}}$

5. Compute the wall distance:

$y = \frac{y^+ \mu}{\rho \, u_*}$