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Y plus wall distance estimation

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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_*}

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