How this calculator works
The wall coordinate scales distance by friction velocity and kinematic viscosity. The offered smooth-wall correlations estimate a representative turbulent skin-friction coefficient from a reference-length Reynolds number. They are average correlations used here as preliminary wall-shear estimates, rather than a resolved local CFD wall stress. A custom coefficient lets you use better local data. Check the resulting y-plus in the solved flow. The full layer thickness is approximately twice the cell-centre distance only for a simple orthogonal first cell.
Formula
Cf choices: 0.455/(log10 Re)^2.58; 0.0776/(log10 Re-1.88)^2+60/Re; 0.055/Re^0.182; 0.074/Re^0.2; 0.427/(log10 Re-0.407)^2.64. tau_w = Cf rho U^2/2; u_tau = sqrt(tau_w/rho); y = yplus mu/(rho u_tau).
Worked example
At U = 10 m/s, rho = 1000 kg/m3, mu = 0.001 Pa s and custom Cf = 0.004, shear stress is 200 Pa and friction velocity is 0.4472136 m/s. Target y-plus 1 needs cell-centre distance 0.00000223607 m.
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Frequently asked questions
Is wall distance the full first layer height?
No. It is the wall-to-cell-centre distance. Twice this value is an approximate full thickness for a simple orthogonal cell.
Which y-plus should I target?
Use the wall-treatment requirements of your turbulence model and solver, then verify y-plus after solving.
Are these local friction correlations?
The built-in choices are representative average smooth-wall correlations. Supply a custom local coefficient when local wall shear is available.