Function incompressible_skin_friction 

pub fn incompressible_skin_friction(
    reynolds: f64,
    relative_roughness: f64,
) -> Result<f64, AeroError>

Incompressible skin-friction coefficient of a fully turbulent boundary layer at Reynolds number reynolds on a surface of relative roughness R_s/L (Niskanen 2009 eq. 3.81):

C_f = 1.48e-2                     R < 1e4
    = 1/(1.50 ln R − 5.6)²        1e4 ≤ R < R_crit     (eq. 3.78)
    = 0.032 (R_s/L)^0.2           R ≥ R_crit, R ≥ 1e4  (eq. 3.80)

As printed, the piecewise form jumps at R_crit: eq. 3.79 is not where eq. 3.78 and 3.80 cross (about +9% for 60 µm on 1 m). hpr keeps the published form.

Errors

AeroError::Domain for a negative or non-finite Reynolds number or relative roughness.