Function skin_friction_coefficient 

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

Skin-friction coefficient with compressibility (Niskanen 2009 eq. 3.82–3.84; Barrowman 1967 eq. 4-12, 4-13):

• M < 1: C_fc = C_f (1 − 0.1 M²) on either branch of incompressible_skin_friction;
• M ≥ 1, turbulent: C_fc = C_f/(1 + 0.15 M²)^0.58;
• M ≥ 1, roughness-limited: C_fc = C_f/(1 + 0.18 M²), but never below the turbulent value at the same Reynolds number.

The subsonic and supersonic corrections differ at M = 1 (0.900 against 0.922 of C_f turbulent, 0.847 roughness-limited), as published; no transonic blend is given.

Errors

As incompressible_skin_friction, and AeroError::Domain for a negative or non-finite Mach number.