Function log_law_velocity_kernel 

pub fn log_law_velocity_kernel<R>(
    y_plus: R,
    kappa: R,
    b: R,
) -> Result<R, PhysicsError>
where
    R: RealField,

Logarithmic law of the wall u⁺ = (1/κ) · ln(y⁺) + B.

Errors on κ = 0 (division by zero) or y⁺ ≤ 0 (log undefined). Standard values: κ ≈ 0.41, B ≈ 5.0. Valid for roughly 30 ≲ y⁺ ≲ 300.