Function peclet_number_kernel 

pub fn peclet_number_kernel<R>(
    u: &Speed<R>,
    length: &Length<R>,
    thermal_diffusivity: R,
) -> Result<R, PhysicsError>
where
    R: RealField,

Peclet number (thermal) Pe = u · L / α. Convective to diffusive heat transport.

Identity: Pe = Re · Pr.