def Terfous(Re):
    """Calculates drag coefficient of a smooth sphere using the method in
    [1]_ as described in [2]_.

    .. math::
        C_D = 2.689 + \\frac{21.683}{Re} + \\frac{0.131}{Re^2}
        - \\frac{10.616}{Re^{0.1}} + \\frac{12.216}{Re^{0.2}}

    Parameters
    ----------
    Re : float
        Reynolds number of the sphere, [-]

    Returns
    -------
    Cd : float
        Drag coefficient [-]

    Notes
    -----
    Range is 0.1 < Re <= 5E4

    Examples
    --------
    >>> Terfous(200.)
    0.7814651149769638

    References
    ----------
    .. [1] Terfous, A., A. Hazzab, and A. Ghenaim. "Predicting the Drag
       Coefficient and Settling Velocity of Spherical Particles." Powder
       Technology 239 (May 2013): 12-20. doi:10.1016/j.powtec.2013.01.052.
    .. [2] Barati, Reza, Seyed Ali Akbar Salehi Neyshabouri, and Goodarz
       Ahmadi. "Development of Empirical Models with High Accuracy for
       Estimation of Drag Coefficient of Flow around a Smooth Sphere: An
       Evolutionary Approach." Powder Technology 257 (May 2014): 11-19.
       doi:10.1016/j.powtec.2014.02.045.
    """
    Cd = (2.689 + 21.683 / Re + 0.131 / Re ** 2 - 10.616 / Re ** 0.1 + 
        12.216 / Re ** 0.2)
    return Cd
