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

    .. math::
        C_D = (2.49Re^{-0.328} + 0.34Re^{0.067})^{3.18}

    Parameters
    ----------
    Re : float
        Particle Reynolds number of the sphere using the surrounding fluid
        density and viscosity, [-]

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

    Notes
    -----
    Range is Re <= 2E5

    Examples
    --------
    >>> Khan_Richardson(200.)
    0.7747572379211097

    References
    ----------
    .. [1] Khan, A. R., and J. F. Richardson. "The Resistance to Motion of a
       Solid Sphere in a Fluid." Chemical Engineering Communications 62,
       no. 1-6 (December 1, 1987): 135-50. doi:10.1080/00986448708912056.
    .. [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.
    """
    return (2.49 * Re ** -0.328 + 0.34 * Re ** 0.067) ** 3.18
