def ut(cd, dp, rhog, rhos):
    """
    Calculate terminal velocity of a single particle based on Equation 28 on
    page 80 in the Kunii and Levenspiel book [1]_ where :math:`C_D` is an
    experimentally determined drag coefficient.

    .. math:: u_t = \\left( \\frac{4 d_p\\, (\\rho_s - \\rho_g) g}{3 \\rho_g\\, C_D} \\right)^{1/2}

    Parameters
    ----------
    cd : float
        Drag coefficient [-]
    dp : float
        Diameter of particle [m]
    rhog : float
        Density of gas [kg/m^3]
    rhos : float
        Density of solid [kg/m^3]

    Returns
    -------
    ut : float
        Terminal velocity [m/s]

    Example
    -------
    >>> ut(11.6867, 0.00016, 1.2, 2600)
    0.6227

    References
    ----------
    .. [1] Daizo Kunii and Octave Levenspiel. Fluidization Engineering.
           Butterworth-Heinemann, 2nd edition, 1991.
    """
    g = 9.81
    tm1 = 4 * dp * (rhos - rhog) * g
    tm2 = 3 * rhog * cd
    ut = (tm1 / tm2) ** (1 / 2)
    return ut
