def Nicklin_Wilkes_Davidson(x, rhol, rhog, m, D, g=g):
    r'''Calculates void fraction in two-phase flow according to the model of 
    [1]_ as given in [2]_ and [3]_.
    
    .. math::
        \alpha = \frac{x}{\rho_g}\left[C_0\left(\frac{x}{\rho_g} + \frac{1-x}
        {\rho_l}\right) +\frac{v_{gm}}{G} \right]^{-1}
        
    .. math::
        v_{gm} = 0.35\sqrt{gD}

    .. math::
        C_0 = 1.2
        
    Parameters
    ----------
    x : float
        Quality at the specific tube interval []
    rhol : float
        Density of the liquid [kg/m^3]
    rhog : float
        Density of the gas [kg/m^3]
    m : float
        Mass flow rate of both phases, [kg/s]
    D : float
        Diameter of the channel, [m]
    g : float, optional
        Acceleration due to gravity, [m/s^2]

    Returns
    -------
    alpha : float
        Void fraction (area of gas / total area of channel), [-]

    Notes
    -----
        
    Examples
    --------
    >>> Nicklin_Wilkes_Davidson(0.4, 800., 2.5, m=1, D=0.3)
    0.6798826626721431

    References
    ----------
    .. [1] D. Nicklin, J. Wilkes, J. Davidson, "Two-phase flow in vertical 
       tubes", Trans. Inst. Chem. Eng. 40 (1962) 61-68.
    .. [2] Xu, Yu, and Xiande Fang. "Correlations of Void Fraction for Two-
       Phase Refrigerant Flow in Pipes." Applied Thermal Engineering 64, no. 
       1-2 (March 2014): 242–51. doi:10.1016/j.applthermaleng.2013.12.032. 
    .. [3] Woldesemayat, Melkamu A., and Afshin J. Ghajar. "Comparison of Void 
       Fraction Correlations for Different Flow Patterns in Horizontal and 
       Upward Inclined Pipes." International Journal of Multiphase Flow 33, 
       no. 4 (April 2007): 347-370. doi:10.1016/j.ijmultiphaseflow.2006.09.004.
    '''
    G = m/(pi/4*D**2)
    C0 = 1.2
    vgm = 0.35*(g*D)**0.5
    return x/rhog*(C0*(x/rhog + (1-x)/rhol) + vgm/G)**-1
r'''Calculates void fraction in two-phase flow according to the model of 
    [1]_ as given in [2]_ and [3]_.
    
    .. math::
        \alpha = \frac{x}{\rho_g}\left[C_0\left(\frac{x}{\rho_g} + \frac{1-x}
        {\rho_l}\right) +\frac{v_{gm}}{G} \right]^{-1}
        
    .. math::
        v_{gm} = 0.35\sqrt{gD}

    .. math::
        C_0 = 1.2
        
    Parameters
    ----------
    x : float
        Quality at the specific tube interval []
    rhol : float
        Density of the liquid [kg/m^3]
    rhog : float
        Density of the gas [kg/m^3]
    m : float
        Mass flow rate of both phases, [kg/s]
    D : float
        Diameter of the channel, [m]
    g : float, optional
        Acceleration due to gravity, [m/s^2]

    Returns
    -------
    alpha : float
        Void fraction (area of gas / total area of channel), [-]

    Notes
    -----
        
    Examples
    --------
    >>> Nicklin_Wilkes_Davidson(0.4, 800., 2.5, m=1, D=0.3)
    0.6798826626721431

    References
    ----------
    .. [1] D. Nicklin, J. Wilkes, J. Davidson, "Two-phase flow in vertical 
       tubes", Trans. Inst. Chem. Eng. 40 (1962) 61-68.
    .. [2] Xu, Yu, and Xiande Fang. "Correlations of Void Fraction for Two-
       Phase Refrigerant Flow in Pipes." Applied Thermal Engineering 64, no. 
       1-2 (March 2014): 242–51. doi:10.1016/j.applthermaleng.2013.12.032. 
    .. [3] Woldesemayat, Melkamu A., and Afshin J. Ghajar. "Comparison of Void 
       Fraction Correlations for Different Flow Patterns in Horizontal and 
       Upward Inclined Pipes." International Journal of Multiphase Flow 33, 
       no. 4 (April 2007): 347-370. doi:10.1016/j.ijmultiphaseflow.2006.09.004.