def density_two_phase(alpha, rhol, rhog):
    """Calculates the "effective" density of fluid in a liquid-gas flow. If
    the weight of fluid in a pipe pipe could be measured and the volume of
    the pipe were known, an effective density of the two-phase mixture could be
    calculated. This is directly relatable to the void fraction of the pipe,
    a parameter used to predict the pressure drop. This function converts
    void fraction to effective two-phase density.

    .. math::
        \\rho_m = \\alpha \\rho_g + (1-\\alpha)\\rho_l
        
    Parameters
    ----------
    alpha : float
        Void fraction (area of gas / total area of channel), [-]
    rhol : float
        Density of the liquid [kg/m^3]
    rhog : float
        Density of the gas [kg/m^3]

    Returns
    -------
    rho_lg : float
        Two-phase effective density [kg/m^3]

    Notes
    -----
    **THERE IS NO THERMODYNAMIC DEFINITION FOR THIS QUANTITY. DO NOT USE THIS
    VALUE IN SINGLE-PHASE CORRELATIONS.**

    Examples
    --------
    >>> density_two_phase(.4, 800, 2.5)
    481.0

    References
    ----------
    .. [1] Awad, M. M., and Y. S. Muzychka. "Effective Property Models for 
       Homogeneous Two-Phase Flows." Experimental Thermal and Fluid Science 33,
       no. 1 (October 1, 2008): 106-13. 
    """
    return alpha * rhog + (1.0 - alpha) * rhol
