def compute_downcomer_area_fraction(F_LV):
    """
    Return the ratio of downcomer area to net (total) area, `A_dn`.
    
    Parameters
    ----------
    F_LV : float
        Flow parameter.

    Notes
    -----
    The fraction of downcomer area is given by [3]_. See source code for details.

    """
    if F_LV < 0.1:
        A_dn = 0.1
    elif F_LV < 1:
        A_dn = 0.1 + (F_LV-0.1)/9
    else:
        A_dn = 0.2
    return A_dn