def epsi_vapor_top(Fr_top):
    """
    Calculates the vapor content of bubble layer at the top of column
    Parameters
    ----------
    Fr_top : float
        The Frudo criterion at the top of column, [dimensionless]
    Returns
    -------
    epsi_vapor_top : float
        The vapor content of bubble layer at the top of column, [dimensionless]
    References
    ----------
    Дытнерский, страница 207, формула 5.47
    """
    return Fr_top**0.5 / (1 + Fr_top**0.5)