def speed_section_top(speed_limit_top, D_top, D):
    """
    Calculates the section speed of vapor
    Parameters
    ----------
    speed_limit_top : float
        The limit speed of the vapor at the top of  column, [m/s]
    D_top : float
        The calculating top diameter of column, [m]
    D : float
        The choosing diameter of column, [m]   
    Returns
    -------
    speed_section_top : float
        The section speed of vapor, [m/s]
    References
    ----------
    &&&&&
    """         
    return speed_limit_top * (D_top / D)**2