def ETM(Iref, Vmean, wind_speed):
    """
    Additional evaluator - evaluates turbulence intensity according to IEC edition 3 extreme turbulence model

    :param Iref: Reference turbulence intensity in percent according to turbine class
    :param Vmean: Annual mean wind speed according to turbine class
    :param wind_speed: 10-minute mean wind speed in m/s
    :returns: Turbulence intensity in percent
    """
    c = 2.0
    return c * Iref * (0.072 * (Vmean / c + 3.0) * (wind_speed / c - 4) + 10.0
        ) / wind_speed
