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

    :param Iref: Reference turbulence intensity in percent according to turbine class
    :param wind_speed: 10-minute mean wind speed in m/s
    :returns: Turbulence intensity in percent
    """
    return Iref * (0.75 * wind_speed + 5.6) / wind_speed
