# IEC:61400 -1 2019 for turbulence intensity in TurbSim

Hi Jason:
I am seeking clarification on the following that is used to compute the turbulence intensity in IEC:61400:1-2019, which is subsequently used in TurbSim.

For NTM it states that:
sigma 1 = IRef(0.75Vhub+b); where b = 5.6 m/s.

Wind turbine classes are given as A+ to C and for example class A, IRef=0.16, so using equation above sigma1 can be determined and the TI found by:
sigma1/Vhub. 1) Is this understanding correct?
Also, it defines the category as A+ for very high turbulence and C for very low.
2) Is there a turbulence criteria that causes one to know whether they are in the high or low category?
I arbitrarily chose B for two site locations I am investigating to arrive at a different TI for each site but have no basis for why I chose B.

Finally I read some notes that said that TI = standard deviation of wind speed/mean wind speed (mean wind speed at hub height in this case).
3)If I have the wind time series can I find, standard deviation/Vhub, and use the percentage obtained as the TI at each site location in TurbSim instead of using formula above? I am essentially trying to determine an appropriate value for TI for both sites given the data that I have.
4) Also, if TI is entered as percentage in TurbSim for 10% do I just enter 10 instead of A, for example?

Thank you.

Regards,
AOAW

Dear Andre,

1. Yes.
2. I’m not an expert on this topic, but a site assessment or site suitability analysis is performed to test the turbulence conditions at an actual site. See section 11 of IEC 61400-1, 2019. Keep in mind that sigma1 in the equation you reference represents the 90% quantile of the turbulence distribution at each wind speed bin.
3. Yes, if you want to use site-specific data rather than a standard turbulence design class.
4. Yes.

Best regards,

Many thanks Jason.

Regards,
AOAW

Hi Jason:

I have a further query on the turbulence intensity being entered as a percentage. The TurbSim theory manual states the following:
If the IECturbc parameter was specified as a percentage instead of as a standard turbulence category, the wind model must be “NTM.”

Does this statement mean that the normal turbulence model is still used to derive the turbulence intensity?

Would be very grateful for some clarification as I thought that if TI was not entered as a category, for example A, but as a percentage, then the NTM would have no longer been utilized.

Regards,
AOAW

Dear Andre,

Input parameter IEC_WindType in TurbSim is either NTM, ETM, or EWM. For the ETM and EWM, you must specify the turbulence class and recurrence probability, which uniquely determines the turbulent conditions. But for NTM, you can specify the TI directly rather than the turbulence class.

Best regards,

Thanks Jason. Need to further clarify.

So does it means that whether you enter a category or a percentage for the turbulence intensity, for the NTM it is still the IEC NTM that is utilized in TurbSim?

I am trying to confirm whether even if a percentage is used, it is the IEC NTM that is used by TurbSim or maybe it is that if the TI as a percentage is different from that given by the NTM then you can enter the percentage and stating NTM is just prodecural. I determined my TI using data for the site and did not use the standard NTM model given in IEC:61400. The TI that I computed were different from those given by the IEC standard model. So trying to see how/if this differentiation can me made in TurbSim.

Hope I am clear.

Regards,
AOAW

Dear Andre,

I would say it is more procedural. If you specify a TI directly, then this will override the standard TI defined by the turbulence class for the NTM model.

Best regards,

Noted Jason.
Thank you.

Regards,
AOAW