Turbsim for Class IV from IEC 61400-1 2nd edition

Wind & Water Inflow Turbulence
1

Hi everybody,

I am using Turbsim for some simulations for a wind turbine for low winds. I wanted to use in Turbsim the former standard Class IV of the IEC 61400-1 second edition, which corresponds to lower wind speeds. Since there is the possibility to use the second edition for the “IECstandard” for the scaling, I was wondering if something similar could be done in “IEC_WindType” whith extreme turbulence “ETM” & “EWM”. If this is not a possibility offered by Turbsim, would it be possible to mannually insert the corresponding parameters of the old class IV through the input file?

Thanks a lot

Best regards

Nicolas

2

Dear Nicolas,

While TurbSim supports the second edition of IEC 61400-1, I don’t see from its documentation that it supports class IV winds. However, you can always specify your own turbulence conditions in TurbSim, e.g., by setting IEC_WindType = NTM and IECturbc to the desired turbulence intensity in percent.

Best regards,

3

Dear Jason,

First of all thanks for the fast answer.

I actually do not have the second edition of IEC 61400-1, I extracted this information from the first edition of the Tony, B., David, S., Nick, J., and Ervin, B., 2001, “Wind Energy Handbook”, Wiley, New York, section 5.1.2 IEC61400-1. I attach the corresponding table where the classes are defined.

With respect to the suggested solution, since I am simulating different hub wind speeds, I am not sure if I can set the desired turbulence intensity with NTM, because, if I understand correctly, then it won’t be able to follow a curve as it was doing when choosing a defined category for IECturbc, like the ones shown in the TurbSim Manual: “Figure 7. Longitudinal wind-speed standard deviation and TI for IEC turbulence categories as functions of the mean hub-height wind speed, Vhub”.

I guess that I should specify then a desired turbulence intensity for each hub wind speed, isn’t it?

Thanks in advance,

Best regards,

Nicolas
IEC_WTClasses.PNG

4

Dear Nicolas,

Correct. TurbSim is run separately for each mean wind speed (etc.), so, set the turbulence intensity appropriate to each mean wind speed you simulate.

Best regards,

5

Dear Jason,

Okay, thanks a lot.

Best regards

6

Dear Jason,

I had some confusion about the “IECturbc” option in the TurbSim module based on the above discussion. Take the normal turbulence model as an example.

If we choose the turbulence intensity categories “A” or “B” or “C” specified in the IEC standard (2005 3rd edition for onshore WT and 2009 1st edition for offshore WT), the default values corresponding to 0.16, 0.14, and 0.12 would be adopted in the simulation for all the wind speeds and the suggested equation, sigma_1=I_ref*(0.75V_hub+5.6) for the onshore WT and sigma_1=V_hub/ln(Z_hub/Z_0)+1.281.44I_15 for the offshore wind turbine, would be employed to define the standard deviation of longitudinal wind speed at hub height. Therefore, it applies to both onshore and offshore wind turbines during the preliminary wind turbine design process even though the standard deviation calculation formula has been updated in the IEC 61400-3-1, 2019 for offshore WT. Right ? However, we choose the turbulence intensity in percent, we should specify the different values corresponding to various wind speeds based on Fig.7 (I consider this picture with normal turbulence model is also depicted based on the same abovementioned equations) ? Then the standard deviation of longitudinal wind speed at hub height is calculated based on the formula, sigma_1=IECturbcu_hub/100, as depicted in the manual. Considering we adopt “IECturbc=A” or “IECturbc=16”, the generate wind files adopting two methods are the same ?

Another problem is related to atmospheric stability. With reference to the Turbsim manual, the neutral, stable, and unstable conditions could be considered for different turbulence models, except the IEC Kaimal and Von Karman models, by changing the value of “Rich_NO”. I want to know whether other settings should be considered when adopting this stochastic turbulence model, as different atmospheric conditions of wind inflow in the paper “multimodel validation…” (doi.org/10.1002/we.1654), and any plan to update the TurbSim? Thank you for your help.

Best regards,
Jian

7

Dear Jian.Zhang,

TurbSim uses the equation

sigma_1=I_ref*(0.75*V_hub+5.6)

that you mention (where I_ref is based on turbulence class A, B, or C), but it does not use the equation

sigma_1=V_hub/ln(Z_hub/Z_0)+1.281.44I_15.

The first equation is dictated by the chosen wind turbine class, as implemented in TurbSim, but the second equation can be used for the assessment of ambient turbulence in the absence of specific measurements, which is not really a functionality of TurbSim.

When a turbulence intensity is specified in TurbSim, I agree with your equation

sigma_1=IECturbc*u_hub/100,

but IECturbc = “A” and IECturbc = 16 are only equivalent when

0.16*(0.75V_hub+5.6) = 0.16V_hub,

i.e., at V_hub = 22.4 m/s.

We’ve used TurbSim to model non-neutral atmospheric flows (e.g., in the paper you reference by Doubrawa et al) by specifying the desired turbulence characteristics like the profile (shear, veer), turbulence spectra (in u, v, w), spatial coherence, and Reynold’s stresses in place of the default IEC model. The second paper you reference by Wang et al is again applicable to the assessment of ambient turbulence in the absence of specific measurements, which again is not really a functionality of TurbSim. NREL is not currently funded to implement other turbulence models in TurbSim.

Best regards,

8

Dear Dr. Jonkman,

Thank you for your reply. I don’t know whether the following understanding is right, which needs your further confirmation.

If the turbulence class (A, B, and C) is preferred, the following equation would be adopted for both onshore and offshore wind turbines? (even though the equation, sigma_1=V_hub/ln(Z_hub/Z_0)+1.281.44I_15, is recommended in the IEC standards for offshore wind turbines in IEC)

sigma_1=I_ref*(0.75*V_hub+5.6)

If the turbulence intensity (values in percent) is chosen, the following formula would be employed for both onshore and offshore wind turbines?

sigma_1=IECturbc*u_hub/100

Therefore, different equations are chosen to define the sigma when selecting “turbulence class” and “turbulence intensity in percent”?

Best regards,
Jian

9

Dear Jian,

My answers to questions 1-3 are yes.

Regarding (1), I would say that the IEC 61400-3 does not recommend that the equation

sigma_1=V_hub/ln(Z_hub/Z_0)+1.281.44I_15

be used to define the design turbulence level; instead, the equations from IEC 61400-1 define that (same for land and offshore). Instead, this equation is stated to estimate the turbulence during a site assessment in which no turbulence measurements are obtained.

Best regards,

10

Dear Dr. Jonkman,

Thank you for your explanation, and it is a great help.

Best regards,
Jian