WAT IEC NTM correction

[Janst1]

Dear WAsP Team,

I am working with WAT to calculate the effective TI values and I do a correction of the Weng 3.0 TI values with the IEC model.
As far as I understand WAT is adding an offset on high wind speeds to match the Weng turbulence results before it is doing the correction to the IEC curve.
How can I incluence this offset factor to do a more or less conservative calcuation?

Thanks for your help in advance

Jan

[Morten]

Dear Jan,


WAT has three methods for modelling representative ambient TI, defined as the 90% percentile of a typical scattered distribution. The simplest of these methods, and the first one published, is to give the IEC model the offset which makes it converge to the WEng estimate at very high wind speed. A quick-and-dirty fix is to change the input from WEng. This is feasible if you select the WEng output in Excel format and manually correct the data in the column called TI. NB: I am not really recommending this, so please read on for better solutions.


The real problem is that the IEC model is designed to be conservative, and it is not quite suitable as a the most likely estimate for the ambient TI. It is probably better to use one of the other WAT methods. Here, you can 1) provide observed TI statistics at one or several reference masts and let WAT translate these conditions to your turbine sites by as described in the WAT help file, or 2) you can prescribe your own modelled TI statistics for each of the turbine sites. Getting observed or modelled TI stats into WAT 3.0 can be a hassle, so please contact WEng support if you have problems with this.


Regards,
Morten

[Janst1]

Hi Morten,

Thanks for your answer. I am just wondering how WAT is modeling the standard deviation. I am refering to the TI Model tab in Report for selected sites in WAT. For instance at 15 m/s it seems to be approx. 0.096 * (ambient TI Weng*correction factor). But it is differnt for other bins and not following the IEC curve in my understanding. Can you explain how the turbulence standard deviation is modeled in WAT tool and on which standard or method it is based please?

Thanks

Jan

[Morten]

Hi Jan,


WAT is using rather crude estimates for this. One method is to use a modified version of the IEC NTM model. Another one is to use measured statistics. Please read the WAT help file section called "modeling with WAT| ambient turbulence models".


Regards,
Morten