Representative and characteristic TI in WAT

[Christian]

Hi everyone,
I just discovered something in WAT I do not understand. I took the characteristic turbulence from the turbulence table in WAT, after that I changed only the IEC to edition 3 and compared the representative turbulence with the characteristic turbulence.

The representative turbulence was showing lower values than the characteristic turbulence. How is this possible?
As I understand the representative ambient turbulence is calculated according to: Irep=Imean+1,28*Istdv and the characteristic turbulence according to: Ichar=Imean+1*Istdv.
Therefore the representative turbulence should be larger than the characteristic, right?

Thanks in advance!
Christian

Finally the settings and the values:
------------------------------------------------------------------------------
[Wake modelling]
Wake effect not included
Calculation with fixed exposure angle
Apply Ct=7/u rule for thrust coefficinet in added-wake-turbulence formula
No correction for wake deflection
Include turbulence form distant turbines

[Turbulence modelling]
Directional wind distribution applied
Directional turbulence distribution applied
Woehler exp not applied for directional averaging
No park effect on ambient turbulence
Variable turbulence intensity within 10-min periods
Correction for 10-min windowing
Add speed dependence on turbulence distribution

[IEC 61400-1 complex terrain turbulence factors]
No complex-terrain effect on ambient turbulence
Assess complex terrain by new IEC 61400-1/A1 rules
Fitted slope may have transversal component
Dont use WEng TI estimates for complex-terrain factors
Preferred TI model is WEng results corrected by ref data
TI model second option is WEng results corrected by IEC model
------------------------------------------------------------------------------
Characteristic 10-min turbulence intensity [%] at WEC_1

u 0
-----------
1 71.3
2 41.2
3 30.5
4 25.1
5 21.7
6 19.4
7 17.8
8 16.6
9 15.6
10 14.8
11 14.2
12 13.6
13 13.2
14 12.8
15 12.5
16 12.2
17 11.9
18 11.7
19 11.5
20 11.3
21 11.1
22 11.0
23 10.8
24 10.7
25 10.6
26 10.5
27 10.4
28 10.3
29 10.2
30 10.1
------------------------------------------------------------------------------
Representative 10-min turbulence intensity [%] at WEC_1

u 0
-----------
1 54.8
2 32.6
3 24.7
4 20.6
5 18.1
6 16.4
7 15.2
8 14.3
9 13.6
10 13.0
11 12.6
12 12.2
13 11.8
14 11.5
15 11.3
16 11.1
17 10.9
18 10.7
19 10.5
20 10.4
21 10.3
22 10.2
23 10.1
24 10.0
25 9.9
26 9.8
27 9.7
28 9.6
29 9.6
30 9.5

[Morten]

Hi Christian,

It is always wise to make this kind of tests, and I fully agree that it is strange that representative TI (IEC 61400-1 Ed.3) results are higher that the characteristic TI (Ed.2) when wake effects are turned off. However, this is actually what you get from WAT with turbulence category A wind turbine classes. I shall explain why.

As a model for background turbulence WAT simply adapts the IEC NTM turbulence model and adjust it by an offset to make it match WEng TI predictions at high wind speeds. The reason for your strange results is simply that IEC 61400-1 (Ed.2) specifies a different wind speed dependence for category A turbulence. If you have edition 2 of the standard at hand, you can see that the so-called slope parameter in the formula of paragraph 6.3.1.3 is a=2 for category A and a=3 is for category B turbulence. In Edition 3 the NTM model is written differently, but you can detect that it corresponds to a=3 for all categories.

My current feeling is that the IEC NTM model is too conservative in most cases. As a supplement the coming WAT 3.0 will offer use of observed turbulence statistics. You could also claim that WEng ought to model these variations in turbulence intensity. These variations are typically caused by variable atmospheric stability and trends in the mean wind signal, so WEng both need improvements in its flow and turbulence models and a new statistical weighting between different stability conditions. All very difficult I think, but we actually have a Ph.D. student working on stability effects on the turbulence model.

Cheers,
Morten