Ambient turbulence model

[Timm]

Dear WAsP Team,

I'm studying Geophysics and Meteorology and
currently i'm working at my Bachelor Thesis.
One topic of the thesis is a survey of the connection between
ambient turbulence and terrain complexity.
In one chapter, i want to describe how the ambient turbulence
is modelled by WEng 3.0. I don't have to know every little step.
But i have to give an overview of the whole process with the main facts.

So the first input is the Generalised Wind Climate(.lib file).
With this information WEng predicts a wind flow over the turbine site,
with the LINCOM model but in a slightly different form.
But how does it works exactly?

I know that the basic equations are the mass and momentum equations.
Then there are lots of conversions until we get to the set of the linear
equations, like in ris-r-900 page 13.
Is the reason for these conversations to transform the nonlinear to linear equations?
These equations are solved with a fourier transformation?!
Do the solutions of these equations represent the predicted wind flow?
Please correct me if there is anything wrong.

With this results the WAT tool models the ambient turbulence TI_WEng?
Which model is used? Is it the one from Mann 1994 (uniform gradient turbulence model)
or from Kaimal 1972 (spectral and exponential coherencemodel) ?
And how does these models work?

I know these are a lot of questions, but it would be very nice if someone could help me.

Best regards
Timm

[Morten]

Hi Timm,


The report you have , Ris-R-900(EN), is a good introduction to the WEng flow model, LINCOM. In addition, I recommend Ris-R-1107(EN) for details on how WEng models surface roughness over water and Ris-R-1179(EN), chapter 2, for details on how the flow equations are solved.


As you say, WEng uses the LINCOM flow results as input to the Mann turbulence model, based on rapid distortion theory. My favourite references to the turbulence model are [1] J Mann (2000) The spectral velocity tensor in moderately complex terrain, J. Wind Eng. Ind. Aerodyn. 94 (2006) 581–602 (explaining the model) and [2] J Mann (1998) Wind field simulation, Prob. Eng. Mech.13, 269-282 (explaining how simulation how the model is tuned to classic text-book spectra like the one of Kaimal and how to simulate pseudo-random time series).


The WEng turbulence predictions are valid for ideal steady-state turbulence, i.e. without trends in mean speed and direction, for neutral atmospheric stability only and without wake effects from buildings and neighboring turbines. The main input is the terrain elevation and surface roughness maps. The modeled turbulent fluctuations over land are proportional to the mean wind speed and thus the modeled turbulence intensity is independent on wind speed. Over water surfaces there is a slight effect of the wind-speed dependent surface-roughness.


The calculations in WAT adapt the WEng turbulence results to the IEC 61400-1 standard. It adds turbulence from neighboring turbines and calculates the so-called effective turbulence intensity, which is the single turbulence level giving the same fatigue damage as turbulent winds from all directions. This averaging depends on mean wind statistics, and that is why you need the *.lib wind atlas file. The turbulence level used by the IEC is actually not the mean turbulence or WEng, but the 90% level of the random distribution of turbulence intensity during random 10-min periods. WAT has various (crude) ways to correct the WEng turbulence predictions. You can either use 1) the same correction as in the IEC standard, or 2) observed turbulence statistics at a reference mast, or 3) supply mean and spread of the turbulence intensity modeled by your own model. See the WAT help file sections "modeling with WAT| ambient turbulence models" and "IEC 61400 standards| IEC 61400-1, Annex D - windfarm turbulence" for further details.


Best regards,
Morten Nielsen

[Timm]

Hallo Morten,

Thank you very much for your detailed explanations.
But i have a few more questions.

If i want to calculate only the ambient turbulence, i don't need the wind atlas file and i don't need WAT?
The wind atlas file is no Input for LINCOM or the turbulence model by Mann?
It is only used for the averaging(effecitve turbulence intensity), as you said?

What is the input for the LINCOM model? The Orography and roughness data?

I hope i get it right this time:
To Predict the ambient turbulence intensity at a site:
Orography and roughness data are used as input for the LINCOM flow model.
The flow results are used as input for the turbulence model by Mann to predict the ambient turbulence intensity. Which also needs the orography and roughness data.
Then WAT adapts the WEng turbulence results to the IEC. And adds wake turbulence and calculates effective turbulence. And therefore i need the wind altas file.
So the WEng turbulence results (TI_WEng) are saved in a .wtr file.
But are this results in the .wtr file, the ambient turbulence intensity in the different sectors at 15m/s?
Or is the ambient turbulence intensity at 15m/s given by: I(15)=(15+a*u)*TI_WEng/(a*u) ?

Best regards,
Timm

[Morten]

Hi Timm,


You got it all right, except the last point. The results in the *.wtr file are for the wind speed you select in the turbulence intensity rose script, and it may differ from 15 m/s depending on your choice. The results of this script are calculated by the Mann turbulence model and not the formula you quote. This is used in of the methods for the mean TI inside WAT. It is actually not the preferred method if you have access to data from a reference mast.


Best regards,
Morten