## Linearization issue using Fast V7.02

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Babak.Gavgani
Posts: 29
Joined: Sat Mar 14, 2020 12:03 pm
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Location: Belgium

### Re: Linearization issue using Fast V7.02

Dear Jason,

I don’t have two distinct models.
At first I find the optimal value for the TSR and Cp of the turbine.
Next I do the calculations in opposite direction: If I set that turbine to rotate at fixed optimal speed (for its corresponding wind speed) and I find the TSR and Cp values, they should be the optimal values that I had on the first step. But it is not!!
Because: TSR = (R*rotor speed)/v OR
rotor speed = (TSR *v)/R

Bests,

Jason.Jonkman
Posts: 4952
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Linearization issue using Fast V7.02

Dear Babak,

To answer your question, I need to understand what the difference is between these two FAST solutions:

Solution 1
At first I find the optimal value for the TSR and Cp of the turbine.

Solution 2
Next I do the calculations in opposite direction: If I set that turbine to rotate at fixed optimal speed (for its corresponding wind speed) and I find the TSR and Cp values, they should be the optimal values that I had on the first step. But it is not!!
Because: TSR = (R*rotor speed)/v OR
rotor speed = (TSR *v)/R

What is the difference between how these two FAST solutions are set up or post-processed?

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Babak.Gavgani
Posts: 29
Joined: Sat Mar 14, 2020 12:03 pm
Organization: ---
Location: Belgium

### Re: Linearization issue using Fast V7.02

Dear Jason,

I want to do linearization around MPP. So I need to have the rotor speed at MPP and then do linearizations.
--But how to find rotor speed at MPP? To do this, I find the optimal TSR and then calculate the rotor speed at MPP for its corresponding constant wind speed. The values are surely correct.

The problem is when:
I just fix the wind speed to constant value and I set the optimal rotor speed to its corresponding wind speed. But there is an inconsistency in the FAST output for Cp and TSR while setting rotor speed to its MPP value.
All DOFs are disabled.
I expect to see that the TSR and Cp are the optimal values but they are not and Cp is lower than Cpmax!!!
But why?
The wind speed is constant and the turbine is set to rotate at fixed rotor speed at MPP. SO the TSR and Cp must be the optimal values.

Kind regards,

Jason.Jonkman
Posts: 4952
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Linearization issue using Fast V7.02

Dear Babak,

To debug this, I need to have the input files associated with Solution 1 and Solution 2 to see what is different between the two solutions. Please attach those.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Babak.Gavgani
Posts: 29
Joined: Sat Mar 14, 2020 12:03 pm
Organization: ---
Location: Belgium

### Re: Linearization issue using Fast V7.02

Dear Jason,

Thanks for your reply.
The solution 1 is just to verify optimal values of TSR and Cp. In different literatures, these values are TSR =5.6 and Cpmax = 0.42. So I have just included the files for solution 2. Please find in the attachment the files. 10kW wind turbine is considered.

All DOFs are disabled.
Wind speed is set to 10m/s and I have fixed the rotational speed to the MPP of its corresponding wind speed (10m/s) and pitch angle is fixed at its optimal value (7.5°).
The rotor speed has been calculated as below:
Rotor Speed at MPP = (Optimal TSR)*(Wind Speed)/Rotor radius
Optimal TSR = 5.6; Wind speed =10m/s; Rotor radius = 2.9m SO rotor speed at MPP = 19.31034 rad/s or 184.4002 rpm
The optimal values for TSR and Cp are 5.6 and 0.422, respectively. These values are surely correct.

I obtain the Cp and TSR values from Aerodyn.
If you debug it, you will see that the Cp and TSR are not the optimal values.

Kind regards,
Babak
Attachments
Files.7z
(9.06 KiB) Downloaded 12 times

Jason.Jonkman
Posts: 4952
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Linearization issue using Fast V7.02

Dear Babak,

I'm not an expert on the SWRT model, and I'm not sure what literature you are referring to, but my guess is this OpenFAST models does not predict the same power coefficient curve that you have found in literature. I would suggest trying to generate Solution 1 yourself using this OpenFAST model to better understand the difference.

I briefly looked at your input files, and nothing sticks out to me as incorrect. I did notice that you've set the fixed blade-pitch angles to be 7.5 degrees, whereas the sample SWRT models provided in the OpenFAST r-test use 11.44 degrees; I'm not sure I know why these angles are different.

Please be aware that if you decide to enable structural DOFs, that OpenFAST does not yet support tail-fin aerodynamics or furling degrees of freedom (NREL has not yet been funded to implement this functionality into OpenFAST). If you need to model these physics, I suggest downgrading back to FAST v7.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

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