## Printing the axial induction factor in FAST

Discuss the theory and modeling of rotor aerodynamics.

Moderator: Bonnie.Jonkman

Jason.Jonkman
Posts: 5727
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Printing the axial induction factor in FAST

Dear Bowen,

In a parked or idling simulation, you should disable induced velocity (WakeMod=0); this will disable the skewed-wake correction. And for cases with high angle of attack, you should also disable unsteady airfoil aerodynamics (AFAeroMod=1). In this case, the angle of attack is determined geometrically by AeroDyn and the results should be simple to confirm by a hand calculation. You say you've pitched the blades to 90 degrees, but Figure 2 shows the airfoil flat into the wind, which implies a pitch of 0 degrees. I would expect angles of attack close to zero degrees in a parked/idling condition without yaw error. I would also verify that the aerodynamic loads computed by FAST/AeroDyn are in fact steady by simulating multiple time steps.

Regarding (1), I would guess the difference in results for different azimuth angles depends on the rotor tilt or wind shear.

Regarding (2), if you are hand-calculating the correct angles of attack, the force coefficients can be predicted by table lookup and the forces can be computed with these coefficients and the dynamic pressure. If you can account for compound angles correctly, I would think your hand calculation would match the result from FAST/AeroDyn. AeroDyn can output more than just the local angle of attack at each cross section, which you can use to double check each step of the calculation.

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

Bowen.Jiang
Posts: 7
Joined: Sat Aug 17, 2019 4:41 am
Organization: Chongqing University
Location: China

### Re: Printing the axial induction factor in FAST

Dear Jason，
In the question1, I made a mistake expressing my yaw angle. I set the“NacYaw” to 90, BlPitch(1), BlPitch(2) and BlPitch(3) to 90. Just in this case, when the “Azimuth” were set to 0, I looked the angle of attack of blade 1 from FAST. I found that the angle of attack was about -280 deg, but the “Airfoil” file just covers -180 deg to 180 deg. Would you please tell me how did FAST simulate the drag coefficient in this situation？In the beginning, I thought whether FAST change this angle (-280 deg) to the range from -180deg to 180 deg.That’s to say, -280+360=80 deg, then FAST obtain drag coefficient by linear interpolation. But in “Airfoil” file, this coefficient should be much larger than those from FAST. I felt so confused.
Sincerely
nacyaw90 azimuth0.png (13.78 KiB) Viewed 1518 times

nacyaw90 azimuth0 AOA.png (11.98 KiB) Viewed 1518 times

Jason.Jonkman
Posts: 5727
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Printing the axial induction factor in FAST

Dear Bowen,

I would expect the angle of attack to be converted (MODULO arithmetic) to between -180 and 180 degrees so that -280 = 80 degrees. I asked Bonnie Jonkman and she said that she recalls the angle of attack being output incorrectly in an older version of AeroDyn v15, which is a bug that has now been fixed. Which version of FAST/OpenFAST are you using? Does switching to the newest release of OpenFAST solve your problem?

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

Bowen.Jiang
Posts: 7
Joined: Sat Aug 17, 2019 4:41 am
Organization: Chongqing University
Location: China

### Re: Printing the axial induction factor in FAST

Dear Jason，

The version of FAST I used earlier was FAST (v8.16.00) and AeroDyn (v15.03.00）.When I switch to OpenFAST-v2.0.0,the problem has been solved.
Thank you very much.

Sincerely