Hi everybody,

I am trying to find the aerodynamic thrust force (perpendicular to the plane of rotation) of a wind turbine. As written in AeroDyne user’s guide, the output parameter ForcN# indicates the element thrust force, so I am trying to use this output parameter.

The modeled wind turbine is three bladed, and I specified ten elements per blade. As the rotor has 3 blades I expected to get time histories of ForcN# for 30 elements, but in the output file I got time histories of ForcN# for 10 elements.

It would be much appreciated if you could let me know these thrust force time histories belong to which blade, and how I can find the whole thrust force on the rotor.

Best Regards

Salehyar,

The blade-element files are mostly for debugging and trying to get a handle on what’s happening. Only data for Blade 1 is output.

The only FAST output I know of is the LSShftFxa (LSS thrust). See Table 35 on page 114 of the FAST user’s guide. Unfortunately, it includes gravitational and inertial forces. I would think it would be representative if the winds were constant, the turbine was rigid, and the shaft tilt was zero. If the tilt is not zero, you can probably compute the gravity load and take out its affect.

Depending what you are trying to do, you may consider using WT_Perf–it outputs pure aerodynamic thrust.

Marshall

Dear Marshall,

Many thanks for your reply to my post.

The modeled wind turbine is rigid, the shaft tilt angle is zero, and the wind speed is constant (it does not change with time) and I have considered a power law vertical wind shear for it. The wind turbine has degrees of freedom in surge and pitch directions, and is subjected to rigid body periodic motions in these two DOFs.

It would be much appreciated if you could let me know ,whether in these conditions the output parameter “LSShftFxa” indicates purely aerodynamic thrust forces or not, and whether it contains any forces due to the control systems of the wind turbine or not.

Best Regards

Dear Salehyar,

The output LSShftFxa (or RotThrust) in FAST includes all loads transmitted across the shaft between the rotor and gearbox, including the applied aerodynamic loads, gravitational loads (from the rotor mass and shaft tilt/tower deflection/platform pitch), and inertial loads (from the rotor mass/inertia and acceleration/deceleration). If the system is oscillating with surge and pitch motions, then the gravitational and inertial terms will not be zero, so, LSShftFxa won’t represent the pure aerodynamic thrust.

I hope that helps.

Best regards,

Dear Jason,

Many thanks for your guidance.

Best Regards

Dear Jason and Marshall,

Based on your comments, I concluded that in my modeled wind turbine , I cannot use the output parameter “LSShftFxa” to find purely aerodynamic thrust forces; Furthermore AeroDyn output for the thrust force is just for the blade number1.

If in the FAST primary input file, I use “AzimB1Up =0” and then run the program for the 3 cases: “Azimuth=0, Azimuth=120 ,Azimuth=240” , and sum the AeroDyn output parameter " ForcN# " for these three cases, does the result indicate the time history of the pure aerodynamic thrust forces on the rotor?

Many thanks for your kind cooperation.

Best Regards

Dear Salehyar,

If each blade is identical, I agree that your approach would result in the pure aerodynamic thrust. Of course, if your rotor has precone or blade deflection, the summation will not be trivial becaues AeroDyn’s output forces, “ForcN#”, are normal to the instantaneous position of the blade element (the vector sum of “ForcN#” would need to be applied if the rotor has precone or blade deflection).

That said, if all you’re after is the average aerodynamic thrust, you could also derive it from LSShftFxa as follows:

*Subtract from LSShftFxa the value of RotMass*Gravity*SIN(ShftTilt), where RotMass is the rotor mass as listed in the FAST summary (*.fsm) file; this will eliminate the average gravitational contribution.

*Then average the result over some time scale much larger than the natural periods of structural oscillation; this will eliminate the inertial contribution, resulting in the average aerodynamic thrust.

I have already added an aerodynamic rotor thrust output to the list of features we plan to add to the code in the future.

Best regards,

Dear Jason,

Can you eliminate the effects of gravity by setting the gravitational acceleration constant in the FAST input file to 0? I assume this would eliminate the effects of gravity on the entire structure and may not acceptable but should also negate the gravitational effects of surge and pitch motions. I have used this approach before just to get an idea of aerodynamic loads for troubleshooting purposes. I would appreciate your thoughts on the validity of this approach. Thank you.

Best Regards,

Subin.

Dear Subin,

Of course, setting Gravity to 0.0 in FAST will eliminate the gravity effect. However, gravity impacts the system response, so, the aerodynamic loads may be different with and without gravity (e.g., gravity could impact system deflections, which may lead to different aerodynamic loads with and without gravity). The magnitude of these differences will depend on your system.

Best regards,

Dear Jason,

Thank you for that quick response. Every little bit helps .

Best Regards,

Subin.

Dear Jason and Subin,

Many thanks for your comments. They are really helpful.

Best Regards

Hello everybody,

I am modeling an upwind turbine, with zero yaw angle. Wind is blowing to the rotor such that it makes a 90 degree angle with the rotor plane. As it can be read in the AeroDyn User’s guide, AeroDyn can give us the aerodynamic forces normal and tangential to the plane of rotation for each blade element. I would like to know the sign convention of the output, what does it mean when the forceN, forceT are negative or positive?

Furthermore, I appreciate it if you could let me know whether the orientation of the tangential force acting on a blade element is fixed with time or it depends on the position of the blade in the rotor plane (I am asking this because I would like to know whether I can sum up the forceT acting on all the blade elements in three blades at a fixed time, and consider it as the total tangential aerodynamic force at that time) .

Best Regards

Sara

Dear Sara,

For the convention of AeroDyn outputs ForcN and ForcT, please see the Appendix C, particularly Figure C2, of the AeroDyn User’s Guide: wind.nrel.gov/designcodes/simula … eroDyn.pdf. These forces are in a body-fixed coordinate, so they move and rotate as the blade deflects. It is only easy to sum them up if the blade is rigid (otherwise, you’d have to do a vector sum considering the instantaneous deflection).

I hope that helps.

Best regards,

Sir,

From the Aerodyne Theory Manual (Moriarty and Hansen, 2005), is it ok to solve Equations 26/27 (a) and 28(a’) without using Newton Raphson, Fixed-Point or Brent? For instance, could I simply use a for-loop algorithm to obtain the induction factors?

Kind regards,

Wesley

Dear Wesley,

I’m not sure what you mean by a “for-loop”, i.e., what is changing in the loop? Regardless, I think any solution algorithm is OK as long as you are converging to a consistent solution to the underlying algebraic equations.

Best regards,