# Difference between FAST and OPENFAST results

Dear @Lin.Ding,

A positive `RootMxc` will compress the leading edge and tension the trailing edge. A positive `RootMyc` will compress the suction side and tension the pressure side of the airfoil.

Best regards,

Dear @Jason.Jonkman

I have a question that I would like to consult you. The formula for calculating the axial stress of a section in Fig.2 can only be used for isotropic material sections, right? The root of the blade is made of composite material, so this formula cannot be used.Is my understanding correct.

Best regards,

Dear @Lin.Ding,

I would say that your classic stress formula is valid for both isotropic and orthotropic materials. But I agree that the formula may not apply for composite materials. See: Bending - Wikipedia.

Best regards,

Dear @Jason.Jonkman

I have a few questions I would like to consult you.

Question 1: Does Load Roses calculate fatigue in all directions,
Question 2: I calculated the fatigue in the 0 to 180 Ā° direction based on Load Roses (rootmxb, rootmyb), and the curve I plotted shows that the short-term DEL is the smallest at 90 Ā°. But at 90 Ā°, there is only rootmyb, which is the force mainly acting on the fan blades in this direction. shouldnāt fatigue be maximum in this direction? This confuses me.

I have attached the data file and test18.mlif used for your reference.

Thank you very much for your answer!

Best regards,

Dear @Lin.Ding,

Hear are my responses:

1. Yes, the load roses computed by MATLAB cover the full orientation around the cross section. The orientation only needs to cover 180 of 360-deg total because the result from 180 to 360 would be identical as the result from 0 to 180 (the loads in the two sectors would be the same, but opposite in sign).
2. The load rose of short-term DELs matches the time series youāve provided where it looks like the oscillation amplitude is much higher in RootMxb than in RootMyb. You havenāt stated what your simulation set up is, so, Iām not sure why this is. Iām also puzzled why the mean of RootMxb is near zero. Are you simulating with high shear?

Best regards

Dear @Jason.Jonkman

In fact, I did not set wind shear. I simulated it based on test 18, and I did not change any other settings except for wind speed.

I have attached the settings for OpenFAST at a wind speed of 3m/s

Best regards

Dear @Lin.Ding,

Overall your settings look fine except for the initial conditions of `BlPitch` and `RotSeed` in ElastoDyn that are not set appropriately for a wind speed of 3 m/s (rather it looks like your initial conditions are more appropriate for cut-out wind speeds). My guess is that your controller is not responding properly, resulting in an unrealistic turbine response, but that will be difficult to confirm without also outputting the rotor speed and blade-pitch angles from ElastoDyn.

Best regards,

Dear @Jason.Jonkman ,

Iām very sorry that the file I provided made you misunderstand. I have matched corresponding BlPitch and RotSeed for different wind speed conditions when using OpenFAST. (The reason for the problem here is that I directly took out the setting file for the 25m/s working condition and simply changed the wind speed to 3m/s. I originally wanted you to see the setting issue, but forgot to modify BlPitch and RotSeed.)

I reapplied the operating conditions with wind speeds of 3, 5, 7, 9, and 11.4m/s, and output BldPitch and RotSpeed.

Best regards,

Dear @Lin.Ding,

OK, thanks for clarifying. That makes more sense. Looking at your results closer, it looks like `RootMxb1` is dominated by the gravitational load, which makes sense why it has a much higher oscillation amplitude than `RootMyb1`, which only oscillates due to the aerodynamic load variation with shaft tilt.

Best regards,