Dear Jason,
Thank you for your answer. I understand what you mean about the platform, so I will keep in mind that the results will be similar but not identical.
However, I had the idea that the global inertial frame of OpenFAST is rotating with the wind propagation direction, based on OpenFAST manual, p.148: “The positions must be specified in the OpenFAST global inertial frame coordinate system, which is located at the tower base and has the x-axis pointing downwind, the y-axis pointing laterally, and the z-axis pointing vertically upward.”
But according to your response on this thread: Coordinate System
“The inertial frame coordinate system used in OpenFAST has the origin at the intersection of the undisplaced tower centerline and mean sea level, with the X-axis along the zero-degree wind/wave direction, the Z-axis vertical (opposite gravity), and the Y-axis directed to the left when looking along the zero-degree wind/wave direction.”
So, according to the first sentence, it is implied that the coordinate system is aligned with the wind at any case, whereas on the second that it is fixed at 0 deg wind and wave direction.
I tried to test this hypothesis via running simulations for the afforementioned configuration:
1.) Steady wind field of 10 m/s and a PLexp of 1.1
2.) HydroDyn,MoorDyn and platform 6 DoFs turned off
3.) The DISCON.IN controller is the one for the IEA 15 MW monopile configuration
4.) Nacelle yaw DoF is turned on
5.) Two wind directions,0 deg and -20 deg. For the latter, I used PropagationDir=-20 deg, NacYaw IC =20 deg and YawNeut=20 deg in ServoDyn.
6.) Simulation time of 600 s (total simulation time 1000 s, where the first 400 s are discarded).
In addition to the mean power produced by the turbine in that windspeed, I wanted to see the response of the turbine and, for that reason I used the flapwise moment at the blade root and the out of plane tip deflection. Below, you can see some zoomed charts with the aforementioned responses.
As far as the mean power is concerned, it is 14.47 MW in the case of a -20 deg wind misalignment and 14.44 MW in the case of the 0 deg wind direction. All three results suggest that, indeed, although there are discrepancies between the two cases, they are more or less identical, so whether the coordinate system rotates or not does not influence much the results.
However my issue is the Wind1VelX output. Currently it is set at the point (0,0,150) (m) as I want to output the wind speed at the IEA 15 MW hub height. In the case of the misaligned case, the mean wind speed drops to 9.4 m/s. I tried to move the point along the x-y plane, but again the wind speed results were the same. In my opinion, it is strange because, not only the coordinates specified in InflowWind are in the global system, but also, due to steady wind, all points in the same height should have the same wind speed. Also, the points do not feel the rotor wake as, if it is considered that the points are set in the global coordinate system, the point (0,0,150) should be behind the rotor, so it would experience lower wind speed, which is not the case.
Therefore, I am a bit confused on explaining the results I get and I would like to have your opinion on these.
Best regards,
Ioannis Voultsos.