[FOWT]Platform pitch and roll motions largely different from Bladed results under parked condition and specified azimuth angle.

[getChaos22]

Hi all,

I am currently comparing results between OpenFAST and Bladed for FOWT through the IEA 15MW wind turbine atop UMaine platform. But I found some differences existed in the platform pitch and roll movements under some specified condition like parked with an azimuth angle of 50 degree.

In the following picture, the results were obtained under still water, steady wind with no wind sheer and the wind direction is -30°. The turbine was parked and the azimuth angle is 50°. The unsteady aerodynamic or dynamic stall were both closed in Bladed and OpenFAST

It seems like Bladed and OpenFAST show different damping effects.

Do you have any ideas or experiences about this kind of problem? Is this related to aerodynamics or hydrodynamics?

Thanks!
Chao Li

[jjonkman]

Dear @getChaos22.

It sounds like you are running a simplified form of DLC 6.2 (with steady wind and still water), but for a parked/idling condition with high yaw error that can result in an aeroelastic stability, leading to growth in response over time. I agree that it appears that your OpenFAST model has more damping than your Bladed model, but I'm not sure I know why that would be (and could depend on various damping levels like structural and hydrodynamic). DLC 6.2 is known to generate a blade-edgewise instability whereas you are showing platform roll and pitch, which is much further down the load path. To isolate where the difference in damping levels are coming from, I would suggest to first start by comparing the blade response with a rigid support structure, and then add complexity in steps (only blade flexibility, then blade + tower flexibility, then blade + tower flexiblity + floater motion).

Best regards.

[getChaos22]

Dear @jjonkman

Thank you very much for your quick reply. Please first apologize for my unclear description. I forgot to mention that in the cases I showed above, rigid tower and blades were employed.
Following your advice, I have tried flexible blades with rigid tower and fixed bottom. In this situation, results of OpenFAST and Bladed do show blade instability. OpenFAST didn't finish the calculation and the error message shows that the Mach number exceeds 1.0.

It is clear that blade responses obtained by Bladed and ENFAST are different. Basing these results and those I showed above, is it solid to say that it is aerodynamic instability causes the differences of platform motion between Bladed and OpenFAST? Even though in the floating case, the blades and tower are rigid, maybe the aeroelastic instability is reflected in platform motion? This is my guesses. If it does not make sense, please correct me.

Thanks again.

[jjonkman]

Dear @getChaos22,

Thanks for clarifying.

Yes, I agree that it is an aerodynamic instability that is coupling with floater motion in your original post and with blade deflection in your second post. Again, I'm not sure why OpenFAST and Bladed do not match better for this case, but I agree that it could be due to different levels of damping. If you disable all DOFs in both OpenFAST and Bladed, do you get similar aerodynamic loads between the models for this case (which would imply that differences in damping are more likely than differences in direct aerodynamic forcing)?

Best regards,