Initial Condition for Rated Speed Region 🙏

[Parrotyeon]

Dear all,
I'm currently working on OpenFAST for OC4 Semi-submersible model.
For fatigue analysis i got to repeat a lot of cases.
Before i kick in the batch processing, i think i should check whether the OpenFAST runs normally with my cases.
So, i ran it "Still water" & "Steady wind velocity of 15.6 m/s".
But i had some problems having to do with three warnings which came out steadily:

1. Turning off UA due to high angle of attack or low relative velocity
2. Mach number exceeds 0.3
3. Mach number exceeds 1.0

I supposed that those warning were due to initial conditions: rotor speed, pitch angle, tip out-of-plane displacement, tip in-plane displacement (i modified later two options after i read this document: https://forums.nrel.gov/t/high-angle-of-attack-at-large-wind-speed/6175/2).
But i still couldn't fix the error ...
Could I get some advice on this issue?

Best regards,
Yeon
---- Here's my settings ---

(this option was the only thing that i could run without fatal error, wind velocity 15.6)


(Every thing is same except those modifications)

[jjonkman]

Dear @Parrotyeon,

As I mentioned in the forum post your cite (as well as other posts), we generally recommend initializing the rotor speed and blade-pitch angles to their expected (mean) values conditioned on the mean wind speed, to minimize start-up transients and to avoid issues with the initiation of the controller. For the NREL 5-MW baseline turbine at 15.6 m/s, these should be around RotSpeed = 12.1 rpm and BlPitch = 12deg, as summarized in Figure 9-1 from its specifications report: https://www.nrel.gov/docs/fy09osti/38060.pdf (values at other wind speeds are also plotted).

It is not typically important to initialize the blade-tip and tower-top displacements because these tend to converge quite quickly after a short start-up transient, but you certainly can initialize these if you want to reduce the start-up transient. The values you've specified for OoPDefl and IPDefl are reasonable close to what I would expect at 15.6 m/s.

For floating systems, the start-up transient can still be significant (due to low natural frequencies of the floater motions) if you don't also initialize the floater displacements, especially surge and pitch. So, I would recommend initializing PtfmSurge and PtfmPitch as well,

Best regards,

[Parrotyeon]

Dear @jjonkman,

Thank you for your quick reply.
I appreciate your comment regarding the platform motion, which I had not fully considered.

By the way, would it be desirable—and beneficial for faster convergence—to use initial conditions corresponding to a given sea-state environmental condition, such as a JONSWAP spectrum with a specified peak period and significant wave height?
I believe this could be advantageous.

Best regards,
Yeon

=== update ===
I ran the OpenFAST with adequate initial conditions with still water sea-state.
I didn't see any error.
Thanks for the advice 👍

[jjonkman]

Dear @Parrotyeon,

I would not expect the spectrum to influence the mean platform motions. There may be some small effect from second-order mean drift loads (first-order waves can't shift the mean), but I would expect these to be much less important than the mean aerodynamic thrust.

Best regards,

[Parrotyeon]

Dear @jjonkman

Thank you for your guidance on how to run OpenFAST.
I truly appreciate getting a direct reply from the main developer.
It means a lot to me.

Have a nice day!

Best regards, Yeon