ACDC without aerodynamics

Wind & Water Structural Analysis
1

I am trying to use ACDC to perform linearization without aerodynamics at different rotor speeds.

My question has to do with Calc Steady. Since the rotational speed now is no longer determined by a balance of torques but is rather defined directly, What does CalcSteady do?

The current behaviour is that it never reaches a steady state and it just linearizes once at the end. But this is not what I want. I have to perform the linearization at many points in a rotor revolution.

I thought about changing the trim case but this is not an option within ACDC (it automatically chooses trim case 2)

I also thought about forcing linearization at specific times after the structure has settled. But this is also not an option in ACDC and would require a script to calculate the times anyway because they would be different for every rotational speed.

Thanks to everyone in advance for the support!

2

Dear @Vasilis.Xenodochidis,

When aerodynamics are disabled, the use of CalcSteady = TRUE, TrimCase = 2 during a linearization solution is correct. While the aerodynamic torque is zero without aerodynamics, TrimCase = 2 will adjust the generator torque down to zero to match and maintain the specified RotSpeed.

Best regards,

3

Thank you for your reply,

But there is still something i do not understand. What is the criterion for a stable operating point in this case? The rotational speed is fixed. When does OpenFAST say, ok, now we have reached a periodic OP and linearization can begin?

I am trying to understand this so that I can effectively troubleshoot the behaviour i mentioned in the original post. (OpenFAST never detects a periodic solution and just forces 1 linearization at the end)

4

Dear @Vasilis.Xenodochidis,

During a steady-state solve, OpenFAST reviews the states over a full azimuth rotation to assess their periodicity. If a steady-state solution is not found, I would review the time series of generator torue, rotor speed, blade/tower deflections, etc. to figure out why. Is the solution converging such that increasing TMax would result in convergence? Is additional damping (Twr_Kdmp, Bld_Kdmp) needed to reach a steady-state solution?

Best regards,

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