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Fixed rotor speed

Posted: Thu May 02, 2019 1:59 pm
by Tomas.Cortes
Hi everybody,

Does anyone know how to run a simulation with a fixed rotor speed? I want to use several subroutines, so the standalone file is not an option. I've defined the rotor speed in elastodyn and turned off the control sub-routine. However, in the output file I keep getting a rotor speed varying. Is there any way to do it?

Another doubt: I run a simulation with a wind speed of nearly 0m/s, (10^-9m/s). Still, it crashed because there was a node in which the speed reached mach number. How is it possible and how can I fix it?

Thanks in advnace,

Tomás Cortes

Re: Fixed rotor speed

Posted: Thu May 02, 2019 8:47 pm
by Jason.Jonkman
Dear Tomas,

You can run a FAST / OpenFAST simulation with a fixed rotor speed by disabling the generator degree of freedom in ElastoDyn (GenDOF = False). Even then you may get a slight variation in rotor speed due to shaft torsion; if don't want that either, disable the drivetrain torsion DOF in ElastoDyn as well (DrTrDOF = False).

A node will only reach mach speed if the model has gone numerically or physically unstable. Perhaps this is because you disabled control while keeping GenDOF enabled? If so, I would guess the problem would go away by disabling GenDOF.

Best regards,

Re: Fixed rotor speed

Posted: Mon May 06, 2019 5:30 am
by Tomas.Cortes
Dear Jason,

Thank you for your reply.
I've managed to solve the problem! I have one more question. How is it possible that the same turbine, rotating with the same rotor speed (9RPM) and in two situations:
a) wind speed=10^-9m/s
b)wind speed= 8m/s

has the same GenPwr and GenTq?

Best regards,

Tomás

Re: Fixed rotor speed

Posted: Mon May 06, 2019 6:42 am
by Jason.Jonkman
Dear Tomás,

Disabling GenDOF can result in nonphysical behavior. When GenDOF = False, the generator will spin at a fixed speed regardless of the torques applied (even if the applied torques are not balanced). The generator model will calculate the torque and power based on the fixed speed, even if there is not enough aerodynamic torque to maintain that torque and power.

Best regards,