Hello everyone,
I did a free decay test in OpenFast using the strip theory instead of potential theory. Before running OpenFast, i omit the linear damping. Then, i did another simulation but i added the linear damping. I was surprised : there is almost no difference between the two responses.
Therefore, the damping here comes only from the drag term in Morison equation right ?
that is what we call it quadratic damping ?
Is there any linear terms in the damping matrix somewhere in OpenFast that accounts for hydrodynamic damping or the only damping is viscous i.e comes from Morison equation ?
Thanks in advance
Best regards,
Riad
Dear @Riad.Elhamoud,
If you’ve entirely disabled the potential-flow solution, then viscous / quadratic damping is the only source of hydrodynamic damping (though there there may be other aerodynamic or structural damping depending on what OpenFAST functionality you’ve enabled).
I would generally expect the viscous / quadratic damping to dominate for large motions and the linear damping to dominate for small motions.
Best regards,
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Dear @Jason.Jonkman ,
Sorry for being so demanding. In fact, i am interested in the free decay responses. That’s mean, i disable AeroDyn. I set potential flow in HydroDyn to zero. So, i am using the strip theory. Also, i put zero the damping you’ve added to account for radiation damping which is not taken into account in strip theory. In this case, there is only two types of damping : the linear and the quadratic damping (comes from the drag term in Morison equation).
However, you have said that the quadratic damping dominate large motions.
In my case, i have small motion. So the linear damping dominates.
What do you think ? Is what i am saying correct ?
Best Regards,
Riad
Dear @Riad.Elhamoud,
It sounds like you’ve enabled the strip-theory solution and you disabled the potential-flow solution and have zeroed out the additional damping matrices; is that correct? The strip-theory solution only accounts for viscous (quadratic) drag; so, there will be no linear damping in this case.
Best regards,
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Dear @Jason.Jonkman ,
That’s correct !!! I enabled the strip-theory solution and i disabled the potential-flow solution and have zeroed out the additional damping matrices.
i succeed in implementing the viscous force and it works like expected i.e the response decays with time. So Thanks a lot !!! 
I have also a couple of questions.
1- i was thinking: let’s assume a wind blowing only on the tower in the foreaft direction. When it becomes to oscillate, i was wondering whether the damping of the blade in flapwise contributes to the damping of the gust. Therefore, the total damping is the sum of the damping of the tower in fore aft and that of the blade in flapwise. am i right ?
2- i would also ask you about the damping in Euler Lagrange formulation. I don’t know whether you are familiar or not. Anyway, I wanna hear your opinion if you don’t mind.
I considered the damping as a non conservative force. In other words, it acts like generalized forces.
I saw a video on YouTube, claiming that the damping in Euler Lagrange equation is a kind of function called “Rayleigh dissipation function”.
So, I am a bit confused.
What do you think ?
Best Regards,
Riad
Dear @Riad.Elhamoud,
I agree with (1), although I would say that tower aerodynamic loads are quite minimal relative to blade aerodynamic loads in most wind turbines when the rotor is operating.
I’m not sure I understand what you asking in (2) to comment.
Best regards,
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Dear @Jason.Jonkman ,
Sorry for the late reply !
In fact, i was writing the EOM (Equations Of Motion). The mistake i made is that when computing the generalized forces in the case of the free decay test : damping forces and the viscous term in Morison equation, i include the damping of the blade in flapwise in Q7. (Q7 represents the generalized forces with respect to the tower foreaft). The response i was obtaining was not decaying with time.
I felt there is something wrong and i imagine the wind turbine as modal masses, with dampers and modal springs and i realized that the dampers are in parallel distribution and not in series. Thats mean that the damping of the blade in flapwise does not contribute to damp the tower in foreaft.
But if one adds a TMD in the tower in fore aft, we should have in Q7 the damping force from the TMD.
I don’t know if i clarify my point of view.
Best Regards
Riad
Dear @Riad.Elhamoud,
I’m not fully sure I understand what you are saying. But certainly OpenFAST considers tower damping associated with blade aerodynamics and tower- or nacelle-based TMDs, when enabled.
Best regards,
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