Hello @Jason.Jonkman,

I have a couple of questions regarding the output list parameters.

In fact, i am looking for the acceleration of tower top in fore-aft direction in the global frame. When using the Excel file named *OutList Parameters.xlsx*, i found in the sheet named *ElastoDyn* a section called Internal degrees of freedom. Among the outputs, there is **Acceleration of 1st tower fore-aft bending mode DOF** named **QD2TFA1**. I just wanna make sure that this output is computed in the global frame (i.e. inertial frame) ?

My second question is : tower top acceleration is equal to nacelle acceleration when the yaw dof of the nacelle is disabled. Do you confirm ?

My third question : is there an output for tower top acceleration relative to the undeflected position and expression in xt-direction ?

Thank you in advance.

Best Regards,

Riad

Dear @Riad.Elhamoud,

I donâ€™t see that ElastoDyn has an output that expresses the absolute tower-top acceleration expressed in the global-inertial frame. The closest to that is ElastoDyn outputs `YawBrTAxp`

, etc., which are the absolute tower-top-acceleration expressed in the tower-top / base-plate coordinate system, which you could reorient to global coordinates based on the rotation/orientation of the tower top. You could also output the position of the tower-top in global coordinates (ElastoDyn outputs `TwHt1TPxi`

, etc., assuming you place a tower-analysis node near the tower-top) and compute its second time derivative.

I agree that the tower-top and nacelle accelerations are equivalent for a point in the nacelle located at the tower top.

Iâ€™m not sure what you mean by â€śtower top acceleration relative to the undeflected position and expression in xt-directionâ€ť, but ElastoDyn output `QD2FA1`

is the second time-derivative of the tower-top degree of freedom (DOF), and ElastoDyn uses relative coordinates for its DOFs (i.e., the tower DOFs are expressed in a frame that moves with the floating platform), so, perhaps that is what you want?

Best regards,

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Dear @Jason.Jonkman,

Thank you for your reply.

In fact, I am looking for what you have meant in your last point. In fact, if i understand well, the internal degrees of freedom in ElastoDyn are computed using the relative coordinate for its DoFs. Thatâ€™s mean the ElastoDyn output QD2FA1 is computed in the xt frame which is located at tower base and moves with the platform. Am i right ?

For platform velocities and accelerations, they are computed with respect to the global frame (fixed at Still Water Level) which does not move with the platform. Right ?

Second, when i want to know about each frame in OpenFAST, i used to looking in FAST v7 documentation. Is that the right place ? If not, could you please tell me where to look ?

Thank you in advance.

Best Regards,

Riad

Dear @Riad.Elhamoud,

I agree with your comments regarding the tower and platform DOFs of ElastoDyn.

The structural model of FAST v7 became the ElastoDyn module of FAST v8 and OpenFAST. So, the theory documented in the â€śunofficial FAST theory manualâ€ť remains the theory document for ElastoDyn, as provided on OpenFAST readthedocs: 4.2.7. ElastoDyn Users Guide and Theory Manual â€” OpenFAST v3.5.2 documentation.

Best regards,

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