FAST8 Linearization

[Jason.Jonkman]

Dear Charlie,

Can you clarify your question? Are you trying to calculate the blade and tower mode shapes from mass and stiffness for use in FAST? Or do you already have a FAST model and you want to visualize the mode shapes--if so, for the full system (blade coupled to tower) or in isolation (blade only and tower only)?

Best regards,

[Charlie.Sheng]

Dear Dr. Jonkman,

Sorry for my unclear wording. Here, I already have completed the modelling in FAST as well as got the blade and tower natural frequencies though the linearization analysis discussed previously. I wanna plot the full system mode shapes, as well as the isolated blade or tower mode shapes. As I want to compare and have a look the difference of tower and blade mode shapes with and without the system couple. Hope I make the question clear.

Best regards,
Charlie

[Jason.Jonkman]

Dear Charlie,

FAST does not currently support visualization of the full-system mode shapes. The CampbellDiagram.xls spreadsheet that you've no-doubt used to interpret the linearization output (after MBC3) is what we've used historically to interpret the full-system modes (i.e. to identify the contribution of each DOF to each full-system mode). That said, we will soon be initiating a project to add visualization functionality of the full-system modes in collaboration with Envision Energy through an extension of the current FAST visualization functionality (involving VTK files).

For the isolated blade and tower modes, the mode shapes are inputs to the ElastoDyn module in the form of polynomial coefficients. You can visualize the mode shape by plotting the polynomial i.e.

y(x) = C_2*x^2 + C_3*x^3 + C_4*x^4 + C_5*x^5 + C_6*x^6

where x is the normalized distance along the blade or tower (0 <= x <= 1) and y(x) is the normalized mode shape (normalized so that y(1) = 1).

Best regards,

[Charlie.Sheng]

Dear Dr. Jonkman,

That really helps me a lot for my concern, thanks your informative reply!

Best regards,
Charlie

[Yu.Zhao]

Dear Jason!
When LinInputs = 1, it can get nine inputs.How can I reduce the number of input variables?In addition,How to get the value of the input variable as a change of time.
Could you help me please ?

Kind regards,
Yu.Zhao

[Jason.Jonkman]

Dear Yu.Zhao,

In FAST v8 and OpenFAST, you can choose in the linearization process to include no inputs (LinInputs=0), the standard set of 9 inputs (LinInputs=1), or all module-level inputs (LinInputs=2). If you only want some of the 9 inputs, simply ignore the columns associated with the inputs you don't want.

I'm not sure I understand your second question regarding the inputs changing with time. Are you asking about the linearization functionality or something else?

Best regards,

[Zhe.Zhang]

Dear Jason,
I have used FAST and MBC3 to calculate the coefficient matrix of the state space equation. My matlab foundation is relatively poor, so I would like to ask how to build this model using matlab. Can you provide a code example?
Best regards,

[Jason.Jonkman]

Dear Zhe.Zhang,

I would encourage you to read the paper that describes how the linearization capability was added to FAST v8: https://www.nrel.gov/docs/fy17osti/67015.pdf. The examples in this paper should hopefully explain the process.

I'm not sure if you are specifically interested in the wave disturbance mentioned in the paper excerpt you posted, but this has recently been added to OpenFAST--see the following paper: https://www.nrel.gov/docs/fy19osti/71865.pdf. This feature is in the final stages of being included in a public release.

Best regards,