constraint equations of the multibody approach

Please correct me if i am mistaken, but I saw that FAST employs a multibody formulation, modelling the tower, generator shaft and rotor blades using flexible body dynamics, while for other components like nacelle and hub applies rigid body dynamics. In that case, you come up with a system of equations that is the equations of motion for the whole wind turbine. But in multibody approaches, you usually have to solve for some algebraic constraint equations at the same time with the aforementioned system.
I didn’t find in any documentation or manual these constraint equations, and I would like to see what constraints are applying between the different components. Is it possible that you direct me to some documentation or even in which module of the code should I look for?

PS: Btw, as its my first post i would like to thank you for this helpful forum and for sharing your in-house software like FAST.

Dear Ethan,

Jason is out for about a week, so I thought I’d point you to some documentation you can look at for now. He has often talked about the FAST theory basis here and searching this forum for “fast theory” may help you find a few more articles that will help you. However, his post of Dec. 15, 2010 in the thread seems to collect much of the information. Please review that posting.


Dear Marshall,

thank you for your response and that thread. I found there in the post from Jason that describes the EoM in the standard multibody dynamics form but with no constraints. I am wondering if that’s the case, because from the following documentation “Modeling of the UAE Wind Turbine for Refinement of FAST_AD (J.M. Jonkman, 2003)” there is a part that is maybe related to the constraints but it’s not that clear to me, for this reason I initiated this thread, but I will keep searching for relevant information since Jason is back and we discuss it further.

Best regards,

Dear Ethan,

A general-purpose multi-body formulation would typically require the solution of equations of motion with constraints. However, the structural-dynamics model in FAST v7 is not general purpose. Instead, the structural-dynamics model of FAST v8 is built around a fixed configuration; it uses appropriately chosen degrees-of-freedom such that the solution does employ constraints.

Likewise, the ElastoDyn and SubyDyn structural-dynamics modules of FAST v8 do not employ constraints. However, the coupling between modules in FAST v8 does impose constraints that must solved as part of the solution process. The modularization framework of FAST v8 is also set up to be able support modules with internal constraints. See the framework overview paper for more information:

Best regards,