Dear @Jason.Jonkman ,
Hope you are doing well.
I wanna ask whether the time step in Hydrodyn is different from elastodyn ?
For the first glimpse, i thought that the time step is the same due to the fact that OpenFast is a hydro-elastic tool.
Moreover, what is the difference between the time step in the fst file and the time step present in elastoDyn ?
Furthermore, in the expression of irregular long crested wave appears a variable x. I don’t remember where i read (maybe in your thesis !) that x is the undisplaced position of the platform (x=0 for OC3 Hywind ?), is that right ?
BR
Riad
Dear @Riad.Elhamoud,
OpenFAST presently requires that all modules communicate with each other at the same rate–as regulated by the OpenFAST driver/glue code–which is the time step (DT) specified in the OpenFAST primary (.fst) input file.
Each module also has its own time integrator based on a time step that is specified at the module level, e.g., DT in ElastoDyn and RdtnDT in HydroDyn. You can specify these time steps at the module-level as “DEFAULT” if you want to use the same time step as the driver/glue code.
OpenFAST currently supports subcycling, which means the module-level time steps can be smaller than the driver/glue code, but does not support oversampling. Moreover, if you have subcycling, OpenFAST currently requires that the module-level time steps be integer divisors of the driver/glue code time step.
I’m not sure I understand your last question; can you share a link to the expression you are referring to?
Best regards,
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Dear @Jason.Jonkman ,
Sorry for the late reply.
Thank you so much for clarifying the different time steps.
I am so sorry for my question whichis related to the variable x in the expression of irregular wave. I was misconfused. So, if i get it, the variable x in the expression of irregular waves generated by HydroDyn is the position of the floater at each time step. right ?
Thank you
BR
Riad
Dear @Riad.Elhamoud,
I’m still not sure I understand what expression you are referring too. Can you share a link or identify the source of the expression you are referring to?
Best regards,
Dear @Jason.Jonkman ,
Sorry for the late reply. The “x” am referring to is in the following figure (The figure is the expression of regular waves):
Could you please explain it to me ?
Best Regards,
Riad
Dear @Riad.Elhamoud,
In the HydroDyn module of OpenFAST v3.4.1 and older, the undisplaced position of the structure was used in the calculation of the wave kinematics in the strip-theory solution. In OpenFAST v4.0 and newer (coming soon), NREL is introducing the new SeaState module that will generate a 4D grid of wave kinematics (three spatial dimensions plus time) from which HydroDyn can interpolate within at each time step when calculate strip-theory loads such that the use of the displaced position can used when calculating wave kinematics in the strip-theory solution.
Best regards,
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Dear @Jason.Jonkman,
Very good news !! I’m glad to hear about this grid !! 
So, in the actual HydroDyn, the undisplaced position is the initial condition right ?
Best Regards,
Riad
Dear @Riad.Elhamoud,
No, by “undisplaced position” I meant the position defined by zero displacements of the structural degrees of freedom (DOFs, e.g., PtfmSurge = PtfmSway = PtfmHeave = PtfmRoll = PtfmPitch = PtfmYaw = 0). The initial conditions may be different from zero, and most likely the transient dynamic response will involve nonzero displacements of the structural DOFs (unless the DOFs are disabled), but until SeaState is introduced in v4.0, HydroDyn always used the wave kinematics at the undisplaced position regardless of the displacement.
Best regards,
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