Hi! Thanks to this forum I was able to find the correct loads to apply to my model in case of considering the RNA as a lumped mass (those loads being RtAeroFxi, RtAeroFyi, RtAeroFzi, RtAeroMxi, RtAeroMyi, RtAeroMzi). Now I’m trying to use OpenFAST to benchmark the displacements I produce in my code (injecting these loads that I mentioned) with an equal setup between models. I came across something weird though. I wanted to check the displacements at the base of the tower and to my surprise they were zero! This displacement I got from the ElastoDyn module using strain gages that matched the tower nodes (channels TwHt1TDxt, TwHt1TDyt, TwHt1TDzt). Now, this zero displacement was right above the transition piece, in this context do OF treat this point as fixed or am I outputting the wrong channel? Also I have almost zero displacement above the transition piece, but when I check the node that I use as joint for TP, it has displacement!
Dear @Filipe.Rocha
The tower displacement outputs you are using (TwHt1TDxt, etc.) are expressed relative to the undisplaced tower in the tower coordinate system that is aligned with the base of the tower, but moves with the platform. So, outputs TwHt1TDxt, etc. won’t report the deflection of the substructure as represented within SubDyn. If you want to know the total displacement of a given tower node, you can use the outputs in the inertial coordinate system instead, e.g. TwHt1TPxi, etc.
Best regards,
Dear Jason, once again thanks for the insightful and quick response. That definitely helped. I wanted to clarify something else though. Since I’m trying to match the displacements between my in-house code and openFAST, it’s very important to apply the loads produced and get the displacements as close as possible between two models. That being said though, after carefully reviewing my code I am stuck in the current situation:
-Top of the wind tower, where I apply the loads, good convergence (node 125);
-Middle of the wind tower, signal remains consistent, but displacements become more deviated (node 91);
-Bottom of the tower, signal remains consistent, but displacements deviate even more (node 58).
I’ve been reading the documentation of OpenFAST and I know I won’t ever be able to have exactly the same outputs because of how much more complex OF is compared to my simulation, but my question is this, is there any option in the simulation that can for some reason mess with the flexibility of the structure that I could try to switch on or off to have a simpler behaviour of the SubDyn (jacket) and ElastoDyn (wind tower). Currently I’m running as an example file the 5MW_OC4Jckt_DLL_WTurb_WavesIrr_MGrowth setup with a few modifications only in terms of geometry for the OC4 and the wind tower.
Dear @Filipe.Rocha,
I’m not sure I fully understand what you are asking, but you can always fix the tower base (making the substructure rigid) by disabling the platform degrees of freedom (DOFs) in ElastoDyn (PtfmSgDOF = FALSE, etc.). And you can always keep the substructure flexible while keeping the tower rigid by disabling the tower DOFs in ElastoDyn (TwFADOF1 = FALSE, etc.).
Best regards,
Thanks for your answer Jason, it wasn’t exactly what I needed, but it helped me figure out something else (so in the end helped a lot!). Now, as I said before I’m trying to mimic the 5MW wind turbine behaviour with a lumped mass matrix with Inertia and mass at the top of the wind tower. I came across an old topic:
https://forums.nrel.gov/t/rotor-and-nacelle-mass-moment-of-inertia-tensors/2015/
In this topic you use a few values for Inertias:
Mass = 110000
Ixx = 4.37E7
Iyy = 1.99E7
Izz = 2.01E7
Ixy = 0
Izx = 8.3E5
Iyz = 0
I checked the excel file, but got a little confused. Since it’s an old topic I was wondering if you can clarify a few points:
- These values that I quoted they are around the CM of the rotor, so if I want to apply them at the Yaw of my structure I need to consider the distances between the Yaw axis and rotor CM?
- Also, for the Nacelle, do you have the correspondent values so I can use the Parallel axis theorem to also apply these values at the Yaw?
- The units you are using according to the spreadsheet are still kg.m²?
I understand this information may be in the spreadsheet, but since benchmarking is essential for me at this point I need to be absolutely sure of the values I will use because at the moment I’m falling to reproduce the displacements OF calculates. Thank you very much for the patience!
Best wishes
Filipe
Dear @Filipe.Rocha,
I don’t recall the source of the some of values reported in this spreadsheet. Can you clarify what you are trying to calculate?
Best regards,
Jason, hi!
I need to match the displacements from my model with the ones produced by OpenFAST, so I’m modelling the OC4 coupled with the 5MW NREL wind turbine. I model the RNA as a lumped mass. I need to apply the mass and Inertias at the highest point of the wind tower, then when I apply the wind loads extracted from FAST (at the top of the wind tower) and the aerodynamic loads along the wind tower coupled with wave forces produced using Morison Equation plot the same displacements and natural frequencies (hopefully) as FAST does. If not the same, something very similar.
Best wishes
Filipe
Dear @Filipe.Rocha,
It sounds like you need the full mass, center of mass, and inertias of the full rotor-nacelle-assembly (RNA) of the NREL 5-MW baseline wind turbine. For that, see my post dated Jun 7, 2013 in the following forum topic: Using Aggregate Mass in ADAMS to Check NREL CS_Monopile.bmi.
Best regards,