Hi All,
I am seeking to replicate the ElastoDyn model for the OC3 Monopile in a structural dynamics solver, OpenSees, where I would compare the results from AeroDyn/ElastoDyn coupling to the AeroDyn/OpenSees coupling. I am thinking of imposing 6DOF aerodynamic forces (RtAeroFxi, RtAeroMyi etc) only at the hub for now, and comparing displacements along the tower in both coupling methods. My question is do these aerodynamic forces include inertial force from the mass of the hub, or it is simply just the imposed aerodynamic loads?
Would this be the recommended approach to compare the two? Thanks very much!
Best,
Elaine
Hi @Elaine.Liu
For me, the aerodynamic forces in OpenFAST includes the elastic deformation of the tower and the blades. This is called aero-elastic calculation. This aero-elastic calculation has other effect like aerodynamic damping.
If your model guarantes all aero-elastic effects, then for me it is fine.
Best Regards,
Riad
Dear @Elaine.Liu,
The AeroDyn outputs RtAeroFxi, etc. only include the applied aerodynamic loads; the structural reaction loads, including both applied loads and weight/inertia terms are outputs of ElastoDyn.
That said, the aerodynamic applied loads calculated by AeroDyn do depend on the structural motions (two-way coupling), so, taking the aerodynamic loads from OpenFAST and putting them in OpenSees (with one-way coupling) may introduce some error unless the structural motions are the same between the solvers.
Best regards,
Hi All,
Thanks for your comments, this perfectly addresses my original questions.
As a follow-up, I am running the OC3 Monopile model as available on Github. It is my interpretation that the hub center of mass is offset from the tower centerline/nodes. I have this eccentricity modeled in OpenSees, and because of this, the tower has an initial static displacement of -0.2m, due to the heavier asymmetric weight from the hub. However, the OC3 monopile code has its displacement starting from 0m with no initial offset. If I do not incorporate this eccentricity, the displacements match the OC3 monopile. Would you know why there is this discrepancy? Let me know if I need to provide more information!
Best,
Elaine
Dear @Elaine.Liu,
I would expect OpenFAST to also show some tower deflection resulting from the overhanging weight of the rotor-nacelle-assembly (RNA) offset from the tower centerline. This initial tower offset is not always specified in ElastoDyn, but it could be to reduce the initial simulation start-up transient.
Best regards,
Hi Jason,
Thanks for that information. Do you know where that initial offset would be specified in ElastoDyn? Thanks so much!
Best,
Elaine
Dear @Elaine.Liu
It seems odd to me that you did not get the static offset.
Normally, i would say that the static offset is the mean of the time series not necessarily at time equals zero seconds.
Best Regards,
Riad
Dear @Elaine.Liu,
The initial tower deflection is specified through ElastoDyn inputs TTDspFA and TTDspSS.
Best regards,
Thank you, all. May I ask, for the OC3monopile, is the hub center of mass located at the rotor apex? Thus, the hub CoM is -5m (accounting for shaft tilt) and 90m above the MSL. Is the nacelle CoM 1.9m (in the opposite direction) and 89.35m above MSL? Just wanted to confirm these details, thanks!
Best,
Elaine
Dear @Elaine.Liu,
Yes, I agree with the values you state.
Best regards,
Thank you, Jason, for the prompt reply. Would an individual blade mass also be 17740 kg? I could not find this value specified in the ElastoDyn file, which is why I am confirming.
Thanks!
Elaine
Dear @Elaine.Liu,
The blade mass is specified in ElastoDyn as a mass distribution and the total integrated blade mass is calculated internally and written to the ElastoDyn summary file. I confirm the value you state.
Best regards,
Dear @Elaine.Liu
You can enable the feature “Print summary file” in ElastoDyn input file and after running a simulation you will find a summary file called “5MW_OC3Mnpl_DLL_WTurb_WavesIrr.ED.sum” where you can find a lot of information including the blade mass. I am using OpenFAST v3.5 and in the summmary file, the blade has a mass of 17536.617 kg.
Best Regards,
Riad
Great! Many thanks, I see the same information. On the presentation available on the OpenFAST documentation, it says that the nacelle node and tower top overlap. So at the tower top is the mass equal to the tower top mass and the nacelle mass? I thought the nacelle node has center of mass (1.9m,0,1.75m) with respect to the tower centerline.
One follow-up too, is the hub displacement the same as tower top displacement (TwrTpTDxi)?
Best,
Elaine
Dear @Elaine.Liu
Could you please share the link for the presentation ? It seems awesome to look at it.
Regarding your question on the nodes, i am pretty sure that the nodes are not those where the mass distributed. They are talking about the origin of the reference frame of the nacelle. That is, the origin of the reference nacelle frame is located at tower top but the nacelle CM is loacted at (1.9m,0,1.75m) measured in the nacelle frame.
Best Regards,
Riad
Dear @Elaine.Liu,
I agree with @Riad.Elhamoud’s comments.
Regarding the hub displacement, it is not identical to the tower-top displacement because there is an additional lever arm from the tower-top to the hub (e.g., tower-top tilt will result in different hub translation than the translation of the tower-top). That said, the hub displacement is not directly output from ElastoDyn, although you could calculate through internal variables or through post-processing of the tower-top displacement.
Best regards,
