model Validation

[Satish.Jawalageri]

Dear Jason,

I have included the wheeler stretching (as you mentioned in the previous post), still I am getting the difference in mudline bending moment (attached plot). Could you please advice me on this.

@Jason.Jonkman

Dear Satish,

OK, thanks for clarifying.

One big difference I see is that the original OC3 Phase I results for load case 5.1 assumed wave linearity (Airy waves). However, I see that you have enabled second-order wave-kinematics (WvDiffQTF = WvSumQTF = TRUE) in your analysis (hence why your wave elevation is not sinusoidal). The second differences is in regards to wave stretching. The original OC3 Phase I results included Wheeler stretching for the wave kinematics, however, this functionality is not available in the version of OpenFAST that you are running. The lack of Wheeler stretching will impact the wave loads and resulting bending moment at the mudline.

Best regards,

Thanks,
Satish J

[Jason.Jonkman]

Dear Satish,

I’m not really sure. How do your wave kinematics compare to those shared with the OC3 Phase 1 data, which are provided on the Google drive linked above?

Best regards,

[Satish.Jawalageri]

Dear Jason,

Thanks for your reply.

I have attached two plots, 1 - Considered +3m to -20m in hydrodyn and 2 - considered +11m to -20m in hydrodyn.
Wave kinematics are matching with the file in google drive (matches with each time step and node).

1. I am not able to validate, could you please suggest on this.

2. When I change the number of nodes in hydrodyn eventually in wave kinematics (attached figure 1 & 2), the BM values changes but wave height and wave period remains same, could I know where I might be doing wrong in this case?

Dear Satish,

OK, thanks for clarifying.

One big difference I see is that the original OC3 Phase I results for load case 5.1 assumed wave linearity (Airy waves). However, I see that you have enabled second-order wave-kinematics (WvDiffQTF = WvSumQTF = TRUE) in your analysis (hence why your wave elevation is not sinusoidal). The second differences is in regards to wave stretching. The original OC3 Phase I results included Wheeler stretching for the wave kinematics, however, this functionality is not available in the version of OpenFAST that you are running. The lack of Wheeler stretching will impact the wave loads and resulting bending moment at the mudline.

Best regards,

Thanks,
Satish J

[Jason.Jonkman]

Dear Satish,

Have you run the model long enough to ensure all start-up transients have died out? The OC3 Phase 1 results on Google drive are provided after all start-up transients die out.

And I don’t really understand your question about the wave height and period. You are using WaveMod = 6, correct? The wave height and period should be as you specified.

Best regards,

[Satish.Jawalageri]

Dear Jason,

Thanks for your reply.

• Yes I did consider TStart as 30 sec.

• What I mean is when I change the number of nodes, I am getting different values of BM.

• I can see values in dynamic pressure column in wave data of google drive file, but you suggested to set it to zeroes unless it is tapered section.

Thanks,
Satish J

[Jason.Jonkman]

Dear Satish,

How many nodes are you using? I would expect the results to converge as the number of nodes is increased. You may also want to increase the structural discretization in SubDyn.

Best regards,

[Satish.Jawalageri]

Dear Jason,

Thanks for your reply.

I have considered 25 nodes (got this from .HD sum file) from +3m to -20m and considered NDiv in Subdyn as 3.

Thanks,
Satish J

[Jason.Jonkman]

Dear Satish,

And how does the result change when you increase the number of nodes?

Best regards,

[Satish.Jawalageri]

Dear Jason,

Thanks for your reply.

Sorry for the confusion, first case I have considered 25 nodes (got this from .HD sum file) from +3m to -20m and considered NDiv in Subdyn as 3; second case I have considered 33 nodes from +11 to -20m.

Here, the wave height of 6m is considered for both cases.

-When I plot against time, I can observe increase in BM for second case when compared to first case.

• But finally, in any of the cases I could not succeed to validate.

Thanks,
Satish J

[Jason.Jonkman]

Dear Satish,

Are you running a case with linear waves (Airy) or with free surface nonlinearity? 3 m above MSL is OK for the former, but not for the latter, where the wave peaks would be higher than 3 m for a 6-m wave height.

Regardless, the solution may still not be converged. We typically recommend using a wave discretization of at last 0.5 m near the free surface. I suggest increasing resolution to see if results improve.

Best regards,

[Satish.Jawalageri]

Dear Jason,

Thanks for your reply.

Yes, I am running case with linear waves.
I have increased number of nodes i.e., from +3m to -20m with 0.5m node in hydrodyn. I could not see much change (attached plot).

Thanks,
Satish J

[Jason.Jonkman]

Dear Satish,

It looks like the main difference between your results for load case 5.1 from OC3 Phase I and those from OC3 are in the mean value. This could come from differences in the mean thrust or wave nonlinearity (mean drift, from wave stretching). I would suggest running load case 3.1 (wind only) and load case 4.1 (wave only) to help isolate the difference in mean value.

Best regards,

[Satish.Jawalageri]

Dear Jason,

Thanks for your reply.

As you suggested,

1. I have run the analysis LC 3.1 by considering water density = 0 and WavMod = 0 in hydrodyn file and considered TStart as 30sec. I am getting different values when compared (attached plot).

2. For the LC 4.1, I have considered air density = 0 in aerodyn module and RotSpeed = 0 in elastodyn , but I am getting following error,

[code]FAST_InitializeAll:FAST_Init:FAST_ReadPrimaryFile:1/VTK_fps is not an integer multiple of DT.
FAST will output VTK information at 15.385 fps, the closest rate possible.
FAST_InitializeAll:AD_Init:ValidateInputData:The air density (AirDens) must be greater than zero.

FAST encountered an error during module initialization.
Simulation error level: FATAL ERROR

Aborting OpenFAST. [/code]

Thanks,
Satish J

[Jason.Jonkman]

Dear Satish,

LC 3.1 of OC3 Phase I involves a land-based turbine. So, you should not be comparing loads at the seabed.

For LC 4.1, it is better to disable aerodynamic loads altogether, rather than to set the air density to zero. Set CompAero = 0.

Best regards,

[Satish.Jawalageri]

Dear Jason,

Thanks for your reply.

• For LC 3.1, I have analyzed by setting WaveMod and water density to zero and I have compared tower top FA BM and have attached plots as they are not matching well when compared.

• For LC 4.1, as you suggested I have considered CompAero = 0 and I have attached the plots of comparison (Mudline BM).

Thanks,
Satish J

[Jason.Jonkman]

Dear Satish,

It looks like these results still have some start-up transients in them. How do the results compare after the start-up transient has passed?

For LC 4.1, how does the bending moment at the mudline look?

Best regards,

[Satish.Jawalageri]

Dear Jason,

Thanks for your reply.

The results I have attached in the previous post and this post are after transient time of 30sec (TStart = 30sec).

I have attached the plot for BM of LC 4.1.

Thanks,
Satish J

[Jason.Jonkman]

Dear Satish,

There still appears to be some transients in both LC 3.1 and LC 4.1. I would suggest increasing the start-up transient period to 60 s and only outputting after that.

Best regards,

[Satish.Jawalageri]

Dear Jason,

Thanks for your reply.

I have considered TStart = 60 sec and attached the plots. It looks LC 4.1 is matching well but not LC 3.1.

Thanks,
Satish J

[Jason.Jonkman]

Dear Satish,

The difference in mean bending moment at the mudline from LC 5.1 that you showed was about 3000 kNm. The difference in mean value of tower-top moment for LC 3.1 that you are showing is about 200 Nm. So, there is still something that is not yet making sense. That said, the moment arm from the thrust to the tower-top force is rather smaller. The shear force at the tower top multiple by the height of the support structure is likely the bigger effect. How does the shear force at the tower-top compare?

Best regards,