No Current Influence about iea-15-240-rwt HOT 6 CLOSED

Dear @danielfrancisconi,

I haven't done this myself, but I imagine you could infer the drag coefficient as you say. In surge, the drag coefficient would be something like AddBQuad*2/(WtrDens*SurgeArea), where SurgeArea is the submerged cross-sectional area of the floater projected in the surge direction.

Best regards,

from iea-15-240-rwt.

Hello,

You would have to post your input file for us to see what might be going on. Also, the OpenFAST forum might be a better spot to get helpful debugging information. See for instance this prior thread.

Cheers,
Garrett

from iea-15-240-rwt.

Hi Garrett,

Thank you for your answer. I've read the thread you suggested but it couldn't help me because I have already turned on and off the 2nd order floating platform forces with no difference on the result.
And sorry for my lack of precision before, I am referring to the HydroDyn input file of the IEA-15-240-RWT-UMaineSemi case available in the master branch of this git:
https://github.com/IEAWindTask37/IEA-15-240-RWT/blob/master/OpenFAST/IEA-15-240-RWT-UMaineSemi/IEA-15-240-RWT-UMaineSemi_HydroDyn.dat
With the only modification of applying any current, for example a Depth-independent current of 1m/s (file attached):
IEA-15-240-RWT-UMaineSemi_HydroDyn_current.txt
I've posted this issue here because it is related to how the IEA-15 was hydrodynamically modelled. Let me be more clear:
When we look to the HydroDyn file of OC4 SemiSub (from NREL5MW):
https://github.com/OpenFAST/r-test/blob/01f51597b8f058d6f08ec67d7b9aeb4a2ffccaad/glue-codes/openfast/5MW_OC4Semi_Linear/NRELOffshrBsline5MW_OC4DeepCwindSemi_HydroDyn.dat
the platform members were created (section "Members"), and its drag coefficients written in the "MEMBER-BASED HYDRODYNAMIC COEFFICIENTS (model 3)" section.
However, for the IEA-15 Hydrodyn file, this section is empty. The only member created (section "Members") refers to section SIMPLE HYDRODYNAMIC COEFFICIENTS (model 1), where the drag coefficients are zero. That is why, I think, the hydrodynamic forces due to current are zero.
So, should one create the members with non-zero drag coefficients in order to observe a current influence on the platform response? Or all the necessary modelling is already implemented?

Best regards,
Daniel

from iea-15-240-rwt.

Dear Daniel,

You are correct. The current-induced loads are zero because the viscous-drag coefficients are zeroed. HydroDyn itself includes a hybrid combination of a potential-flow solution and a strip-theory solution. However, this HydroDyn model of the UMaine Semi for the IEA Wind 15-MW reference turbine appears to only include the potential-flow solution. (The joints and member do not seem to be set up physically.) You'll have to update the HydroDyn file to set up the strip-theory solution (in terms of joints, members, diameters, and drag coefficients) in order to simulate current-induced loads on this model.

Best regards,

from iea-15-240-rwt.

@jjonkman Thank you very much for the complete response.

Daniel- A strip-theory friendly version of the Hydrodyn input file with member layout specification is something we are also working towards. I cannot promise when it would be available though.

from iea-15-240-rwt.

Thank you @jjonkman and @gbarter for the explanations and update, it was very helpful for me.

Now, I wonder if it is possible to infer a drag coefficient for the whole platform using the data presented on the UMaine documentation (https://www.nrel.gov/docs/fy20osti/76773.pdf). The viscous damping matrix presented on table-5 (p. 12) was obtained with OpenFoam simulations and is also written in the Hydrodyn file section "PLATFORM ADDITIONAL STIFFNESS AND DAMPING" as an "Additional quadratic drag". Would it be possible to infer a drag coefficient on surge direction from the first element of this matrix and apply it on the HydroDyn section "SIMPLE HYDRODYNAMIC COEFFICIENTS (model 1)"?

Best regards,
Daniel

from iea-15-240-rwt.