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Questions regarding Cp values and hydrodynamic file generation for the OC4 platform in QBlade
Dear QBlade team,
I am currently developing an optimization framework for a VAWT mounted on the OC4 semisubmersible platform using QBlade.
Among other parameters, I use the dimensions of the floating substructure (column diameters and outer column distance) as optimization variables.
One of the main challenges I’m facing is updating the drag coefficients used in Morison’s equation.
I have been trying to replicate the results from the OC4 Platform Definition paper [1], as well as the OC4 model available on the QBlade website.
I was able to very accurately reproduce the water depth over Reynolds number curves (Fig. 4-1) [1].
However, I am getting different values for the depth-averaged Reynolds numbers used in the Re-to-Cp interpolation (Fig. 4-6) [1].
I have thoroughly checked my implementation and calculations, but I still can’t seem to reproduce the correct results.
The only other resources I could find that describe the calculation of depth averaged Cp values are other platform papers by NREL.
Were the Cp values directly taken from the OC4 paper, or did you derive them yourself?
If so, could you provide any guidance on how to derive these values, or point me to relevant resources that describe the process?
Additionally, I encountered difficulties with the hydrodynamic input files.
Initially, I attempted to use Nemoh to generate the radiation and excitation files, but QBlade returned an unspecific error while reading the ExcitationForce.tec file, and I was unable to resolve it.
Next, I tried using pyHAMS (as also used in RAFT that is used for QBtoWEIS). This time, I encountered an error with the Buoy.1 file.
To match the format of the radiation.1 file from the QBlade OC4 model, I had to remove certain coupling terms from the added mass and damping matrices.
The deleted values are relatively small.
After scaling all values by 10³, the resulting hydrodynamic plots closely resemble those in the OC4 paper (Fig. 4-2, 4-3) [1].
While these “fixes” lead to reasonable results, I’m unsure whether they are valid or merely coincidentally correct.
Do you have any guidance on how to properly generate the hydrodynamic input files for QBlade?
Thank you very much in advance!
Best regards,
David
[1] Robertson, A. et al., Definition of the Semisubmersible Floating System for Phase II of OC4, Technical
Report No. TP-5000-60601, NREL, 2014
Hello David,
I assume by Cp you are actually referring to the drag coefficient (Cd).
The morison drag coefficients used for the OC4 Semisub are based on the HydroDyn definition files for the OC4 model that are provided by NREL. You can find the HydroDyn input file on the OpenFAST GitHub.
Note that the axial coefficients in OpenFast are defined slightly different (with a trailing 1/4 instead of 1/2) so QBlade only uses 1/2 of the value for the axial drag coefficients.
The pyHAMS files should be fully compatible, as they are currently used in QBtoWEIS. Did you encounter problems when trying to load them, or was the issue solely related to scaling?
For reference, in the WAMIT format (which pyHAMS uses), there is a ULEN parameter that controls scaling for .1 and .3 files. Depending on the parameter used to generate the files, you can set ULEN in QBlade via: UNITLENGTH_WAMIT
This parameter allows you to specify a WAMIT unit length different from the default 1.0. If not specified, 1.0 is assumed. The unit length is used to scale WAMIT Excitation (.3), Radiation (.1), and Hydrodynamic Stiffness (.hst) data files when they are loaded into QBlade.
Best regards,
David
