Scale Platform files

Hi,
I have the FAST model of a floating offshore wind turbine where the platform and the relative mooring lines are modeled in, respectively, HydroDyn and MoorDyn. Starting from given scale factors I’m trying to build the FAST model of the scaled wind turbine and the relative platform to perform numerical simulations (like in “FAST Code Verification of Scaling Laws for DeepCwind Floating Wind System Tests”). Right now I’m facing problems trying to scale the HydroDyn and MoorDyn files. In particular it seems the scaled still turbine (no wind, no waves, no rotor rotation) mounted on the platform scaled with simple scaling rules is not equilibrated. Have you some suggestions to get this task done? Are the files used in the paper above available to be studied?
Thank you in advance,

Alessandro

Dear Alessandro,

The paper you refer to describes the approach we applied. If your model is not an equilibrium, there must be an error in one of your scaling conversions. I would start by checking the equilibrium of forces in the heave (vertical) direction i.e. check that the external buoyancy from displaced water balances with the weight of the system (including the weight of the rotor-nacelle assembly, tower and substructure) and mooring system pretension.

The results of that paper were generated by a student at NREL and I don’t personally have access to the files, but they were for an older version of FAST, and so, may not be too useful for you anyway.

Best regards,

Dear sir,
I would like to ask, I want to use FAST to calculate the scaled floating turbine model to obtain numerical simulation data. I would like to ask the following questions,
1.Whether the scaled floating turbine model can still be calculated by FAST
2. How to scale the nacelle interior control system, and what data need to be changed to make the floating turbine work normally
3.What problems do I face if I scale the model in FSAT
Best regards,

Dear @Hairui.Jiang,

Yes, you can model a scaled floating wind turbine in FAST, as described in the article reference above. You can find a link to that article here: https://www.nrel.gov/docs/fy12osti/54221.pdf. That said, it is more common to numerically model a scaled floating wind turbine at full scale and scale up the measured data from a wave tank to full scale for code-to-data validation.

One thing to note when modeling a scaled wind turbine in OpenFAST is that the dimensions will be small and the natural frequencies will be large, requiring very small time steps (that said, the simulation length will also be short due to the scaling laws).

Best regards,

Dear sir,
Thank you for your reply, I have read the article you sent me, but still have a few questions, 1. As the article said the control module is not activated in the simulation, so I should disable the pitch control and yaw (set VSContrl and YCMode to 0)? 2. Because the blades have different airfoil parameters, how should the blades be scaled 3. How should the mooring be scaled, is it scaled according to λ?
Best regards,

Dear @Hairui.Jiang,

Here are my answers:

  1. You can enable the control if you want, but you’ll have to apply similar scaling laws to the parameters used within the controller (time, length, mass, etc.)
  2. You’ll need to use airfoil data (lift, drag, pitching moment versus angle of attack) that is appropriate for the Reynolds number at the scale you want.
  3. The mooring parameters should be scaled applying similar scaling laws for the turbine variables (time, length, mass, etc.).

Best regards,

Dear sir,
Thank you for your reply. As mentioned in the article, under the condition that the Reynolds number of the prototype and scale model are the same, how should I scale the blade? In other words, the blade has different airfoil parameters at different distance,If I keep the Reynolds number of the scaling model constant(Same as the prototype), do I just scale the blade length and the chord length?Are there other factors to consider?
best regards,

Dear @Hairui.Jiang,

If the airfoil data is unchanging, the only aerodynamic blade data left are geometry related (lengths). So, these should be scaled the same as other lengths (by lambda).

Best regards,

Dear sir,
Thank you for your reply. When scaling the blade, I still have questions. The following questions are based on the fact that the Reynolds number of the scaled blade is consistent with the prototype (5MW),

  1. In the Blade.dat file , whether to regenerate DISTRIBUTED BLADE PROPERTIES and BLADE MODE SHAPES for the scaled blades, in the DISTRIBUTED BLADE PROPERTIES module, is it necessary to scale “BMassDen”, “FlpStff” and “EdgStff” according to the scale ratio, and other parameters("BlFract ",“PitchAxis”,“StrcTwst”) remain unchanged?
  2. Where are the chord length of the blade, “RNodes” and “DRNodes” modified?
  3. Whether the .txt file of the airfoil needs to be scaled

    image
    Best regards,

Dear @Hairui.Jiang,

Here are my answers:

  1. I agree that BMassDen, FlpStff, and EdgStiff should be scaled following the scaling laws and BlFract, PitchAxis, and StrcTwst are unitless, and so, need not be scaled. Note that the PitchAxis input is not used if AeroDyn v15 is used in place of v14.
  2. The blade aerodynamic discretization is specified in AeroDyn. RNodes and DRNodes are set within the AeroDyn v14 input file and the equivalent for AeroDyn v15, BlSpn, is specified within the AeroDyn v15 blade input file.
  3. All inputs in the airfoil coordinates data file are unitless, and so, need not be scaled. The airfoil coefficient data (lift, drag, pitching moment) should be adapted to the appropriate Reynolds number as discussed before.

Best regards