Question about low-resolution domein setting values in FAST.Farm

Wind & Water Wind Farm Modeling
1

Dear community,

I am currently using FAST.Farm to analyze the wake of the IEA 15MW semi-submersible floating wind turbine. During this process, I encountered some questions about setting parameters for the low-resolution domain.

  1. About DT_Low
    • According to the FAST.Farm User’s Guide and Theory Manual, DT_Low depends on the hub-height wind speed. However, in the WES paper, “Investigating the interactions between wakes and floating wind turbines using FAST.Farm,” DT_Low is fixed at 3 seconds.
    • Question: Is it necessary to adjust DT_Low according to the hub-height wind speed? Also, could you explain the reason for fixing DT_Low at 3 seconds in this study?
  2. About DY_Low and DZ_Low
    • The User’s Guide states that DY_Low and DZ_Low should be modified according to the hub-height wind speed, but in the WES paper, these values are fixed.
    • Question: Is it necessary to adjust DY_Low and DZ_Low based on the hub-height wind speed?
  3. About the DT_Low formulas in Modeling Guidance (Section 4.18.6.3.1)
    • In Section 4.18.6.3.1 “Low-Resolution Domain” of the Modeling Guidance, there are two different formulas for DT_Low. For example, with a hub-height wind speed of 18 m/s for a 15MW semi-submersible floating wind turbine, these two formulas would yield significantly different DT_Low values. Additionally, using the formula that includes dr would result in a much smaller DT_Low.
    • Question: Which formula should be used in this case? For reference, I am using Wake_Mod = 2.

Thank you and best regards,
Rikuto.

2

Dear @Rikuto.Nitta,

As mentioned in the paper you reference, the:

discretization is a bit coarser than current modeling guidance, which evolved over the course of this project; however, a subset of cases were run with finer discretization that follows current modeling guidance, and the results were generally found to be consistent with the present results, but the present results show a small underprediction of fatigue loads at low ambient turbulence levels.

In general, I would recommend following the modeling guidance for the spatial-temporal descretizations to ensure that the FAST.Farm solution is converged and computationally efficient.

Regarding (3), we recommend using the second formula for DT_Low (based on dr) with Wake_Mod = 2.

If you are using the python scripts from the OpenFAST Toolbox for FAST.Farm model set up and inflow generation: openfast_toolbox/openfast_toolbox/fastfarm at main · OpenFAST/openfast_toolbox · GitHub, all of this spatial-temporal discretization will be taken into account automatically.

Best regards,

3

Thank you for the detailed explanation and recommendations. I apologize for the delayed response.

I will adjust the parameters using the OpenFAST Toolbox for FAST.Farm

Thank you once again for your support.

Best regards,