Having high resolution grid on any plane in FAST.Farm

Dear All,

I have been working on a FAST.Farm project but I am facing some issues I can not resolve.

My goal is to use FAST.Farm to generate a disturbed wind plane that I will later use as an input for other simulations. First I tried to extract this plane using vtk file on the low resolution grid, but I need a good resolution and it is costly to have a highly resolved low grid on the full domain, especially as I only need one plane of this domain.

The solution I came up with was to keep my low grid with a large resolution but to use a High resolution grid at the plane of interest. For what I know, the only way to generate a high resolution plan in FAST.Farm is to have a wind turbine in the middle of it. However I want to have my wind field undisturbed so I can’t have any object in this plane, so I made a “ghost turbine” that would be transparent to the wind but would enable be to generate a grid at this place.

The issue I encountered is the following : when I tried to run simulations with my “transparent” turbine, It appears that it still has an impact on the flow.

Here I ran 3 simulations :
-first one with two 5MW WT
-second one with only one 5MW WT at the end of the domain
-last one with the 5MW WT at the end of the domain and my “transparent” turbine at the place of the first 5MW

If my “transparent” turbine was actually transparent, the wind field in the second and third simulation should be exactly the same, however the mean values are different.

Can you please tell me if first there is a better approach to solving my problem and second how can I succesfully make a turbine that is transparent to the wind, or have a High res grid in my domain without a turbine inside ?

To give more details on what I did to make a transparent turbine, I set these values in its Aerodyn file :

------- AERODYN v15 for OpenFAST INPUT FILE -----------------------------------------------
NREL 5.0 MW offshore baseline aerodynamic input properties.
======  General Options  ============================================================================
False         Echo               - Echo the input to "<rootname>.AD.ech"?  (flag)
"default"     DTAero             - Time interval for aerodynamic calculations {or "default"} (s)
          0   WakeMod            - Type of wake/induction model (switch) {0=none, 1=BEMT, 2=DBEMT, 3=OLAF} [WakeMod cannot be 2 or 3 when linearizing]
          1   AFAeroMod          - Type of blade airfoil aerodynamics model (switch) {1=steady model, 2=Beddoes-Leishman unsteady model} [AFAeroMod must be 1 when linearizing]
          0   TwrPotent          - Type tower influence on wind based on potential flow around the tower (switch) {0=none, 1=baseline potential flow, 2=potential flow with Bak correction}
          0   TwrShadow          - Calculate tower influence on wind based on downstream tower shadow (switch) {0=none, 1=Powles model, 2=Eames model}
False         TwrAero            - Calculate tower aerodynamic loads? (flag)
False         FrozenWake         - Assume frozen wake during linearization? (flag) [used only when WakeMod=1 and when linearizing]
False         CavitCheck         - Perform cavitation check? (flag) [AFAeroMod must be 1 when CavitCheck=true]
False         Buoyancy           - Include buoyancy effects? (flag)
False         CompAA             - Flag to compute AeroAcoustics calculation [used only when WakeMod = 1 or 2]
"unused"      AA_InputFile       - AeroAcoustics input file [used only when CompAA=true]
======  Environmental Conditions  ===================================================================

And this is the corresponding blade file :

------- AERODYN v15.00.* BLADE DEFINITION INPUT FILE -------------------------------------
NREL 5.0 MW offshore baseline aerodynamic blade input properties; note that we need to add the aerodynamic center to this file
======  Blade Properties =================================================================
         2   NumBlNds           - Number of blade nodes used in the analysis (-)
  BlSpn        BlCrvAC        BlSwpAC        BlCrvAng       BlTwist        BlChord          BlAFID
   (m)           (m)            (m)            (deg)         (deg)           (m)              (-)
0.0000000E+00  0.0000000E+00  0.0000000E+00 0.0000000E+00  1.3308000E+01  3.5420000E+00        1
6.1499900E+01 -3.2815226E-04 -1.7737470E-01 0.0000000E+00  1.0600000E-01  1.4190000E+00        1

With this Airfoil File :

! Table of aerodynamics coefficients
          3   NumAlf            ! Number of data lines in the following table
!    Alpha      Cl      Cd        Cm
!    (deg)      (-)     (-)       (-)
  -180.00      0.000   0.0     0.0
     0.00      0.000   0.0     0.0
   180.00      0.000   0.0     0.0
! ------------------

Best regards,

Dear @Maximilien.Andre,

Your approach to zero-out the aerodynamic loads on the “ghost” turbine seems OK, but it would probably be easier to disable AeroDyn for this turbine altogether, by setting CompAero = 0.

That said, can you clarify what difference you are seeing in the “mean values”; I don’t see any visible difference in your visualization of the flow from the 2nd and 3rd simulations.

Best regards,

Dear Jason,

Thank you for your answer.

To clarify what I meant by mean values, I have represented the velocity magnitude at height 90m on the graphs and even if it looks visually similar, I have some differences when I compute the mean value of the velocity magnitude (I have displayed it in the title of the figures). If my ghost turbine were to be properly transparent, I would expect to have rigorously the same wind fields in the second and third simulation.

I tried your suggestion of disabling AeroDyn but it seems FAST.Farm does not allow it :

Warning: Turning off Unsteady Aerodynamics because polar has constant data. (node 3, blade 3)
Warning: Turning off Unsteady Aerodynamics because polar has constant data. (node 4, blade 3)
Running InflowWind.
Running ServoDyn.
Running ServoDyn Interface for Bladed Controllers (using Intel Visual Fortran for Windows).
Using legacy Bladed DLL interface.

Farm_Initialize:Farm_ValidateInput:OutFmt produces a column width of 10 instead of 20 characters.
Farm_Initialize:T1:Farm_InitFAST:FWrap_Init:AeroDyn (v15) must be used in each instance of FAST
for FAST.Farm.

 Aborting FAST.Farm.



Dear Jason,

I just performed a test in order to find the cause of this difference in the wind speed magnitudes. I have changed the length of my blades from 61.5 to 1 meter and I ran the fast farm simulation again. The difference between the case with the “ghost” turbine and no turbine was much smaller, so I guess whatever is affecting my wind speed is coming from the blades.

I now see a solution to simply set the blades length to a ridiculously small value and I should retrieve an undisturbed wind field, but if you have something else to recommend or if you find this abnormal I will be happy to have your thougths.



Dear @Maximilien.Andre,

Ah yes, I recall this limitation of FAST.Farm now.

I’m not sure I understand yet why your approach to model a “ghost” turbine is resulting in a small (but nonzero) wake effect. I would have expect if the aerodynamic loads are zero, then the wake deficits would be zero. Regardless, I’m glad you found a suitable workaround.

Best regards,