Confusion about FAST.Farm result simulating a step change in wind speed

Wind & Water Inflow Turbulence
1

Dear community,

I am puzzled by the result of FAST.Farm, when it simulates a step change in the wind speed as the change apparently arrives at wind turbine 2 (downstream) as the same time as at turbine 1.

Simulation setup

  • As a starting point I used glue-codes/fast-farm/TSinflow/FAST.Farm.fstf from [1], where two wind turbines are located downwind (more or less).
  • Of course, “…/5MW_Baseline/ServoData” contains the compiled DISCON.dll, DISCON_WT1.dll and DISCON_WT2.dll.
  • Set in IW.dat and IW_WT.dat the parameter “WindType” to 2 and “Filename_Uni” to “UniformWind.dat” including (similar to [2] to simulate the step change):
# UAE Phase VI (Ames) wind for for a simple power curve.
# Time   Wind    Wind    Vert.   Horiz.  Vert.   LinV    Gust
#        Speed   Dir     Speed   Shear   Shear   Shear   Speed
00.0  8.0     0.0     0.0     0.0     0.0     0.0     0.0
70.0  8.0     0.0     0.0     0.0     0.0     0.0     0.0
70.1  9.0     0.0     0.0     0.0     0.0     0.0     0.0
999.9 9.0     0.0     0.0     0.0     0.0     0.0     0.0
  • Remark: WindType etc. in IW_WT.dat should be ignored as described in Sections 3.4 and 3.5 in FAST.Farm User’s Guide, see [3].

Observation

The output file (.out) contains:

  Time       RtVAmbT1    RtVAmbT2    RtVRelT2   ...
  ...
   69.0000	 7.967E+00	 7.969E+00	 5.031E+00	...
   72.0000	 8.963E+00	 8.965E+00	 5.877E+00	...
  ...

I would not have expected that the effect of the step change arrives at the second turbine at the same time as at the first turbine (see RtVRelT2). There should be a time delay as the step change has to propagate. Am I misinterpreting something?

Since I have not been working with FAST.Farm for long, I would like to ask if a more experienced user can reproduce a similar behavior. I really wonder if I have missed something.

Best regards,

Florian

References:
[1] GitHub - OpenFAST/r-test
[2] Confusion about TurbSim
[3] Jason Jonkman and Kelsey Shaler: FAST.Farm User’s Guide and Theory Manual, Technical Report, National Renewable Energy Laboratory, 2021.

2

Dear @Florian.Buergel,

The result you are showing is what I’d expect to see when using InflowWind’s WindType = 2 inflow type in FAST.Farm. The WindType = 2 represents inflow that is time-varying, but spatially uniform. The wind does not convect downstream, like it does with turbulent inflows using TurbSim, Bladed, or HAWC formats. As such, the WindType = 2 is useful for single wind turbine simulations (in standalone OpenFAST), but not as useful for wind farm simulations in FAST.Farm, even though this option is supported.

Best regards,