NumPlanes in version 5 of FastFarm

[mat-poli]

Good morning, I am trying to simulate the following wind farm in FAST.Farm:

--- WIND TURBINES ---
4 NumTurbines - Number of wind turbines (-) [>=1] [last 6 columns below used only for Mod_AmbWind=2 or 3]
WT_X WT_Y WT_Z WT_FASTInFile X0_High Y0_High Z0_High dX_High dY_High dZ_High
(m) (m) (m) (string) (m) (m) (m) (m) (m) (m)
240 0 0 "WT.T1.fst" 96 -145 10 4 5 5
2040 160 0 "WT.T2.fst" 1896 15 10 4 5 5
3840 320 0 "WT.T3.fst" 3696 175 10 4 5 5
5640 480 0 "WT.T4.fst" 5496 335 10 4 5 5

with the following parameters:

--- WAKE DYNAMICS ---
1 Mod_Wake - Switch between wake formulations {1:Polar, 2:Curl, 3:Cartesian} (-) (switch)
240 RotorDiamRef - Reference turbine rotor diameter for wake calculations (m) [>0.0]
5 dr - Radial increment of radial finite-difference grid (m) [>0.0]
73 NumRadii - Number of radii in the radial finite-difference grid (-) [>=2]
17 NumDFull - Distance of full wake propagation, expressed as a multiple of RotorDiamRef [>0.0] or DEFAULT [DEFAULT=15]
2 NumDBuff - Length of wake propagation buffer region, expressed as a multiple of RotorDiamRef [>=0.0] or DEFAULT [DEFAULT=5]
0.05 f_c - Cutoff (corner) frequency of the low-pass time-filter for the wake advection, deflection, and meandering model [recommended=1.28*U0/R] (Hz) [>0.0] or DEFAULT [DEFAULT=12.5/R, R estimated from dr and NumRadii, not recommended].

The turbines are IEA 15 MW Reference Wind Turbines.

--- AMBIENT WIND: INFLOWWIND MODULE ---
2.5 DT_Low - Time step for low-resolution wind data interpolation; will be used as the global FAST.Farm time step (s) [>0.0]
0.25 DT_High - Time step for high-resolution wind data interpolation (s) [>0.0]
336 NX_Low - Number of low-resolution spatial nodes in X direction for wind data interpolation (-) [>=2]
73 NY_Low - Number of low-resolution spatial nodes in Y direction for wind data interpolation (-) [>=2]
33 NZ_Low - Number of low-resolution spatial nodes in Z direction for wind data interpolation (-) [>=2]
-480 X0_Low - Origin of low-resolution spatial nodes in X direction for wind data interpolation (m)
-720.0 Y0_Low - Origin of low-resolution spatial nodes in Y direction for wind data interpolation (m)
10.0 Z0_Low - Origin of low-resolution spatial nodes in Z direction for wind data interpolation (m)
24 dX_Low - Spacing of low-resolution spatial nodes in X direction for wind data interpolation (m) [>0.0]
20.0 dY_Low - Spacing of low-resolution spatial nodes in Y direction for wind data interpolation (m) [>0.0]
20.0 dZ_Low - Spacing of low-resolution spatial nodes in Z direction for wind data interpolation (m) [>0.0]
73 NX_High - Number of high-resolution spatial nodes in X direction for wind data interpolation (-) [>=2]
59 NY_High - Number of high-resolution spatial nodes in Y direction for wind data interpolation (-) [>=2]
59 NZ_High - Number of high-resolution spatial nodes in Z direction for wind data interpolation (-) [>=2]
"./IW_WT.dat" InflowFile - Name of file containing InflowWind module input parameters (quoted string).

In the log file, I get the following message:

Plaintext
Time: 420 of 1000 seconds. Estimated final completion at 11:52:07 (in 0.013 days).

T1:FARM_UpdateStates:WD_UpdateStates: The number of wake planes of turbine 1 exceeded the allowed
number (150). Excess plane(s) removed.
WD_UpdateStates: Turbine 1 wake plane 140 (x_plane=2606.9) has overtaken wake plane 141
(x_plane=2606.9). Offending wake plane removed. Reduce f_c to prevent planes from passing each
other.
WD_UpdateStates: Turbine 1 wake plane 131 (x_plane=2448.4) has overtaken wake plane 132
(x_plane=2448.2). Offending wake plane removed. Reduce f_c to prevent planes from passing each
other.
T2:FARM_UpdateStates:WD_UpdateStates: Turbine 2 wake plane 79 (x_plane=1319.8) has overtaken wake
plane 80 (x_plane=1319.8). Offending wake plane removed. Reduce f_c to prevent planes from
passing each other.
T3:FARM_UpdateStates:WD_UpdateStates: The number of wake planes of turbine 3 exceeded the allowed
number (150). Excess plane(s) removed.
T4:FARM_UpdateStates:WD_UpdateStates: Turbine 4 wake plane 104 (x_plane=2202.4) has overtaken
wake plane 105 (x_plane=2201.7). Offending wake plane removed. Reduce f_c to prevent planes from
passing each other.

However, I cannot find the NumPlanes parameter in the .fstf file (i'm using Fast Farm v5). I can only modify the values of NumDFull and NumDBuff. By visualizing the vtk files in ParaView, it can be observed that the wake of Turbine 1 does not reach Turbine 3 (it is as if the planes are being deleted and the wake recedes). This happens despite the values of NumDFull=17 and NumDBuff=2 (T1 and T3 are 15 D apart).

Thank you in advance.

Matteo and Stefania from Politecnico di Milano :)

[jjonkman]

Dear @mat-poli,

As of FAST.Farm within OpenFAST v5, input parameter NumPlanes was replaced with NumDFull and NumDBuff such that the number of wake planes is dynamically allocated to extend the desired distance downstream, including a buffering region where the wake deficits are tapered away. Within FAST.Farm, the maximum number of wake planes (used to preallocate array sizes) is estimated based on NumDFull, NumDBuff, and C_Meander as 15 * ( NumDFull + NumDBuff )/ C_Meander, which is 150 for your case, which should be more than enough wake planes if the modeling guidance for specifying DT_Low is followed.

Regarding the visualization in ParaView, can you share an animation of what is happening with the wakes?

We are currently tracking down an issue with the new dynamic wake plan algorithm and perhaps your issue is related to that.

Best regards,

[mat-poli]

Perfect.
Yes, to select the Dt_Low we checked the userguide and we satisfied the equation in paragraph 4.19.6.3.1: Dt_Low=2.5<4.8.
Here attached the animation of our simulation.
GIF.zip
If you are unable to open it, I will try to send it to you in a different way.

Thank you in advance,

Matteo and Stefania

[jjonkman]

Dear @mat-poli,

I would guess our internal calculation of 15 * ( NumDFull + NumDBuff )/ C_Meander is not large enough, so, the maximum number of wake planes has been reached and the wake is truncating prematurely. We plan to increase the 15 to 18 to resolve this issue.

Moreover, we discovered that this equation is only valid for Mod_Wake = 1 (polar). For Mod_Wake = 2 or 3 (curl, cartesian), the maximum number of planes should increase. We will implement the default for those cases to 54 * ( NumDFull + NumDBuff ).

Best regards,

[mat-poli]

Thank you so much.
How soon will these modifications be implemented? Will you be releasing an updated version of the software?

Best regards,

Matteo and Stefania

[jjonkman]

Dear @mat-poli,

We are working on this updates now and it should be released soon.

In the interim, you can always increase NumDFull to increase the number of wake planes.

Best regards,