Query Regarding Discontinuous Wake Visualization in FAST.Farm Simulation

[6Mr-Y]

Hello. When performing post-processing visualization of FAST.Farm results recently, I noticed an abnormal wake pattern which appears fragmented and discontinuous. I am writing to ask what may cause this phenomenon. I suspect it could be related to the configuration of the low-resolution grid.Similar behavior has occurred in my earlier simulations, yet I have not been able to identify the exact source of the issue. I have attached the relevant parameters from my .fstf file together with my VTK visualization video for your reference.Thank you very much for your guidance. I look forward to your reply.
Best regards
wake vtk.zip

.fstf:
--- AMBIENT WIND: INFLOWWIND MODULE --- [used only for Mod_AmbWind=2 or 3]
0.6 DT_Low - Time step for low -resolution wind data interpolation; will be used as the global FAST.Farm time step (s) [>0.0]
0.3 DT_High - Time step for high-resolution wind data interpolation (s) [>0.0]
625 NX_Low - Number of low -resolution spatial nodes in X direction for wind data interpolation (-) [>=2]
145 NY_Low - Number of low -resolution spatial nodes in Y direction for wind data interpolation (-) [>=2]
49 NZ_Low - Number of low -resolution spatial nodes in Z direction for wind data interpolation (-) [>=2]
-240 X0_Low - Origin of low -resolution spatial nodes in X direction for wind data interpolation (m)
-720 Y0_Low - Origin of low -resolution spatial nodes in Y direction for wind data interpolation (m)
1 Z0_Low - Origin of low -resolution spatial nodes in Z direction for wind data interpolation (m)
10 dX_Low - Spacing of low -resolution spatial nodes in X direction for wind data interpolation (m) [>0.0]
10 dY_Low - Spacing of low -resolution spatial nodes in Y direction for wind data interpolation (m) [>0.0]
10 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]
73 NY_High - Number of high-resolution spatial nodes in Y direction for wind data interpolation (-) [>=2]
73 NZ_High - Number of high-resolution spatial nodes in Z direction for wind data interpolation (-) [>=2]
"InflowWind.dat" InflowFile - Name of file containing InflowWind module input parameters (quoted string)
--- WIND TURBINES ---
2 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 0.0 0.0 "15MW1.fst" 60.0 -180.0 1.0 5.0 5.0 5.0
1200.0 0.0 0.0 "15MW2.fst" 1020.0 -180.0 1.0 5.0 5.0 5.0
--- WAKE DYNAMICS ---
2 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]
16 dr - Radial increment of radial finite-difference grid (m) [>0.0]
25 NumRadii - Number of radii in the radial finite-difference grid (-) [>=2]
550 NumPlanes - Number of wake planes (-) [>=2]
DEFAULT 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]
DEFAULT C_HWkDfl_O - Calibrated parameter in the correction for wake deflection defining the horizontal offset at the rotor (m ) or DEFAULT [DEFAULT= 0.0 ]
DEFAULT C_HWkDfl_OY - Calibrated parameter in the correction for wake deflection defining the horizontal offset at the rotor scaled with yaw error (m/deg) or DEFAULT [DEFAULT= 0.0 if Mod_Wake is 2, 0.3 otherwise]
DEFAULT C_HWkDfl_x - Calibrated parameter in the correction for wake deflection defining the horizontal offset scaled with downstream distance (- ) or DEFAULT [DEFAULT= 0.0 ]
DEFAULT C_HWkDfl_xY - Calibrated parameter in the correction for wake deflection defining the horizontal offset scaled with downstream distance and yaw error (1/deg) or DEFAULT [DEFAULT= 0.0 if Mod_Wake is 2, -0.004 otherwise]
DEFAULT C_NearWake - Calibrated parameter for the near-wake correction (-) [>1.0 and <2.5] or DEFAULT [DEFAULT=1.8]
DEFAULT k_vAmb - Calibrated parameters for the influence of the ambient turbulence in the eddy viscosity (set of 5 parameters: k, FMin, DMin, DMax, Exp) (-) [>=0.0, >=0.0 and <=1.0, >=0.0, >DMin, >=0.0] or DEFAULT [DEFAULT=0.05, 1.0, 0.0, 1.0, 0.01]
DEFAULT k_vShr - Calibrated parameters for the influence of the shear layer in the eddy viscosity (set of 5 parameters: k, FMin, DMin, DMax, Exp) (-) [>=0.0, >=0.0 and <=1.0, >=0.0, >DMin, >=0.0] or DEFAULT [DEFAULT=0.016, 0.2, 3.0, 25.0, 0.1]
DEFAULT Mod_WakeDiam - Wake diameter calculation model (-) (switch) {1: rotor diameter, 2: velocity based, 3: mass-flux based, 4: momentum-flux based} or DEFAULT [DEFAULT=1]
DEFAULT C_WakeDiam - Calibrated parameter for wake diameter calculation (-) [>0.0 and <0.99] or DEFAULT [DEFAULT=0.95] [unused for Mod_WakeDiam=1]
DEFAULT Mod_Meander - Spatial filter model for wake meandering (-) (switch) {1: uniform, 2: truncated jinc, 3: windowed jinc} or DEFAULT [DEFAULT=3]
DEFAULT C_Meander - Calibrated parameter for wake meandering (-) [>=1.0] or DEFAULT [DEFAULT=1.9]
--- CURLED-WAKE PARAMETERS [only used if Mod_Wake=2 or 3] ---
DEFAULT Swirl - Switch to include swirl velocities in wake (-) (switch) [DEFAULT=TRUE]
DEFAULT k_VortexDecay - Vortex decay constant for curl (-) [DEFAULT=0.0001]
DEFAULT NumVortices - The number of vortices in the curled wake model (-) [DEFAULT=100]
DEFAULT sigma_D - The width of the vortices in the curled wake model non-dimesionalized by rotor diameter (-) [DEFAULT=0.2]
DEFAULT FilterInit - Switch to filter the initial wake plane deficit and select the number of grid points for the filter {0: no filter, 1: filter of size 1} or DEFAULT [DEFAULT=1] [unused for Mod_Wake=1] (switch)
DEFAULT k_vCurl - Calibrated parameter for scaling the eddy viscosity in the curled-wake model (-) [>=0] or DEFAULT [DEFAULT=2.0 ]
DEFAULT Mod_Projection - Switch to select how the wake plane velocity is projected in AWAE {1: keep all components, 2: project against plane normal} or DEFAULT [DEFAULT=1: if Mod_Wake is 1 or 3, or DEFAULT=2: if Mod_Wake is 2] (switch)

[6Mr-Y]

Additionally, I have another question. Where can I check the actual wall-clock simulation time for OpenFAST / FAST.Farm runs? Thank you for your guidance.

[jjonkman]

Dear @6Mr-Y,

I would guess the streaks you are seeing in the wake are the result of a value of the low-pass time-filter cut-off frequency (f_c) that is too high for the rotor motion you are simulating. If these are IEA Wind 15-MW RWTs atop floating platforms e.g., the VoltunUS semisubmersible, you should set f_c below the rigid-body natural frequencies to avoid unrealistic wake response, e.g. as discussed in related OpenFAST issue: #3103.

The actual wall-clock time is labled as "Total Real Time", which is written to the command prompt when OpenFAST and/or FAST.Farm are executed.

Best regards,

[6Mr-Y]

Dear @jjonkman ，I calculated the cutoff frequency f_c（f_c=0.0853） using the recommended formula provided in the .fstf file: recommended = 1.28*U0/R, and updated the corresponding parameter accordingly. However, the wake field still shows obvious striped artifacts in visualization. This simulation uses uniform inflow wind of 8 m/s with the IEA 15 MW reference turbine.
You previously suggested that f_c should be set below the turbine’s rigid-body natural frequencies. I have reviewed the report Definition of the IEA Wind 15-Megawatt Offshore Reference Wind Turbine, yet no rigid-body natural frequency data can be found inside. Could you tell me where I can obtain these frequency values? I look forward to your further guidance.
Thank you very much.
Best regards,

[jjonkman]

Dear @6Mr-Y,

The recommended value of f_c is too high for your case, resulting in streaks in the wake, likely due to the presence of low-frequency vibration modes in your model. Are you simulating a floating offshore wind turbine, e.g., the IEA Wind 15-MW RWT atop the VolturnUS semisubersible? If so, you should set f_c below the surge and pitch natural frequencies.

Best regards,

[6Mr-Y]

Thank you for your reply @jjonkman . I will try adjusting the value of f_c（recommended=1.28*U0/R right？）.
Regarding the Total Real Time metric: I run 14 cases sequentially via a script. However, this information is not printed in the command prompt window, and I am unsure why this occurs. Apart from reading this output from the terminal, are there any alternative methods to retrieve the wall-clock runtime for each individual simulation case?
Thank you very much for your guidance. I look forward to your reply.
Best regards

[andrew-platt]

To capture that output, you will want to modify your script to pipe the output to a text file for each of the runs. You can either use tee (linux, or powershell on windows), or use the > to redirect the output.

[6Mr-Y]

Thank you for your reply @andrew-platt. However, all 14 cases have already finished running. Could you advise whether there is any way to extract the wall-clock runtime of each case after completion, without re-running the simulations?

[andrew-platt]

There isn't any way I know of to get that information afterwards. It isn't stored in the output files.

[6Mr-Y]

@andrew-platt I am confused why the actual simulation runtime is not displayed for my 14 cases. I run all cases sequentially via a .bat script in CMD, yet this information never gets printed. Besides, I would like to ask how to implement the output redirection using the > operator that you mentioned. Are PowerShell and CMD similar in Windows for this purpose? I am concerned that the same missing-output issue may persist even after enabling output redirection.

[andrew-platt]

If your run a single FAST.Farm simulation by itself, does it print something like this at the end?

 Time: 120 of 120 seconds.  Estimated final completion at 17:42:22 (in 0.000 days).

  Total Real Time:       2.9653 minutes
  Total CPU Time:        2.8943 minutes
  Simulation CPU Time:   2.8345 minutes
  Simulated Time:        2 minutes
  Time Ratio (Sim/CPU):  0.7056

  FAST.Farm terminated normally.

[6Mr-Y]

In my previous simulations, the Total Real Time information was printed normally. However, none of my current 14 cases output this message. Each case has a simulated time of 4000 seconds. Another issue is that the output history in CMD is truncated. Only the logs of the last 9 cases remain visible in the command window, while output from the earlier cases has been lost. I intend to compare the wall-clock runtime of each case to analyze computational efficiency under different configurations, yet I am blocked by these limitations.