Hello everyone.
I am using FAST.Farm to do research on wakes caused by wind turbines.
I performed simulations under the following conditions to investigate the direction of wake deflection and velocity deficit rate when nacelle yaw is varied.
・Wind speed →18m/s steady
・Wave length →still water
・Nac yaw →30deg
・Mod_Wake →1(Polar wake model)
・IEA15MW turbine and UMaine VolturnUS-S Reference Platform
As the wind passes through the turbine, the velocity vector in the direction of the axis of rotation decreases and the velocity vector in the direction perpendicular to the axis of rotation does not change, so the wake would be deflected in the sway negative direction.
Below the rated wind speed, the wake was deflected in the negative Sway direction as expected, but above the rated wind speed, the wake was deflected in the positive Sway direction as shown in the figure.
I would like to know about the following two questions.
・Does the direction of wake deflection change when the wind speed is above or below the rated wind speed?
・Are there any papers that summarize the behavior of wake flow above rated wind speeds?
Best regards,
Dear @Yuki.Ogura,
I would not expect the direction of the wake deflection to change between below and above-rated wind speeds. For positive nacelle-yaw (counterclockwise, when viewed from above), I would expect the wake deflect to the right when looking downwind.
I’m not sure why your results are not showing this. Can you clarify your simulation set-up (share your FAST.Farm input file)?
FYI: The curled wake model is known to generate a more accurate wake in skewed (yawed) inflow conditions. I’m not sure why your polar wake model solution is not giving you the results you expect, but we generally recommend using the curled wake model under skewed inflow conditions regardless.
Best regards,
Dear Jason,
Here are the fstf and ElastoDyn files I use.
Please let me know if there is any other information you need.
------- FAST.Farm for OpenFAST INPUT FILE -------------------------------------------------
FAST.Farm input file, using two turbines separated by 3D downstream and an offset of 30m, with a turbulent inflow given by one turbsim box
--- SIMULATION CONTROL ---
False Echo - Echo input data to <RootName>.ech? (flag)
FATAL AbortLevel - Error level when simulation should abort (string) {"WARNING", "SEVERE", "FATAL"}
4500 TMax - Total run time (s) [>=0.0]
False UseSC - Use a super controller? (flag)
2 Mod_AmbWind - Ambient wind model (-) (switch) {1: high-fidelity precursor in VTK format, 2: one InflowWind module, 3: multiple instances of InflowWind module}
2 Mod_WaveField - Wave field handling (-) (switch) {1: use individual HydroDyn inputs without adjustment, 2: adjust wave phases based on turbine offsets from farm origin}
0 Mod_SharedMooring - Shared mooring system model (switch) {0: None, 3=MoorDyn}}
--- SUPER CONTROLLER --- [used only for UseSC=True]
"unused" SC_FileName - Name/location of the dynamic library {.dll [Windows] or .so [Linux]} containing the Super Controller algorithms (quoated string)
--- SHARED MOORING SYSTEM --- [used only for Mod_SharedMoor>0]
"" SharedMoorFile - Name of file containing shared mooring system input parameters (quoted string) [used only when Mod_SharedMooring > 0]
0.04 DT_Mooring - Time step for farm-level mooring coupling with each turbine (s) [used only when Mod_SharedMooring > 0]
--- AMBIENT WIND: PRECURSOR IN VTK FORMAT --- [used only for Mod_AmbWind=1]
30.0 DT_Low-VTK - Time step for low -resolution wind data input files; will be used as the global FAST.Farm time step (s) [>0.0]
1.0 DT_High-VTK - Time step for high-resolution wind data input files (s) [>0.0]
"unused" WindFilePath - Path name to VTK wind data files from precursor (string)
False ChkWndFiles - Check all the ambient wind files for data consistency? (flag)
--- AMBIENT WIND: INFLOWWIND MODULE --- [used only for Mod_AmbWind=2 or 3]
3.0 DT_Low - Time step for low -resolution wind data interpolation; will be used as the global FAST.Farm time step (s) [>0.0]
0.1 DT_High - Time step for high-resolution wind data interpolation (s) [>0.0]
90 NX_Low - Number of low -resolution spatial nodes in X direction for wind data interpolation (-) [>=2]
51 NY_Low - Number of low -resolution spatial nodes in Y direction for wind data interpolation (-) [>=2]
51 NZ_Low - Number of low -resolution spatial nodes in Z direction for wind data interpolation (-) [>=2]
-408 X0_Low - Origin of low -resolution spatial nodes in X direction for wind data interpolation (m)
-500 Y0_Low - Origin of low -resolution spatial nodes in Y direction for wind data interpolation (m)
5.0 Z0_Low - Origin of low -resolution spatial nodes in Z direction for wind data interpolation (m)
54.0 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]
52 NX_High - Number of high-resolution spatial nodes in X direction for wind data interpolation (-) [>=2]
52 NY_High - Number of high-resolution spatial nodes in Y direction for wind data interpolation (-) [>=2]
52 NZ_High - Number of high-resolution spatial nodes in Z direction for wind data interpolation (-) [>=2]
"InflowWind_01.dat" InflowFile - Name of file containing InflowWind module input parameters (quoted string)
--- WIND TURBINES ---
1 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)
0.0 0.0 0.0 "FFTest_WT1_01.fst" -255 -255 5.0 10.0 10.0 10.0
--- WAKE DYNAMICS ---
1 Mod_Wake - Switch between wake formulations {1:Polar, 2:Curl, 3:Cartesian} (-) (switch)
5.0 dr - Radial increment of radial finite-difference grid (m) [>0.0]
80 NumRadii - Number of radii in the radial finite-difference grid (-) [>=2]
136 NumPlanes - Number of wake planes (-) [>=2]
0.007 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 parameter for the influence of ambient turbulence in the eddy viscosity (-) [>=0.0] or DEFAULT [DEFAULT=0.05 ]
DEFAULT k_vShr - Calibrated parameter for the influence of the shear layer in the eddy viscosity (-) [>=0.0] or DEFAULT [DEFAULT=0.016]
DEFAULT C_vAmb_DMin - Calibrated parameter in the eddy viscosity filter function for ambient turbulence defining the transitional diameter fraction between the minimum and exponential regions (-) [>=0.0 ] or DEFAULT [DEFAULT= 0.0 ]
DEFAULT C_vAmb_DMax - Calibrated parameter in the eddy viscosity filter function for ambient turbulence defining the transitional diameter fraction between the exponential and maximum regions (-) [> C_vAmb_DMin ] or DEFAULT [DEFAULT= 1.0 ]
DEFAULT C_vAmb_FMin - Calibrated parameter in the eddy viscosity filter function for ambient turbulence defining the value in the minimum region (-) [>=0.0 and <=1.0] or DEFAULT [DEFAULT= 1.0 ]
DEFAULT C_vAmb_Exp - Calibrated parameter in the eddy viscosity filter function for ambient turbulence defining the exponent in the exponential region (-) [> 0.0 ] or DEFAULT [DEFAULT= 0.01]
DEFAULT C_vShr_DMin - Calibrated parameter in the eddy viscosity filter function for the shear layer defining the transitional diameter fraction between the minimum and exponential regions (-) [>=0.0 ] or DEFAULT [DEFAULT= 3.0 ]
DEFAULT C_vShr_DMax - Calibrated parameter in the eddy viscosity filter function for the shear layer defining the transitional diameter fraction between the exponential and maximum regions (-) [> C_vShr_DMin ] or DEFAULT [DEFAULT=25.0 ]
DEFAULT C_vShr_FMin - Calibrated parameter in the eddy viscosity filter function for the shear layer defining the value in the minimum region (-) [>=0.0 and <=1.0] or DEFAULT [DEFAULT= 0.2 ]
DEFAULT C_vShr_Exp - Calibrated parameter in the eddy viscosity filter function for the shear layer defining the exponent in the exponential region (-) [> 0.0 ] or DEFAULT [DEFAULT= 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]
2.1 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]
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] (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)
------- ELASTODYN v1.03.* INPUT FILE -------------------------------------------
IEA 15 MW offshore reference model on UMaine VolturnUS-S semi-submersible floating platform
---------------------- SIMULATION CONTROL --------------------------------------
False Echo - Echo input data to "<RootName>.ech" (flag)
3 Method - Integration method: {1: RK4, 2: AB4, or 3: ABM4} (-)
"default" DT Integration time step (s)
---------------------- DEGREES OF FREEDOM --------------------------------------
True FlapDOF1 - First flapwise blade mode DOF (flag)
True FlapDOF2 - Second flapwise blade mode DOF (flag)
True EdgeDOF - First edgewise blade mode DOF (flag)
False TeetDOF - Rotor-teeter DOF (flag) [unused for 3 blades]
False DrTrDOF - Drivetrain rotational-flexibility DOF (flag)
True GenDOF - Generator DOF (flag)
False YawDOF - Yaw DOF (flag)
True TwFADOF1 - First fore-aft tower bending-mode DOF (flag)
True TwFADOF2 - Second fore-aft tower bending-mode DOF (flag)
True TwSSDOF1 - First side-to-side tower bending-mode DOF (flag)
True TwSSDOF2 - Second side-to-side tower bending-mode DOF (flag)
True PtfmSgDOF - Platform horizontal surge translation DOF (flag)
True PtfmSwDOF - Platform horizontal sway translation DOF (flag)
True PtfmHvDOF - Platform vertical heave translation DOF (flag)
True PtfmRDOF - Platform roll tilt rotation DOF (flag)
True PtfmPDOF - Platform pitch tilt rotation DOF (flag)
True PtfmYDOF - Platform yaw rotation DOF (flag)
---------------------- INITIAL CONDITIONS --------------------------------------
0 OoPDefl - Initial out-of-plane blade-tip displacement (meters)
0 IPDefl - Initial in-plane blade-tip deflection (meters)
0 BlPitch(1) - Blade 1 initial pitch (degrees)
0 BlPitch(2) - Blade 2 initial pitch (degrees)
0 BlPitch(3) - Blade 3 initial pitch (degrees) [unused for 2 blades]
0 TeetDefl - Initial or fixed teeter angle (degrees) [unused for 3 blades]
0 Azimuth - Initial azimuth angle for blade 1 (degrees)
5.65 RotSpeed - Initial or fixed rotor speed (rpm)
30 NacYaw - Initial or fixed nacelle-yaw angle (degrees)
0 TTDspFA - Initial fore-aft tower-top displacement (meters)
0 TTDspSS - Initial side-to-side tower-top displacement (meters)
15.36 PtfmSurge - Initial or fixed horizontal surge translational displacement of platform (meters)
0.1 PtfmSway - Initial or fixed horizontal sway translational displacement of platform (meters)
-0.4 PtfmHeave - Initial or fixed vertical heave translational displacement of platform (meters)
0.3 PtfmRoll - Initial or fixed roll tilt rotational displacement of platform (degrees)
2.6 PtfmPitch - Initial or fixed pitch tilt rotational displacement of platform (degrees)
0.28 PtfmYaw - Initial or fixed yaw rotational displacement of platform (degrees)
---------------------- TURBINE CONFIGURATION -----------------------------------
3 NumBl - Number of blades (-)
120.97 TipRad - The distance from the rotor apex to the blade tip (meters)
3.97 HubRad - The distance from the rotor apex to the blade root (meters)
-4 PreCone(1) - Blade 1 cone angle (degrees)
-4 PreCone(2) - Blade 2 cone angle (degrees)
-4 PreCone(3) - Blade 3 cone angle (degrees) [unused for 2 blades]
0 HubCM - Distance from rotor apex to hub mass [positive downwind] (meters)
0 UndSling - Undersling length [distance from teeter pin to the rotor apex] (meters) [unused for 3 blades]
0 Delta3 - Delta-3 angle for teetering rotors (degrees) [unused for 3 blades]
0 AzimB1Up - Azimuth value to use for I/O when blade 1 points up (degrees)
-12.098 OverHang - Distance from yaw axis to rotor apex [3 blades] or teeter pin [2 blades] (meters)
0.0 ShftGagL - Distance from rotor apex [3 blades] or teeter pin [2 blades] to shaft strain gages [positive for upwind rotors] (meters)
-6.0 ShftTilt - Rotor shaft tilt angle (degrees)
-5.125 NacCMxn - Downwind distance from the tower-top to the nacelle CM (meters)
0.0 NacCMyn - Lateral distance from the tower-top to the nacelle CM (meters)
4.315 NacCMzn - Vertical distance from the tower-top to the nacelle CM (meters)
0.0 NcIMUxn - Downwind distance from the tower-top to the nacelle IMU (meters)
0.0 NcIMUyn - Lateral distance from the tower-top to the nacelle IMU (meters)
0.0 NcIMUzn - Vertical distance from the tower-top to the nacelle IMU (meters)
4.3495 Twr2Shft - Vertical distance from the tower-top to the rotor shaft (meters)
144.386 TowerHt - Height of tower above ground level [onshore] or MSL [offshore] (meters)
15 TowerBsHt - Height of tower base above ground level [onshore] or MSL [offshore] (meters)
0 PtfmCMxt - Downwind distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
0 PtfmCMyt - Lateral distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
-14.400 PtfmCMzt - Vertical distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
0 PtfmRefzt - Vertical distance from the ground level [onshore] or MSL [offshore] to the platform reference point (meters)
---------------------- MASS AND INERTIA ----------------------------------------
0 TipMass(1) - Tip-brake mass, blade 1 (kg)
0 TipMass(2) - Tip-brake mass, blade 2 (kg)
0 TipMass(3) - Tip-brake mass, blade 3 (kg) [unused for 2 blades]
69131 HubMass - Hub mass (kg)
969952 HubIner - Hub inertia about rotor axis [3 blades] or teeter axis [2 blades] (kg m^2)
1836784 GenIner - Generator inertia about HSS (kg m^2)
644857 NacMass - Nacelle mass (kg)
32929058 NacYIner - Nacelle inertia about yaw axis (kg m^2)
28249 YawBrMass - Yaw bearing mass (kg)
1.7838E+07 PtfmMass - Platform mass (kg)
1.2507E+10 PtfmRIner - Platform inertia for roll tilt rotation about the platform CM (kg m^2)
1.2507E+10 PtfmPIner - Platform inertia for pitch tilt rotation about the platform CM (kg m^2)
2.3667E+10 PtfmYIner - Platform inertia for yaw rotation about the platform CM (kg m^2)
---------------------- BLADE ---------------------------------------------------
50 BldNodes - Number of blade nodes (per blade) used for analysis (-)
"../IEA-15-240-RWT/IEA-15-240-RWT_ElastoDyn_blade.dat" BldFile1 - Name of file containing properties for blade 1 (quoted string)
"../IEA-15-240-RWT/IEA-15-240-RWT_ElastoDyn_blade.dat" BldFile2 - Name of file containing properties for blade 2 (quoted string)
"../IEA-15-240-RWT/IEA-15-240-RWT_ElastoDyn_blade.dat" BldFile3 - Name of file containing properties for blade 3 (quoted string) [unused for 2 blades]
---------------------- ROTOR-TEETER --------------------------------------------
0 TeetMod - Rotor-teeter spring/damper model {0: none, 1: standard, 2: user-defined from routine UserTeet} (switch) [unused for 3 blades]
0 TeetDmpP - Rotor-teeter damper position (degrees) [used only for 2 blades and when TeetMod=1]
0 TeetDmp - Rotor-teeter damping constant (N-m/(rad/s)) [used only for 2 blades and when TeetMod=1]
0 TeetCDmp - Rotor-teeter rate-independent Coulomb-damping moment (N-m) [used only for 2 blades and when TeetMod=1]
0 TeetSStP - Rotor-teeter soft-stop position (degrees) [used only for 2 blades and when TeetMod=1]
0 TeetHStP - Rotor-teeter hard-stop position (degrees) [used only for 2 blades and when TeetMod=1]
0 TeetSSSp - Rotor-teeter soft-stop linear-spring constant (N-m/rad) [used only for 2 blades and when TeetMod=1]
0 TeetHSSp - Rotor-teeter hard-stop linear-spring constant (N-m/rad) [used only for 2 blades and when TeetMod=1]
---------------------- DRIVETRAIN ----------------------------------------------
100 GBoxEff - Gearbox efficiency (%)
1 GBRatio - Gearbox ratio (-)
69737644900 DTTorSpr - Drivetrain torsional spring (N-m/rad)
49418406 DTTorDmp - Drivetrain torsional damper (N-m/(rad/s))
---------------------- FURLING -------------------------------------------------
False Furling - Read in additional model properties for furling turbine (flag) [must currently be FALSE)
"unused" FurlFile - Name of file containing furling properties (quoted string) [unused when Furling=False]
---------------------- TOWER ---------------------------------------------------
20 TwrNodes - Number of tower nodes used for analysis (-)
"IEA-15-240-RWT-UMaineSemi_ElastoDyn_tower.dat" TwrFile - Name of file containing tower properties (quoted string)
Thank you for your help,
Dear @Yuki.Ogura,
I believe the issue is that the DEFAULT wake-deflection correction parameters–especially the DEFAULT value of C_HWkDfl_xY = -0.004/deg is not appropriate for the wind turbine (IEA Wind 15-MW RWT) or case you are using. With a -30 deg yaw error, a value of C_HWkDfl_xY = -0.004/deg will yield a deflection of Y = +480 m at 4000-m downstream of the rotor. The DEFAULT wake-deflection correction parameters were derived for a different wind turbine (NREL 5-MW baseline wind turbine) and different cases (including different mean wind speed and turbulence conditions).
Best regards,
Dear @Jason.Jonkman ,
Thank you for your reply.
I tried C_HWkDfl_xY=0 and got the following figure.
The figure shows that the direction of deflection is changing as you said.
However, I don’t know how to determine if the set C_HWkDfl_xY is a reasonable value.
Is there an equation to determine C_HWkDfl_xY?
Best regards,
Dear @Yuki.Ogura,
The horizontal wake-deflection correct in FAST.Farm is meant to be used for calibration of the wake deflection of the polar model to known solutions, e.g., from large-eddy simulation or experimental data. There is no “equation” to determine it.
We were hopeful that our original calibration of the wake-related parameters of FAST.Farm for the NREL 5-MW turbine against LES data (from the following paper: https://www.nrel.gov/docs/fy18osti/70533.pdf), from which the DEFAULT settings in FAST were defined were generally valid, but apparently this is not the case for C_HWkDfl_xY.
Regardless, as I suggested before, we generally recommend applying the curled wake model in situations with skewed inflow anyway.
Best regards,
Dear @Jason.Jonkman ,
Thank you for your kind reply.
I am aware that the value of C_HWkDfl_xY can be obtained from LES simulations and actual experimental data.
As you suggested, I will use the curled wake model from now on.
Best regards,
