Unable to reproduce the tower side-side instability in DLC 6.2

[Melancho7ee]

Dear @jjonkman ,

I'm trying to study the tower stability problem recently. As mentioned in your Ph.D dissertation "Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine", Tower Side-to-Side Instability of Land-Based Wind Turbine is found in DLC 6.2a for land based wind turbines.

However, I failed to reproduce this phenomenon. Could you please provide some tips on how to reproduce it? Thanks in advance!
Some important settings are as follows:
Blade pitch angles are set to 90 deg.
GenTq = 0; GenPwr = 0.
Wind speed = 50 m/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]
True          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)
False          PtfmSgDOF   - Platform horizontal surge translation DOF (flag)
False          PtfmSwDOF   - Platform horizontal sway translation DOF (flag)
False          PtfmHvDOF   - Platform vertical heave translation DOF (flag)
False          PtfmRDOF    - Platform roll tilt rotation DOF (flag)
False          PtfmPDOF    - Platform pitch tilt rotation DOF (flag)
False          PtfmYDOF    - Platform yaw rotation DOF (flag)

and

---------------------- 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)
          0   RotSpeed    - Initial or fixed rotor speed (rpm)
          -35   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)
          0   PtfmSurge   - Initial or fixed horizontal surge translational displacement of platform (meters)
          0   PtfmSway    - Initial or fixed horizontal sway translational displacement of platform (meters)
          0   PtfmHeave   - Initial or fixed vertical heave translational displacement of platform (meters)
          0   PtfmRoll    - Initial or fixed roll tilt rotational displacement of platform (degrees)
          0   PtfmPitch   - Initial or fixed pitch tilt rotational displacement of platform (degrees)
          0   PtfmYaw     - Initial or fixed yaw rotational displacement of platform (degrees)

[jjonkman]

Dear @Melancho7ee,

I would expect to see this blade-edgewise / tower side-side instability in any idling rotor in high wind with large skew, but which wind turbine are simulating in OpenFAST?

The inputs you shared look fine, except that I don't see the blade-pitch angles set to 90 degrees (they are set to zero instead).

Presumably you've disabled torque and pitch control in ServoDyn or disabled ServoDyn altogether?

Have you disabled induction and unsteady airfoil aerodynamics in AeroDyn?

Best regards,

[Melancho7ee]

Dear @jjonkman ,

Sorry for the late reply. I used NREL 5MW onshore wind turbine (OpenFAST 3.5.0). I have set GenTq=0 and blade pitch angle = 90 deg, using Matlab/Simulink as the controller.

As for the question "Have you disabled induction and unsteady airfoil aerodynamics in AeroDyn?", The settings on Aerodyn15.dat is presented below.

Do you mean that I should set "WakeMod = 0" and "AFAeroMod = 1"?

------- 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)
          1   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]
          1   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}
True          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  ===================================================================
1.225         AirDens            - Air density (kg/m^3)
1.464E-05     KinVisc            - Kinematic viscosity of working fluid (m^2/s)
335           SpdSound           - Speed of sound in working fluid (m/s)
103500        Patm               - Atmospheric pressure (Pa) [used only when CavitCheck=True]
1700          Pvap               - Vapour pressure of working fluid (Pa) [used only when CavitCheck=True]
======  Blade-Element/Momentum Theory Options  ====================================================== [unused when WakeMod=0 or 3]
          2   SkewMod            - Type of skewed-wake correction model (switch) {1=uncoupled, 2=Pitt/Peters, 3=coupled} [unused when WakeMod=0 or 3]
"default"     SkewModFactor      - Constant used in Pitt/Peters skewed wake model {or "default" is 15/32*pi} (-) [used only when SkewMod=2; unused when WakeMod=0 or 3]
True          TipLoss            - Use the Prandtl tip-loss model? (flag) [unused when WakeMod=0 or 3]
True          HubLoss            - Use the Prandtl hub-loss model? (flag) [unused when WakeMod=0 or 3]
True          TanInd             - Include tangential induction in BEMT calculations? (flag) [unused when WakeMod=0 or 3]
False         AIDrag             - Include the drag term in the axial-induction calculation? (flag) [unused when WakeMod=0 or 3]
False         TIDrag             - Include the drag term in the tangential-induction calculation? (flag) [unused when WakeMod=0,3 or TanInd=FALSE]
"Default"     IndToler           - Convergence tolerance for BEMT nonlinear solve residual equation {or "default"} (-) [unused when WakeMod=0 or 3]
        100   MaxIter            - Maximum number of iteration steps (-) [unused when WakeMod=0]
======  Dynamic Blade-Element/Momentum Theory Options  ============================================== [used only when WakeMod=2]
          2   DBEMT_Mod          - Type of dynamic BEMT (DBEMT) model {1=constant tau1, 2=time-dependent tau1, 3=constant tau1 with continuous formulation} (-) [used only when WakeMod=2]
          4   tau1_const         - Time constant for DBEMT (s) [used only when WakeMod=2 and DBEMT_Mod=1 or 3]
======  OLAF -- cOnvecting LAgrangian Filaments (Free Vortex Wake) Theory Options  ================== [used only when WakeMod=3]
"unused"      OLAFInputFileName  - Input file for OLAF [used only when WakeMod=3]
======  Beddoes-Leishman Unsteady Airfoil Aerodynamics Options  ===================================== [used only when AFAeroMod=2]
          3   UAMod              - Unsteady Aero Model Switch (switch) {2=B-L Gonzalez, 3=B-L Minnema/Pierce, 4=B-L HGM 4-states, 5=B-L 5 states, 6=Oye, 7=Boeing-Vertol} [used only when AFAeroMod=2]
True          FLookup            - Flag to indicate whether a lookup for f' will be calculated (TRUE) or whether best-fit exponential equations will be used (FALSE); if FALSE S1-S4 must be provided in airfoil input files (flag) [used only when AFAeroMod=2]
======  Airfoil Information =========================================================================
          1   AFTabMod           - Interpolation method for multiple airfoil tables {1=1D interpolation on AoA (first table only); 2=2D interpolation on AoA and Re; 3=2D interpolation on AoA and UserProp} (-)
          1   InCol_Alfa         - The column in the airfoil tables that contains the angle of attack (-)
          2   InCol_Cl           - The column in the airfoil tables that contains the lift coefficient (-)
          3   InCol_Cd           - The column in the airfoil tables that contains the drag coefficient (-)
          4   InCol_Cm           - The column in the airfoil tables that contains the pitching-moment coefficient; use zero if there is no Cm column (-)
          0   InCol_Cpmin        - The column in the airfoil tables that contains the Cpmin coefficient; use zero if there is no Cpmin column (-)
          8   NumAFfiles         - Number of airfoil files used (-)
"../5MW_Baseline/Airfoils/Cylinder1.dat"    AFNames            - Airfoil file names (NumAFfiles lines) (quoted strings)
"../5MW_Baseline/Airfoils/Cylinder2.dat"
"../5MW_Baseline/Airfoils/DU40_A17.dat"
"../5MW_Baseline/Airfoils/DU35_A17.dat"
"../5MW_Baseline/Airfoils/DU30_A17.dat"
"../5MW_Baseline/Airfoils/DU25_A17.dat"
"../5MW_Baseline/Airfoils/DU21_A17.dat"
"../5MW_Baseline/Airfoils/NACA64_A17.dat"
======  Rotor/Blade Properties  =====================================================================
True          UseBlCm            - Include aerodynamic pitching moment in calculations?  (flag)
"../5MW_Baseline/NRELOffshrBsline5MW_AeroDyn_blade.dat"    ADBlFile(1)        - Name of file containing distributed aerodynamic properties for Blade #1 (-)
"../5MW_Baseline/NRELOffshrBsline5MW_AeroDyn_blade.dat"    ADBlFile(2)        - Name of file containing distributed aerodynamic properties for Blade #2 (-) [unused if NumBl < 2]
"../5MW_Baseline/NRELOffshrBsline5MW_AeroDyn_blade.dat"    ADBlFile(3)        - Name of file containing distributed aerodynamic properties for Blade #3 (-) [unused if NumBl < 3]
======  Hub Properties ============================================================================== [used only when Buoyancy=True]
0.0   VolHub             - Hub volume (m^3)
0.0   HubCenBx           - Hub center of buoyancy x direction offset (m)
======  Nacelle Properties ========================================================================== [used only when Buoyancy=True]
0.0   VolNac             - Nacelle volume (m^3)
0,0,0 NacCenB            - Position of nacelle center of buoyancy from yaw bearing in nacelle coordinates (m)
======  Tail fin Aerodynamics ======================================================================== 
False         TFinAero           - Calculate tail fin aerodynamics model (flag)
"unused"      TFinFile           - Input file for tail fin aerodynamics [used only when TFinAero=True]
======  Tower Influence and Aerodynamics ============================================================ [used only when TwrPotent/=0, TwrShadow/=0, TwrAero=True, or Buoyancy=True]
         12   NumTwrNds         - Number of tower nodes used in the analysis  (-) [used only when TwrPotent/=0, TwrShadow/=0, TwrAero=True, or Buoyancy=True]
TwrElev        TwrDiam        TwrCd          TwrTI          TwrCb !TwrTI used only with TwrShadow=2, TwrCb used only with Buoyancy=True
(m)              (m)           (-)            (-)           (-)
0.0000000E+00  6.0000000E+00  1.0000000E+00  1.0000000E-01  0.0
8.5261000E+00  5.7870000E+00  1.0000000E+00  1.0000000E-01  0.0
1.7053000E+01  5.5740000E+00  1.0000000E+00  1.0000000E-01  0.0
2.5579000E+01  5.3610000E+00  1.0000000E+00  1.0000000E-01  0.0
3.4105000E+01  5.1480000E+00  1.0000000E+00  1.0000000E-01  0.0
4.2633000E+01  4.9350000E+00  1.0000000E+00  1.0000000E-01  0.0
5.1158000E+01  4.7220000E+00  1.0000000E+00  1.0000000E-01  0.0
5.9685000E+01  4.5090000E+00  1.0000000E+00  1.0000000E-01  0.0
6.8211000E+01  4.2960000E+00  1.0000000E+00  1.0000000E-01  0.0
7.6738000E+01  4.0830000E+00  1.0000000E+00  1.0000000E-01  0.0
8.5268000E+01  3.8700000E+00  1.0000000E+00  1.0000000E-01  0.0
8.7600000E+01  3.8700000E+00  1.0000000E+00  1.0000000E-01  0.0
======  Outputs  ====================================================================================
True          SumPrint            - Generate a summary file listing input options and interpolated properties to "<rootname>.AD.sum"?  (flag)
          9   NBlOuts             - Number of blade node outputs [0 - 9] (-)
          11,12,13,14,15,16,17,18,19   BlOutNd             - Blade nodes whose values will be output  (-)
          0   NTwOuts             - Number of tower node outputs [0 - 9]  (-)
          1,          2,          6    TwOutNd             - Tower nodes whose values will be output  (-)
                   OutList             - The next line(s) contains a list of output parameters.  See OutListParameters.xlsx for a listing of available output channels, (-)
"RtAeroFxh"				- Total rotor aerodynamic load (force in x direction)
"RtAeroFyh"				- Total rotor aerodynamic load (force in y direction)
"RtAeroFzh"				- Total rotor aerodynamic load (force in z direction)
"RtAeroMxh"				- Total rotor aerodynamic load (moment in x direction)
"RtAeroMyh"				- Total rotor aerodynamic load (moment in y direction)
"RtAeroMzh"				- Total rotor aerodynamic load (moment in z direction)
"RtArea"				- Rotor swept area
"RtAeroCt"			    - Rotor aerodynamic thrust coefficient
"RtAeroCq"			    - Rotor aerodynamic torque coefficient
"RtAeroCp"			    - Rotor aerodynamic power  coefficient
"RtTSR"
"RtVAvgxh"
"B1Azimuth"                             - Azimuth angle of blade 1
"B2Azimuth"                             - Azimuth angle of blade 2
"B3Azimuth"                             - Azimuth angle of blade 3
"RtAeroPwr"
END of input file (the word "END" must appear in the first 3 columns of this last OutList line)
---------------------- NODE OUTPUTS --------------------------------------------
          3   BldNd_BladesOut  - Blades to output
         99   BldNd_BlOutNd   - Blade nodes on each blade (currently unused)
              OutList     - The next line(s) contains a list of output parameters.  See OutListParameters.xlsx, AeroDyn_Nodes tab for a listing of available output channels, (-)
"VRel"      - Relvative wind speed at each node
"AxInd"     - Axial induction factor at each node
"TnInd"     - Tangential induction factor at each node
"Alpha"     - Angle of attack at each node
"Theta"     - Pitch+Twist angle at each node
"Phi"       - Inflow angle at each node
"Cl"        - Lift force coefficient at each node
"Cd"        - Drag force coefficient at each node
"Cm"        - Pitching moment coefficient at each node
"Cx"        - Normal force (to plane) coefficient at each node
"Cy"        - Tangential force (to plane) coefficient at each node
"Cn"        - Normal force (to chord) coefficient at each node
"Ct"        - Tangential force (to chord) coefficient at each node
"Fl"        - Lift force per unit length at each node
"Fd"        - Drag force per unit length at each node
"Mm"        - Pitching moment per unit length at each node
"Fx"        - Normal force (to plane) per unit length at each node
"Fy"        - Tangential force (to plane) per unit length at each node
"Fn"        - Normal force (to chord) per unit length at each node
"Ft"        - Tangential force (to chord) per unit length at each node
END of input file (the word "END" must appear in the first 3 columns of this last OutList line)
---------------------------------------------------------------------------------------

Best regards,

[jjonkman]

Dear @Melancho7ee,

Correct; by "disabl[ing] induction and unsteady airfoil aerodynamics in AeroDyn", I meant setting WakeMod = 0, AFAeroMod = 1. In your case, it looks like you are using WakeMod = 1, but blade-element/momentum theory is not applicable for an idling rotor.

Best regards,

[Melancho7ee]

Dear @jjonkman ,

Thanks for your advice, and I have changed to WakeMode = 0. However, I still failed to reproduce the stability problem mentioned by your Ph.D dissertation.

Part of the ElastoDyn.dat is shown below. I have tried NacYaw = 30 and -30, separately. But NO instability of tower-top side-side motion is observed.

------- ELASTODYN for OpenFAST INPUT FILE -------------------------------------------
NREL 5.0 MW Baseline Wind Turbine for Use in Offshore Analysis. Properties from Dutch Offshore Wind Energy Converter (DOWEC) 6MW Pre-Design (10046_009.pdf) and REpower 5M 5MW (5m_uk.pdf)
---------------------- SIMULATION CONTROL --------------------------------------
True         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]
True          DrTrDOF     - Drivetrain rotational-flexibility DOF (flag)
False          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)
False          PtfmSgDOF   - Platform horizontal surge translation DOF (flag)
False          PtfmSwDOF   - Platform horizontal sway translation DOF (flag)
False          PtfmHvDOF   - Platform vertical heave translation DOF (flag)
False          PtfmRDOF    - Platform roll tilt rotation DOF (flag)
False          PtfmPDOF    - Platform pitch tilt rotation DOF (flag)
False          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)
          0   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)
          0   PtfmSurge   - Initial or fixed horizontal surge translational displacement of platform (meters)
          0   PtfmSway    - Initial or fixed horizontal sway translational displacement of platform (meters)
          0   PtfmHeave   - Initial or fixed vertical heave translational displacement of platform (meters)
          0   PtfmRoll    - Initial or fixed roll tilt rotational displacement of platform (degrees)
          0   PtfmPitch   - Initial or fixed pitch tilt rotational displacement of platform (degrees)
          0   PtfmYaw     - Initial or fixed yaw rotational displacement of platform (degrees)

Time series are shown below. The first one is NacYaw = -30, and the second one is NacYaw = 30.

Also, I tried to set GenDOF = true and NacYaw = -30, but there is also no instability.

[jjonkman]

Dear @Melancho7ee,

You mentioned two cases, but then share results from three; what is different in the third case? Presumably you've enabled GenDOF at some nonzero yaw angle? I would normally expect GenDOF to be enabled for an idling rotor for this case and it could also be that the instability only happens at certain azimuth angles, which are not reached when GenDOF = FALSE,

Regardless, I do see signs of the instability, with rapid growth in response near the start of the simulation. Instead of completely blowing up, it appears that your simulation is reaching a limit-cycle oscillation, likely due to nonlinearities in the model.

You haven't said what level of structural damping you've set in the blades or tower, but lowering or raising the structural damping would likely increase or decrease the potential for instability.

Best regards,

[Melancho7ee]

Dear @jjonkman ,

My former expressions were a little unclear.

The FIRST pic is that NacYaw = -30 + GenDOF = FALSE.

The SECOND pic is that NacYaw = +30 + GenDOF = FALSE.

The THIRD pic is that NacYaw = -30 + GenDOF = TRUE.

As you mentioned, "it could also be that the instability only happens at certain azimuth angles". All my simulations set 0 Azimuth - Initial azimuth angle for blade 1 (degrees), I have not tested other azimuth angles. I would test more cases to see whether the instability will be reproduced.

P.S. I did not change any structural damping of the blades or tower. All corresponding parameters remain default.

Best regards,

[jjonkman]

Dear @Melancho7ee,

Actually I'm suggesting that GenDOF should be set to TRUE for this case, where an idling rotor is expected.

Best regards,

[Melancho7ee]

Dear @jjonkman ,

Soon after I submitted the first reply, I realized that I misunderstood your meaning at first. And the reply has been modified by myself.

The THIRD pic is that NacYaw = -30 + GenDOF = TRUE. But no instability is shown.

[jjonkman]

Again, I do see signs of an instability in all cases. In the third case, RotSpeed and YawBrTDyt show rapid growth near the start of the simulation, then reach a nonlinear limit-state oscillation. You haven't shown any blade-tip deflection.