Dear Jason,
It makes sense to me that the tower damping under the idling condition would be lower than the damping under the operational condition because I would expect more aerodynamic damping under the operational condition.
Best regards,
Hello Dr. Jonkman,
I am using the stochastic subspace identification method (with Multiblade coordinates transformation and harmonic removal method) to study the modal parameters of the 5MW wind turbine in a onshore configuration.
This is the stabilization diagram that I obtained from analyzing the acceleration data.
The majority of modes can be successfully obtained.
However, I am not sure about two things:
- the damping ratio (number besides the points) is not correlated well with the data stated in the report (Aeroelastic Instabilities of Large Offshore and Onshore Wind Turbines
Gunjit Bir and Jason Jonkman). Especially for the modes corresponding to the blade (Lower than the reference number). However, there is an explanation for this that the assumptions of OMA are more or less violated for the operating wind turbine. How do you think about this? - there are two modes around 0.1Hz with high damping ratios ( the number besides the point, around 50~60%) . I am not sure which modes they represent. Can you give me some hints?
Thank you
Best regards,
Xiao
Dear Xiao,
I’m not sure I know enough about you are doing and your figure to comment much.
I will say that the paper by Bir and Jonkman that you reference was published some time ago and was led by Gunjit Bir, who has since left NREL; I was only loosely involved in the paper and cannot attest to all the published results. You may want to compare your results to other sources and/or to other FAST analyses e.g. running your own FAST linearizations.
The lowest structural natural frequency of the land-based NREL 5-MW turbine is around 0.3 Hz, for the tower fore-aft and side-to-side bending modes. The only effects at around 0.1 Hz that I’m aware of for the land-based NREL 5-MW turbine are (1) this is roughly the rotor speed (6.9 rpm) at cut-in wind speed and (2) this is roughly the target natural frequency (0.6 rad/s) of the second-order drivetrain response of the baseline PI-based collective blade-pitch controller for wind speeds above rated.
Best regards,
Hello Jason,
Thanks for the explanation.
The figure was obtained at mean wind speed of 16 m/s. The two stable modes were also found for 12 m/s but not for 8 m/s and 4 m/s. Thus, I think it is related to the drivetrain response to pitch controller. But why the damping is so large?
Dear Xiao,
As explained in the NREL 5-MW baseline turbine specifications report: nrel.gov/docs/fy09osti/38060.pdf, the target damping ratio of the second-order drivetrain response is 70%, but this is for an ideal (rigid) drivetrain. The actual damping obtained from a more advanced model (like FAST) is likely different, but I would guess it would still be high.
Best regards,
OK, I see. I conducted a linearization analysis. And the damping of the generator (around 0.1Hz) is approaching infinite.
Dear Jason,
I performed both linearization study and my OMA study on the original 5MW wind turbine and the turbine with trailing edge flap.
I assume the flap only increase the mass of the blade and does not change the stiffness of blade.
From both studies, I found the 1st rotor flap modes are reduced sightly, and 2nd rotor flap and 1st lag modes are dramatically increased for all the wind speed I simulated. I expected the sightly reduction is reasonable but why it increase so dramatically?
flap1 reg 0.648->0.474
flap1 col 0.726->0.614
flap1 prog 0.851->0.710
lag1 reg 0.97->4.419
lag1 prog 1.20->4.68
flap2 reg 1.965->6.285
flap2 col 2.08->6.422
flap2 prog 2.136->6.53
The figure shows my results for the turbine with flaps.
Dear Xiao,
That doesn’t sound correct, but I’m not sure what the problem might be.
Best regards,
Hi, Jason,
The flaps cover 30% of the blade from the tip. Although it only increases the total blade mass by 5%, the local sectional mass is increased by 40%.
This significantly changes the mode shape of the blade (estimated by Bmodes ).
I test the setting with the original 5MW turbine. Both linearization and OMA can match the reference data in the report.
Then I conduct the linearization and OMA on the turbine with flaps. The results from different methods can match each other.
But they are weird compared to the original case. Could it be influenced by the altered beam mode shape?
This is a input script for my linearization. In region 2, I trim the case to obtain a constant generator torque. In region 3, I trim the case to obtain a constant pitch.
linear.fst
--------------------------------------------------------------------------------
------- FAST INPUT FILE --------------------------------------------------------
NREL 5MW Wind Turbine Updated by Xiao Sun, MTU, Feb, 2015.
Compatible with FAST v7.02.00.
---------------------- SIMULATION CONTROL --------------------------------------
True Echo - Echo input data to "echo.out" (flag)
1 ADAMSPrep - ADAMS preprocessor mode {1: Run FAST, 2: use FAST as a preprocessor to create an ADAMS model, 3: do both} (switch)
2 AnalMode - Analysis mode {1: Run a time-marching simulation, 2: create a periodic linearized model} (switch)
3 NumBl - Number of blades (-)
800.0 TMax - Total run time (s)
0.001 DT - Integration time step (s)
---------------------- TURBINE CONTROL -----------------------------------------
0 !YCMode - Yaw control mode {0: none, 1: user-defined from routine UserYawCont, 2: user-defined from Simulink/Labview} (switch)
9999.9 !TYCOn - Time to enable active yaw control (s) [unused when YCMode=0]
0 PCMode - Pitch control mode {0: none, 1: user-defined from routine PitchCntrl, 2: user-defined from Simulink/Labview} (switch)
0 TPCOn - Time to enable active pitch control (s) [unused when PCMode=0]
1 VSContrl - Variable-speed control mode {0: none, 1: simple VS, 2: user-defined from routine UserVSCont, 3: user-defined from Simulink/Labview} (switch)
1173.7 !VS_RtGnSp - Rated generator speed for simple variable-speed generator control (HSS side) (rpm) [used only when VSContrl=1]
43093.55 !VS_RtTq - Rated generator torque/constant generator torque in Region 3 for simple variable-speed generator control (HSS side) (N-m) [used only when VSContrl=1]
0.0255764 !VS_Rgn2K - Generator torque constant in Region 2 for simple variable-speed generator control (HSS side) (N-m/rpm^2) [used only when VSContrl=1]
10 !VS_SlPc - Rated generator slip percentage in Region 2 1/2 for simple variable-speed generator control (%) [used only when VSContrl=1]
2 !GenModel - Generator model {1: simple, 2: Thevenin, 3: user-defined from routine UserGen} (switch) [used only when VSContrl=0]
True !GenTiStr - Method to start the generator {T: timed using TimGenOn, F: generator speed using SpdGenOn} (flag)
True !GenTiStp - Method to stop the generator {T: timed using TimGenOf, F: when generator power = 0} (flag)
9999.9 !SpdGenOn - Generator speed to turn on the generator for a startup (HSS speed) (rpm) [used only when GenTiStr=False]
0.0 !TimGenOn - Time to turn on the generator for a startup (s) [used only when GenTiStr=True]
9999.9 !TimGenOf - Time to turn off the generator (s) [used only when GenTiStp=True]
1 !HSSBrMode - HSS brake model {1: simple, 2: user-defined from routine UserHSSBr, 3: user-defined from Labview} (switch)
9999.9 !THSSBrDp - Time to initiate deployment of the HSS brake (s)
9999.9 **TiDynBrk - Time to initiate deployment of the dynamic generator brake [CURRENTLY IGNORED] (s)
9999.9 **TTpBrDp(1) - Time to initiate deployment of tip brake 1 (s)
9999.9 **TTpBrDp(2) - Time to initiate deployment of tip brake 2 (s)
9999.9 **TTpBrDp(3) - Time to initiate deployment of tip brake 3 (s) [unused for 2 blades]
9999.9 **TBDepISp(1) - Deployment-initiation speed for the tip brake on blade 1 (rpm)
9999.9 **TBDepISp(2) - Deployment-initiation speed for the tip brake on blade 2 (rpm)
9999.9 **TBDepISp(3) - Deployment-initiation speed for the tip brake on blade 3 (rpm) [unused for 2 blades]
9999.9 !TYawManS - Time to start override yaw maneuver and end standard yaw control (s)
9999.9 !TYawManE - Time at which override yaw maneuver reaches final yaw angle (s)
0.0 !NacYawF - Final yaw angle for yaw maneuvers (degrees)
9999.9 !TPitManS(1) - Time to start override pitch maneuver for blade 1 and end standard pitch control (s)
9999.9 !TPitManS(2) - Time to start override pitch maneuver for blade 2 and end standard pitch control (s)
9999.9 !TPitManS(3) - Time to start override pitch maneuver for blade 3 and end standard pitch control (s) [unused for 2 blades]
9999.9 !TPitManE(1) - Time at which override pitch maneuver for blade 1 reaches final pitch (s)
9999.9 !TPitManE(2) - Time at which override pitch maneuver for blade 2 reaches final pitch (s)
9999.9 !TPitManE(3) - Time at which override pitch maneuver for blade 3 reaches final pitch (s) [unused for 2 blades]
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 !BlPitchF(1) - Blade 1 final pitch for pitch maneuvers (degrees)
0 !BlPitchF(2) - Blade 2 final pitch for pitch maneuvers (degrees)
0 !BlPitchF(3) - Blade 3 final pitch for pitch maneuvers (degrees) [unused for 2 blades]
---------------------- OK ENVIRONMENTAL CONDITIONS --------------------------------
9.80665 Gravity - Gravitational acceleration (m/s^2)
---------------------- OK FEATURE FLAGS -------------------------------------------
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)
True CompAero - Compute aerodynamic forces (flag)
False CompNoise - Compute aerodynamic noise (flag)
---------------------- OK INITIAL CONDITIONS --------------------------------------
0.0 OoPDefl - Initial out-of-plane blade-tip displacement (meters)
0.0 IPDefl - Initial in-plane blade-tip deflection (meters)
0.0 TeetDefl - Initial or fixed teeter angle (degrees) [unused for 3 blades]
0.0 Azimuth - Initial azimuth angle for blade 1 (degrees)
6.9 RotSpeed - Initial or fixed rotor speed (rpm)
0.0 NacYaw - Initial or fixed nacelle-yaw angle (degrees)
0.0 TTDspFA - Initial fore-aft tower-top displacement (meters)
0.0 TTDspSS - Initial side-to-side tower-top displacement (meters)
---------------------- OK TURBINE CONFIGURATION -----------------------------------
63 TipRad - The distance from the rotor apex to the blade tip (meters)
1.5 HubRad - The distance from the rotor apex to the blade root (meters)
1 PSpnElN - Number of the innermost blade element which is still part of the pitchable portion of the blade for partial-span pitch control [1 to BldNodes] [CURRENTLY IGNORED] (-)
0.0 UndSling - Undersling length [distance from teeter pin to the rotor apex] (meters) [unused for 3 blades]
0.0 HubCM - Distance from rotor apex to hub mass [positive downwind] (meters)
-5.0191 OverHang - Distance from yaw axis to rotor apex [3 blades] or teeter pin [2 blades] (meters)
1.9 NacCMxn - Downwind distance from the tower-top to the nacelle CM (meters)
0 NacCMyn - Lateral distance from the tower-top to the nacelle CM (meters)
1.75 NacCMzn - Vertical distance from the tower-top to the nacelle CM (meters)
87.6 TowerHt - Height of tower above ground level [onshore] or MSL [offshore] (meters)
1.96256 Twr2Shft - Vertical distance from the tower-top to the rotor shaft (meters)
0.0 TwrRBHt - Tower rigid base height (meters)
-5.0 ShftTilt - Rotor shaft tilt angle (degrees). Negative for an upwind rotor.
0.0 Delta3 - Delta-3 angle for teetering rotors (degrees) [unused for 3 blades]
-2.5 PreCone(1) - Blade 1 cone angle (degrees)
-2.5 PreCone(2) - Blade 2 cone angle (degrees)
-2.5 PreCone(3) - Blade 3 cone angle (degrees) [unused for 2 blades]
0.0 AzimB1Up - Azimuth value to use for I/O when blade 1 points up (degrees)
---------------------- OK MASS AND INERTIA ----------------------------------------
0.0 YawBrMass - Yaw bearing mass (kg)
240000 NacMass - Nacelle mass (kg)
56780 HubMass - Hub mass (kg)
0.0 TipMass(1) - Tip-brake mass, blade 1 (kg)
0.0 TipMass(2) - Tip-brake mass, blade 2 (kg)
0.0 TipMass(3) - Tip-brake mass, blade 3 (kg) [unused for 2 blades]
2.6079e6 NacYIner - Nacelle inertia about yaw axis (kg m^2)
534.116 GenIner - Generator inertia about HSS (kg m^2)
115926 HubIner - Hub inertia about rotor axis [3 blades] or teeter axis [2 blades] (kg m^2)
---------------------- OK DRIVETRAIN ----------------------------------------------
100.0 GBoxEff - Gearbox efficiency (%)
94.4 GenEff - Generator efficiency [ignored by the Thevenin and user-defined generator models] (%)
97.0 GBRatio - Gearbox ratio (-)
False GBRevers - Gearbox reversal {T: if rotor and generator rotate in opposite directions} (flag)
28.1162e3 HSSBrTqF - Fully deployed HSS-brake torque (N-m)
0.6 HSSBrDT - Time for HSS-brake to reach full deployment once initiated (sec) [used only when HSSBrMode=1]
"unused" DynBrkFi - File containing a mech-gen-torque vs HSS-speed curve for a dynamic brake [CURRENTLY IGNORED] (quoted string)
8.67637E+08 DTTorSpr - Drivetrain torsional spring (N-m/rad)
6.215E+06 DTTorDmp - Drivetrain torsional damper (N-m/(rad/s))
---------------------- *** OK SIMPLE INDUCTION GENERATOR ------------------------------
9999.9 SIG_SlPc - Rated generator slip percentage (%) [used only when VSContrl=0 and GenModel=1]
9999.9 SIG_SySp - Synchronous (zero-torque) generator speed (rpm) [used only when VSContrl=0 and GenModel=1]
9999.9 SIG_RtTq - Rated torque (N-m) [used only when VSContrl=0 and GenModel=1]
9999.9 SIG_PORt - Pull-out ratio (Tpullout/Trated) (-) [used only when VSContrl=0 and GenModel=1]
---------------------- *** OK THEVENIN-EQUIVALENT INDUCTION GENERATOR -----------------
9999.9 TEC_Freq - Line frequency [50 or 60] (Hz) [used only when VSContrl=0 and GenModel=2]
9998 TEC_NPol - Number of poles [even integer > 0] (-) [used only when VSContrl=0 and GenModel=2]
9999.9 TEC_SRes - Stator resistance (ohms) [used only when VSContrl=0 and GenModel=2]
9999.9 TEC_RRes - Rotor resistance (ohms) [used only when VSContrl=0 and GenModel=2]
9999.9 TEC_VLL - Line-to-line RMS voltage (volts) [used only when VSContrl=0 and GenModel=2]
9999.9 TEC_SLR - Stator leakage reactance (ohms) [used only when VSContrl=0 and GenModel=2]
9999.9 TEC_RLR - Rotor leakage reactance (ohms) [used only when VSContrl=0 and GenModel=2]
9999.9 TEC_MR - Magnetizing reactance (ohms) [used only when VSContrl=0 and GenModel=2]
---------------------- OK PLATFORM ------------------------------------------------
0 PtfmModel - Platform model {0: none, 1: onshore, 2: fixed bottom offshore, 3: floating offshore} (switch)
"NREL5MW_Platform.dat" PtfmFile - Name of file containing platform properties (quoted string) [unused when PtfmModel=0]
---------------------- OK TOWER ---------------------------------------------------
20 TwrNodes - Number of tower nodes used for analysis (-)
"NREL5MW_Tower.dat" TwrFile - Name of file containing tower properties (quoted string)
---------------------- OK NACELLE-YAW ---------------------------------------------
9.02832E+09 YawSpr - Nacelle-yaw spring constant (N-m/rad)
1.916E+07 YawDamp - Nacelle-yaw damping constant (N-m/(rad/s))
0.0 YawNeut - Neutral yaw position--yaw spring force is zero at this yaw (degrees)
---------------------- OK FURLING -------------------------------------------------
False Furling - Read in additional model properties for furling turbine (flag)
"unused" FurlFile - Name of file containing furling properties (quoted string) [unused when Furling=False]
---------------------- OK ROTOR-TEETER --------------------------------------------
0 TeetMod - Rotor-teeter spring/damper model {0: none, 1: standard, 2: user-defined from routine UserTeet} (switch) [unused for 3 blades]
0.0 TeetDmpP - Rotor-teeter damper position (degrees) [used only for 2 blades and when TeetMod=1]
0.0 TeetDmp - Rotor-teeter damping constant (N-m/(rad/s)) [used only for 2 blades and when TeetMod=1]
0.0 TeetCDmp - Rotor-teeter rate-independent Coulomb-damping moment (N-m) [used only for 2 blades and when TeetMod=1]
0.0 TeetSStP - Rotor-teeter soft-stop position (degrees) [used only for 2 blades and when TeetMod=1]
0.0 TeetHStP - Rotor-teeter hard-stop position (degrees) [used only for 2 blades and when TeetMod=1]
0.0 TeetSSSp - Rotor-teeter soft-stop linear-spring constant (N-m/rad) [used only for 2 blades and when TeetMod=1]
0.0 TeetHSSp - Rotor-teeter hard-stop linear-spring constant (N-m/rad) [used only for 2 blades and when TeetMod=1]
---------------------- *** OK TIP-BRAKE -----------------------------------------------
0.0 TBDrConN - Tip-brake drag constant during normal operation, Cd*Area (m^2)
0.0 TBDrConD - Tip-brake drag constant during fully-deployed operation, Cd*Area (m^2)
0.0 TpBrDT - Time for tip-brake to reach full deployment once released (sec)
---------------------- OK BLADE ---------------------------------------------------
"NRELOffshrBsline5MW_InternalBlade48.dat" BldFile(1) - Name of file containing properties for blade 1 (quoted string)
"NRELOffshrBsline5MW_InternalBlade48.dat" BldFile(2) - Name of file containing properties for blade 2 (quoted string)
"NRELOffshrBsline5MW_InternalBlade48.dat" BldFile(3) - Name of file containing properties for blade 3 (quoted string) [unused for 2 blades]
---------------------- OK AERODYN -------------------------------------------------
"NREL5MW_AD_Sin_ORI_Lin.ipt" ADFile - Name of file containing AeroDyn input parameters (quoted string)
---------------------- OK NOISE ---------------------------------------------------
"unused" NoiseFile - Name of file containing aerodynamic noise input parameters (quoted string) [used only when CompNoise=True]
---------------------- OK ADAMS ---------------------------------------------------
"unused" ADAMSFile - Name of file containing ADAMS-specific input parameters (quoted string) [unused when ADAMSPrep=1]
---------------------- OK LINEARIZATION CONTROL -----------------------------------
"5MW_Linear.dat" LinFile - Name of file containing FAST linearization parameters (quoted string) [unused when AnalMode=1]
---------------------- OK OUTPUT --------------------------------------------------
False SumPrint - Print summary data to "<RootName>.fsm" (flag)
1 OutFileFmt - Format for tabular (time-marching) output file(s) (1: text file [<RootName>.out], 2: binary file [<RootName>.outb], 3: both) (switch)
True TabDelim - Use tab delimiters in text tabular output file? (flag)
"ES10.3E2" OutFmt - Format used for text tabular output (except time). Resulting field should be 10 characters. (quoted string) [not checked for validity!]
30 TStart - Time to begin tabular output (s)
1 DecFact - Decimation factor for tabular output {1: output every time step} (-)
5.0 SttsTime - Amount of time between screen status messages (sec)
-3.09528 NcIMUxn - Downwind distance from the tower-top to the nacelle IMU (meters)
0 NcIMUyn - Lateral distance from the tower-top to the nacelle IMU (meters)
2.23336 NcIMUzn - Vertical distance from the tower-top to the nacelle IMU (meters)
1.912 ShftGagL - Distance from rotor apex [3 blades] or teeter pin [2 blades] to shaft strain gages [positive for upwind rotors] (meters)
0 NTwGages - Number of tower nodes that have strain gages for output [0 to 9] (-)
0 TwrGagNd - List of tower nodes that have strain gages [1 to TwrNodes] (-) [unused if NTwGages=0]
3 NBlGages - Number of blade nodes that have strain gages for output [0 to 9] (-)
5, 9, 13, BldGagNd - List of blade nodes that have strain gages [1 to BldNodes] (-) [unused if NBlGages=0]
OutList - The next line(s) contains a list of output parameters. See OutList.xlsx for a listing of available output channels, (-)
"WindVxi,WindVyi,WindVzi"
"OoPDefl1 , IPDefl1 " - Blade 1 out-of-plane and in-plane deflections and tip twist
"RootFxc1"
"RootFyc1"
"RootFzc1"
"RootMxc1"
"RootMyc1"
"RootMzc1"
END of FAST input file (the word "END" must appear in the first 3 columns of this last line).
--------------------------------------------------------------------------------linear.dat
--------------------------------------------------------------------------------
---------------------- FAST LINEARIZATION CONTROL FILE -------------------------
5 MW offshore Reference wind turbine linearization input properties.
---------------------- PERIODIC STEADY STATE SOLUTION --------------------------
True CalcStdy - Calculate periodic steady state condition {False: linearize about initial conditions} (flag)
2 TrimCase - Trim case {1: find nacelle yaw, 2: find generator torque, 3: find collective blade pitch} (switch) [used only when CalcStdy=True and GenDOF=True]
0.01 DispTol - Convergence tolerance for the 2-norm of displacements in the periodic steady state calculation (rad ) [used only when CalcStdy=True]
0.01 VelTol - Convergence tolerance for the 2-norm of velocities in the periodic steady state calculation (rad/s) [used only when CalcStdy=True]
---------------------- MODEL LINEARIZATION -------------------------------------
36 NAzimStep - Number of equally-spaced azimuth steps in periodic linearized model (-)
2 MdlOrder - Order of output linearized model {1: 1st order A, B, Bd, C, D, Dd; 2: 2nd order M, C, K, F, Fd, VelC, DspC, D, Dd} (switch)
---------------------- INPUTS AND DISTURBANCES ---------------------------------
2 NInputs - Number of control inputs [0 (none) or 1 to 4+NumBl] (-)
3,4 CntrlInpt - List of control inputs [1 to NInputs] {1: nacelle yaw angle, 2: nacelle yaw rate, 3: generator torque, 4: collective blade pitch, 5: individual pitch of blade 1, 6: individual pitch of blade 2, 7: individual pitch of blade 3 [unavailable for 2-bladed turbines]} (-) [unused if NInputs=0]
5 NDisturbs - Number of wind disturbances [0 (none) or 1 to 7] (-)
1,2,3,4,5 Disturbnc - List of input wind disturbances [1 to NDisturbs] {1: horizontal hub-height wind speed, 2: horizontal wind direction, 3: vertical wind speed, 4: horizontal wind shear, 5: vertical power law wind shear, 6: linear vertical wind shear, 7: horizontal hub-height wind gust} (-) [unused if NDisturbs=0]Then the attached files are the results and the excels from MBC3, I am not sure if I interpret them correctly.
Thanks.
Xiao
ExFlap_Linear_16ms.txt (661 KB)
ExFlap_CampbellDiagram_16ms.xls (259 KB)
Dear Xiao,
I’m sorry, but I’m not sure I understand your question. I don’t see anything obviously wrong in the input or output files you’ve attached.
I would expect an increase in the section mass near the tip to reduce the flapwise and edgewise bending stiffness. I do see this for the first flapwise mode, but the 1st edgewise and 2nd flapwise modes are higher in your results. I’m not sure I know enough about what you’ve done to comment as to why.
You say the section mass change has a significant impact on the blade mode shapes. Can you explain more (perhaps visually)?
Best regards,
Dear Dr. Jonkman,
Thanks for your comment. I should have explained it more clearly.
These three figures show the comparison of the bending mode shape between original blade and blade with flap.
I expected the mode shape is correct since the paper also plot a similar mode shape for a similar rotating blade with a tip mass ( Fig.4 from “Effect of balance weight on dynamic characteristics of a rotating wind turbine blade” J Eng Math (2016) 97:49–65) , which is essentially the same according to my simplified assumption that flap only add mass to the tip. In the figure, r is the ratio of tip mass over total blade mass, r0 is the radial location of the tip mass over the blade length.
However, it seems it significantly change the mode shape of the original blade.
My questions are:
- For the FAST input, does the mode shape has strong influence on the natural frequencies?
- In the Campbell diagram excel file (last poster), there are some modes that is not clear to me(2.08 Hz~3.10 Hz).
The flapwise 2nd is clearly from 5.78 Hz~6.18 Hz.
I associated the 4.16 Hz~4.62 Hz to the edgewise 1st.
And the 0.429 Hz~0.786 Hz to the flapwise 1st.
Did I interpret the results right?
Thank you,
Best Regards,
Xiao Sun
Dear Xiao,
The data in the first Figure looks like I would expect. But in your second figure, the 1st edgewise mode for the case w/ flap looks like a 2nd bending mode and the 2nd edgewise mode for the case w/ flap looks like a 3rd bending mode. I don’t think you’ve interpreted these modes correctly.
To answer your direction questions:
- Yes, the mode shapes input into FAST have a strong effect on the natural frequencies predicted by FAST.
- I would guess modes 8, 9 and 10 in the CampbellDiagram spreadsheet you linked earlier correspeond to the free-free drivetrain torsion, second tower fore-aft bending, and second tower side-to-side bending modes, respectively.
Yes, I agree with your other interpretations.
Best regards,
Dear Dr. Jonkman
Thank you for your help.
I see what was wrong with my results.
Best regards,
Xiao
Dear Mr Jonkman
I have a question concerning the first Page of this topic and your uploaded NRELOffshrBsline5MW_Platform_Monopile_DS.dat and NRELOffshrBsline5MW_Tower_Monopile_DS.dat
Are the values for the coefficients in the NRELOffshrBsline5MW_Tower_Monopile_DS.dat file gained when in BModes the parameter radius was set to the entire tower length like:
radius=tower+monopile ? Or is radius=Height of tower above MSL? I want to model the DS foundation.
Then, what is the effect of the file NRELOffshrBsline5MW_Platform_Monopile_DS.dat ? Does it bring the Morison Equation into the Model? I thought UserTwrLd_DS.f90 would implement it already, and I thought the Platform would be used for modeling the CS foundation, what do I understand wrong?
How should I set PtfmModel in the main .fst file to generate the DS model?
I really appreciate Your help,
best regards,
Dear Lasse,
Regarding the BeamDyn model: the “radius” should the distance from the MSL to the tower top / yaw bearing. The distance from the MSL to the bottom of the pile should be specified as the “draft”.
Regarding the FAST model: Please keep in mind that in the structural model of FAST v7 for fixed-bottom offshore support structures, the “tower” is really the tower + monopile and the “platform” is the lower end of the monopile. The sample NRELOffshreBsline5MW_Platform_Monopile_DS.dat file is used to:
- make the lower end of the monopile a free end (enabling 4 of the 6 platform DOFs),
- define the draft of the monopile below the MSL,
- enable the user-defined tower loading model (for modeling the Morison’s equation and distributed springs), and
- set the wave and current conditions.
As shown in the sample NRELOffshreBsline5MW_Platform_Monopile_DS.dat file, the platform loading model is not enabled, which means there are no reaction loads at the free end of the pile (instead the pile reactions through the DS model are distributed along the tower through the user-defined tower loading model).
You should set PtfmModel = 2 to use this DS model.
Best regards,