by switching VBSContrl to 1 and CalcStdy to True with TrimCase =3 linearization converged and i got 0.12 Hz for PtfmYawDOF.
I set
12.74235216 VS_RtGnSp - Rated generator speed for simple variable-speed generator control (HSS side) (rpm)
43090 VS_RtTq - Rated generator torque/constant generator torque in Region 3 for simple variable-speed generator control (HSS side) (N-m)
212.6790715 VS_Rgn2K - Generator torque constant in Region 2 for simple variable-speed generator control (HSS side) (N-m/rpm^2)
10 VS_SlPc - Rated generator slip percentage in Region 2 1/2 for simple variable-speed generator control (%)
For 0mps Windspeed that works well but for the others Igot small angel assumption violated warning due to large tower deflection (I tried different start settings for Initial Platform positions withoutsuccess) followed by the error, that due to smllRottrans() Catenary cannot solve quasi-static mooring
Are you saying that your time-domain solution is now not working? You should be using VSContrl = 2 in time-domain simulations within FAST v7.02 in order to use the torque controller from the Bladed-style DLL.
The FAST User’s Guide describes how to use VSContrl = 1 with TrimCase = 3. Normally, I’d expect TrimCase = 3 to be used only above rated wind speed, whereby for the NREL 5-MW turbine I would set VS_RtTq = 43093.55 N-m (and VS_RtGnSp = VS_Rgn2K = VS_SlPc = small numbers). Below rated, I’d use TrimCase = 2.
thanks for clarifying. I ran the linearization with Trimcase =2 for Rotspeed <12rpm (rated). As the steady state solution was not found I tried it with compaero=false.
Now it works and the PtfmYaw DOF is found at 0.12 Hz. But I think that without air the results are not really useful, am I’m right?
Also there are some modes missing. Shouldn´t the Modes from Blade 1-3 the same? So 1st flw Bld1 = 1st flw Bld2 …
Is there a way to check the linearized model for errors?
I lowered the wind speed to get a higher TSR (now for sure higher than TSR at cp_max) but with CompAero = True no steady state solution is found.
With CompAero= false there is no problem for FAST finding a steady state solution and linearize, apart from that I think the model is different then and results are not reasonable anymore, right?
When I set GenDOF = false and CompAero = true the steady state solution is also found. Is it reasonable to set the VS_RtTq to an appropriate value for the windspeed / rotor speed and then assume a fixed speed turbine (GenDOF = false) to get the linearization of this variable speed turbine?
Does the steady-state solution with TrimCase = 2 seem to be converging such that increasing TMax would help, oscillating periodically such that increasing damping would help, or diverging?
I’m not sure what you mean by “reasonable”, but the linearization solution should be valid for small deviations around the operating point.
If you fix the generator DOF during linearization, I suspect you’ll get differences relative to a linearization with the generator DOF enabled in modes that involve coupling to the rotor rotation e.g. tower side-to-side bending, drivetrain torsion, and blade-edgewise bending.
Rotor speed, GenTq and the residues are oscillating periodically. I tried to increase TwrFADmp(1) and BldFlDmp(1) and the other tower and blade damping factors as well as the hydrodynamic damping matrix B in spar.1 by more than one order of magnitude but apart from rated condition the same problem is still showing up.
Tmax is already = 2000s so I don’t think I need to increase it further.
Even with very low wind speeds (5 rpm, 0.1 mps) the simulation is not converging.
Is anything else I can do to get a converged steady state condition without fixing the Gen DOF which you say would change the coupling between rotational modes.
I suggest that you debug by disabling structural DOFs. Does the rotor speed converge with TrimCase = 2 when only the generator DOF is enabled? If so, which DOF when enabled is causing the solution to not converge?
I was able to solve the error by switching IndModel in AeroDyn from NONE to SWIRL.
I disabled all structural DOFs but the error remained for larger wind speeds. So I tried changing aerodynamic settings. Now the steady state solution is found with all DOFs turned on again and with appropriate wind speed (still lower than for optimal TSR).
I hope someone find this helpful.
When looking for the eigenfrequencies with Getmats.m, MBC.m and the excel-sheet the blade modes for the individual Blades are not identical.
I assume that these are asymmetric modes ? For 1st edw bld 1 there is no mode fitting. Can someone help with identifying the blade modes?
1st flapwise I expect at 0.7 Hz 1st edgewise at 1Hz and 2nd flapwise at 2Hz.
Is there a way to get collective Blade modes?
Best regards,
Simon FAST.7.02_rpm07.00_lin.txt (3.43 MB)
I’m glad you identified what was causing the lack of convergence. Just FYI – Normally, IndModel = NONE in AeroDyn is meant for simulations involving a parked/idling rotor. IndModel should be set to WAKE or SWIRL for operational rotors.
Collective modes have all blades deflecting in-phase. Asymmetric modes have one blade deflecting out-of-phase with the other(s). I have not reviewed your results, but you can find several examples on this forum where you can find interpretations of the eigensolution e.g.:
These examples should help you as you interpret your eigensolution.
I think no everything regarding identifying the frequencies is clear. But I have some question:
I observe a decreasing PtfmYaw DOF eigenfrequency from 0.12 Hz (0 RPM and Wind speed = 0 m/s) to 0.09 Hz (12.1 RPM and Wind speed = 10 m/s) for OC3-Hywind Spar.
For (12.1 RPM and 0 m/s) or (12.1 RPM and 10 m/s and PtfmPitch = False) I got 0.12 Hz for PtfmYaw DOF.
Is that a valid behavior and what is the reason for that?
In Bladed 4.4 I can´t reproduce this effect. I wonder if in Bladed there is no PtfmPitch angel for the linearized solution.
Do I need to specify control inputs (CntrlInpt) and wind disturbances ? or is this only necessary when analyzing the controllers behavior?
Is it right, that there is no fundamental difference between FAST 7 and 8 regarding the linearization technique?
I have not performed such an analysis myself, so, I can’t confirm the behavior you are seeing. But it could be that the platform displacements (surge, pitch) induced by the rotor thrust are effecting the yaw stiffness of the mooring system. I can’t comment on the linearization approach used within Bladed.
If you are only interested in eigenanalysis of the state matrix “A”, this matrix is not influenced by the perturbations of the control inputs or wind disturbances, which only effect the “B” and “D” matrices of the linearized model.
The differences in the linearization approaches implemented between FAST v8 and FAST v7 are summarized in the section titled, “Linearization Files” in the FAST v8 ReadMe file: nrel.gov/docs/fy17osti/67015.pdf.
The “README_FAST*” document has a bullet point list of differences between Fast 7.02 and Fast 8.16 in Table 1. You should have downloaded the document when you installed FAST 8.16.
In FAST v8, you can simply to choose to pause the simulation at any time in the time-domain solution and linearize the model at that time. Usually I’d set simulation settings that will result in a steady-state condition e.g. steady wind speed and fixed control. You can review the response time series to ensure that the solution is in a (periodic) steady state (periodic due to the rotor rotation). I would normally linearize at many time steps around a given rotor revolution (e.g in 10-degree increments) from which you can apply MBC3 (for a 3-bladed rotor) and azimuth-average the resulting matrices to arrive at a linear time-invariant model.
