I am using FAST to simulate the NREL 5 MW turbine (using Test #18 from CertTest as a baseline) under a variety of loading conditions. I am also using Sandia’s 5 MW blade definition (found here: http://prod.sandia.gov/techlib/access-control.cgi/2013/132569.pdf). For the majority of load cases (ECD, EWS, low-speed turbulence) the computations are working well. However, under higher speed turbulence I am getting errors, like the following, early on in the simulation:
FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 9, blade 3):BEMT_UnCoupledSolve:DeterminePhiBounds:There is no valid value of phi for these operating conditions! Vx = 0.28338, Vy = -20.782, rlocal = 28.159, theta = 0.27689
I understand this to be a fairly general error indicating overall numerical instability. To that end, I have followed the recommendations posted on the topic [url]Initial conditions for tower and rotor] which suggest, at a minimum, having the initial blade pitch and rotor speed appropriately set. The wind case in question is the normal turbulence model (NTM) at a speed of 23 m/s generated by TurbSim. Usually I am using an admittedly coarse grid both spatially and temporally (10x10 grid points and dt = 0.5 s) for the sake of file size and initial testing, but I have also used a finer grid (25x25 grid points with dt = 0.05 s) for this case and it did not seem to help. My initial blade pitch is set to 21.0 degrees and initial rotor speed is 12.25 rpm. Interestingly, if I use the original NREL 5 MW blade properties with initial blade pitch of 0 degrees then the simulation is successful, but the simulation is not successful if I use the original NREL 5 MW blade properties with an initial blade pitch of 21.0 degrees. I believe the blade models should behave similarly with Sandia’s definition being slightly stiffer, although that slight difference seems to be problematic.
I have plotted a few variables from the failed simulation to try to get a better idea of what is going on:
To me, it appears as if the structural simulation is the first to become unstable. That instability is then reflected in the blade pitch control, which is in turn reflected in the overall simulation. I happy to provide more information about this particular simulation to try to diagnose what is going if anybody has any thoughts about additional parameters I should be looking at.
In general, does anybody have any suggestions of additional strategies I could use to avoid these kinds of failures? Perhaps providing initial tower and/or blade deflections would be beneficial for these more extreme cases? I plan to use these simulations for blade optimization procedures, so the reliability of the simulations is fairly important to maintain for a spectrum of blade properties. Perhaps it would be wise to find the steady-state conditions for the original blade design and to use these as initial conditions throughout optimization? I had hoped to not have to go that direction but it wouldn’t surprise me if it ended up being helpful.