I’m using Aerodyn v15 with FAST v8.16 to generate a Cp surface for various values of pitch angle and tip speed ratio for the NREL 5MW turbine. I’m aware this has been done before and the curves have been made available on this forum, but I would like to set this up myself now with Aerodyn v15.
I’ve created a set of ElastoDyn input files, in each of which the initial rotor speed is set to the appropriate value with regard to the 8 m/s uniform wind speed which I’ve specified, in order to give the desired tip-speed ratio for each simulation. I’ve disabled all of the DOFs in the ElastoDyn input file including GenDOF which of course causes the rotor speed to remain constant at its initial value for the duration of the simulation. This seems to be the recommended set up after reading other posts on the topic on this forum. The pitch angles are then specified via a Matlab script and FAST is executed via the S function in Simulink.
Using the recommended aerodynamic modelling settings for an operational rotor as outlined in the AeroDyn v15 user manual, any simulations I try to run with a non-zero blade pitch angle only complete successfully for TSRs up to around 7.5 (this number varies depending on pitch angle). For example, my simulation at a TSR of 7.5 and pitch angle of 4° fails with the error message:
Error reported by S-function ‘FAST_SFunc’ in ‘OpenLoop/FAST Nonlinear Wind Turbine/S-Function’:
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:BEMT_CalcOutput(node 18, blade 2):Compute_UA_AirfoilCoefs:UA_CalcOutput:Mach number exceeds 1.0. Equations cannot be
By disabling unsteady aerodynamics (setting AFAeroMod to 1) all of my simulations complete successfully for TSRs in the range 3 to 16 and blade pitch angles in the range 0° to 30°. Taking the mean of the last 100 Cp values from each simulation and looking at the resulting Cp curve for pitch angle 0°, several values exceed 0.5. The highest value is 0.5029 at a TSR of 8. This is considerably higher than the 0.482 mentioned here: http://forums.nrel.gov/t/nrel-5-mw-reference-turbine-cp-cq-ct-coefficients/456/1
With unsteady aerodynamics (UA) enabled I do manage to calculate the Cp curve at pitch angle 0°. Here I get a value of 0.484 for Cp_max. This seems like the more valid result.
Is it possible, or even advisable to simulate all operating points using unsteady aerodynamics?
If so, how can I avoid the error I’ve been getting?
Thanks for any help.