I am translating to OpenFAST a HAWC2 model provided by DTU of a 25kW upwind, geared, 3 bladed turbine with asynchronous generator and 13m rotor diameter. I am seeing strange behaviour in the power output when I turn on the DrTrDOF. See the figure below for wind stairs from 5 m/s:
The blue curve is with DrTrDOF turned off, and the red is when I turn it on with the values I calculated, and the black is when GenIner is 100x higher than my calculated value. First of all the response is very slow for some reason for my calculated values of DTTorSpr and DTTorDmp. Secondly the power level seems to scale with GenIner.
I am wondering if it is something about the combination of parameters? I have calculated the GenIner as follows:
GenIner = GenIner_LSS/GBRatio^2 = 190.81/23.25^2 = 0.353
and DTTorSpr, and DTTorDmp according to the equations in the post here: How are torsional spring and torsional damper calculated?
G: Modulus of rigidity for the shaft material
d1 and d2 are diameters of LSS and HSS, respectively;
l1 and l2 are lengths of LSS and HSS, respectively.
G [N/m2] 7.93E+10 for stainless steel
d1 [m] 1.20E-01
d2 [m] 6.00E-02
l1 [m] 7.00E-01
l2 [m] 5.00E-01
L = 4.33E+03
DTTorSpr (K)= 3.73E+02
DTTorDmp can be calculated as follows:
Cc is critical damping coefficient =2sqrt*(K*rotor inertia);
ζ is fraction of critical damping or the percentage of damping.
Rotor inertia = 3448.517 from ED.sum
Cc = 2.27E+03
ζ = 0.6
DTTorDmp = 1.36E+03
fixed-free fn (in Hz) = SQRT( DTTorSpr/RotIner )/(2*pi) = 0.05 Hz
The resulting fixed-free natural frequency also seems to be very low, although I don’t have any reference for what it should be for this size and type of turbine, I guess it should be more like ~2 Hz?
Thanks in advance.
Northwind Engineering OÜ