Wind Energy

18 W. Bierbooms and D. Veldkamp
3.4 Load Calculations Based on Normal and Constrained
Wind Field Simulations
The (un)constrained wind fields have been used as input for two standard
wind turbine design tools: Flex5 and Bladed. Both packages feature among
other things dynamic inflow, stall hysteresis and tip losses. Logarithmic wind
shear is assumed and tower shadow is taken into account. In Flex5 the NEG
Micon NM80/2750–60 is considered and in Bladed a generic 2 MW turbine.
Both are pitch-regulated variable speed and have a diameter of 80 m, hub
height of 60 m; the rated power is 2,750 kW and 2,000 kW respectively.
As example the time series of the calculated blade root flapping moment
is shown in Fig. 3.3 based on the wind fields of Fig. 3.2. The response based
on the “measured” wind is assigned “measured” load. The 1P (Ω=1.9 rad/s)
response is clearly visible. The mean response of the constrained simulations
gets more detailed, with increasing number of anemomenters / constraints,
and the standard deviation decreases since the wind fields gets more fixed.
Comparison with the filtered “measured” load shows that the constrained
response captures the low frequency phenomena well.
In order to consider fatigue, the damage equivalent load ranges are considered
of the blade root flapping moment (S–N slope 12). The load ranges
and the frequencies of occurrence are found with the usual rain flow counting
procedure.
It is anticipated that constrained simulation results into a smaller scatter
of the obtained equivalent damage. As measure for the scatter the COV
(coefficient of variation) is taken. The convergence of the COV has been
checked by doing 100 simulations for a particular situation. It turned out
that about 20 simulations are enough; the uncertainty in the COV is about
0.1 to 0.2.
Blade root flapping
moment [Nm]
1.5 2
2.5 x 10 6
0.5
0
0 10 20 30 40 50 60
1
x 10 6
1.5 2
2.5
0.5
0
0 10 20 30 40 50 60
1
Time (s)
1.5 2
6
x 10
2.5
0.5
0
0 10 20 30 40 50 60
1
x 10 6
1.5 2
2.5
x 10 6
0.5
0
0 10 20 30 40 50 60
1
1.5 2
2.5
0.5
0
0 10 20 30 40 50 60
1
Time (s)
Fig. 3.3. Generic turbine: Left, top: “measured” flap moment; bottom: filtered
flap moment. Right, from top to bottom: mean flap moment (plus/minus standard
deviation) for 0, 1 and 7 anemometers/constraints. Note low frequency similarity